CNC CRP-2448 Auto Gantry Square

I finally got my new CRP-2448 CNC machine up and running. I’ve managed to cut several things quite successfully, none of them Speaker related. But, since I’ve solved a few crucial issues I though I’d share what I’ve learned here. Eventually I’ll set up a CNC blog for these articles but for now they are here.

First, I purchased the proximity sensor kit from cncrouterparts and it works fine. However, I did need to change the auto square code under the “Ref All Home” button. It comes with code to home each axis individually starting with Z, then X then Y then A. I found it works better using the refcombination(x) code rather than the default Z-X-Y-A so I changed the code under the button to:

DoButton( 24 )
DoButton( 22 )

Your setup may be different. The stock CRP-2448 and CRP-4848 XML file from cncrouterparts has the X axis defined as the long axis (on the 2448) and the Y as the short axis and has A slaved with X. I changed the setup on mine so I’m slaving the Y axis with A. I defined my axes as X horizontal (short axis) and Y the long axis. My “Y” proximity (home and limit) sensor is on the same side as the Y motor and the “Prime” sensor (A home only) is on the same side as the A motor. You can check this by entering “Config->Ports and Pins->Motor Outputs” and removing the green check from the A motor. Then jog the slaved axis (Y or X) and you should see only the master motor running. It will pull the gantry sideways so don’t jog it very far. Just make sure the prime sensor is on the same side as the A motor and you’ll be fine.

Gantry Not Square in “neutral” position?
Next, once I got the gantry so it will square itself (I move the quill to 0,0 then make a mark at 0,0 then move the gantry to 26,0 and check the Y position for square) I noticed that several of my pieces had lost steps or lost gantry square while cutting. It only happened if I had hit eStop or changed configuration during the run. After a long and drawn out debugging I finally figured out it was happening when I hit “eStop” after the axes had been homed without re-homing them. What happens is that when the gantry squares to the slave side it then pulls the gantry sideways placing it under torsional stress. An eStop releases the holding torque on the motors allowing the gantry to move freely. If the gantry is not nearly square before Ref All Home is run it will have some torsional stress on it at all times after the Ref All Home and during all cutting. Normally the stepper motor holding torque is enough to keep it from moving out of sync with the slave but when you release the holding torque, the gantry moves back to its non-square “neutral” position possibly compromising your work on a restart. The solution is to square the gantry with Ref All Home, then with holding torque applied to the motors, attach pipe clamps across the outside of the bearing blocks to maintain bearing adjustment pressure and keep the bearings in contact with the steel, then loosen all 16 gantry riser bolts. When you loosen the 9/16ths/14mm bolts it relieves any torsional stress on the gantry from the auto square procedure making the now “square” position the new “neutral” position. When you re-tighten the bolts it will lock the gantry in a much more “neutral” position such that it won’t move out of position if eStop is pressed. At least that’s the theory. You may need to manually square the gantry before using “Ref All Home” just to be sure. Otherwise it becomes necessary to “Ref All Home” every time the motors loose holding torque or you’ll end up with a gantry that is no longer square. Be aware that loosing square is a very subtle release often obscured by the sound of the motors so its easy to miss. It took me a while to track down that issue.

Hope this helps you with your new CNC machine.

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