Clearance for split cotter screws

[ericc]

Hi. There is something I don't quite understand about these instructions for making a simple boring head:

Lathe Boring Head - HomemadeTools.net

A small boring for the lathe made many years ago and has performed very well. It can be easily made on the lathe from M/S as was this one, and as I

www.homemadetools.net

It seems that the holes for the split cotter retaining clamps for the boring tool shanks are too close to the center line of the holes for the shanks. If my calculations are correct, there is only 1/16" for the screws holding the clamps to clear. Of course, I didn't figure this out until right before this step. It seems that the split cotter screw size was not specified. It certainly is not #10. Maybe #8, but that still would not clear. It seems that one would need to go down to #5, but that would still be tight. Clever grinding on the central threads of the screws; would that do the trick? A good use for a Dremel type tool?

 

[jwmelvin]

That is kind of skimpy. 1/8” diameter to (barely) clear. I like the idea of relieving the central portion of the screw and using something like #10-32.

 

[whitmore]

Hi. There is something I don't quite understand about these instructions for making a simple boring head:

Lathe Boring Head - HomemadeTools.net

A small boring for the lathe made many years ago and has performed very well. It can be easily made on the lathe from M/S as was this one, and as I

www.homemadetools.net

It seems that the holes for the split cotter retaining clamps for the boring tool shanks are too close to the center line of the holes for the shanks.

Would it help if the tightening screws weren't on-center? You'd want circa 1/32 inch clearance,0.031", or the
threading operation could telegraph to the outer wall of the cotter, and a #10 is about 0.192" diameter...
I like collets, but the four-jaw chuck can come off the shelf for this.

 

[ericc]

An off center split cotter clamp is a very interesting idea. That would probably do it.

I just got done with a slight scale-down of the plans. I went with 5/16" round and a #8 screw. It works just fine. Those split cotter clamps hold well, but the do take up some space.