Using a collet on shiny things, how to preserve the surface finish?

[WobblyHand]

Sorry to bombard the forum with these simple questions. I have some 25mm OD carbon fiber tubing I'd like to hold in a 25mm collet. The collet is in a square collet block so I can drill holes in the tubing at right angles. The tubing obviously isn't exactly 25mm in diameter. I measured 24.99mm at one end and 25.04 at the other. (Using my junk calipers, since that's what at my desk.) The collet just scrapes the shiny gloss finish in places when I insert the tubing. Is there something I can use to minimize the gloss finish damage? Should I use lube? A wrap of thin film like saran wrap? Something else? Slightly spring the collet? Thanks.

 

[Reddinr]

How about you make a mandrel for the ID of the tube and hold the mandrel and not the tube?

 

[WobblyHand]

Because, I didn't think about that. That's a good idea. Not sure how to make a mandrel, but I can look that up. Hopefully it's possible without a mill. edit: Well that's not too hard!

I need to drill through two things actually, the CF tube and an insert piece I machined to fit inside the tubing. I think I want to hold the tubing on the OD, but will think some more about a mandrel. If I used the mandrel, the mandrel could be held by a different collet. I'm using a rectangular collet block as a poor man's 90 degree indexer. The only issue I see is the stick out will be a little long using a mandrel.

 

[extropic]

5C collets are commonly available up to 1.125". You could use a 1.125" collet and make a "finger sleeve" to adapt. Make the sleeve out of Delrin?

By "finger sleeve" I mean:
Turn OD, ID and face length to match grip length of collet. Leave a flange on one end to ease removal from the collet.
Saw slit from one end, lengthwise, 4 places equally spaced and leaving about 1/8 uncut.
Saw slit from the opposite end, 4 more places, dividing the cylinder into 8 total fingers.

Alternatively, brass emergency collets (example) are available which you can bore to fit your workpiece.
Nylon emergency collets (example) are also available.

 

[WobblyHand]

5C collets are commonly available up to 1.125". You could use a 1.125" collet and make a "finger sleeve" to adapt. Make the sleeve out of Delrin?

By "finger sleeve" I mean:
Turn OD, ID and face length to match grip length of collet. Leave a flange on one end to ease removal from the collet.
Saw slit from one end, lengthwise, 4 places equally spaced and leaving about 1/8 uncut.
Saw slit from the opposite end, 4 more places, dividing the cylinder into 8 total fingers.

Alternatively, brass emergency collets (example) are available which you can bore to fit your workpiece.
Nylon emergency collets (example) are also available.

Is it possible to use a 1" collet and use shim stock? 1" = 25.4 mm. That's 0.4mm to take up. That's just under 0.008" shim wrapped around the tube just under one revolution. Around 3" of shim stock. Could use aluminum? or brass? How thick is an aluminum can?

 

[WobblyHand]

To answer my own question, about 0.004", so two wraps. That's doable. Paper is close to this value as well.

 

[SLK001]

Is it possible to use a 1" collet and use shim stock? 1" = 25.4 mm. That's 0.4mm to take up. That's just under 0.008" shim wrapped around the tube just under one revolution. Around 3" of shim stock. Could use aluminum? or brass? How thick is an aluminum can?

This is the way I would do it. Get some copper shim stock and wrap it around the tube ONE time (or slightly less than one time). The copper is soft enough to not scratch your tube and hard enough to be able to hold it tight in the collet.

 

[Tozguy]

Scotch tape is good for this job. It is thin enough so you can use a number of wraps to get a nice fit in the collet.

 

[stupoty]

Because, I didn't think about that. That's a good idea. Not sure how to make a mandrel, but I can look that up. Hopefully it's possible without a mill. edit: Well that's not too hard!

I need to drill through two things actually, the CF tube and an insert piece I machined to fit inside the tubing. I think I want to hold the tubing on the OD, but will think some more about a mandrel. If I used the mandrel, the mandrel could be held by a different collet. I'm using a rectangular collet block as a poor man's 90 degree indexer. The only issue I see is the stick out will be a little long using a mandrel.

If you use a block of wood (round dowl sort of thing) as the madrel you can just drill through it.

Have you tried a wrap of paper around the tube in the collet block ?

 

[WobblyHand]

Lots of good ideas! I think I'm all set. If one idea doesn't quite work, I'm sure one of the others will. Thanks everyone. I'll start with the simplest and go from there.