Setting Y axis offset from SKR TFT display, and losing about 5 mm over 1.5 m?

I have finished setting up my LR3, and am now onto squaring and calibrating. I have an SKR that I bought here, and I am running the V1CNC_SkrPro_DualLR_2209 firmware. Do I understand correctly that there is some way to set the Y axis offset from the SKR TFT display to square the machine? I looked through the menus, but I couldn’t figure out how to do it.

Also, when cutting my struts I discovered that what should have been a 1400 mm strut, ended up being about 1395 mm. If I move the machine 1400 mm from the display, it also only moves about 1395 mm. This happens on both the X and the Y axis. Any suggestions for where I might start looking to figure out why my machine isn’t moving quite as far as it should be?

Thank you!

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Typical V1 build uses 16T pulleys at 16X microstepping, on a 1.8° per step motor, for 100 steps/mm.

1400/1395=1.00358.

Changing your steps/mm to 100.358 will make the difference.

Before you do that:

Make sure that you are measuring the actual machine movement. For the Y axis, measure from the edge of the table to a fixed point on the Y plate before and after the move. For the X axis I would suggest that you measure from the edge of the core (Or plate on a LR2) to the YZ plate before and after. If you measure the length of a cut, or size of a piece cut out, you have to take the endmill diameter into consideration.

So if you had a 14" endmill, and you tell it to cut a square:

G92 X0 Y0 Z0
G1 Z-1 F180
G1 X1400 F1800
G1 Y1400 F1800
G1 X0 F1800
G1 Y0 F1800
G0 Z3 F360

Although it looks like this should make a 1400mm square, the reality is that you should end up with a 1393.65mm square, because of the kerf of the 1/4" endmill.

If you are measuring actual machine movement, and it’s still out by this much

  1. Ensure that there isn’t excess slack in the belts. Tighten them up a bit, and it might make the difference.

  2. 0.358% isn’t that large an error. You might never notice this on a 3D printer, for example. Maybe your batch of belt was just off? Or maybe your batch of 16T pulleys? So long as it’s consistent, it’s fixable though by altering the steps/mm. I would be correcting this if it were my machine for certain, (And probably getting obsessive over making sure that the error is consistent.)

  3. So… Make sure that the error is consistent. when measuring your fixed points, maybe break down that 1400mm move into 5 moves of 280mm each, which should show individual errors of 1mm each. I’d probably also do the cam for a bunch of smaller squares and use the calipers to detect smaller errors to be sure that they don’t change for small increments.

Edit: Squaring the machine is done with the M666 command, which sets up an offset for the 2 Y endstops. (And 2 Z endstops for levelling the LR.) I can’t really help with specifics on the command usage, but it takes the argument of the axis and an amount eg: M666 Y1.5

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Thank you for such a detailed response, it is greatly appreciated!

The error is very consistent. The first three struts that I cut were all exactly the same length. I was very impressed with the repeatability. I’ve also measured it a few times with a pen, and the actual movement of the machine is reliably about 5 mm shorter than expected over 1400 mm.

I’m not sure how much more I should tighten my belts? As a point of reference, with the steppers engaged, I can only force the LR to move about 1-2 mm back and forth along the Y axis (my belts along the Y axis are about 1600 mm long). That seems pretty good to me? While I’m sure that I could tighten the belts a little more, I wouldn’t want to overstretch them?

If it sounds like my belts are tight enough, I guess the next step is to change the steps/mm? I had been holding off on that as I haven’t seen many posts where there have been problems with the firmware, but I suppose you’re right that it could be an issue with the actual belt itself.

Thank you for the reminder of how to square the machine with the G-code commands. In the guide, Ryan mentions “If you are over 1mm in difference you can use the terminal on your SKR Pro or a USB connection and repetier host to add a homing offset.” I assumed that the “terminal on your SKR Pro” refers to the display that is used to control the machine? However, looking through the menus on the display, I couldn’t figure out how to add a homing offset on the Y axis, so maybe it means something else? I would love to have the option of setting the offset from the display, but if I can’t, it’s not a huge deal.

Thanks again!