Lathe Tailstock Tooling

[randyc]

Part One

(These are a collection of quick-fix anecdotes written several years ago and mainly intended for entertainment. Because readers vary in levels of experience and knowledge, I'm aiming at the novice. More experienced readers will have devised their own means for performing the operations that I've included. This is a post that I've been working on for a while and it is long-winded. I've tried to break it down into logical sections to avoid boredom.)

Introduction

The tailstock is a necessary but unglamorous feature of an engine lathe. We use it for drilling, reaming, tapping, centering and sometimes as a light arbor press but otherwise give little respect to the cast-iron appendage parked at the end of the ways. A bed-turret ? Now we're talking - THAT is something a home machinist can get excited about, but I get a lot of use from the tailstock.

The best machinists that I've worked for (or with) were characterized by invention in addition to their obvious knowledge and skill. The ability to configure an unconventional setup for a special task … to create a "fix" that reduces time, improves finish or precision … design a special tool for a specific job.

These often make the difference between a successful job shop run by an inventive owner or manager and one that goes belly-up after a year. (I worked in both environments, as a young man. Preferred the shop with a constant pay check, LOL, although frankly I didn't learn as much from the experience.)

After retirement and relocation from a major urban area, I established a home business. My work was engineering consultation, most often proposal work: specification analysis, creating a system architecture to provide the specified performance, estimation of development and production costs and sometimes detailed designs - all electronically transmitted to customers located in the San Francisco Bay Area.

Sounds nice, right ? Clean hands, nothing much to buy except copier paper, LOL. BUT sometimes a customer required measured performance data - prototypical "proof" of the key design aspects on which the system performance might depend. This involved making, assembling and testing microwave circuits comprised of fairly small parts, usually machined from non-ferrous materials (One bedroom of my house has been configured as an electronic test lab). Microwave "circuits" don't normally look like how most of us envision electronics.

Here are two examples of microwave circuits, photographed from a nineteen-sixties textbook. Note that the technology has come a LONG way beyond this and not just by decades ! One aspect of these circuits hasn't changed: the precision required. The two bandpass filters shown below were produced to locational tolerances of +/- .002 and thickness/cavity dimensions of +/- .0005 - fairly typical for state-of-the-art at that time.

These examples are representative of vertical mill operations but lathe operations were also required to produce microwave circuits to these tolerances. These examples were produced long before digital readouts and on manual machinery by skilled operators using techniques that were once common knowledge. (Sorry about the quality of the drawings - my digital camera is the copying machine.)



The following photo depicts the LARGEST single part that I've made in years:



Before relocating, my machine work was subcontracted to quick-reaction (= small) local shops. In my present semi-rural location, those resources weren't available so I had to tool up and resurrect old skills so that I could make the critical parts myself, LOL. The equipment in my small shop reflects my needs (small lathes, small mills, small welders, small inspection equipment, small everything)

But occasionally parts outside the normal capability or work envelope that the machine designers intended are required. I had some machinery and welding equipment before moving but required additional capability (mainly the two mills + tooling). Given my limited floor space - shared with a wood shop - unconventional solutions to some problems were suggested.

"Centerless" Operations

My first "real" machine tool was a used 6 x 18 Atlas lathe purchased around 1968, configured for rebuilding motors and generators. The keyless tailstock "commutator chuck" opened to 3/4 (or maybe an inch) diameter and had bronze jaws that resembled rounded vee-blocks. Journals of motor and generator shafts (or similar rotating workpieces) could be secured in the tailstock, free to rotate. The chuck functioned like a small steady rest but was much quicker to install, required minimal adjustment and produced good results - which gave me the following idea.

For working small diameter centerless ground shafting and the like, a convenient substitute for the commutator chuck (or for a center) is to turn a brass or bronze bushing that fits the tailstock drill chuck then bore/ream a running fit to the shaft to be centered/supported. (It's a good idea to cross-drill the bushing for an oil hole so that the work can be lubricated while turning if the operation involves substantial pressures or lengthy duration. This can also be a convenient means of ejecting short parts, with a jet of air.)

Eventually, I made rotating tailstock chuck using an old Jacobs 18N chuck, a hardened ball and a long oilite bushing. At first glance it looks like a conventional drill chuck. Looking closely, one can observe that the shank of the chuck is actually in two pieces. One of the three set screws that ride loosely in a groove can be seen. The purpose is to hold the two pieces together.



Note that this type of end support isn't normally used for long turning operations, where substantial amounts of taper might result from a slightly misaligned drill chuck. However the practice can be useful for short lengths, when requirements are not critical. In the example that follows, a length of ground shafting is supported so it is possible to fine-tune the tailstock alignment by sweeping the work with a DTI if greater precision is required. Those learning this craft should not be averse to changing tailstock alignment, which may be necessary for many operations - tapering for example - after which, re-centering will be required.

Shown below is a length of 7/16 diameter ground shafting supported in an oilite bushing that I was fortunate to have on hand - didn't have to make a bushing. The end of the shaft was de-burred and lightly sanded before insertion into the bushing. These are inexpensive and convenient to have around the shop - add a couple of them to each order from your normal tooling distributor and a respectable inventory will soon accumulate. The purpose of this particular operation, BTW, was to produce two "O" ring grooves near the end of the shaft. Drill bushings can be even more useful since they don't wear rapidly as do bronze bushings.



Incidentally, this can be a fairly good way to part off and collect short workpieces, instead of digging through the chip pan trying to find them - for occasional work. The bushing in the tailstock fully supports the parted off work so the little "nib" remaining on the work is almost non-existent. The tailstock is run up to the work so that the workpiece is partly enclosed in the bushing (it's unnecessary to clamp the tailstock to the ways - moving the tailstock by hand allows rapid engagement and withdrawal). A little slot or hole in the bushing allows ejecting the cutoff part, if required with a short blast of air.

