Grabbing on the inside

[Rex Walters]

My friend @jimsehr showed me something a while back that rocked my world. Jim's got a few years on me, and I've learned to pay attention when old-timers take the time to show you something.

How many parts have you made that looked something like this:



Now, how many parts have you made that are the exact opposite: a single OD and multiple IDs? Any at all?

If you're anything like me, about 90% of the parts I make on the lathe look something like the above with lots of OD features and fairly simple ID (most often just a single bore). I almost always want to face the left and right sides of the part so they are exactly parallel, too.

So why do we almost exclusively use a 3-jaw chuck to hang onto the OD of our parts?!

A scroll chuck is certainly convenient, but I suspect many of us hardly ever take it off our spindle, which is a shame. I strongly encourage even hobbyists to spend some time getting acquainted with other forms of work holding. We all need to use the 4-jaw more often. Turning between centers is still the most accurate way to use a lathe, and can be surprisingly convenient sometimes. Soft jaws are great for some tasks. 5C and other OD collets are terrific for accurate work.

But this post is about hanging onto the ID of the work.

If the bore is big enough, you can use the outside of the jaws on your scroll-chuck to hang onto the part, but you'll have to put up with al the scroll-chuck limitations. As we all learn soon enough, it's hard to hold concentricity to less than a few thou any time you remove a part from the chuck. If you hold it on the OD to bore the hole, then you'll have to indicate it in on the 4-jaw to hold it accurately on the ID for subsequent machining.

And what if the bore is smaller than the 1" or so minimum diameter your chuck jaws can hold on the ID?

The options most of us think of include:

1. Leaving some extra material on the left side to hold it on the chuck, then do all machining without removing the part from the chuck, parting it off at the end. [This works for the part above, but you'll probably still want to face the side you parted off to clean it up, which requires flipping the part and holding it accurately. And sometimes you want to use a die to thread both ends.]
2. Turning a stub arbor out of a piece of scrap, and then either clamping or super-gluing the part to the arbor. [This also works, but making the arbor and tapping a hole for the clamp is a pain. You often need to make another part to use as the washer or clamp to hold it, and superglue is messy and a pain to clean up after you remove the part.]

Guys with some experience might think of using expanding or tapered arbors of some sort and turning between centers. That's a great option if you've got the right sized arbor (or are willing to spend the time to make one).

The smarter pros would use commercial expanding collets like these. Those things are the fershizzle, but they don't give them away!

This post is about how to make a "ghetto" version of your own expanding or ID collets.

Here's an arbor in the holder prior to loading the part:



And here it is with the part loaded:



Here's the holder:



And finally, here's a sample sacrificial arbor (you make these a half dozen or so at a time):



Jim's system comprises at least three parts:

1. A holder of some sort that you make once. I'll describe other options later, but above I used a 3MT arbor blank with a soft head ready for machining since my spindle arbor is a #3 morse taper. I also made one out of a 2MT arbor blank so I can move parts between the lathe (using a 3MT to 2MT sleeve) and my rotary table without removing it from the arbor.
2. Sacrificial expanding arbors, made in batches in whatever sizes and materials are convenient (I like using leaded steel in various diameters, but aluminum or even 1018 also works).
3. 1/4"-20TPI socket-head cap screws with the heads turned down at a taper (30 degree included angle). These provide the force to expand the arbor (just a quarter or half turn provide all the force you'll need to hold parts securely).

Here's how you use it:

1. Bore the central hole in the part using the 3-jaw chuck or whatever to hold it.
2. Remove the chuck and replace it with your expanding/ID collet holder.
3. Find a sacrificial arbor that's a slightly larger diameter than you need and screw it into the holder.
4. Tighten the SHCS until you get a small amount of expansion, about 0.005" radially. You can either use an indicator or do what I do and just squeeze the arbor with your fingers and tighten the screw until you feel the sides expand slightly.
5. Now put a LH turning tool in your tool post and turn down the diameter until you get a very close fit to your part. Take care to face the shoulder accurately. If your part has a square shoulder on the ID, make sure you undercut the shoulder on your arbor slightly else the nose radius on your turning tool can prevent your part from registering accurately.
6. Unscrew the SHCS a half-turn.
7. Place the part on the arbor, and tighten the SHCS until the part is secure.

The mating tapers at the end of the sacrificial arbors and the SHCS heads provide much more expansion than you'll ever need, so you might be tempted to slack off when turning the arbor to fit in step 5. Don't do this! To maintain accuracy, it's important to turn the arbor to a tight fit. If you turn it undersized and use the screw to expand the arbor to fit, you'll only grab on the outside edge. You want the arbor to hold over as much area as possible.

Holding parts this way has some wonderful benefits:

• You can remove and return the part as often as you want and still hold tenths accuracy.
• Unlike 5C OD collets, you can maintain axial location between parts — the shoulder you cut does not move. 5C collets will pull parts into the spindle a different amount if the diameters vary even slightly.
• You can access the entire OD of the part without needing to remove or flip the part.
• But you can easily flip the part without losing concentricity if it's convenient for some reason.
• If you want to ensure both faces are perfectly parallel, you don't need to locate to a shoulder, just turn a section of the arbor long enough to hold the part, and you can now face both ends without removing the part.
• It's cheap! The sacrificial parts can be made a half dozen or so at a time, and can be reused.
• The arbors are sacrificial, there is never a concern about cutting too far (you can even part narrow rings/washers without the parts going flying.

