Tail Stock Alignment

petertha

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I had to use a half ground dead center today that I haven't brought used that much. The part I was machining was simple shaft ~4" long, held in pretty accurate 3J chuck & center drilled for tailstock dead center. I spotted a taper on first measurements about 0.003" larger diameter on headstock side vs. tailstock across the 4". This taper remained about the same as I turned down the diameter. Not happy about this, but I finished off the part just correcting diameter with a file on final finishing. I figured maybe this center was wonky because normally I use live center & did not recall anywhere near that much taper before.

So lathe cleaned off I chucked a DTI in 3J & measured ID of my MT3 quill by rotating chuck. I get: TOP=0.000", NEAR = -0.0055" BOT = 0.000 FAR =+0.0050". I did this in a few positions down the inside of the MT taper, always the same. I got thinking maybe my MT bore is elliptic but that is way too much & lathe never abused. I inserted dead center into MT socket & measured around the point section & also back on the full circular section just outside the quill lap. Same readings.

Now it may well be that the chuck jaws are gripping the DTI stem off a thou or 2 so zeroing DTI with the dial facing up initially (what I call TOP) may influence clock relative readings, but what explains this measurement mapping & the taper cutting I'm seeing? I know I can screw the tailstock in/out laterally to adjust taper, but shouldn't I be seeing some hint of this on the readings corresponding to the cut taper? I can repeat this with a 5C collet & hold the circular DTI stem in that for better concentricity than the chuck. But I figure once its rotating, it should all be relative about headstock axis, no?
 
It sounds like you just have to move your tailstock over by bout .005". Vertically you are OK and your horizontal readings are equal but opposite which is good. Your method of measurement is indeed true to the spindle axis. The fact that you measure a .005" offset with the DTI but only see .003" taper in turning probably is indicating some twist in your bed. Turning the shaft is essentially the two collar test, as applied to use of the tailstock center and is a combination of alignment of the bed ways with the spindle axis and tailstock alignment.

If you aligned the tailstock using the two collar test or Rollie's Dad's Method, you would automatically correct for both and you should turn a uniform diameter.
 
RJ, it's reading an "egged" shaped diameter. That was my first impression too. Had to re-read his post.

Peter,

Scan over the OD of the tailstock spindle and see if you get the same readings. If so, then lets look at the headstock of your lathe.

Ken
 
Well it was an eventful & educational day. I'd even call it mostly successful.

First off the tailstock. Not sure if addressing that was the proper starting point or root of the problem but I thought maybe easier to rectify. I had to sit down with my DTI, pencil & paper & figure out what deflection direction meant what. I'm pretty sure I befuddled myself by measuring OD vs ID of the quill & misinterpreting direction before. This time I marked quadrant points with a felt pen so the DTI ball would always land on the same spot. I broke the problem into 2 directional issues kind of like a 4-jaw chuck.

First was vertical. I zeroed the DTI at TOP (12-oclock), then went round 180-deg & measured bottom (6-oclock). I helped to use a small mirror..which of course is kind of looking at things bass-akwards so you have to pay attention to dial direction. I did the quill OD, then ID & got prefect repeat every time. They are concentric. The tailstock barrel sits 0.002" high of lathe rotation axis. Tried this at different positions, barrel extensions, various clamping, always the same. I hear that high is normal & not a lot I can do anyway.

Next was lateral in & out 3-oclock & 9-oclock, same method. Doesn't matter which position gets zeroed first but it made it simpler on my simple brain to note which side the tailstock was sitting. On my lathe I unscrew 2 set screws on end of tailstock (where hex wrenches are located) which I think clamp against the gib. Then unscrew the 2 main clamping screws (yellow arrows). With everything loose it was a simple matter of bumping the upper body until I got same DTI reading side to side, then tighten up arrow screws. Now the new learning bit,. Good thing I left the DTI in. Tightening the gib set screws can easily move the setting off several thou even with the main screws set. I'll have to look at the manual to see what this is about. So I kind of snugged alternatively but always watching to preserve my zero DTI reading. Now with everything is tight & accurate &bang on. The alignment graduation sticker cant be trusted, no shock there.

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Now the tailstock situation improved & moved out of position for now, I chucked a poorly chosen chunk of CRS (no tailstock centering) & started down the path of: skim cut, measure diameter difference between head/tail stock side & shim adjust the appropriate lathe foot accordingly. I'm within 0.001" diameter on ~5" length. But I think I'm chasing my tail now so pausing to ask some more questions.

It may well be the crappy steel quality of bar I chose. I blued it & took a spring pass at same depth but different traverse gear, but no real amount of excess material came off. The measurement change seemed rather muted, not like 1 leg turn = 0.0005" correction progressively towards some zero convergence. Maybe my stand is twisting but its not getting applied, or to the same degree, in the lathe bed itself? I can jack the feet up/down independently, no issue there. Its 2" square tubing 0.125 wall. You can see the lathe lugs are bolted to a cross member of the stand, but at a shorter lateral spacing. ie. the stand feet are spaced wider than the lugs. Do you think jacking the padded feet still is correctly applying corrective twist to the lathe bed in a consistent predictable manner? Then I did another cut & suddenly got same 0.001" deviation except this time it flipped direction & opposite end of bar was larger. wtf???? Are you supposed to adjust & then wait for a settling period?

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Hi Peter, yes a settling time is a good idea, then rinse n repeat! Quality lathe makers used to 'season' castings for a year or more, rough machine and leave for stresses to work out before finish machining n grinding, adjustments for bed twist can take a day or two to settle, depending on the heft of the castings ( heavier take longer) - I leave a couple of days between adjustments, 2 tons of iron takes a while to relax!