It's helpful to grind the parting tool with a slight angle. Looking downward on the tool from above, the right edge (closest to tailstock) of the tool should part off the workpiece before the parting tool removes the nib from the stock held in the headstock chuck or collet. I tried to photograph this operation but the details were fuzzy; here's a sketch that might convey the shape of the parting tool better. As noted above, the stock needs to be de-burred before insertion into the support bushing but that should be an automatic reaction during parting operations.



Incidentally, if drill bushings are used in an application like the one shown above, cross-drilling holes for air ejection isn't practical. There are other possibilities for the HSM, the most common is the use of cheap diamond tooling in a die grinder (or Dremel tool). Both 1/8 and 1/4 shank diamond tools are available from most importers at throwaway prices (e.g. $6/dozen).

Measuring From The Tailstock

A simple measurement tool originally constructed for the vertical mill has also been useful in the lathe tailstock. Here's a description, a photo of the tool and the reason for making it. The tool consists of a 1/2 inch arbor (turned eccentric from 3/4 stock), bored to accept the stem of a travel indicator and slotted for clamping with a small socket head cap screw. (The shank must be clearance drilled about 1-1/4 inches deep so that the indicator spindle can travel up into the arbor.)



The original intent was to be able to adjust a workpiece, held in the milling vise - to a shallow angle with better precision than a spirit-level protractor head (when sine bar, gage blocks or angle plates weren't available or when time was limited). The process commences with the workpiece lightly clamped and adjusted to the approximate angle with the spirit level, then fine-tuned by gentle tapping with a soft hammer, monitoring progress with the indicator (it is helpful for the lowest end of the part to be supported in the vise with a small spacer or the end of a parallel, providing a "pivot point"):



The travel indicator is first zeroed at one end of the workpiece, then the "X" axis is offset by a known distance (determined by handwheel graduations or DRO) while holding the indicator spindle retracted by hand (so that it doesn't deflect from friction). Gently lowering the spindle of the indicator until it contacts the workpiece after the table has been offset, the indicator displacement is noted. (Although a "ball end" is shown in the photo, a flat end is OK for this application.)

The angle is determined by taking the arctangent (tan-1) of the two known distances. In the example above, the table has been displaced exactly 2.000 inches, according to the X axis dial count while the travel indicator shows a vertical displacement of .356 so the angle = tan-1 (.356/2.000) = 10.093 degrees.
[For the photographic example, an imported 10 degree angle plate was supported by a pair of imported parallels.

I didn't expect too much from the angle plate but thought that it should be more accurate than the measured 5.5 minutes (.093 degrees) of error. Removing the angle plate, I swept the parallels with the travel indicator and found an error of .0015 (the vise contributes to that error, too). If I subtract the error from the .356 previously measured, I get an angle of 10.05 degrees and an error of 3 minutes which is slightly better. After initially writing this, I looked at the catalog specs of the Enco angle plates and the measured angle is likely typical of the product.]

The example indicates the utility of a simple, inexpensive tool to determine angular accuracy (shallow angles) when one has neither sine bar nor gage blocks. The example also illustrates that imported tools, while seemingly good value for the money, need to be examined carefully when used in a setup that requires precision. (One of these days, I'll repeat the test using a sine bar and gage blocks, as an exercise.)
(Oh yeah … when the correct tangent reading is obtained, the vise is tightly clamped, checking one more time to be sure that the angle is correct.)

I've found other uses for the indicator/arbor … like setting stops on a drill press or precisely adjusting quill travel stops on a milling head. The indicating tool allows fairly accurate machine settings while installation/removal is quick and convenient. One of the most common uses is checking the level of work installed in the mill vise - slip the tool into a 1/2 collet and quickly run it front-to-back across the workpiece to check that it hasn't lifted enough in the vise to be problematic. (Any DTI can do this; it happens that the length is convenient for the purpose.)

Another application for this tool - similar to the last one - is setting a parting tool parallel with cross-slide travel. The measurement tool is installed into the collet/chuck in the headstock, and the cross-slide moved while the runout of the parting tool is observed with the indicator which rests along the top edge of the blade, adjusting the blade until there is no runout before clamping.



Sorry for the divergence - returning to topic, a convenient application for this device is in the lathe tailstock chuck. In above paragraphs discussing parting, I was reminded of how handy this measurement tool has been when parting off rods of various lengths in small quantities. The indicator is held in the tailstock and zeroed against the freshly parted face of the stock material (carriage is locked throughout all of the parting operations). Loosening the workholder (headstock chuck) slightly, the stock is extended until the indicator reading is the desired part length + the width of the parting tool. After tightening the 3-jaw chuck, the tailstock is retracted and the piece parted off to a precise length.



This is most useful when a number of parts of differing lengths are required and the tailstock is not required for supplementary operations such as drilling. If a number of parts of identical length are required, there are simpler means of performing the parting operation. In fact, extending that concept to its logical conclusion, a drill bushing secured in the chuck could provide both production stop and work support during parting.

Reviewing the sketch shown previously concerning the parting tool and the tailstock bushing supporting workpiece conveys the idea, I hope. (In addition to deburring the leading edge of the workpiece it's necessary to vacuum or blow out the bushing from time to time, freeing any small chips that may have inadvertently found their way inside.) Both operations can be performed simultaneously with a puff of shop air if a small hole is cross-drilled near the tailstock end of the bushing.



In short, an indicator tool like the one described, whose spindle is exactly in line with the 1/2 diameter shank, can be useful several times a day. Imagination is the only limit and I'm still finding applications for this simple device.

Tailstock As Drill Press

Securing, guiding and moving tools and workpieces has always required imagination combined with skill and experience. In cramped quarters, such as tiny workshops in small deepwater ships, sometimes only an engine lathe with a milling attachment can be accommodated so the drill press function is often performed by the lathe.



A friend was a machinist's mate on a U.S. Navy destroyer escort (during the Vietnam thing). I've enjoyed and learned from his anecdotal experiences of repair operations performed using a 10 inch South Bend, a milling attachment, welding equipment, hacksaw, chisels and files and his personal ingenuity. Some of the notes in this discussion resulted from conversations over a pint of beer or two.

