Making an ER collet holder for VN 12 spindle. How close is close enough?

The collet holder I am making is to fit an old VN C taper- The mill spindle has been ground and is within .0002" runout.

The toolholder has an accurate taper and the 8 degree ER recess was bored and faced on the mill.

Now it is on the lathe to turn threads for the collet nut. As the nut has a taper for the collet, the effort is to get the threads very accurate, so the nut pulls up square.

The face is running .0003" out. The concentricity is within .0002. The piece is parallel with the lathe centerline on one axis and within .0002" when rotated 90 degrees.

The toolholder is in a four jaw chuck, and there are two .005" paper shims to get it aligned, on one side and end of the toolholder shank.

I can measure closer (barely) , but can't physically adjust the part any closer-

Are there techniques for making minute adjustments on this sort of thing?

I have never tried to work to this sort of accuracy before. And considering what this is for, it is likely good enough. Chasing accuracy is a good illustration of the Law of Diminishing Returns....

Update on the Van Norman #12 collet holder.

The completed holder is holding under .0008" TIR, 4 1/4" from the mill spindle face, at the end of a 7/16" x 2" endmill. Not bad for a machine born in 1942!

It uses a Maritool ER25 nut and a Maritool collet.
It has two diameters, English and metric threads, two different tapers, a keyway and drive dog slots.
A good challenge for a non machinist.

Accuracy tricks?

The biggest trick I can think of would be to do both the taper and the thread in ONE set up. That would probably have been on the lathe, but it could also have been done on the mill if it were CNC.

At the point where you are, in the lathe I would chuck up a scrap of metal (steel or aluminum) and machine it into a mount for your collet holder. That would ensure the best concentricity possible. Without disturbing the chuck or that temporary mount, install the collet holder on it and do the thread. I think this would be the best way at the point where you are with the taper already cut.

The two tapers on an ER collet are at different angles. The rear taper has a smaller angle and is a longer length than the front taper. So the front taper is designed to push the collet into the rear taper. The rear taper will have the largest influence on the position and angle of the collet and it is the most important where the accuracy is concerned. So I would not worry too much about the threads that mount the nut which has that front taper. A few tenths should be OK there and the rear taper can easily overcome that much error. The nut will probably just seat a bit off the center of those threads.

EPAIII said:

Accuracy tricks?

The biggest trick I can think of would be to do both the taper and the thread in ONE set up. that was my original plan, but due to lack of skill I was unable to set it up with necessary accuracy. Part of the problem was getting the already cut machine taper and shank to run concentric- I think I could do it now, as some folks have pointed out some tips to using a center.That would probably have been on the lathe, but it could also have been done on the mill if it were CNC.

At the point where you are, in the lathe I would chuck up a scrap of metal (steel or aluminum) and machine it into a mount for your collet holder. I am not clear on this- do you mean a mount that would replicate the machine spindle, that the toolholder would fit in and then the whole works be put in the chuck and cewntered?That would ensure the best concentricity possible. Without disturbing the chuck or that temporary mount, install the collet holder on it and do the thread. I think this would be the best way at the point where you are with the taper already cut.

The two tapers on an ER collet are at different angles. The rear taper has a smaller angle and is a longer length than the front taper. So the front taper is designed to push the collet into the rear taper. The rear taper will have the largest influence on the position and angle of the collet and it is the most important where the accuracy is concerned. So I would not worry too much about the threads that mount the nut which has that front taper. A few tenths should be OK there and the rear taper can easily overcome that much error. The nut will probably just seat a bit off the center of those threads.

Click to expand...

I am thinking the threads and necessary clearance probably have the most play of anything on this effort, so a very slight discrepancy would be OK.
Thanks for the reply! Would most work like this would be rough turned, hardened and then go to a cylinder grinder for finish? So as to ensure concentricity all the way through?

This was made from a "blank" turned up with the shank, keyway, dog slots and drawbar threads already cut- and an unturned chunk of bar on the business end. I was making some tooling a few years ago and turned up some extras, as the setup was already in place.