This is probably related but what could be causing the following in the attached picture?

In the bottom cut zero was just above the plywood surface and marked at 1st zero. As it cut the z got progressively deeper a small amount each line until it plunged deep. Once I stopped it and hit go to zero it moved to the spot marked zero after and was buried in the wood.

2nd try at cutting I lowered the Z velocity and let it go. It was still cutting just barely deeper each pass until it plunged and continued. When I sent it back to zero it went to the spot marked zero after and again buried in the wood. It did not run as far before unexpectedly plunging but I expected it to zero in the wrong place to the right of my 1st zero not the left of it.

Could it be velocity causing to to be off like this?

If so that still doesn't explain the plunging at random points in the same unchanged g-code file.

(edit: offending attachment removed, thread should be fine again - scottn)

Something wrong with this thread as I am not able to view the last responses, so not sure if this will show correctly.

Does your machine have servos or steppers? Do you have any information on the drives/motors? I looked at the site you linked to but good info is sparse.
Hood

Scott fixed it.

Steppers and the part numbers are in the first post.

I Did a simpler cut yesterday and it went much better. It was supposed to be 8" tall (y axis) and 6 1/4"wide (X axis). Ended up 8" exactly tall but 6 3/8" wide.

Going to try cutting it several more times to see how it does.

Ok easiest way, assuming you dont know the pitch of the screws or microstepping of the drives would be to use the Set Step button on the settings page. It will be as accurate as your measuring so if you have means of measuring accurately then you will get close.
Hood

Just got back from work so I'm going to give it a shot.

The spec sheet I was able to find for these steppers says the screws are 1.8 degree pitch.

I think that will be the motors it is referring to rather than the screws. Steppers are usually 1.8 degree or in other words 200 steps per rev.
But you also have microstepping in the drives to take into account and then any gearing plus pitch of screws.
Hood

Ahh, I'm learning more as I go along. My tech guy tells me if I can figure it out I can probable get the micro steps engaged and get even more precision but I'll wait until I get this nailed down first.

I would imagine your drives already have the microstepping set, depends on the drives but usually the Chinese drives have adjustable microstepping and it is set with dip switches.
Hood

oK, Maybe this is a stupid question but if I have metric steps from the CNC sales person and I have mm selected under "Select Native Units" when I run the step calibration and it asks for the distance to travel is it asking for mm?

Yes, basically it is asking for distance in Units, so if you are set in metric then a unit is a mm, if set Imperial then a unit is an inch, if its a rotary axis then a unit is a degree (well normally on the rotary, could be a rev depending on your setup )

Hood