Hood,

I watched it closely this time.  The tool position was exactly where the toolpath display said it should be.

This is what happened today:

The first two parts ran flawlessly

The third part started off ok but then after a tool change mach3 executed the z move first, milling down into the part in the wrong location and then plowed through it to the proper x and y coordinate where it proceeded to mill the pocket

This is just one example sometimes the random moves can be far from their proper position.

Would it be possible for you to try something. Can you put
M5
M1
M3

In your code just before a tool change and then in General Config set it to ignore tool change and see if it runs ok. Obviously you will have to remember you have done that as all code you load with ignore tool change set will just run straight past a tool change unless you do as above on every single tool change.

With the above the spindle will stop and so will the code, once you have changed tools pressing start should start the spindle and the file will carry on.
Hood

Well I just made the changes you suggested and ran 5 parts without any problem.  That just might be the solution.

I am curious what is going on here behind the scenes.  Is the M6 command contributing to the random moves?

Another interesting thing happens with your code, After a tool change the spindle now starts at the previous speed and noticeably changes to the new tool speed. I don't remember this happening before.

I am thinking it may be something in the M6End macro that is being ignored  during the toolchange. The M6End macro is only used when in the Stop Spindle and wait for Start mode.
 It will likely need a bit of time before we can be sure this has solved matters but fingers crossed.
Regarding the spindle speed change, if you have always got a spindle speed and start after a tool change then you wont need to put the M3 in as I said above but it is a safety thing in case your code doesnt have a new spindle speed called. With the M3 there the spindle will start at the last commanded speed and then when another speed is called in the code it will do that speed. If there were no M3 in the lines I said to add then the spindle would start, when commanded later in the code, at the new speed. As mentioned however missing the M3 out could lead to trouble if there were no new M3 call further in the code.
 Hood

I have similiar problems every now and then. Always after a tool change. Usually the table will go to correct x/y position then before the z move it fires off in the wrong direction and cuts the part 6 or so inches away from the correct position. I just press stop, jog to a safe z then run from the last tool change and all is well. It never looses position it anything just has a brain fart

Hood, Thank you very much. I think telling Mach3 to ignore tool changes has solved the problem.  It has run lots of  parts now without a hitch. Thanks again.

Good to hear it seems to be working
Hood