I'm probably forgetting other benefits but as Jim said, if you build it I"m sure you'll like it.

 

[Rex Walters]

If you have or are willing to make a cam/lever-actuated drawbar, you can even use the system for parts that don't have a through-hole.

Instead of something like that 3MT arbor blank as the ID collet holder, you can use something that mounts like a chuck. I have a thread-on chuck mount, so I could have just welded a plate onto a "thread protector" that you use with collets.

I only have one thread protector, so I chose to machine a plate that mounts onto my faceplate instead:


Here's the backside. The plate just locates on the faceplate via the spigot and screws (the holder doesn't need to be exactly concentric since you always turn a fresh face on the arbor every time you use it anyway.



The drawbar in the first image has a handwheel instead of lever-actuator, so I can't really use it to expand the arbor (the SHCS will turn as I turn the handwheel). I've got an idea for how to make a cam-actuated drawbar, though, that should work fine.

 

[Rex Walters]

I'll eventually provide some dimensioned drawings to this thread, but it's pretty easy to figure out from the photos.

The sacrificial arbors are a straightforward turning job:

1. Saw a half dozen or so pieces of scrap to length, and face both ends.
2. Turn down a 1/2" long stub on one end that is 7/8" in diameter.
3. Cut some relief at the shoulder, then single point to 7/8"-20TPI.
4. Drill a 1/2" diameter hole to a depth of about 1"
5. Remove the chuck and install the ID collet-holder
6. Insert the threaded end of the blank you just made into the collet-holder
7. Drill a 1/4" hole through
8. Set the compound to 15 degrees
9. Cut a taper at the end of the 1/4" hole you just drilled until the larger rightmost diameter is just larger than the head of a 1/4" SHCS
10. While you have the compound set, turn a taper on the head of a few 1/4-20 socket-head cap screws.

Once you've made up a few blanks, you need to cut the cross slits.

I used Jim's suggestion of a fixture on a horizontal bandsaw. I just made mine out of scrap:



Because the bandsaw blade levers down, you want the fixture to have a slight tilt to match the angle. I just drilled and tapped some holes to hold a strip of material at one edge to provide the tilt. I also added a cap screw to locate the fixture each time I use it:



The only other thing is to index the part to make the cross slot. Since I used 7/8-20 threads, the thread pitch is 1/20 or 0.050". A quarter turn is thus 0.0125". I just inserted a piece of 0.010" shim stock between the blank and the fixture to index.

One other small but important detail! In order not to saw through your fixture, you'll need to rig up a stop of some sort. I just sawed off a length of black pipe that I place over the normal stop on my saw when I'm using the fixture. You can just barely see it at the top of the first photo in this post (just to the top left of the blade).

 

[Mini Cooper S]

Great idea! I have certainly made my share of expanding arbors (like in your first picture) over the years. I use a tapered pipe plug for the expanding screw, works like a charm.

Richard

 

[Janderso]

fershizzle?
I watched you build the Quorn with plenty of input from Jim S.
You galloped to the top of the ability chart using your new learned skills.
Very useful thread! Thanks for sharing.

 

[ddillman]

Thank you for that write up. it was very interesting

 

[Jimsehr]

Rex
Thanks for the nice write up. One thing I should point out is you want a polished finish on the tapers on both mating angles. Or they sometimes want to stick together. Another thing is you used your machines 3MT Morse taper As a base to mount the soft expandable collets. There are other ways to make a mount. One is to
just chuck a piece of stock in 3 jaw and face drill and thread a 7/8 -20 so mount your soft collet that way. Or you can get some ideas by looking at Rovi collets web site. It is nice to be able to move collet from the lathe to the mill or drill press.
Again Rex did a great job of showing how to make and use Id collets.

 

[Jimsehr]

Also I posted a pic of another way to make the mount for holding Id collets in the Group photos section. I bought a couple of 1 1/2 threaded nuts That fit the Logan spindle and made Id collet holders from them.
Jim Sehr

 

[Rex Walters]

Great idea! I have certainly made my share of expanding arbors (like in your first picture) over the years. I use a tapered pipe plug for the expanding screw, works like a charm.

Yup, pipe plugs are also a good trick, but I think Jim's system is still superior for a few reasons:

• I make a lot of small parts. I think NPT plugs (and taps) start at about 1/2", so you'd need at least a 5/8" to 3/4" bore or so to use one. The head of a 1/4-20 SHCS is only 0.375" diameter so I can hold onto bores about an eighth of an inch smaller.
• You can use a lever closer with the smooth-walled taper on a cap screw head (good for parts without a through-hole).
• Making a custom arbor as a one-off is always a bit of a PITA. The time it takes to build the arbor (including drilling and tapping) is a distraction from the project at hand. The beauty of Jim's system is that you do most of the distracting work ahead of time, and make the arbors in bulk. During a project, I just need to grab one of the screws that I already tapered the head on and an arbor that's close to the right size, then turn down the latter a bit. Easy peasy.

For me, at least, that third bullet was enough of a pain that I only did it as a last resort. Now I use ID collets all the time.

Here's a photo Jim just sent me of a holder he made that screws onto the nose of a Logan lathe (instead of the 3MT arbor blank I used):

 

[Jimsehr]

One thing a pipe expanding plug can’t do is load a part with a blind hole. Plus if you have a lever collet you can load parts in seconds. The first time I made a Id collet was because we had to run bunch of castings with a outer surface that no good way to grip the part. And the bore was blind. That was more then 50 years ago.
Jim Sehr