Expanding Collet design question

[90LX_Notch]

I need to hold a .098 bore so that I can turn the od of the part concentric to that bore. My plan is as follows:

1) Hold .375 dia Al in a collet.
2) Face and center drill.
3) Engage center from tailstock.
4) Turn to .098
5) While still in the lathe cut the slot(s) using .025 cutoff disk in a Dremel.

The idea is to use the center in the tailstock to expand the collet rather then a screw. Reason being that an 0-80 screw is .060 od, that would only leave less then a .020 wall.

My questions are:

Do I need to make two slots at right angles or would one slot suffice? Two versus four expanding sections.
Do I need to have cross holes at the end of the slot? I am only doing less then 5 parts.
Is this a sound method using the tailstock to expand the collet?

 

[moconnor]

Hello,

I think that is too small of a diameter for what you are planning. You might consider a tapered arbor or lathe mandrel (.0005" per inch) to hold your part. The smallest standard commercially available mandrel is 1/8" diameter, but your .098" diameter only has to be slightly longer than your workpiece. It does not necessarily have to be a between centers mandrel, you could chuck a 1/4" or 5/16" dia. piece of stock and then turn a .0005"/in. taper on the end for a length slightly longer than your workpiece.

Regards,
Mike

 

[BillC]

How about using loctite to hold the parts to your .098 arbor? Then slight heat will loosen and remove each part. I have loctited to a push fit - so you can easily maintain concentricity....No need to make it an expanding collet.

BillC

 

[90LX_Notch]

Thanks for the replies.

The problem is the part is plastic and the hub is .390 wide. That's why I want to use an expanding collet. I don't want to scratch the bore because it used in a rotating assembly.

 

[Bernd]

Sounds to me like it would work. Now here's another question to ask yourself, since you mentioned the medium your working with, will it expand the OD to the point were once released from the collet the dia. of the part might be smaller than you wanted after turning?

You could also make an arbor with a thread on the end and use a washer and nut to hold the part against a shoulder that is toward the chuck to hold the part. Might want to use some sand paper between the shoulder and the part to keep it from spinning. Sounds like you'll be taking small cuts with plastic.

Bernd

 

[90LX_Notch]

Bernd,

You're right about the small cuts. It's an injection molded part that I want to true up with very light skim cuts with a sharp tool.

-Bob

 

[MachineTom]

I believe that the center idea will not work as you describe. The tailstock at that position on the bed would need to be within .0005 of center measured at the taper of the live center, which is possible but not likely. Any misalignment will wobble that tiny AL shaft like plastic, which would kill your skim cut objective.

A steel shaft turned while in the collet with a threaded end to take a nut and washer as already state sounds good. Or center drill a steel shaft Longer by .100" than the hub lenght, tap 0-80,slit from the the center of hub to shaft end, then make an oversize 0-80 screw using an adjustable die. The idea being to expand the end of the shaft a bit to drive the work.

 

[Ken I]

Turn up a mounting shaft - at the end turn a short eccentric - make an eccentric ring to suit.

When "lined up" your part slips on - as soon as you try to rotate the part it "locks" - do your light machining - rotate forwards to unlock and remove.

I have never tried this at such a small scale but this method is commony used for mounting cardboard tube spindles to driveshafts on slitting lines etc (here the drive ring is usually knurled).

Ken

 

[90LX_Notch]

MachineTom: I only turn with dead centers.

Ken I: Cool idea but I already made an arbor with a 0-80 screw to sandwich the part. Now I just have try it.

-Bob