Using an emergency collet

[DellParticipant @dell]

I find I need one a 2.1 collet for something on my Pultra P type & it has to be one of the very few collets missing in my collet box so as I have about 20 emergency collets I decided to make one, yes I could use 5/64 as I have a full set of imperial but where is the fun in that & anyway it’s something I haven’t done it will be a learning experience.

I have already put a 2mm drill through the collet & then a 2.1mm end mill & checked it with a 2.1 pin gauge so happy so far, next comes how to hold it in the rotary table to cut the three slots, my thoughts are to make an extra long MT2 arbor to fit into the centre of the rotary table with a M10X1.5 thread into the small end of MT2 arbor & on the other end a M10X1 thread to screw the collet into with a hole in the side to thread a dog point grub screw into to sit in the slot of collet to stop collet turning, but before I start making the MT2 arbor I thought I would ask on here if anyone has any better thoughts.

Dell

[DC31kParticipant @dc31k]

To make any proposal we need to know the dimensions of the collet.

What is the diameter – can we assume 10mm?

How long is it? How far from the front do the slots need to extend? An alternative formulation of this question is how much unslotted length is there to grip?

To me, gripping the threaded end in an ER hexagon collet block seems plausible.

[DellParticipant @dell]

The collet is for a Pultra 10mm lathe the reason for cutting in the rotary table is to cut the three slots @ 120 degrees & it has a small hole at the end of the slot & I want it to fit into the MT2 arbor to then fit into the rotary table for cutting.

[DellParticipant @dell]

This picture may help to give an idea of my thinking, the MT2 arbor is for Illustration purposes as it’s the one I use to centre RT on mill quill

[JasonBModerator @jasonb]

Should work. I would add a sacrificial ctr plug so you can use some tailstock support too.

It will need quite a small diameter saw to go all the way through to the 2.1mm bore yet avoid cutting into the far side of the cross holes and your arbor.

[DellParticipant @dell]

I am probably being thick but what is a ctr plug ?

because I will be cutting from the side I will use an angle block against the other side of the cut

[JasonBModerator @jasonb]

Just something to put in the 2.1mm hole so you can locate the tailstock centre in without risk of running the saw into the centre. that will support it both vertically as well as horizontally.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dell, ctr is often short for centre.

Regards Nick.

[PeteParticipant @pete41194]

ctr = center plug in the 2.1 mm collet bore. With the length of the unsupported cut, chatter and vibration is likely. You might also machine something like a horse shoe shaped clamp to allow the slotting saw access while still holding the end of the collet rigidly against the angle plate might help? But I think Jason’s idea is even better.

[MacolmParticipant @macolm]

Assuming the need is for a one off job, the (very satisfactory) fudge I use in these circumstances is to turn a sleeve, ID 2.1mm, OD to fit an available larger bore collet. Clearly both diameters must be turned at the same time without disturbing the sleeve so as to ensure concentricity. Cut the sleeve to length, then split it lengthwise from one end with a junior hacksaw nearly but not quite all the way along. Now cut it similarly from the other end with the cut at 90 degrees, again nearly all the (opposite) way. Deburr etc. It is best made from aluminium.

Fit it into the larger collet so that none of the slots coincide. Tighten the larger collet onto the 2.1mm diameter work. It will be found to grip well and be nearly as concentric as the unavailable collet would have been.

[Macolm]

On Macolm Said:

Cut the sleeve to length, then split it lengthwise from one end with a junior hacksaw

With a small diameter sleeve, would we need to consider slit width, and hence the tool used to produce the slit?

The ID of the sleeve here is 2.1mm. Its perimeter is about 6.8mm.

What is the kerf width of a junior hacksaw blade?

It is not clear to me if midway aong the sleeve there would be two cuts or four cuts. With four kerf-widths removed, there may not be much of the circumference remaining.

Maybe the OP could comment on the slot width that is used on OEM collets of similar diameter.

On some collets I have, at 1/16″ size, the main slit stops short of the bore and a much thinner slit continues.

[JasonBModerator @jasonb]

DC31K, Like malcom I have made many simple sleeves, just a single lengthways cut is usually sufficient provided the OD is not too large. Piecing saw would have a narrow cut if Junior too big. Or hold in a vertical vice and run a 0.3 or 0.4mm slitting saw down it.

how much circumference do you think is needed to drive the part? if held in a 3-jaw you are not going to have much contact area so even a 1mm wide slot or 3 would give plenty to grip. Assuming it is not a tube wher other factors come into it.

[DC31k]

On DC31k Said:

On some collets I have, at 1/16″ size, the main slit stops short of the bore and a much thinner slit continues.

When commercially manufactured collets are made they are not fully split until after heat treatment and grinding, they are then finally slit through with an elastic wheel hence the different width at the end.

Andy.

[MacolmParticipant @macolm]

I was just outlining the general method, 2mm being near the lower limit if using common hand tools, but clearly any form of narrow cut tool is a possibility. The aim should be that the sleeve is made as a push fit on both diameters. The poorer this detail, the poorer the achievable concentricity, but with a bit of care it can be very good.

[DC31k]

On DC31k Said:

[…] Maybe the OP could comment on the slot width that is used on OEM collets of similar diameter. […]

Mmmm … He has already provided a useful photo

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Here’s a cropped version of Dell’s useful photo

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MichaelG.

[Author]

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