Tapers

Short tapers can be easily cut using the compound-slide although there might be more unsupported tool overhang than we'd like. Long tapers are usually turned by offsetting the tailstock if the lathe has no taper attachment. Many newcomers to the craft don't care to disturb the alignment of their tailstock but only a few moments are required to re-align the ram by sweeping the Morse taper bore with a DTI held in the headstock (provided that the lathe ways are not overly worn and that the headstock is properly aligned).

Similarly, it's not difficult to offset the tailstock using a travel indicator to determine the proper offset (after first determining the length of the taper and solving for the tailstock offset). Use the appropriate trigonometric function of your pocket calculator to calculate the offset: tangent if the centerline length is known, sine if the tapered length is known. (If the travel indicator is left in place and not disturbed by the cutting operation, the tailstock can be returned to original position, after the taper has been turned, in less than a minute.)

Note that the "coincidence" of tailstock and spindle is a complex definition. Concentricity, axial/angular alignment of the two largely depends on where the tailstock is located, along the ways and how the machine is established on its foundation. This may not concern home machinists … because the tolerances we need to produce, the age of the machinery that we own and the lack of a production schedule allows us to take our time and produce acceptable fits with mating parts in the mechanisms that we design or modify. The alignment of headstock to tailstock to carriage has been discussed many times, most recently in this thread:

http://www.hobby-machinist.com/threads/two-collar-test-how-close-is-close-enough.31435/

In some setups, a hardened and ground steel ball is a better center than the standard 60 degree configuration provided that the cuts are not heavy. An expression frequently used when describing a tailstock offset to turn a taper is that the tailstock center "wallows out" the center-drilled hole, distorting the configuration. For shallow angles, this may not be critical but for larger angles a hardened ball can be a better idea.



In the above arrangement, a tapered arbor has been bored for a standard drill bushing (the drill bushing clamping method isn't shown but will be discussed later). By changing the size of the inexpensive drill bushing, various sizes of hardened balls may be accommodated, depending on the size of the workpiece. This is a versatile tool because drill bushings can also be used to support long workpieces (as described in the "centerless" paragraph above) when it's not possible to use a tailstock center. Both drill bushings and hardened balls can be readily and inexpensively obtained from your normal tooling distributor.



Many ingenious ideas have been implemented for offset taper turning. The most common seems to be replacing the normal tailstock center with a boring head. With a small 60 degree straight-shank center installed in the boring head, adjustment of the taper offset with precision is not difficult. Except for the large diameter fine pitch threads, it's not particularly challenging to make an arbor for the boring head with the appropriate taper for the lathe tailstock, keeping a couple of points in mind.

The possibility of the entire assembly rotating in the tailstock must be considered and some means to prevent this should probably be devised because the load imposed by the cutting tool may encourage torsional movement at the tailstock. Rotation isn't as likely as another problem: flexing of the center, which isn't good for accurate work. If the center is offset from the ram - as it will be when held in a boring head - the center will not be located along the load axis and deflection is unavoidable. It may or may not be a problem, depending on the accuracy required, material, cutting load and the load imposed by the tailstock ram.

Boring heads are designed to support centrifugal displacement due to their mass, the negligible weight of the cutting tool and a modest cutting load - they aren't intended to replace the function of the tailstock center, especially when supporting heavy weights combined with heavy DOC and feeds. I prefer the previous arrangement (tailstock ball center and offsetting the tailstock) because it avoids all of these problems.

As mentioned previously, adjusting tailstock offset and restoring it on center is not a difficult or lengthy procedure using a DTI. If one keeps a test "dumb-bell" handy, it's literally a matter of a minute or less. A test dumb-bell is a workpiece that has been turned between centers to align the lathe or the tailstock. If the two ends are exactly the same diameter - as they should be - then moving the carriage with DTI attached, comparing the two diameters allows quick, accurate centering of the tailstock. (This was discussed extensively in the post that was referenced above.)

Thread Chasing

One of the most useful tailstock applications for small lathes is chasing threads (small diameters and/or long lengths) although this is a function more often performed by turret lathes and speed lathes. The shop-made tool shown below passes over the carriage with adequate length to work up to the headstock. Short pieces of steel rod, salvaged from under the bandsaw, are welded to both ends of a length of steel pipe.

One end is turned to suit the tailstock taper and the other end is turned concentric in the same set-up. Two opposed slots are milled through the pipe, just behind the die head location, to allow clearing chips. I performed a primitive stress relief in the kitchen oven after welding and before finish machining the tool. (Three cycles from room temperature to 400 degrees F, allowing plenty of time to stabilize at temperature and plenty of time to cool.)

The headstock end is bored so that the die is centered on the lathe spindle when the tailstock is properly aligned with some clearance (provision is made for variability in inexpensive hex dies by including three adjustment screws). A crossbar is attached to the tailstock end, to prevent rotation and slipping in the tailstock. The tool produces threads many times more quickly than single-pointing, utilizing standard, inexpensive HSS hex dies for thread diameters up to 1/2. The adjustment screws included for centering the die are rarely used - I prefer a little float.



This is a simple tool for a simple task; I usually thread at 120 RPM (using pipe threading oil), pushing the tailstock by hand which is easy on my small Emco "Compact Eight". I've encountered no problems using hex or circular dies to produce full depth threads up to 1/2 inch diameter provided that the major diameter is turned to the low side of the "unified" thread standard tolerances. Most of my previous chasing work was nonferrous although after retirement, it is mostly mild steel.

FWIW, a rough rule of thumb for turning the major diameter is 2% below the nominal diameter. In other words, multiply the nominal diameter by 0.98 to obtain the diameter to which the rod must first be turned. For example, 1/2 - 20 UNF threads require that the nominal diameter be turned to .50 x .98 = .490 diameter before chasing the threads. This will minimize stress on the machine and wear on the die.

