By plunging, do you mean like parting, or do you mean facing to a small diameter? I've had good success with the latter but not the former.
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Tips/Techniques Light cuts and tiny diameters
- Thread starter DPittman
- Start date
Tips/Techniques
By plunging, do you mean like parting, or do you mean facing to a small diameter? I've had good success with the latter but not the former.
The issue I see with the Joe Pi technique is: OK, you hog the material down to a sliver in one pass. Fine. Now are you are going to hit a 0.001" diameter tolerance AND finish AND no taper on the first go? Unlikely. So if you back up the DOC & do a secondary/finishing pass, it ends up being no different that properly staged turning. Plus, depending on material, DOC, cooling... that big chip load must make more heat, also not conducive to hitting dimensions.
DPittman
Ultra Member
He was also using brass which I think is much easier to do what he was doing.
yes like parting
Well, that assumes that you can back up and do another cut at all. For the few times I've ever done this, you only get one crack at it. Anything you try after making the needle bends it and totally destroys the objective. A good finish usually requires finger sanding.
I'd have to go back to my notes to see DOC, but the picture in post #11 was done exactly this way. If I followed my usual recipe, final 2 DOC were maybe 0.005" on that particular insert, striving to keep the last 2 passes relatively equal to hit dimension, using cutting fluid. That was stainless, similar results with O1. The finish is what you see. No appreciable taper. Yes, the next stage was abrasive/lapping on that particular part & achieve -0.0005" along the length, which is why it was important to have pretty close dimensions & no taper from the turning operation.
Former Member
Guest
I'm surprised none of you use the support of a follow rest or the like to offset the forces applied during cutting on small diameters.
This is no different than a long stick out of larger diameter material where the end can not be supported by the tail stock.
This is no different than a long stick out of larger diameter material where the end can not be supported by the tail stock.
Generally speaking we are not talking parts long enough to use a steady. Right off the cuff, I can't imagine what I would be making that could. After all, we are mostly talking about small pins here. Bigger longer stuff is a different game where a steady is an obvious choice.
DPittman
Ultra Member
Hypodermic needles.
The follow rest principle is the same, but its more a matter of cramped real estate, depending on the part of course. Some people have come up with integrated tool post cutter/follower mechanisms. Even so, the follower position must be reset adjusted every DOC pass. Also the follower slide/beak occupies an unmachined area, which, depending on the part, might also have to be machined up to a shoulder or transition which is more PITA. The followers are better suited for controlling tool deflection on long skinny shafts.
Can't get closer to my 3 jaw than about 1.5" or so with my steady. Might get within a 1/2 " with my 5C Collet Chuck.
The smallest diameter I can get in my steady as designed is about 1/8" but even so, no way to get a tool in there at the same time. Just guessing, but I would think someplace around 3/8" is the smallest diameter I could practically use.
They prolly sell smaller steady rests, but most likely I'd have to make one and it would not have bearings to achieve a 16th.
For me and my lathe, I'm not gunna be able to use a steady on a needle.
OK, so this is the best I could do without breaking the needle. The needle measures 35 thou with a target length of 1/4 inch. Anything smaller than that broke off before I could get to length. This is two passes.
For some reason the needle looks bent in the photo but it isn't.
So I tried again with a very sharp HSS tool. This is 20 thou.
And here is 15 thou.
Trying for 10, but not doing well
This is 12. Very hard to hit target. Again, Photo is misleading.
Tried and tried. Giving up on 10 for now. Usually breaks at about 150 thou length. Good thing I have bags of 1/4 inch screws. Might try honing the cutting edges a bit later. Also compressed air to clear chips.
I have zero idea how anyone could use a steady on this task. You even look at that needle and it breaks!
DPittman
Ultra Member
Very good sir.
DPittman
Ultra Member
Ya I was dreaming.... about 1.5" is the closest I can get with my follow rest.
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Ya I was dreaming.... about 1.5" is the closest I can get with my follow rest.
No worries. Even if you could get closer, you can't hold needles with it!
Your note is timely. I've been noodling the problem of tiny diameter turning.
I think I should reduce the back clearance on the tool. Say to 1 degree or less. That way it's harder for a chip to get in there and Lever off the needle. Also thinking I need to take a bigger last cut. At 1/8" which is only 1/16th on each side, the stub probably moves around too much which could knock the needle off too. I plan to try a last cut of 3/32nds instead. My goal is a 10 thou needle.
DPittman
Ultra Member
I haven't tried past my last attempt and I quit at about .020" cuz I could see it flax and taper, however I'm fairly confident that if I support with tailstock I could shave off the tiniest bits.
Personally, I don't think so. Too tiny a column to be held by a tailstock. Unless we invent something cool......
Your limit may also be set by the titanium. We will see......