Re: STG2 - Latest Developments

[Jon Elson <jmelson-at-artsci.wustl.edu>]

JohnDRoc-at-aol.com wrote:

>   I still think that the +/-
> settings are backwards, because the count should go (+) when the table moves
> to the left and (-) when it moves to the right.  I think it may have to do
> with the fact that it is a "CNC-made" machine and not a retrofit - the motors
> are opposite where they would be on a retrofit.

Well, the real reason, assuming a machine similar in general to a Bridgeport,
is that the nut/screw mechanism is reversed on a Bridgeport manual machine.
So, on the X axis, the screw is attached to the table, and the nut is fixed
to the saddle.  On the Y axis, the screw is fixed to the knee, and the
nut pulls the saddle front and back.  If both screws are right hand thread,
the motion is going to be positive in both axes for clockwise rotation of
the screw, from the front for Y and from the right for X.  Now, if you
mount the encoder on the LEFT side of the table, that will see a
COUNTERCLOCKWISE rotation for positive motion.
The combination could be different for different machine layouts.

So, this is one way that opposite rotation of the encoder could result
from motion in the positive direction.

Note that on CNC systems, the positive direction is considered from
the PART, so if the lower left corner of the part is (0,0) the upper
right would be (+X, +Y).  This would require the table to move
TOWARD you and to the LEFT, with you standing in front of the
milling machine, to go from (0,0) to (+X, +Y).

Obviously, some of this is reversed for gantry mills and machines of
different construction from a knee mill.

So, stop worrying about the signs of the encoder counts, and either
reverse the A & B wires on any channel that reads backwards, or
leave the minus sign in the encoder scale parameter.
Note that if you make a change like this at only one place on
an axis that works correctly, only backwards, you will also have
to reverse the motor wires and tach wires, to keep all signs
consistant.  If you change the encoder scale sign AND reverse
the A & B wires on the encoder, those changes cancel out.

Jon