In use, the tailstock is not locked to the ways and is fed by hand (pushing on the rear of the tailstock). It will self-feed almost immediately after die engagement and only slight pressure on the rear of the tailstock is required to overcome friction and to prevent the taper from disengaging. It's only half as fast as a turret die head because the tool has to unthread (in reverse) over the work, instead of snapping open the dies when chasing is completed. (It's quicker to disengage the tailstock from the die and spin the die off by hand.)

I made this tool about ten years ago and each time that I use it, I wonder why it took so long before I thought of it. It is my next most often used tailstock tool, the drill chuck being the first and a dead center being the third. The 3/8 rod that prevents the tool from spinning in the tailstock has been modified since it was photographed to add a roller at the end so that the tool rolls along the vee way.

I've purchased several inexpensive die holders of different diameters for round dies; these are intended to be held in tailstock drill chucks, they are OK for their cost and for non-critical applications requiring short lengths of threads. (Most don't come with a means to prevent rotation and slippage so tack-welding a short rod to them is useful.) I think that these two came from Enco.



For electronic work - particularly microwave electronic work - small threads are the rule and threads from #0-80UNF (.060 major diameter) up to #4-40UNC (.112 major diameter) are the most common sizes in use at the component level. These are not easily single-pointed due to workpiece deflection so die-threading is the universally preferred method.

Hand Rotating The Spindle

This isn't really on topic but the thread chasing discussion reminded me that - when single-pointing external threads without a relief groove - I've been accustomed to rotating the spindle by hand for the last revolution or two. This is a personal routine, useful when forming tools or any other operation requiring a large amount of contact between workpiece and cutting tool is required, mainly because of the limitations of my small lathe.

A small diameter, shallow-depth, cross-drilled hole is also a neat way to terminate a single-pointed thread crisply - I believe this was illustrated in the original South Bend lathe book, many decades ago. By hand-rotating the spindle, it's possible to control the pull-out of the threading tool to any practical degree of accuracy, although the chip pile-up at the end of the cut may need to be removed after each pass (unless the cross-drill technique is used).

(Even simpler is starting the threading tool at the end of the thread, reversing the leadscrew rotation with the cutting tool upside-down and threading toward the tailstock. Because of the simple design of my smallest machine, an additional change gear is required to reverse leadscrew rotation so I have lost the habit of threading in reverse. I'm a thrifty guy, my little lathe has served me well for decades and old habits - if successful ones - aren't easily discarded when there is no production pressure. So the Emco 8 x 18 is still the most often-used lathe in my garage shop.)

There are other operations where hand-rotating the lathe spindle can be useful so I made a simple tool that engages the spindle bore (to ease the procedure for my physical comfort). I must exercise care when rotating the spindle by hand with this tool for safety considerations. If the lathe was inadvertently powered up during the hand-rotation process (however unlikely) there probably wouldn't be a problem. The 1/2 ratchet wrench shown in the photo would free-wheel but if the lathe was accidentally powered up in REVERSE a very dangerous condition could result !



The tool used in the above photo to rotate the spindle was described in detail in a previous thread.

Reaming

One way to utilize large reamers on a small lathe is to use a dead center in the tailstock, engaging the center in the shank of the reamer. (Chucking reamers are very convenient but aren't likely to be concentric with the spindle axis when secured in a drill chuck.) A lathe dog - or any similar expedient - bearing against a solid surface (like the inside surface of the ways), prevents the reamer from rotating as it is fed and guided by the dead center.

The tailstock center must be precisely aligned with the spindle centerline, throughout the travel required, if consistent, accurate results are expected. I made a simple tool to align, or confirm alignment, of a dead center in my tailstock. Aligning the tailstock takes less than a minute provided that a means of reference is available (the reason for making the tool).



Note that chucking reamers have a very long shank - to accommodate slight misalignment between tailstock and workpiece. Engaging the leading edge of the reamer in the workpiece, with the tailstock snug but not fully clamped to the ways, the lathe is activated at a low spindle speed.

While feeding the reamer (pushing the tailstock by hand into the workpiece) the other hand is free to brush coolant on the reamer and to power down the lathe when the reamer reaches required depth. Which reminds me - not for the first time - that a foot switch would be a convenient feature for certain lathe operations - for small lathes - larger ones normally have foot-operated spindle clutches.

The setup below is similar - the dead center is engaged with the center hole in the chucking reamer shank and the ram is used to feed the reamer into the work. One additional step has been included to help the reamer start and run true. Just behind the reamer flutes, a small steady rest supports the shank, aligning it with the spindle axis.

The steady has been adjusted to center the reamer by indicating the reamer flutes with a DTI until the tool is concentric with the spindle. This setup takes a while to establish and isn't justified in most situations. In my opinion, taking the time to set up a reamer in this manner is wasted unless the pilot hole has been bored - a drilled pilot hole won't be coaxial with the spindle centerline and the reamer will try to follow the drilled hole.

In the photo below, a reamer is prevented from rotation by securing the shank in a 5C collet block with a scrap of 4 x 4 resting on the ways of the lathe under it. Clearly there are dozens of ways to do this, depending upon tools and materials on hand and the time available.

In a hurry, one might be tempted to reach for a pair of vise-grips to clamp the shank of the reamer, resting the vise-grips on a scrap of wood laid across the ways. If I paid for a quality reamer, my inclination is to grip it in a manner that doesn't deform the shank. When chip load is light - and it always should be - substantial gripping force is not normally required.

Parts Two and Three to follow

 

[randyc]

Part Two

As mentioned, a reamer is better used to dimension a bored hole rather than a drilled hole. The reason for this is that the reamer will always follow the pilot hole and a drilled hole can be off axis while a bored hole will always align with the axis of the spindle.

By personal preference, I allow very little clearance for the reamer (depending on diameter and length of hole and how it was produced), feed it rapidly by hand by pushing against the tailstock and liberally flooding the tool with lubricant (s-l-o-w RPM but rapid feed). BUT I am not a machinist by trade and my machining work has been restricted to small diameters - most frequently in nonferrous material.

