Hi everyone. Newbie to cnc.  When I run the same program twice, the system takes different cnc patterns.  That would be ok except they didn't exactly match. In effect I ended up with two sets of parts when there was suppose to be one.  Sometimes the program would start in the left corner...when running the second time... the next pattern go to the right corner. Any suggestions? 

Unless the G-code got executed in reverse I can't see that happening. Put single block on and strep through the start of the program to see where it goes. If you created the program in a Cam program and told it to make a pattern of multiple instances then it might start the next part at a different place, but you would see the part starting out in the middle of the program not the beginning.

Understand that the "same" program is a set of identical instructions that the controller ( Mach ) executes, but, the controller will take into account
what state it is in and where it is when those instructions are given.

I think you need to have a read of the Mach Manual, understand Machine Coordinates and Part Coordinates and work offsets, how to setup a job, etc.

CNC is not plug and play........

Read the manuals, watch the videos, etc to get a basic understanding.

RICH

Rich,
You bring up a good point, some things are modal and once changed stay that way. So a subsequent run of a program that left something modal in effect could cause the program to work differently during the next run.

^ I was thinking that. If you set and modify parameters during a program they will persist to the next run.
You need to explicitly set parameters to their desired starting point at the beginning of each cycle.
Also, offsets and fixture positions. Cancel all these in the first few lines, then set the ones you want. Cancel them again at the end of the run as a matter of good practice.