Re: [CAD_CAM_EDM_DRO] Re: Encoders on the opposite end of the shaft

jmw@... wrote:

> Thanks, Brad, re the lore on your Tape 'O' Matic. If it's good enough
> for P&W it's good enough for me. Could you take a look at the
> connection between the encoder and the screw? I assume the encoder(s)
> is rigidly mounted to the underside of the table and coupled to the
> screw with some sort of flexible coupling (what kind?)--or maybe the
> other way around--rigidly coupled to the screw and mounted to the
> table on rubber. How did the P&W guys do it?

Rubber is no good. It can develop a 'set' that can affect accuracy. If
the two shafts (leadscrew and encoder) are not very accurately concentric,
then a cyclical error will develop, even with most types of couplings.
The bellows coupling is the one most resistant to this.

I used a one-piece helical-slitted coupling for my encoders. It is working
very well, and will probably outlast the rest of the machine.

There are some other reasons to not have encoders on the opposite end
of the leadscrew from the motor. Torsional resonances can develop at
various frequencies, which change depending on where the nut is.
That makes it hard to compensate the servo loop, so you end up
reducing bandwidth substantially.

Jon