Dave H. (the other one)
 
Peter, which end was larger when you first found the .001'' difference over 5''?
I would make sure that the cutting tool was super sharp, had sufficient relief angles, and was perfectly on centre for this test. Even at that and with no twist in the ways, I would expect the free end to be slightly larger due to deflection.

Ideally, the workpiece should be turned slightly smaller in the middle section so that you only have two hubs to cut for the final pass. Bear in mind that removing .001'' creates very little chips so if there are any chips at all do another spring pass.

Padded feet, flex in the stand and stiffness of the lathe will tend to 'mute' the impact of adjusting the feet.
 
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Tozguy, I'm reducing diameter by 0.005" each test pass (0.0025" DOC) & locking the carriage Y-axis before traverse each time. For the most part my headstock diameter was larger than my tailstock diameter so my interpretation was to continue tweaking. Last measurement HS=1.0410", TS = 1.0420", so 0.001" larger on end. So my reasoning now: if its deflecting a bit on the end because its unsupported (steel is held cantilever in chuck, no tailstock center engaged) then maybe I'm pretty close to true parallel? I really want to check the bed with a good level now (sourcing). I feel like the tailstock problem was the overriding issue.

I'm going to let it rest as-is & repeat this test with a chunk of 1.25" 6061. I have some sharp inserts that impart very little cutting load & leave much better finish. The cold roll steel was a quick grab thing & probably a poor choice. It took a couple cuts before I even got stable readings which I attribute to the skin & I'm always leery about stress relieving.

So the dumbbell test bar shape with reduced diameter in the middle - I just assumed that was a time saving thing so you are not cutting continuously from end to end. Is that the case or maybe I misunderstood? My logic was cutting the whole section takes a bit more time but now I can mic along the way & see if there is any funny business going on like a bell shape or whatever.
 
I had to use a half ground dead center today that I haven't brought used that much. The part I was machining was simple shaft ~4" long, held in pretty accurate 3J chuck & center drilled for tailstock dead center. I spotted a taper on first measurements about 0.003" larger diameter on headstock side vs. tailstock across the 4". This taper remained about the same as I turned down the diameter. Not happy about this, but I finished off the part just correcting diameter with a file on final finishing. I figured maybe this center was wonky because normally I use live center & did not recall anywhere near that much taper before.

So lathe cleaned off I chucked a DTI in 3J & measured ID of my MT3 quill by rotating chuck. I get: TOP=0.000", NEAR = -0.0055" BOT = 0.000 FAR =+0.0050". I did this in a few positions down the inside of the MT taper, always the same. I got thinking maybe my MT bore is elliptic but that is way too much & lathe never abused. I inserted dead center into MT socket & measured around the point section & also back on the full circular section just outside the quill lap. Same readings.

Now it may well be that the chuck jaws are gripping the DTI stem off a thou or 2 so zeroing DTI with the dial facing up initially (what I call TOP) may influence clock relative readings, but what explains this measurement mapping & the taper cutting I'm seeing? I know I can screw the tailstock in/out laterally to adjust taper, but shouldn't I be seeing some hint of this on the readings corresponding to the cut taper? I can repeat this with a 5C collet & hold the circular DTI stem in that for better concentricity than the chuck. But I figure once its rotating, it should all be relative about headstock axis, no?

Peter, It is my impression that you aligned the tailstock by mounting a test indicator in the headstock and rotating the spindle to check the runout. This is a true indication of the alignment of the tailstock spindle for that particular tailstock position, quill extension, and measurement datum runout. Changing any of those could change the alignment.

Additionally, if you have a twist in the ways or headstock misalignment, these do not enter into the equation. Consequently, you can have a well aligned tailstock but still turn a taper when turning between centers

For taperless turning, I would mount a cylindrical test bar between centers and measure the position in the x direction with a test or dial indicator near the headstock center and the tailstock center. The position is measured relative to the cross slide/compound position so you don't want to change them while making the measurements. If the test bar is round, the measured position of the axial center of the test bar at that point on the bar is average of the minimum and maximum readings (ref. Rollie's Dad's Method). Without changing the cross slide or compound, move to the second position and remeasure. If the two averages are the same, you should be able turn a taperless bar.

If you use a test bar such as this, the maximum and minimum readings should be identical as the test bar is theoretically straight and the centers are located at the true center of the test bar.

http://edgetechnologyproducts.com/tailstock-alignment-bar/?utm_medium=googleshopping&utm_source=bc

Note that this measurement uses the position of the cross slide relative to the spindle axis so, although the tailstock alignment is true for that particular setup, it takes into account all of the factors which affect that alignment. If there are misalignments in one or more of the contributing factors, any change can affect that. To get around that, I would insert the work piece and turn collars to the same diameter, as measured with a micrometer. near the centers, using the cross feed if necessary. I would then go through the measurement procedure described above to adjust the tailstock for the same averages. Since your workpiece is already in position, none of the setup will change and you should be good to go.

Note 1. This is a procedure to quickly align the lathe for turning a taperless bar. It is not a substitute for a full lathe alignment. The advantage of this method is that it eliminates the "turn" from the "turn, measure, adjust" sequence.

Note 2. If you read the background of Rollie's Dad's Method, you will see that the only requirement is that the test bar be round. You can actually measure two different diameters as long as you include the diameter of measurement point in the equation. With this, you don't even have to adjust the crossfeed to turn equal diameters when making the second cut on your workpiece.

Note 3. While this measurement procedure will theoretically turn a uniform diameter, it does not account for tool deflection or workpiece flexing, as mentioned above.
 
I think the dumbbell shape is so you don't put necessary wear on your cutting tool as you move from one point to the next. Using this method, you definitely don't want to change any settings on the crossfeed/compound.
 
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