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CNC Router Max Speed and Acceleration Testing

Once the main core of the electronics were installed and working, it was time to test and tune the speed and acceleration values.

First, the acceleration is set to a low value (750 mm/sec/sec) and then speed is gradually increased, testing each axis individually and also in concert with the others until the motors start stalling or missing steps. Another item that might limit max speed is if the ball screws start whipping.

With the Leadshine AM822 drivers, if the motors stall or lose steps the drive will set a fault and stop motion immediately. That makes it easy to detect when there is an issue and quickly find the maximum speeds.

In this case, the maximum speed on a single axis was 40000 mm/min or 1575 inches/minute (ipm). It was slightly less when driving X and Y together: 38000 mm/min (1500 ipm). 

Here’s a quick video showing this maximum speed, which was achieved not only using a Raspberry Pi to control the machine, but operating the Pi wirelessly over remote desktop from a laptop computer. It’s possible it would have run faster with a NUC computer running LinuxCNC.

The next step in tuning is to slow the speed down to around 50-75% of the maximum and then find the maximum acceleration that can be used without faulting the drives.

The operational limits for maximum velocity and acceleration will be set significantly below the maximum to leave a safety buffer. On a machine this size, there is no need to ever go faster than 500-750 ipm for rapid moves. Most cutting feed rates will be under 350 inches per minute, based on the recommended cutting speed and chip loads for the material and tool being used.

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Workholding and First Cuts

I came up with an idea for a simple way to quickly hold things in place on the bed of the machine. This should work for any size object that I want to work with, in any position on the bed.  I attached 2 T-tracks to the bed, outside the working area. Then I made a sled with 90 degree corners with another T-track mounted on it. This will hopefully allow easy clamping of any size stock at any location on the bed. I plan to use a few different styles of clamps and/or stop blocks to attach to the t-tracks. It will be easier to show pics than try to describe the setup and the many options it will allow:

Workholding T-tracks

On the other end of the bed, I attached a T-track with some low-profile cam clamps, that wedge the spoil board in place. I will eventually mount this t-track to the longer tracks, similar to the image above.

While attempting my first cuts with the machine, I learned how much pulling force an upcut end mill has. I will need some clamps to hold down my spoilboards/fixtures. In addition to the material being pulled up towards the router during cutting, the wedges I used (only hand tight) vibrated loose and the stock moved during cutting.

Switching gears from the dust shoe components that I had planned to cut out, I modeled some clamps in Fusion 360 based on a few designs I had seen. Here is video I made showing the first successful project with this machine and the finished clamps that it produced! Still not having clamps for this operation, I hammered wedges in place to get them very tight and I also used longer screws to hold the stock down more securely.  Everything worked out well.

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Added Spoilboard and First Video!

I added a spoil board and am just finishing up a design for a dust shoe. I hope to cut the dust shoe parts out on the machine shortly.

I finally got around to uploading a video of the machine performing a run through the air. This was one of the example files that came pre-loaded with LinuxCNC. It is cutting a 3D profile of a penguin (the LinuxCNC logo). I didn’t change any settings or edit anything in the G-code, just ran the file as it was.

The workbench that I have the machine sitting on is pretty wobbly when the machine starts accelerating quickly, but the the machine itself is very solid.