There are many opinions about material allowance for reaming - common sense is your best guide. If you are using a drilled hole, a considerable amount of material might have to be removed to make the hole round and remove scoring from the twist drill. A bored hole requires a minimal amount of material allowance - just enough to allow cutting (rather than burnishing) and this will depend on the material alloy.



Since the shank of the reamer is not chucked, the tailstock cannot be used to withdraw the tool from the workpiece and that's not necessarily a bad thing for precision work. In the above example, after the reamer has been fed to the required depth, the lathe spindle rotation is stopped and the reamer can be carefully withdrawn by hand, using whatever means was employed to prevent tool rotation (the square collet block, for example, in the photo). Good practice includes rotating the spindle - also by hand if a small lathe is used - while retracting the reamer.

This is tricky stuff and the opinions about how best to produce a reamed hole are many - some insist that it is not possible to produce an accurate, straight hole with a reamer.

Reamers are a quick way of accurately sizing a small hole previously bored in the same setup, insuring that the hole is concentric with the spindle centerline. The accuracy could be achieved by making repeated passes with a small boring tool but might take a while due to deflection of the tool and the need for making many precise bore measurements.

Bore measurements are subject to error, especially for small diameters and they are dependant on the gauging technique. If there are variations in the hardness of the material being bored, be aware that the bore dimension will vary accordingly. This can be encountered with some corrosion-resistant alloys and also with cast materials.

Performing small boring work on an old machine can be especially frustrating when working close to the headstock. Because the ways are typically "scalloped" in this area, it may not be possible to bore to size and to full length. Using a reamer, one need only make the bore to establish CONCENTRICITY, not to establish dimension. (Bore slightly undersize then let the reamer establish diametrical accuracy.)

Thread Tapping

A similar method (to reaming) is employed when tapping under power. In this photo the tap wrench is located on spindle axis by the tailstock dead center, the sliding handle is adjusted to contact the lathe ways, preventing the tap wrench from rotating - some prefer allowing the handle to rest against the inside surface of the ways. (A scrap of wood is normally placed between the tap wrench handle and the lathe bed - I omitted this step when taking the photo. This setup allows a considerable amount of "float" which might be desirable for some parts.)



For large taps, a tap wrench is unnecessary - the tap is centered with the tailstock dead center. An appropriately-sized wrench is secured across the flats of the tap, resting on a wood block across the lathe ways to prevent tap rotation. In all cases, the lathe spindle is rotated as slowly as the operator considers safe (if the machine doesn't have a back gear, the spindle can be rotated by hand).

Using drill chucks to hold a tap in the lathe tailstock, however convenient, isn't good practice. The likelihood of a tap spinning in the chuck is probable and it's not likely that drill chuck jaws are concentric to - or axially aligned with - the spindle axis.

Axial/concentricity errors in a large (1/2 inch or 5/8) drill chuck may or may not be important, depending on the tap size. If the thread size is small (#0 - 80UNF up to maybe #4 - 40UNC) then the likelihood of breaking the tap AND ruining the workpiece is high. Larger taps held in a chuck that isn't concentric probably won't break but the quality of the tapped hole will be only as good as the quality of the tap drill held in the same (or similar) tailstock drill chuck regarding concentricity and straightness. One might consider performing an accurizing operation on drill chuck arbors as described in a previous thread.

Broaching

Keyway/slotting operations are sometimes mentioned as operations performed in a home shop with an unpowered engine lathe. These are "shaping" operations with a HSS tool, ground to size and configuration, in the lathe toolpost. It's a useful expediency when one lacks other tools for producing a keyway, splines or the like. Stressing the headstock bearings is possible, however so perhaps this operation should be reserved for soft materials or when alternatives are really limited. (It's theoretically possible to brace a chuck against broaching pressure, preventing the load from being transferred to the headstock thrust bearing but this might be risky. Note that the support mechanism might do as much harm to the headstock as the broaching operation.)

The operation is usually performed by taking slight cuts, feeding with the carriage handwheel with the lathe disabled and spindle secured from rotation. This can be time consuming but the process can be speeded up if the tailstock ram is used to "assist" the carriage, allowing heavier cuts. The tailstock is designed for high axial pressure while the carriage isn't optimum for heavy loads on small machines. Here's an example …

My shop-made boring bar accepts 1/2 inch shank standard brazed-carbide tools. A 1/2 inch adaptor, slotted for 1/4 inch square HSS lathe tool bits, took only a few minutes to make and that size (1/4) is convenient for many broaching chores because not much excess material need be ground away for narrower slotting tools. Here's a photo of tool bit, adaptor and boring bar:



The following photo shows the tool bit cutting an internal keyslot. This is a l-o-n-g process on my small lathe. I can feed about .003 inches per carriage stroke and even then have to take a half-dozen quick "spring cuts". This is tedious, even if only one part is required and the part is aluminum.



This photo is the same operation except the tailstock assists the carriage so that heavier feeds can be made. The tailstock ram, resting against the rear of the boring bar, is cranked to exert axial force while the carriage is hand-fed with just enough pressure to accommodate tool movement. The screw threads of the tailstock ram have more mechanical advantage than the carriage gearing and might be cheaper to replace in the event of breakage. The technique is helpful for another reason - because the tailstock pressure is axial, there is less tendency to push the boring bar away from the cut, which occurs when the cutting load is provided only by carriage feed.



Cranking the tailstock ram to leverage the cut allows feeds as heavy as .020 to be made in my small lathe. The number of spring cuts may increase but they do not require the tailstock for assistance and don't take much time. A rough estimate is that the shaping process is speeded up by a factor of three. On large lathes, with beefier carriages, perhaps one might not achieve a substantial improvement. In any case, I prefer that the tailstock acme threads impose the load rather than the carriage gearing. A great deal more area is engaged in the acme threads compared to the "line-contact" of the carriage gearing, greatly reducing unit stress.

