Re: [CAD_CAM_EDM_DRO] Re: sprockets?

In a message dated 12-Jul-02 09:51:09 Central Daylight Time,
dougrasmussen@... writes:

> The little bit of investigation I've done regarding backlash-free connectors
> indicated to me they require some very serious cash to get one with a high
> enough torque rating for a machine in the B'port size range. Or have I
> looked in the wrong places?<<

Doug: Yes, those "Helical" type, or even the "metal bellows" type by
Servometer, are WAY out of line! I used those a couple of times where the
customer had put no limit on material-cost for application-specific plotters,
and they ARE a solution, but the cost is generally so insane as to be
prohibitive. Using a "solid" or "stiff" shaft-coupler, even the "clamp kind"
vs. "set-screw kind" requires the end-machining of the screw and the
motor-shaft to be exceedingly-accurate both in diameter/fit, and "zero angle"
(or would that be "exact 180° angle"?). HARD to do.

>
> I'd like to hear of your bad experiences with timing belt drives<<

At the outset here, I must admit I am thinking about the "rubber toothed
belts with cloth impregnation" not metal links similar to recent
automobile-engine timing-belts. I have used Berg "Min-E-pitch cable-chain"
with NO "slop" nor backlash at all, but that stuff is ENGINEERED at the
outset to do this! I built some small woodworking machinery, once (years
back) with "rubber timing-belts", and after an hour or so of operation, after
"new belts" were installed, the belts "loosened up" and things had to be
re-adjusted. NOT a good plan. BUT, that was for 3600 RPM kinda speed, NOT
"stepper-motor speed".

The only lead-screw motors I have ever seen on a CNC Bridgeport were #42
steppers (with the cast-iron, ribbed housing), and those were "direct drive",
not "belt/chain connected". All the little CNC stuff I have done is "direct
drive", but the motor bearings themselves "double" as the screw-bearings for
the "motor end", the other end having a "deep-groove" bearing.

I imagine it's basically a matter of "to each his own", depending upon
experience, skill, patience, etc....

Jan Rowland


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