Reboring Emergency Collets

[Clemson]

I have several used 5C Hardinge "emergency collets." These collets were apparently shipped with three pins in the top holding the collet fingers in fixed position while the collet was drilled out to a specific dimension.

I would like to redrill or rebore some of these collets. How do I proceed? Can anyone tell me the size of pins that Hardinge would have used on 5C emergency collets? Do I insert 3 pins, draw up the collet, and simply drill away?

Clemson

[Victor_R]

Hi Clemson,

Yes, you have the procedure correct. I use some 5Cs but do not have any emergency types so I'm not sure of the exact pin size. However, if I were in your position, I'd gauge the size using drill bit shanks (fractional or number) to select a slip fit or just a tiny bit looser, but not much. Then I would make pins from a small diameter steel rod turned to the gauged size if needed.

You want the collet to compress a little, but not a whole lot. Also, some pins I've seen in the catalog pictures have a small head on them to prevent them from falling through. So you might want to make yours in the same fashion.

To size the opening of the collet I would definitely bore with a HSS tool bit rather then drill if at all possible. Carbide might not like the interrupted cut. Boring will produce the most concentric and correctly sized hole. If you must drill, I'd use a short drill to keep the rigidity and prevent the drill from flexing. And one that was either new or correctly and evenly sharpended, and feed it slowly. If the original hole was off center or out of round, drilling will probably only exacerbate the condition. Boring will correct it.

Victor

[Harold_V]

Victor lead you straight. Do not drill your collet(s)---the hole won't turn out very functional due to the two flute drill cutting on three segments. You're very likely to end up with a hole that is not round, and will mar parts when used, even if you can get them in and tightened. Collets should be used within a narrow margin of size variation, something like ± .001", so the collet grips uniformly, front to back.

Every emergency collet I've ever used was drilled and reamed for 1/8" pins. You can use a short dowel pin, which well be straight and uniformly sized. That's important if you want your collet to run properly when you're finished boring it.

To bore your collet, set it up in the same fashion you would if it was being used, so you duplicate operating conditions. After boring, remove the pins and clean the collet and closer well so you don't have chips interfering with the fit. Be mindful of machining a step if your parts must have length control. If you hold the diameters constant, a shoulder can act as a good stop, but diameter variations equate to different lengths.

Harold

[Gopher Baroque]

I would like to redrill or rebore some of these collets. How do I proceed? Can anyone tell me the size of pins that Hardinge would have used on 5C emergency collets? Do I insert 3 pins, draw up the collet, and simply drill away?

Post #16 of Thread: Emergency collet machining https://www.practicalmachinist.com/vb/g ... ng-359744/
notes that Hardinge pins are 0.125 and import pins tend to be 3mm / 0.119 dia.

[Clemson]

Well, it has been 16 years, but I still have those collets, the same old South Bend lathe, and am happy to confirm that the 1/8 inch pins are the right ones to be using!

Thanks!

Clemson