how do you measure the fit of a collet?

[FamilyTradition]

That makes perfect sense but I can't see anything but flat when I hold a straightedge up to the taper on our Hardinge 5C collets, even in bright light. If there's a radius it's darn small. I should blue it and see what hits what. OTOH, there are three definite ridges in our spindle that correspond to the slots in the collets. Probably should grind that thing, but I haven't worked up the courage. It can't be good for new collets to be running in an old non-round spindle.

I think thermite was saying they are ground with a radius radially, not axially. I.e. the collets are not perfectly round. The line of pressure is axial as a result.

 

[MCritchley]

That makes perfect sense but I can't see anything but flat when I hold a straightedge up to the taper on our Hardinge 5C collets, even in bright light. If there's a radius it's darn small. I should blue it and see what hits what. OTOH, there are three definite ridges in our spindle that correspond to the slots in the collets. Probably should grind that thing, but I haven't worked up the courage. It can't be good for new collets to be running in an old non-round spindle.

Thermite is fondly recalling his days as a Victorian collet designer when collets may have been ground to a radius.

The taper is straight.

To the op- perhaps place a tight fitting gauge pin in the holder and blue up the collet. Insert into the tool holder and gently spin, check the blueing contact. Leave the nut on the bench.
One thing to think about is what happens to the interface after you tighten the shit out of the nut? You are distorting everything in a non concentric scenario.

You may also check run out on the same pin close to the collet and down the length of the pin. You could do some trig and figure out the angular disparity between the spindle’s CL and your pins true axis.

I really try to avoid using ER collets for anything besides drill bits. You will never know how well everything is seated. Endmills last longer in concentric holders.

 

[rimcanyon]

OK, bluing might provide useful information, but like the ability to move something held in a collet, it is a qualitative measure. I actually was hoping to hear from one of the metrology experts: how are collets verified for fit and for adherence to spec? Collets are strange in that you can't measure the collet alone, you have to measure it as part of a system.

 

[MCritchley]

OK, bluing might provide useful information, but like the ability to move something held in a collet, it is a qualitative measure. I actually was hoping to hear from one of the metrology experts: how are collets verified for fit and for adherence to spec? Collets are strange in that you can't measure the collet alone, you have to measure it as part of a system.

You could always mount the collet on a precision ground arbor ground to the exact bore and mount it on a set of precision sine center’s. That would be good for the taper.

Then spin the collet on the arbor to check the TIR. This assumes the bore is ground perfectly straight. You can always check the bores taper with a set of Deltronic gauge pins in tenths increments.

 

[Conrad Hoffman]

IMO, the best time to measure a collet is before they slot it.

 

[VeryNear Operator]

Have you thought of using pressure paper? Apply it between the collet and the holder or the part and the holder. Tighten it down, loosen and remove and see where it grips and where it does not. We used it to check the mounting of large SCRs and heating.

 

[Conrad Hoffman]

Interesting stuff; didn't know about it. Damn expensive though.

 

[michiganbuck]

I used to regrind collets by strapping rubber bands around and grinding one half of OD length, move the rubber bands and grind the other half to my grease pencil marking.. the set-on mandrel needs to be between centers and run .0002 or less. Good if the take is only a few tenths. 60gt white wheel is good/ok.

Very careful de-bugging inside and outside is a must-do. De-bugging new collets is important.
Most best quality collets run about .0002 max but that doesn't mean a held part will run that close.

 

[Conrad Hoffman]

Cool! I've often thought about making a collet, just to see if I could and how good I could get it. Also wondered how "special" those very expensive Levin collets are and what magic they use to manufacture them.

 

[Conrad Hoffman]

Hey, I have a Jacobs #58B, so all I need is a pin vise! Says right in the vintage Jacobs catalog that it can replace expensive collet systems.