Re: [CAD_CAM_EDM_DRO] Re: making ball nuts

From Smoke the gunsmith, ex-engineer:

Cutting a square thread on a lathe is no more difficult than cutting an acme thread! The hardest part here is grinding the correct geometry on your tool bit. and setting the tool post correctly. The same applies to the ball screw thread cutting geometry.

The biggest problem you'll run into during hardening is warpage of the shaft. We tend to cut custom chamber reamers over size, harden them and then grind to final size due to warpage in heat treating.

If your only making the nut, the best way to harden is to mount the part in a drill press, and turn it on. While the part is rotating, heat it with a torch until cherry red all over and then RAISE a bucket of oil around the part while it's still turning. This all works for small shafts and helps eliminate warpage.

Tempering in the oven works great.

Heck, give it a try...maybe it will work. At any rate you'll learn something that you can past along to the rest of us.

Smoke


-----Original Message-----
From: jmw@... <jmw@...>
To: CAD_CAM_EDM_DRO@egroups.com <CAD_CAM_EDM_DRO@egroups.com>
Date: Thursday, October 26, 2000 8:54 PM
Subject: [CAD_CAM_EDM_DRO] Re: making ball nuts


I started thinking about making ballnuts after inspecting the MSC
catalog and discovering that preloaded ballnuts are likely to cost
about $300 each. The current catalog actually doesn't list 1-1/4 .200
preload nuts, but similarly-sized non-preload nuts are going for
multiple benjamins.

Not everything worth doing is worth doing well--just to the point
where marginal benefits equal marginal costs. It may be that ballnuts
must be made to very high standards of accuracy and with a precise
thread form in order to get decent CNC service from them. Or maybe
not -- perhaps the gothic arch form just extends the life usefull
life from x to 2x. If x is 1000, well that's enough for me; the next
poor devil can worry about hours 1001+.

I've got an old Roper 20 ton bench punch which has a recirculating
ball screw. You want a 1/2" hole in 5/16" mild steel stock? No
problem. Just offer up the stock, crank the old girl around about 270
degrees and whammo. This tool dates from a time when religion tended
to be a more important part of people's lives, but I doubt it has
gothic arch threads. The screw does not; I'm not sure about the
threads in the punch body. Anyway this unit has been working for
about 75 years.

Suppose, however, that arch threads are a sine qua non. As Jon points
out, grinding the gothic arch shape with a form wheel on a long
spindle tool grinder definitely is not a machine shop 1A project. Now
shaping a toolbit to the arch shape shouldn't be too bad, especially
not for someone with a fine touch (I'm not claiming to have one), an
optical comparitor, and actual knowledge of the required shape. It
might be necessary to vivisect a donor nut to figure out the thread
profile, plus a lot of time with measuring wires and recollection of
trig identities.

The next thing that comes to my mind is, given a good lathe that will
cut a .200 lead, how hard will this form be to turn. I have never cut
a square form thread, but all commentators agree, "You don't want to
do this; square is a royal pain, try acme instead." Maybe the gothic
form presents the same kinds of problems. I just don't know.

The recirculation problem might not be so hard as cylinder head
porting. Porting is hard because of pulsations in the inlet channel,
compressibility of the air-gas mixture, the profound desire of the
gasoline to drop out of the air stream, keeping velocity up at the
bottom end, the long aspect ratio of the shape to be sculpted, etc.
My intuition is that hand porting an exit and re-entry ramp for a
train of balls would not be that bad. But again, I just don't know.

I don't think hardening would be a real problem. It would seem that,
as a practical matter, there're only be two choices of material: O1
or A2 drill rod. Given a decent furnace, I guess I'd go for A2 and
hope for better dimensional stability with an air quench. But O2 is
more fun: heat with a big, soft flame till non-magnetic, plunge
straight down into a bucket of used motor oil, then temper in the
kitchen oven till the whole house smells like a highway paving
project.

Maybe, just maybe, a good enough part could be made without grinding,
or by "lubing" some (slightly undersize?) balls in your favorite
compound and running the nut back and forth on a sacrificial length
of screw. When the fork comes out clean, you're done.

So--that's the plan. Utter foolishness, or maybe not? Pls advise.

--Jack


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