WRONG: Program exceeds machine minimum on axis

[torinwalker]

I've had an EMC 2.4.6 machine for about two years now and I've run dozens of parts through it, but recently I've been commissioned to mill a 11" x 10.5" x 4.875" positive mould of (half) of a helmet, and I've run into a big problem.

The Z axis on my machine has a range of 5.75" - I know this because not only did I set it up myself, but also I verified it last night while checking over my machine for something I might be doing wrong. I can move the quill down to its soft min_limit, touch off z = 0, then move up to the max_limit. Distance travelled = 5.7499" - so, I know I have the range.

So I put the workpiece on a jig, moved my tool down to the jig surface (on which the workpiece sits) and touched off to zero, and moved the quill back up to ensure adequate clearance of, oh, a 1/2" or so.

I see the program laid out in AXIS and it clearly shows the plot as being contained entirely within the soft limit envelope (none of the axis indicators are showing red), and the entire workpiece shows 5.25" including all moves which is what I programmed into my CAM program. Which is also within the limits of my machine - supposedly.

Yet, when I run the program, I get "Program exceeds machine minimum on axis z", and despite what I do, I can't seem to fix it. I spent all sorts of time screwing with my machine to increase the limits (moving limit switches further apart), adjusting parameters, double checking my configurations... nothing seems to matter.

I read a post somewhere indicating it might have something to do with the size of the program. My program is three parts - ~5500 lines of roughing code, and another ~4000 lines of two types of finishing passes. I've also broken it down into two programs, roughing and finishing with the same results.

I'm looking for clues. If you have any anecdotal solutions to "exceeds limits on axis" problems you've encountered, I would be very glad to hear them.


Torin...

[lwill]

If you are touching off zero to the top of your work, then all your cuts are going to be negative.
In your .ini file your MIN_LIMIT must be set to -5.25. (or possibly -5.7499 in your case?)
What is yours set to?

I believe this will change your soft limit, so you will have to be careful if you re-zero lower you do not crash into the table. This is what your limit switches should be there to prevent.

[Andre' B]

Does your program activate a tool length offset?
Look for "G43 Hn" just after a tool change.
If you are touching off the end of the tool the tool length needs to be zero or not active.

[torinwalker]

To answer both questions at once, I am touching off on top of the jig which is the Z=0 for the workpiece. The workpiece is roughly 12x12x5" high and sits on top of the jig. If I touch off the top of the workpiece, then I would touch off Z = 5.000. (I don't normally touch off the top of my work because the reference point gets milled away.)

Secondly, I either touch off with a probe that I use as the zero reference and then do a tool change, or I do a T326 M6 prior to touching off and then touch off to the zero of the workpiece (jig reference).

What's really odd is that after loading the program and touching off, AXIS still showed the tool path as being entirely contained within the machine's working envelope so I am very confused as to why the program thinks it would exceed the min length when axis clearly doesn't agree!

Do you guys know if the AXIS envelope remains aligned with the physical dimensions of the MIN/MAX limit axes parameters, or does it float around? If I remember correctly, it is only the program tool path that moves relative to this envelope. I.e. If the program fits in the envelope, then it should be fine.

[chester88]

Have you even tried telling AXIS to ignore the limits and run the program ( I think that option comes up)? I would try it on air without tool first

I remember the developers discussing that some times AXIS doesn't calculate the limits properly. It may be improved in linuxcnc 2.5 due out at the end of March. I can't remember the details.

[torinwalker]

Well as it turns out, I was able to get it to run finally. I touched off with a tool probe (whose length happens to be zero which makes it easy if I forget to do a T1 M6 G43 H1), then ran the program. What's weird is, the tool path was half above and half below the min_limit line depicted by the axis work envelope, but when I set the machine this way (and expected it not to run), it simply said "Enter tool 326 and press continue". It worked!

To think all this time I was trying to get the toolpath to sit within the envelope only to have it tell me it's outside the limits - I should have done the complete opposite, contrary to logic.

Anyway, I managed to mill the left half of this guy's helmet mould for his upcoming thesis, and I will be on time with the delivery.

Thank you everyone. Believe it or not, posting here actually helped me think a little more clearly about the problem and got me trying stuff I wouldn't have tried otherwise.


Torin...

[lwill]

A couple more suggestions to try:
Do you have home switches and home the machine before starting?
Setting zero is only good for the coordinate system you are currently in which does "float around" inside the machine system which is what the soft limits refer to.
Under "View" menu check "Show machine position" vs "Show relative position".
On the DRO tab what do the G54 vs XYZ coordinates say?
Have you tried removing the work and bit and setting zero at a different place and still get the error?
With work and tool removed (for extra room) have you tried overriding the limits and test cutting air to see where it goes wrong?

Try writing a short program that just moves your tool to the maximum points you are trying to mill and see if you still get the error. Then adjust Z in the code until it goes a way to help find where the machine seems to think the limit is and compare to your settings. Maybe it is a bug?

When I get that error, I shut down and re-home everything because I don't always re-home like I should at power up (I know, I know!) and have worked my way past the limits.
I recently ran a file with 68,000+ lines, I don't think there is a limit except possibly the amount of memory you have?

Funny, after all the time I've used axis I never zoomed back far enough to notice the envelope box. But I recently upgraded, is that new(ish)? So I can not comment on the envelope.

[lwill]

Opps! I posted while you were.

[torinwalker]

It seems we overlapped each other. Yes, I actually tried all that stuff you mentioned today just before you mentioned it. I was up until 12:30am, all bleary eyed, banging my head against the machine (hey, I figured if nothing else would fix it...) and what I needed was a moment to reflect.

The guy who I'm doing work for has a thesis due at the end of the month and needed these pieces this friday so he could start moulding this weekend, so I was pretty panicky last night. A night of sleep, and trying to explain stuff here to you guys helped me think of new things to try.

Thanks for your suggestions.

Torin...