Machining 5C Step Collets?

[rake60]

I was asked today if I could machine some 3" face 5C collets at work to try a new application at work.
I told them I could, but have never really done that before.

I have a 5C collet block.

This is an already machined 2" face collet in the block.

The import collets we buy do not come with pins so I will need to shim the split lines before tightening
the new collets in the block. That block will just catch corners on the face of the 4 jaw chuck for the lathe.

I need to bore them for a 2.020" fit about 5/8" deep.

Any suggestions or tips?

Rick

 

[rkepler]

I'd be worried about the centers of the square block and the 5C taper in it - you might think about chucking it up with not collet, centering it on the taper and comparing the outside to see if they're concentric. If so you can use the sides of the collet block to center the collet in the holder (the cheap collets have some amazing errors). It would be better if you had a collet chuck in the lathe.

Too bad about missing the pins. What I would do would be to measure the gaps in the collet and choose a shim about .001/.002 *thinner*, cutting pieces exactly the same in length. Put a little crimp them them so they stay in place in an unclamped collet and flush them up as the collet clamps. This will make the bored hole a little large from the spring when the collet is loose.

Note that you're not going to get a bunch of clamping force with this type of collet - you're gripping something 2" round in a collet really designed for 1" max. What you should be using is a step chuck and closer, those apply the grip from the outside of the part in as a regular 5C collet does. When using it try keeping the cutting forces down and don't let a gorilla tighten things as it can actually bend things between the taper and face. Some of these can be hardened after machining, you might check with the manufacturer (assuming that there's a name on it somewhere).

 

[Blogwitch]

Rick,

As you know, collets are only really used for very accurate work, say within 0.0005", and how you are attempting to do it, I don't think you will be anywhere near that figure, just too many variables. I just don't think you can set up accurately with a collet block and four jaw. In fact, you might have done better using the collet block standing square upright in the mill and boring it that way, after centering up on the collet block taper. I suppose you could do the same in the four jaw, centering up on the taper, but getting the collet in afterwards, with both methods, will cause you some head scratching.

As RK has mentioned, you really do need to do this using the collet chuck on the lathe it is to be used in, unless of course it is to be used in a non rotating milling fixture, then what you are doing might be OK, as long as you centre up on the internal machining you have done on the collet to the mill, and not onto the outer face.

I only use these external collets for holding thin washer type items, never as deep as you have gone, so I can't really comment about the grip you will get, but if you aren't going to be doing any real heavy machining on the piece part, you should be OK.

John

 

[rake60]

Boring the collets in the machine they will be used in isn't really an option.
They will be used in old Hardinge DSM-A automatic turret lathes that don't
have the tooling to do that.

The parts this collet is being made to hold, just get a 1/16" wide X 1/16" deep
groove cut into them. There shouldn't be a lot of tool pressure.

I'm not positive I can make it work, but I'm going to give it a shot.

RIck

 

[rake60]

I realized another problem with this change over.
Theses machines were never intended to process parts larger than about 1-1/2".

The cross slides rapid in until an adjustable rod trips a microswitch.
That signals a hydraulic valve to change from rapid to feed.
That rod did not have enough adjustment on it to get the slide to change to feed
at the larger diameter.

I made a longer rod to fit the assembly.

Not I can adjust it to switch to feed at up to a 4" diameter.
When it's in the machine and set for smaller parts, there is quite a bit of it sticking out.

We'll need to be careful working around that.
(I need to get a phone that takes better pictures!)

Rick

 

[maverick]

A 5C step collet will hold tighter than you think, especially with a 5/8ths depth. A 1/16 wide cut should be no problem.
Watch out for the interrupted cut when boring and use a sharp corner or corner relief on the bottom. As Bogs and RK mention, concentricity could be a problem.

Good luck.
Mike

 

[rake60]

I can't post a picture of the part, but it isn't a critical thing.
It's kind of a jaw clutch thing that gets a groove cut into it for a retaining ring.

The groove is on the OD. That is the only machining this part gets.
Diameter tolerance is .010" and concentricity allowances are pretty generous.
The material is powdered metal, so it cuts like grey cast iron.

It's a production application that we are trying to do in house.

Rick

 

[Blogwitch]

At those sorts of battleship plating tolerances Rick, you should be more than OK.

Don't keep us in suspense, let us know how you get on.


John

 

[rake60]

The collets arrived Thursday. The split lines mic .062.
All of my shim stock is brass. That may not be the best material for this application,
but it's what I have.

Only 3 shims are needed but I could drop a black football is an all white room and
loose it, so I made 6 of them.

Rick

 

[Blogwitch]

Rick,

You could use a couple of pieces of 1/16" steel welding rod or silver steel in the gaps if you want it to be a little more stable.


John

 

[rake60]

FAIL!

It cut just fine but there was a little banging around with the interrupted cut.
I took it to within .030" and reindicated the block in to finish it.

The finished collet:

The parts to be machined fit great and will tighten up with hand pressure on the collet.

Then I mounted the collet in one of the automatic turret lathe spindles.
.005 TIR. DAMN!

They found another guy who is going to try it. He has a 5C collet chuck but it's too big
for his lathe so he chucks it up in a 3 jaw. He's not sure that will work any better.

I believe I have them talked into buying me a collet chuck for that lathe.
I gave them two options. Option 1 or Option 2

We'll have to wait and see where that goes.

Rick

 

[rake60]

We had an outside machinist bore another collet for this application.
His ended up a little closer on the concentricity, but it sprung after the very first part was run through it.

After reviewing the tolerances we saw that .005" run out in the one that I had bored wasn't really a problem.

NEW PROBLEM:

These old automatic turret lathes are built to be, well, automatic.
We need to load it manually, close the collet, turn the spindle on then actuate the rear cross slide only.

The slides are supposed to be actuated by cam turning on the programing drum tripping a microswitch.

The little knurled knobs above those cams are for set up purposes.
When the machines were new, you would turn that knob and it would raise a little lever to trip the switch.
They do not work all of the time now, so you had to reach in there and bump the switch with your finger.

I made a safer alternative to that today.
A heavy duty momentary contact push button switch that was salvaged from another dead machine,
mounted in an inexpensive plastic handy box.

Wired into the same connections that the microswitch wires are connected to.

It will work for now.

We have run 800 parts through my home made collet and it's holding up so far.
I'll be replacing it with a step chuck and closer soon.


Rick