Do you want to hold it using  the large area surfaces or the 'thin' way?.

And relative to this what is the direction of the axis of the 40nm torque ? (are you making a thing  on a stick like a lollipop on a shaft. or more like a mushroom ?)

I have for example in the  past. super-glued things onto a a chuck plate. machined them and then dissolved the glue away in hot acetone.  This is probably no good for you, but With a better understanding of hte direction of the forces and which surface can be touched, better methods may become apparent.
mike.

Thanks for the reply.

The way I'd describe it is that the torque is applied in the axis perpendicular to the flat surface of the part.

I'm not sure if that makes any sense so I attached a picture for reference. 

Which way it is held doesn't matter provided that the top surface remains clear.

So the "sides" can be used and the bottom can be used.

But I can't make any physical changes to the part like drilling holes etc. 

OK, 'mushroom' then ;-)  the 'obvious' is to lay the flat area and clamp onto the circumference, but I presume this is what you are doing, and the jaws are damaging the outer edge ?
If the diameter is very fixed then a collet chuck and a spacer column a mm or so less in diameter  beneath to set the height may be an answer.
(example images  taken from    www.rdgtools.co.uk/.../14-15mm-ER32-Collet-2622.html)

plus

This is both cheaper and  less damaging than a traditional 3 jaw chuck

But maybe you could avoid all this and just use the glue trick if precision you need is not great. ?
Mike.

There are others on here of a more mechanical inclination, it may be worth waiting to see what other folk suggest. !

I've been using a 5C collet in a pneumatic fixture to clamp it. It had a step machined so no need for a spacer column. We even got diamond coating put on it to improve the grip but the part is still twisting which results in damage.

I thought about using a chuck too but the part is quite small and it's a production issue because I imagine the operators might struggle and the process wouldn't be very controlled if different operators over or under-tighten the grip on the jaws.

I don't think the glue trick is an option for me unfortunately because I need a process that's quick and repeatable.

Either way, thanks for your input.

Dagmara Sajdak said:

I've been using a 5C collet in a pneumatic fixture to clamp it

Aah. Now the 5c collet is only single ended  split I think, which means it grips very well at the front, but less well further back. This may not matter on a thin sample but the accepted wisdom is that double split collets like ER and TG give a more parallel gripping action  for cutters shanks etc. I am not sure how much this would matter for holding a disk. 

How good is the finish of the circumference of the disk?  - I can imagine diamond only working well  on a smooth surface.
I can imagine an all-round paralell grip is best
Mike

(https://mellowpine.com/blog/collet-types/)

Dagmara Sajdak said:

I've been using a 5C collet in a pneumatic fixture to clamp it

Aah. Now the 5c collet is only single ended  split I think, which means it grips very well at the front, but less well further back. This may not matter on a thin sample but the accepted wisdom is that double split collets like ER and TG give a more parallel gripping action  for cutters shanks etc. I am not sure how much this would matter for holding a disk. 

How good is the finish of the circumference of the disk?  - I can imagine diamond only working well  on a smooth surface.
I can imagine an all-round paralell grip is best
Mike

(https://mellowpine.com/blog/collet-types/)

I did come across that information at some point, but given that I'm interested in gripping 3 mm of the part I didn't think it would make enough difference to warrant buying a new collet since from my understanding the ER collet clamps down in its entire length which I don't think would solve my problem.

The finish on the part is good, probably no more than a couple microns. Some of them are also plated up to 4 microns thick.

I might look into the TG collets if I don't find an overall better solution.

ER collets don't work with short lengths unless there is a balancing piece of almost identical diameter inserted in the back of the collet. 5C is a sensible choice for this. The problem is there is nothing to support the piece longditudanly and if you are applying pressure with the rod the piece is probbably twisting. That's why I suggested a pot chuck below (the thread structure of this Forum is not very good :-( ) This will also take the longditudinal pressure and it shouldn't be difficult to get one turned in aluminium that will fit your existing 5C collet.

The forum is a bit confusing. Occasionally, I get notifications about my own replies too rather than other users?

Anyway, let me clarify what do you mean. Are you saying create the fixture, split it and then use it in a normal 3 jaw chuck or fit it in a collet with the pneumatic fixture? 

Also, won't there be wear very quickly on the aluminium fixture?

Thanks for your reply by the way.

Yes, it will work in a 3 jaw or a collet chuck. Here is what I have in mind:

If nothing slips aluminium should work well enough to show if it works or not. The clamping pressure can be increased by extending the slit across the centre.

What are you doing, friction welding?

That makes sense. I might try that out next week.

Not friction welding. I'm developing an SOP for product assembly and this is one of the parts - it has to torque into another part and that's where the grip problem comes in.