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Mach4 precision problems
« on: April 29, 2020, 12:59:44 PM »
The attached image shows a piece I cut with Mach3 on the left, and the same piece I cut with Mach4.  As you can see, I suffered an extreme loss of precision.

I have a homemade 3-axis mill.  I replaced the Mach3 controller board with a pokeys57cnc motion controller for mach4; otherwise, it is the same machine and the same part files.

Any ideas?
Re: Mach4 precision problems
« Reply #1 on: April 30, 2020, 10:49:06 AM »
Two thoughts.  Are you using USB?  Many people with pokeys are having issues with USB.  Second, are you sure both are setup with CV on or off?


Re: Mach4 precision problems
« Reply #2 on: April 30, 2020, 11:42:22 AM »
Thanks for the ideas.

I am using ethernet, but I cannot get it to not autoconfigure.  I get a warning when Mach4 loads.  I have spent hours trying to fix this, and I am just resigned to living with it.  Do you think it could affect performance?  I have the network card set as a static address, and I have configured the pokeys to "use this address" but it autoconfigures anyway.

What is CV?

The problem seems almost random.  It's like the machine is drunk.  At first I thought my tool change Lua script had a problem because the Z axis was wildly off between tools.  I slowed down all the velocities and accelerations, and thought maybe that helped, but now it doesn't even cut a straight line.

I'm at a total loss.
Re: Mach4 precision problems
« Reply #3 on: April 30, 2020, 11:59:45 AM »
From a CNC zone post: CV stands for "Constant Velocity", called out by the G64 command. That tries to keep the tool moving at the same speed no matter what's happening in the toolpath. So when it gets to a corner, just like a car that's going fast, it's going to swerve around it instead of one that slows down to make a sharper turn. The less "look-ahead" you have set, or the more speed it's told to run at, the less adjustment it will be able to make. If you want it to follow your toolpath exactly, without cutting any corners, you can set it to "Exact Stop" (G61)

First check to see if you're default (Configure->Mach Motion mode is set to Constant velocity or Exact Stop.  If set to constant velocity you might have run the cv and cv tuning wizards.  If your code is being generated by a cam post processor check to see whether it is using g61 or g64. If not, turn default Motion Mode to exact stop and run air tests on simpler gcode, rectangles..circles.  Next run the same code with cv on.  Next run with spindle on to see if noise is being generated and causing the problem.

If none of that works contact PoLabs and NFS.


Re: Mach4 precision problems
« Reply #4 on: April 30, 2020, 07:04:28 PM »
I thought it meant constant velocity, but I wasn't sure.

When I used Mach3 it would sometimes flip over into exact stop, and I would switch it back.

I always run in CV.

My look ahead is set at 20.