NOTE that sharpening and lubricating the cutting tool properly is even more important for shaping/broaching as for turning, although that might be overlooked. The surface finish of an internal keyway may be of little importance, however the technique could also be used to produce sliding splines - external and internal - for example. For that purpose, the finish might be very important. Surface finish can be no better than the tool used to shape the workpiece (unless considerable supplemental handwork is justified). Careful dressing of the cutting tool will produce a nicer finish, requiring less pressure and extending tool life.

A MUCH better and easier method is to use a rotary broach as described in the following thread:

http://www.hobby-machinist.com/threads/a-shop-made-compact-rotary-broach.32219/#post-272748

Tailstock As Work Surface

(The following resulted mostly from discussion with an old friend who worked as a machinist onboard a small naval vessel back in the early seventies.)
A useful piece of tooling (in a small, tool-deprived shop) is a plate that can be installed in the tailstock perpendicular to the spindle centerline. If the only machine tool available is a lathe, then this tailstock worktable is useful for drill press operations and boring of irregular shapes and can be fabricated in an afternoon. Braze or weld an arbor (purchased or shop-made to fit your tailstock taper) to a steel plate. (The arbor should be turned to fit a hole previously drilled in the steel plate, shouldering against the back side of the plate.)



Most lathes have a Morse taper in the headstock (and in the tailstock) for centers. One of the first pieces of tooling acquired for a small lathe should be a taper adaptor that adapts the tailstock taper to the headstock taper. The easiest way of truing the steel plate shown above is to install the taper adapter over the workpiece. (The photo is of the completed work table with adaptor installed over the tapered arbor - imagine that the plate is unfinished, possibly warped and definitely not perpendicular to the lathe spindle centerline.)



Place the assembly + adaptor into the headstock taper and apply pressure from the tailstock center to lock the tapers then face the plate with the cross-slide. The perpendicularity of the plate to the lathe spindle will be as good as the capability of the machine and the taper adaptor (and can be improved with hand work).



The above photo was taken from the tailstock end of the lathe. The brass rod between live center and workpiece applies just enough axial pressure on the work to insure that the tapers remain locked - there is no other purpose. Because the workpiece was too large to swing over the small lathe carriage, a normal cutting tool couldn't be used so a boring bar was used to face the part.

The little table can be a useful fixture for miscellaneous odd-configuration work, drilled/tapped or slotted as necessary to clamp workpieces to the plate. Faceplates (mounted on spindle) were once a common means of performing precision drilling and boring operations for those lacking a jig-borer. Faceplates required finicky setups to balance them (or else the operations needed to be performed at low speeds) but were capable of fine, precise work.

Because the tailstock fixture doesn't rotate, there are no out-of-balance concerns (with the exception of a boring bar mounted in the headstock). Before owning a drill press (a milling machine was not even in my dreams) a primitive tailstock "table" like this one was useful for drilling workpieces clamped to the table with drills secured in a three-jaw chuck in the headstock or in a Morse taper adaptor. Additionally, some drilling/boring operations involving work too large to swing on a faceplate could be performed by mounting to the tailstock plate.

A boring bar is east to set up in a small 4-jaw chuck. Provided that precision under .0005 isn't required, the boring bar can be adjusted with the chuck screws, using a DTI held in the toolpost to monitor the movement of the cutting edge of the boring bar. Surprisingly good work can be done in temporary set-ups like this - especially when a lathe is the only tool available !

A scrap of wood (or a similar precaution) should be used to prevent tailstock table rotation since the arbor, ways and/or tailstock taper could be damaged if the arbor spun. However, one shouldn't be unduly concerned about losing orientation from minor rotation since all operations are performed with tailstock and headstock aligned and cutting tool concentric to the spindle axis. In all operations, the tailstock ram should be snugged to take up clearance while still allowing the workpiece to be "fed" by the tailstock handwheel.

When drilling from the headstock, the tailstock taper will usually be locked from axial feeding pressure. When boring operations are required, the table should be secured to prevent the taper from becoming unlocked from machine vibration. (There is little axial pressure imposed on the tailstock from the fine feed usually associated with boring.) This doesn't have to be sophisticated - short bungee cords stretched between table and tailstock will do an adequate job of preventing the taper from vibrating loose.



In the above example, I could have used a faceplate to drill the 1-1/4 inch eccentric hole but inserting the "table" into the tailstock was many times faster than removing the chuck from the headstock, mounting the faceplate, clamping and balancing the workpiece then re-mounting the chuck after the operation was performed.

Note that the tailstock table is an unusual contrivance these days. Most home machinists have tools far beyond the capability of those that my old friend, machinist's mate on the destroyer escort, had to work with. I'm very happy that I no longer have to use expediencies like these but they were once very useful to me.

CAUTION ! The tooling was "posed" for the above photo. An actual operation would have involved successively larger drills until the desired diameter was produced. More importantly, there is no back-up material shown in the setup. A backup must always be secured behind the workpiece for several reasons. One reason is so that the worktable is not damaged by the cutting tool, LOL. But the primary concern is SAFETY - when a large diameter cutting tool "breaks through" the workpiece, there is a substantial amount of torque applied to the work and the entire setup can be torqued, loosened and trashed in a few milliseconds. It should also be obvious that the operator can be injured from the resulting crash !

Not only in this particular setup but in all setups, one needs to be aware of this problem. When the tool is completely engaged with the work, all is well and the process is almost self-correcting when feeding by hand. When interrupted cuts or break-out cuts are involved, particular care must be exercised to prevent the tool catching the work with catastrophic results ! Breakthrough scenarios require that the feed of the tool be greatly reduced, "feeling" your way carefully through this most critical part of the operation.

Chances are that many setups, tools and workpieces are damaged by inattention to the end portion of the machining process - the breakthrough. A backup plate can reduce the possibility of spin-out, a hardwood or MDF backup is adequate for drilling; for precision work, like boring, an aluminum backup would be preferred.

Another use for the tailstock work table might be to "float" a small reamer so that it follows a hole that may not necessarily lie on the axis between headstock and tailstock. There are many ways of doing this and a simple one is to hold the reamer in a toolmaker's vise, holding the vise flat against the worktable while feeding the reamer with the tailstock handwheel using a very slow spindle RPM.



