Re: [CAD_CAM_EDM_DRO] Re: fitting ballscrew ends

Leslie Watts wrote:

>Marcus,
>
>I think the cnc hobbyist would be reluctant to pay even $50/end for
>end machining of his surplus ballscrews. That's $300 for an xyz
>machine.(feel free to correct me on that :) )
>
>So he would want a way to make his own accurate ends on a
>HF mini lathe or something.
>
>He could have the enjoyment of turning these end caps
>in his own shop.
>
>
>

This is really pushing the envelope of a 4" - 7" lathe. But, it CAN
be done. What you have to do is sacrifice some of the screw, just enough
to hold in the chuck. You will need a good supply of cutting tools,
and Carbide is the only thing that will handle the hard surface of these
screws. You will need to make or buy a steady rest for the lathe, and
set it up so it doesn't tear up the screw. Probably wrapping a piece of
fairly thick brass shim stock around the screw and clamping it in place with
two hose clamps will do. Then, the steady rest will support the end of the
screw between the hose clamps. You could also turn a sleeve that just
barely slips over the screw, slot it with a hacksaw and clamp in place as
above. You will likely have to remove the tailstock from the lathe.

Grip the screw end in the chuck, and make any adjustments to get that end
of the ball track centered. Set the threading gears to match the screw
pitch
and you can ride a dial indicator clamped to the carriage up and down
the track
by turning the spindle.

When the centering is good enough for you, carefully get the lathe
spinning and
watch out for any signs of "whip" in the free end of the screw. This is
where too
small a lathe really becomes a problem. You don't want to be forced to too
low a spindle speed to prevent the free end from whipping. And, don't
underestimate
how much damage this can do!

Mount the carbide cutter, and try to take as heavy a cut as you can, to
get under the
hardening in as few passes as possible. Yes, it will make horrible
noises, and the
carbide cutter won't last real long. But, indexable inserts can be had
around one $
if you know where to look. (I used to get them for ~ $0.50 each, but
not any more.)
Because of the ball tracks, the hardening is not all at one diameter, so
you will have
to make several hard passes before you get under the root diameter of
the thread.
Then, turning will be much easier, and you can get down close to the
final diameter.
Because of the whip problem, you are not likely to be able to run the
lathe at the
optimum speed, and will have a rough surface.

So, adapt a Dremel, die grinder, toolpost grinder, or whatever you have
to make the
final passes. (Remember to protect the lathe from the grinding grit.)

You may need several steps in the diameter if you are adapting the end
for a pair
of angular contact bearings. The first step should be a clean, square
shoulder
going down to just under the thread minor diameter, which will match an
available
bearing ID. The two bearings will stack close to each other, or
possibly in direct
contact with some bearing sets. Then you will need a threaded portion,
where a
clamping nut or sleeve will press the two inner races against each
other. Finally, there
may be another step down in ID, where the drive pulley or coupling will
attach to the
screw.

Finally, when that end is all done to your satisfaction, you cut off the
stub that the chuck
was holding on to with a hacksaw or bandsaw.

Jon