Re: CNC Router How-To's

Jonty50@... wrote:

> In a message dated 99-05-10 02:17:47 EDT, Gar writes:
> >> Yeah, and it's used on several largish gantry systems I've seen on the
> >> Web. Me bein paranoid, I worry about losing a count, and racking the
> >> gantry, rather than the racking forces of possible imbalance on the
> >> guides. Guess we all have our boogie-men to deal with.
>
> Encoders on the linear drivers?
>
> Drive one end of the gantry with a linear driver riding on leadscrew and then
> use a figure 8 loop of aircraft cable to drive the other end of the gantry.
> With ball bearing pulleys and 3/16" cable tensioned to near its rated load
> (300 lb-ish I think), this works pretty well. I've seen the cable loop trick
> done on a torch cutting rig that was used to cut large shapes from steel
> plate up to 6" thick.

On big gantry bed mills, VBMs and similar machines, they use what they call
a tandem servo. What this is, is two completely separate servo drives, with
encoders, limit switches, home switches, etc. The CNC control is set up
such that it keeps the two axes aligned at all times. They apparently use a
bearing joint at one end, and a bearing in a slot at the other end, so the
machine won't be damaged if the two servo get out of sync. After doing
a home operation, they should remain sync'ed at all times. The software
on my 1970's Allen Bradley 7320 control was set up for this on the Z
axis, which confused me for a while.

Jon