A toolmaker's vise usually has a vee-groove precisely ground into the jaw - sometimes two grooves - orthogonal to the ground exterior surfaces. Vee-grooves can be used to position the reamer square with the vise surfaces (might be useful to wrap a short length of copper or brass shim stock around the shank). By holding the vise securely against the tailstock table, the reamer can "wander" a bit from concentricity but still be fed squarely against the work. Note that a block of wood rests on the ways underneath the vise, just in case it is dropped.

When using this technique, a smaller vise is preferred so that it's easier to "feel" lateral reamer movement unless the possibility of spinning exists (such as when cuts deeper than a few mils must be made). With the four inch vise shown above, the length rests against the 4 x 4 on the ways of my small lathe and the vise cannot rotate.

It's possible to perform good work with this simple tailstock arrangement; the limits are mostly the capability of the operator. At one time, South Bend produced a similar (smaller?) tailstock fixture called a "crotch-drilling post" or something like that. It was designed for cross-drilling round stock but useful for many drill press operations and gave me the original idea for making a tailstock table.

 

[randyc]

Part Three

Other Useful Tailstock Tools

FAST metal removal = Morse taper drills for the tailstock - lots of performance at modest cost. Silver and Deming (the trademark description) drills have the potential for fast metal removal but their reduced shanks (1/2 inch diameter) are subject to slippage. Morse taper shank drills become more tightly locked as axial pressure is increased - tailstock drill chuck shanks can lock under pressure but the gripping jaws have no such capability.

A headstock/tailstock Morse taper adaptor allows these drills to be used under power in the headstock, too. Here's a photo (without a lathe dog attached to the shank of the drill to prevent the unlikely possibility of the drill spinning in the tailstock). If there is adequate horsepower and no drive slippage, these drills are the best means of removing CUBIC INCHES of material quickly in a home shop ! (Lack of drive belt friction - rather than horsepower - limits the use of the larger drills in my lathe.)



Another useful tailstock tool is a "bull center" (large live center), shown in the following photos steadying a length of heavy wall aluminum tubing. These are helpful when standard steel pipe needs to be worked (de-burring the I.D. first). The bull center is also a convenient means of centering a workpiece when adjusting a steady rest or when truing workpiece O.D. for a steady rest. Often the tailstock obstructs other operations that must be performed on the tube or pipe (facing or boring) and the steady rest becomes the only practical solution.





A bull center requires that bearings are appropriately scaled to suit larger diameters and cutting loads. If only light cutting is required or only a centering function (adjusting the steady rest, for example) then it is practical to make a slip-on accessory for a small live center. At one time a number of lathe center products manufactured in Poland were available and of excellent value. I believe that they were sold under the "Bison" brand.

Occasionally it's desirable to turn a one-off part with no obvious means of chucking or centering. An example might be making discs from sheet metal (even elliptical parts in special cases). In the following exercise, a roughed-out sheet metal workpiece is clamped between two pieces of aluminum rod - one piece chucked in the headstock, the other supported by the tailstock center. The tailstock part, on the right, was first centerdrilled on the far side to accommodate a live center before being parted from the stock material.



In the next photo, a piece of sheet metal, roughly band-sawed to shape is clamped between the two pieces of scrap - clamping pressure is exerted by the live center, from the tailstock ram. The part is starting to acquire a circular shape:



The finished part:



Some might question the use of this technique rather than simply parting the discs from bar stock - a reasonable point, considering the size of the disc arbitrarily used in the above example. However, it's not uncommon to produce much larger discs using this technique - diameters that are too large for reasonable parting operations - three to five inches. Also, thin parts sometimes acquire a "cupped" shape because of the parting operation although this can sometimes be useful for their spring characteristics, LOL.

There are other applications: the guy who taught me this method repetitively made ten discs simultaneously in a similar setup. Punched sheet metal blanks, dropouts from another product, were stuck together with double-back tape (carpet tape), clamped in the "spud" setup above and turned together to precise outside diameters. (The parts were separated by immersing in a solvent.)
The same technique can also be implemented for irregular shapes, such as turning this aluminum bearing cap from sawed rectangular bar stock:



After turning, the part can be reversed, chucked on the finished diameter so that an o-ring groove can be trepanned and then blind-bored for a bearing. Four mounting holes need to be drilled on the flange and the small part de-burred. Note that the mounting flange is too thin to support the part in a four-jaw chuck securely.



When using friction to secure a part for turning, it's necessary to take light cuts using high spindle speeds to prevent the tool from digging in and trashing the part and the setup. If the "spud" setup is to be used more than once or twice, scraps of sanding belt can be epoxied to the clamping faces, grit surface facing outward (engaging the workpiece). This will greatly enhance the holding power of the setup and allow higher feed rates with less chance of slippage.

I haven't used this technique except when working softer materials (e.g. aluminum, brass, plastics, zinc alloy castings). It mightn't be unreasonable to turn steel parts in this manner but possibly troublesome to de-burr - not necessarily problematic but maybe a rock tumbler could be helpful.

Other Interesting Tailstock Setups

In the early seventies, a job requiring blind-drilling 3/8 diameter holes in aluminum bars to a depth of almost two feet was bid and won by the machine shop where I worked part-time. The owner of the machine shop devised an interesting solution. He removed the tailstock ram mechanism from a Clausing lathe and made a steel sleeve to replace the ram. The sleeve was about twelve inches long and could be secured with the ram lock. The sleeve was clear drilled through, then bored at each end to accommodate a standard 3/8 hardened drill bushing.

Workpieces were set up in the lathe (with a cat's head and steady-rest). A shallow starter hole was bored (for spindle concentricity) from the carriage. With the spindle rotating, a long "electricians drill (?)", chucked in a 3/8 drill motor, was fed through the tailstock sleeve drill bushings by hand and the two foot long hole was drilled quickly, with "grunt" applied by the operator standing behind the tailstock.



