How do you hold a small screw to machine a dog point?

[AlexK]

I need to machine a dog point on a 10-32 screw that is 1/2' long. In other words I need to cut down some threads off the end of the screw down to it's minor diameter.
What is the best way for securely holding a small screw in a lathe?

[Ian B]

Various ways; machine a recess in soft jaws to match the head.

Or bore a hole through a piece of round stock the diameter of the thread, counterbore the rod to the head diameter, leaving a small amount still at thread diameter. Then internally thread the counterbore, slip the screw in and screw a grub screw into the counterbore to bear on the head.

Ian

[roundrocktom]

Very carefully! Couldn't resist.

1. For 10-32, I'll machine up a special holder. Just a 1/2" rod, counter sunk for the screw head and tapped 10-32 on end. Once you make that, use a 0.020" slitting saw and cut down one side. Now with my three jaw, it will pinch down on that gap, holding the crew in place.

2. For something 1/4" and larger, I'll cut slit a nut lengthwise, and use the three jaw. Admitted I might induce a 0.001" error, but all I'm doing is leaving a clean end so is is easier to start back into a hole.

3. If it fits into a collet, from the back side, just use some shim stock around the threads. Once the collet is tightened, it will hold nicely.

[PeteM]

Also, a soft collet machined to fit.

With 1/2" of thread you could also use a 1/4" threaded standoff -- and chuck the standoff.

[gellfex]

One more way would be to bore a piece of 1/2" AL for tapping the 10 thread, then counterbore for a 3/8 thread so you screw it in, then screw in a 3/8 set screw to hold the head tight inside the counterbore. Hold the whole thing in a collet. Done well it should be more accurate than the slit and 3 jaw.

[JCHannum]

Double nut it.

Unless the point needs to be precisely centered, run two nuts down on the bolt, draw them up very tightly and chuck on the rear one. Take light cuts, of course. Most hardware nuts are stamped, and the threads might not be concentric to the hex, but they will be close enough for most purposes.

[Mike Hunter]

Drill & tap an emergency 5C Collet

[tylernt]

With 1/2" of thread you could also use a 1/4" threaded standoff -- and chuck the standoff.

Wouldn't the screw tend to unthread itself out of the standoff, towards the headstock, as soon as the cutting tool applied pressure?

Maybe I have my directions reversed though.

[beanbag]

Wouldn't the screw tend to unthread itself out of the standoff, towards the headstock, as soon as the cutting tool applied pressure?

Maybe I have my directions reversed though.

Cut it on the back side (away from the operator) then.

[oil mac]

Some months ago, I had a model engine to rebuild, that required 5B.A. screws (British for model and instrument work) about 1/2" total length, with a 1/8" parallel length turned down to the root dia of thread
The method i adopted, was small length of 3/8" dia brass in three paw chuck drill &tap five B.A. Screwed in screw with lock nut, set up in my instrument lathe &by using a hand graver cut each parrallel section down to size, about three minutes each component
Certainly it was a small batch excercise
Dan.

[RobbieKnobbie]

Cut it on the back side (away from the operator) then.

I've actually done this - cutting on the front side even!

I took light cuts and everythiing worked out OK. If I were doing more than one or two of them, I'd probably try that slit collar idea. Thanks guys

[38_Cal]

You've got a 1/2" long screw, and need just a short section turned, right? Thread the screw into a 13/16" (or other convenient size) round die and increase the pressure on the threads, clamp in your 3-jaw or even in a collet, turn the end, take it out and repeat.

David

[SGW]

John Wilding (the clock guy) had a design for a small screw holder for just that sort of job. I'll try to describe it.

Imagine an arbor, 1/2" shank on one end, the other end larger and threaded, say, 1"-16, with pains taken to have the thread concentric and the end of the arbor flat and square.

You have a nut threaded 1"-16 to match the arbor, but one side of the nut has a 1/8" thick plate silver soldered or welded to the nut so that side of the nut is blocked off. The back surface of the plate (the one inside the nut) should be accurately perpendicular to the arbor's axis.

Chuck the arbor in the lathe so it has minimal runout (e.g. hold it in a collet), screw on the nut, and bore a 1/2" hole through the plate covering the front of the nut. Don't bore into the end of the arbor, which should be touching the back side of the plate. If you scuff it up a little, face it off again when you're done boring the hole.

Now you make a bushing out of 3/4" dia. rod. Turn down a section of 3/4" rod to 1/2" dia., the length of the 1/2" dia. section being the length of the screw you want to modify minus the amount you want sticking out. Drill a hole in the center of the bushing equal to the clearance dia. of your screw and part off the bushing leaving maybe a 1/16" wide 3/4" dia. shoulder.

Drop the bushing into the hole you bored in the cover plate of the nut, from the inside. Drop the screw into the hole in the bushing. Screw nut+bushing+screw onto the arbor, until the head of the screw hits the end of the arbor. Tighten nut to lock screw in place with end sticking out the hole in the bushing.

A friend of mine took the trouble to make a whole set of various-length bushings for screw sizes from #2 to #10. He has them all in a rack, nicely organized, so whenever he wants to work on a screw he just grabs the appropriate bushing and he's set.

Making such a set would be quite a commitment in time, but once you were done it sure would be handy.

[lane]

Just put it in a 3/16 collet.

[randyc]

You've got a 1/2" long screw, and need just a short section turned, right? Thread the screw into a 13/16" (or other convenient size) round die and increase the pressure on the threads, clamp in your 3-jaw or even in a collet, turn the end, take it out and repeat.

David

Good idea !