RE: [CAD_CAM_EDM_DRO] Re: Timing Pulleys for CNC Work - OK or Not ???

Professional spark eroders use belts and ball screws. Some of the ball
screws are as course as 5 tpi, and these machines repeat to 5 micron with
ease, and better than 3 if you are careful. (Note these machines have a
temperature controlled bath that every thing sits in. The only difference
with a mill is that there are much greater loads, but to keep the ball screw
back lash under control, many spark eroders also have very tight ball
screws. And before someone sakes, the encoder is mounted on the motor, not
the screw, which theoretically should make things worse.

You may note some discussions on here regarding belt stretch, this does
exist in machines I build, but the belt is 10 meters long and only 5mm wide,
so one would expect a little stretch, and it's not much, about 5mm as it
beds in.

With a short drive belt on a machine, you will need a slotted motor mount to
tension it, but after that should be no problem.

As an indication of size, bigs commercial mills with a 5 tpi ball screw, and
6KW servo use 1.5 inch HTD belt.



Regs Mark



_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of turbulatordude
Sent: 25 November 2006 03:52
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Timing Pulleys for CNC Work - OK or Not ???



--- In CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com, "Paul Alciatore"
<paulalci@...> wrote:

>
> I have been a lurker here for some time but now is the time to get
> more
> active as I have some CNC projects to work on.
>
> I am presently working on my first CNC adaption on a SB lathe. I
> plan
> to use steppers with a gear train to provide finer control and more
> torque with less expensive steppers. I know that many designs just
> attach a 200 step stepper directly to the lead screws, but I believe
> this approach will be better and perhaps not cost a lot more in
> materials.
>
> Anyway, my question is about the connection of the final shaft of the
> gear train to the lead screw. I would like to avoid an exposed gear
> (the main gear train can be enclosed) so I was wondering about using
> a
> pair of same size timing pulleys to transmit the motion. I know that
> the belts are reinforced and do not appear to stretch much but are
> they
> really suitable for this kind of service? What kind of errors could
> I
> expect from such a linkage? Does anyone here have any experience
> with
> then in CNC work? If suitable, how would I calculate the pulley size
> to use based on the expected torque? Or what should I base the size
> on?
>

The accuracy and power transmisstion of timing belts is extreemly
high. Should not be a concern. Since gears will have some play,
they will have much more error than the belts.

Visit Stock Drive SDI.com and check out the technical data sheets
they have. You can get a belt that will handle your lathe.

I have a 9" and made a bracket that replaced the banjo and mounted a
stepper on that, then used a belt on the leadscrew.

I never did threading with it, but it can spin the leadscrew easily.

Manual pulse generator so I can set the feed rate. The 9" uses the
origional screw and half nuts so the errors are too large now for
considdering going to CNC as yet.

Dave





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