The hand-drill allowed rapid feeding and withdrawal of the drill to clear chips, coolant was applied at the end of the workpiece in the drill path. Since the tolerance requirements didn't justify a slow, expensive gun-drilling operation, the setup that my boss devised was cost-effective and we were able to produce a number of these long parts economically using an apprentice-level operator (me).

More importantly, the setup design allowed the use of a normal toolroom lathe (3 feet between centers) rather than a six foot lathe. That setup has stayed in my mind for a while because of three or four good ideas that were suggested by a very good machinist !

An Imitation

I copied the previous idea for a different operation just a few years later when I was working for a tiny (five person) start-up company in San Jose. The company manufactured microwave voltage controlled oscillators and was experiencing a temperature stability problem with one of the products. A caucus decided that a possible solution was to make some of the frequency-determining coils from an alloy called "Invar". (If you are not familiar with this material, it is an iron-nickel alloy that has the unique characteristic of dimensional stability over a very wide range of temperatures.)

The problem was that the material could be obtained in limited sizes (1/4 and 1 inch diameters, as I recall - wire being unavailable). What was required was something around .025 diameter. I made a contraption that replaced the ram in the tailstock of our SB 10L model shop lathe. It consisted of a length of brass rod, turned to the same size as the ram and clamped in the tailstock with the ram locks. A hole drilled through the rod accommodated a four foot length of brass tubing (obtained from a local hobby shop) which was soft-soldered to the rod. Into the brass tube was fed a four foot length of teflon tubing ("spaghetti tubing"). Sorry if this isn't completely clear but perhaps it will be when I describe the actual operation.

This is the basic set-up although several details are not shown because the sketch would become very "busy". A six foot length of 1/4 diameter invar rod is fed through the spindle bore and secured with a collet, as shown in the following sketch. A follower rest (which is not shown) is adjusted to support the invar rod. Also not shown is a support made of plastic that supports the invar rod at the back of the spindle so that it doesn't "whip".

The objective is to turn the diameter of the 1/4 rod down to about .030 diameter in a single pass and accumulate a length of invar wire not less than four feet long.



The operation commences by turning a short length of the invar wire to a length of several inches after which the lathe is powered down without disengaging the carriage feed. The tailstock is then carefully moved up and the loose, turned end of the invar rod fed into the teflon tubing (contained within the brass tube). The tailstock is not locked at any time during this process. The lathe is powered up and the tailstock is gradually pushed by hand to steady the turned wire behind the cutting tool as the carriage advances.

Note that the brass/teflon tubing was simply a guide that supported the rotating wire and prevented it from breaking as it slowly fed into the guide. Ideally, the guide would also have rotated along with the spindle (like a glass lathe) but that was beyond my ingenuity and the time constraints at the time, LOL.

Although the sketch implies that the operation was continuous, it actually took place in separate operations, turning about one foot at a time then powering down the lathe without disengaging carriage feed. I found experimentally that one foot was the maximum unsupported length extended from the collet (even with the follower rest supporting the 1/4 rod). Any length much beyond that caused the invar rod to begin to "whip" and the part was ruined.



After the maximum length of wire had been turned, it was clipped at the cutting tool. The free end was then carefully extracted from the teflon/brass tubing guide and taped to a wood lath. After a half-dozen or so lengths were turned and taped to the lath, they were sent to the plating shop for nickel followed by hard gold plating. There was quite a bit of breakage but the result was reasonably satisfactory.

Burnishing

Around 1972, a modification of 100 parts (drawn and coined from "four-nines" pure silver - 0.9999 % - varying from approximately .090 diameter to .500) was required overnight by my employer for an "instant" engineering evaluation. The length of the parts needed to be slightly reduced and a flat, high quality surface produced on one face - superior to the coining finish provided by the manufacturer. (The quick-reaction machine shops to which I normally contracted this type of work could have produced the work easily but not overnight !)

It was simpler to do the work myself since I had small model-shop capabilities for small work in my large and very untidy office. The pure silver was difficult to work, tearing easily, and the operation was difficult because contaminants (ANY coolants) were prohibited. What I finally ended up doing was as simple as I could imagine and got me home before midnight.

After grinding a small tool (tinkering with angles and lathe RPM for an hour or two) I faced each part slightly oversize in the tiny lathe then burnished the parted face with a highly polished drill rod held in the tailstock drill chuck. About .002 tailstock travel applied at finger pressure burnished tool marks left by the parting tool and produced a smooth, flat surface. (For admirers of Emco-Maier machinery of the sixties/seventies, these operations were performed on a "Unimat" - useful for working the tiny precious-metal parts encountered at that particular job.)

The polished drill rod was "subcontracted" by the way, to the wafer fabrication folks down the hall from me. They were accustomed to lapping better than mirror finishes on hard materials so when I asked them if they could produce a 2 microinch finish on drill rod, they gave me a look that suggested I might be living several centuries behind the rest of the world, LOL.

(As a matter of interest, after thorough chemical and ultrasonic cleaning, Gallium Arsenide semiconductor chips - "Gunn" diodes - were eutectically bonded to the polished surface of the silver rods then combined with other ceramic and metal parts. The resulting miniature assembly was part of an inexpensive Doppler radar transmitter/receiver commonly used in traffic speed detectors of that era. I probably acquired a speeding ticket or two as a result of that project !)

Recently, I acquired an ER-40 collet system, mainly for use in my small horizontal mill (later adapted for the lathe as a headstock collet system). Seemed like a good system after using it for a while so I bought an ER-40 collet chuck with a morse taper for the tailstock of my lathe. The following photo shows a ball end mill held in an ER-40 collet used for reaming a ball socket joint:



With careful tailstock alignment, the ER collet system in the tailstock is superior to drill chucks for drilling, reaming, tapping and so forth. I've used this set-up for counterboring with 4-flute end mills - something that I never had much success doing when the end mill had to be gripped in a drill chuck, LOL (very low spindle speed is required, BTW).