Re: [CAD_CAM_EDM_DRO] Machining Ball Screws

Graham,

First, some random thoughts on your problem, then I'll talk about what I do.

1) The balls roll on the inner track of the threads. If the outer edges
were marred by the lathe chuck, it *MIGHT* not matter.

2) Chucking the screw by the outer surface will have one of the potential
(unproven) problems of the solution I use to reduce ball screw diameter.

3) I always have trouble finding the exact center for my lathes tailstock.

4) The inexpensive ball screws I get from McMaster-Carr look like they are
black-oxide finished, appear to be at least case hardened.

5) I am concerned about applying strong radial forces to the ball screws,
as I don't want to bend them.

So, with this in mind, I was machining a ball screw yesterday.

I need to turn the 5/8" ball screw down to 1/2" for a coupler and bearing
assembly. My toy lathe is not nearly good enough for this, so I'm using a
bench grinder and a fixture I welded together.

The ball screw lies in a piece of 1"x1/8" steel angle, which is supported
on one end by an attachment point to the grinder's tool rest, and at the
other by a clamp to the worktable to which the grinder is attached. There
is a notch cut in the angle for the grinding wheel. There is a stop in the
angle to limit the length of the cut.

To grind down the ball screw, you must rotate it slowly in the angle
trough, and move it past the grinding wheel. Grind it gently, taking off a
couple of thousandths at a time, especially at you approach your target
diameter.

I realize that this is a kludge, that it only approximates a centerless
grinder, and that the precision is limited by several factors:

1) evenness of the operator hand pressure compounded by stiffness of the
fixture
2) concentricity of the outer (uncontrolled and unspecified) part of the
ball screw with the true axis of the threads
3) surface finish

The prototypes I've ground are good enough for my purposes, however.

I've done this with the inexpensive McMaster-Carr rolled ball screws, and
with "hardware store" threaded rod. I haven't work with premium ground
ball screws.

Again, I'm certainly not claiming this is ideal -- maybe not even good or
aesthetic -- but it is working for me.

-- Carl


At 10:59 AM 9/30/2002 +0000, you wrote:

>I am about to start working proper on a CNC coil winding project I
>have had in mind for a while now. I recently discovered that
>McMaster-Carr do nice ball screws that they sell by the inch. This
>seems perfect as I need only a very small length.
>
>One question I have concerns machining the ballscrew ends, one for
>the stepper and one for the support bearing. I have a flexible
>coupler for the stepper motor end but will need to reduce the
>diameter of the lead screw. What is the best way to hold the lead
>screw in the lathe while it is being turned down. I assume it
>shouldn't just be gripped as is. I considered just putting a length
>of heatshrink over the threads but getting it wrong and damaging the
>threads would be costly in time and money.
>
>Any advice appreciated.
>
>Graham
>
>p.s. Does anyone have any leads on short (<10") ballscrews with 1mm
>pitch at low prices? Also cheap lead screw support bearings.

Carl Mikkelsen
Oasis Semiconductor, Inc.
201 Jones Road
Waltham, MA 02451
(781) 647 8775