Turning long thin shafts

[KellyJones]

I've tried turning long, thin shafts before with varying degrees of success. In this case, I am defining a long thin shaft as .188 diameter by 2.30 inches long, turned from a .25 diameter drill rod.

I know it can't be done all in one pass, and the preferred method is to turn it incrementally from the far end toward the head stock. In other words, turn about .50 inches of length to size, then another .50 inches and so forth until the full length is complete.

I have never been satisfied with the results of this technique. If I leave the entire 2.30 inches extend from the chuck, the deflections at the end never allow me to actually get to an accurate size, and if I readjust the amount sticking out of the chuck between every step, the finished part invariably has multiple eccentric sections (because you can never recenter it well enough in the chuck.

How do professionals get such nicely turned pins?

Thanks

[tornitore45]

You can use a "follow rest" or a number of trick more or less effective such as axial feed where the tool is ground to cut on the face placing minimal radial force or a tool that stick out into hole that act as a 360* follow rest.
The idea of axial feeding is to hit final dimension from the start, and go all the way in one pass. Some people swear on it, I never got good results.

I assume you must need some 0.25" diameter feature at some point, otherwise you would not turn down a 1/4" to 3/16".
My favored solution is to avoid the issue altogether and start with a drill rod of the correct size (3/16th), then thread, silver braze or shrink fit the a piece of 1/4".

[Inspector]

Are you using the tailstock?

Pete

[BadDog]

Yes, a follow rest would by the common answer. Some mount on the cross slide, but I've seen a few that were meant for really small stuff and built to mount on a tool block (maybe Gadgetbuilder?). But some years ago I bought a turret "box tool" with such use in mind. It's basically a tool with the follow rest built in. And rather than a Turret, the 1" shank fits my boring bar block, allowing the work piece to pass right through.

I believe such a setup would be near ideal for what you are doing, though I'm not sure if you could do 2.3" in full pass or not, that's an awful long section, and I suspect that cutting forces could be too much for the span. It's going to have to be running pretty fast, and if it bows it's liable to turn into a jump rope. Multiple very small passes might work, but I don't know enough to say for sure. So there are still details to work out, but I think it's pretty much as good as it gets for such things on a lathe.

[Harold_V]

Explore the use of a center. Even if the piece in question doesn't permit one, by using a #1 (or smaller) center drill, you can support the piece well enough to make long turns, then remove the center after the turns are complete. Also, using HSS for cuts, with adequate positive rake, to keep cutting pressure low.

H

[BadDog]

Wow. I've used a center many times, but I wouldn't have thought tailstock center support would be adequate turning to 0.188 for a 2.3" (+ for final) length. Even with razor sharp positive rake (not going too far or risk pulling in for under size), it just doesn't seem like it would be reliably on size. Or are you talking about incremental turning with the center supporting the developing final diameter section(s)?

I've never gotten close to something like that, so I'm curious about the outcome.

[spro]

Thinbits by Kaiser are extremely thin and perfect. They require a perfect tool holder.

[SteveM]

Soft jaws will relocated that part within tenths.

There are ways to attach sacrificial pieces of aluminum to your chuck jaws to create soft jaws.

This is one such approach:


Steve

[BadDog]

Only if the part being fed through the jaws is straight, round, and of consistent size. The chuck itself must also be of high quality and in very good shape. Even if all those things aren't completely true, they will still get you closer than otherwise, though holding tenths (or even low thou's) is growing ever more remote.

[Harold_V]

Wow. I've used a center many times, but I wouldn't have thought tailstock center support would be adequate turning to 0.188 for a 2.3" (+ for final) length. Even with razor sharp positive rake (not going too far or risk pulling in for under size), it just doesn't seem like it would be reliably on size. Or are you talking about incremental turning with the center supporting the developing final diameter section(s)?

The majority of the 18 months I spent working for United Precision (now defunct) was running a small lathe, turning out small work. They did a lot of sub-contract from Univac (previously known as Sperry Utah, where I was trained), and I am quite comfortable with small work, so they turned to me for pretty much all of it.

Included in the myriad of parts I machined over that prolonged time were custom screws, generally made of 303 S stainless. Some had reduced diameter grip, while others may have had threads full length. I had to machine them from bar stock (billet, to those who think anything that isn't a casting is a billet), and they were often long. Threading wasn't much of a problem, as we had a small die head that worked exceedingly well, so it wasn't difficult to hold pitch diameter, but turning the body diameter could prove to be challenging, especially when the screws were quite long, and they often were. Fine feed is a requirement, to keep cutting pressure low, as is considerable positive rake. Really helps to have full speed control, too, which I did (Clausing lathe with vari-drive). When you have the right combination, it's surprising how well it works. Keep in mind, there will likely be minor deflection in the center, but that's what polishing cloth is for. Alternately, such diameters can be ground, but there was no cylindrical grinder at United.

You would be hard pressed to turn out work like this on your 17" lathe, for obvious reasons. It's really wise to size lathes to the intended work.

H

[BadDog]

Yes, certainly not a job for my lathe! Or any of my previous for that matter. But my bigger lathe does have the benefit of speed over my older smaller lathes. It can run ~2k topped out, which is quite nice at times. My previous Rockwell 11 topped out at ~1500, though neither is well suited for this type of thing.

I don't do a lot of small work, though when it comes along, I manage well enough for my needs. But my needs don't encompass work such as the OP is asking about, and I imagined deflection would border on intolerable without a follow of some sort. But clearly doing so is possible when you master the balancing act . As always, thanks for the explanation.

[Harold_V]

One thing I should have mentioned. For those who use live centers, keeping them properly tightened is really important when you're fighting chatter. when the center is too tight, the part likes to sing. I generally adjust mine while taking a cut, making sure it doesn't get backed off to the point of danger. It's surprising how a little adjustment will often silence the cut. And, it's an ongoing battle, as the part expands while being machined. You must remain vigilant.

H