I have a worm gear where the spur has a notch taken out of it (robbed from a sattelite dish positioner and the notch i nthe gear housed a couple limit switches).
I have used this worm gear to build a rotary axis but my problem is that when I move the axis, I need to choose the direction in which to move so as not to drive the spur gear off of the worm.
I am writing custom software to track the location of the axis and write the g-code, but I am not sure how to get Mach3 to choose direction. All ideas welcome.

You will be far better served to FIX the hardware or replace it  Rather than trying to create a software work around.

(;-) TP

Software is not a work-around, or not much anyway. I have a five-axis cnc saw which will need software to calculate the offset of the sawblade relative to the cut. This makes it easy to track the whereabouts of the blade regardless of my dodgy gears. You see as the blade pivots and rotates it dis-aligns itself with the z-axis. Hope this is clear.

Anyway, back to the question, is there a way to choose positive or negative rotation within Mach?

It depends on what mode you intend to run in Abs/Inc.

Inc,  +numbers run one way and -numbers run the other way.

ABS,  higher numbers run one way and lower numbers run the other way.

It is not always as easy as it sounds(;-) especially if you let it wrap the axis beyond 360 deg. Best to stay inside the First 360 deg of rotation.

(;-) TP

Thanks TP, inc is good, and no, the saw will never run more than 360 degrees since it can't ;-)
So the stupid question is:-   is there a gcode for + or - or is it a setting in mach? I take it that "g0 B0" will run in the shortest route to zero?
Thanks for your help so far, Jason.

Hi Jason, just curious, what type of saw is it ?
A 5 axis Bridge Saw ? Or ... ?

If HOME is at one end of travel, say "0",  then you could set the active state of the DIR pin to make all moves the dir you want from home (in degrees).
Just program in degrees and ignore + and - . All positions will be between 0 and 300 (or whatever the max is).
Seems simple, but I might not be seeing it all.
Regards,
Russ

Yes it is a bridge saw, with home around the mid-point of travel. The saw will move 180 degrees positive and 120 degrees negative.
I just cant risk the saw moving the "long way round" to go home.

You could (if you have homing enabled with a home switch) set a Home Off value then all moves would be in the same direction. Then modify your PP or programming to suit.
Maybe ? ? ?
TP'll fix ya up.
That is a neat machine, from the pics I've seen.
Maybe show us yours ?
Regards,
Russ

Yes it is a bridge saw, with home around the mid-point of travel. The saw will move 180 degrees positive and 120 degrees negative.
I just cant risk the saw moving the "long way round" to go home.

Make sure this is not checked in Gen. Config and it will not roll over, it will always go back toward "0".

Grrr, didn't want to show this as it is for work. What the hell, if it gets the point across!

The finished product will cut rafters for timber framed building roofs. You can see the notch in the gear, and as TP says, it is a fault on the machine.