From Dr. Shopbot

My ShopBot is not moving the correct Distance

Generally speaking, there are three reasons your tool may not be traveling the expected distances: first, the software calibration values...the unit values...that are stored in the control software might not be correct for your tool; second, there may be some sort of mechanical problem; third, there may be an electrical problem. If the motors on your tool seem to be running smoothly, you are using the default speed settings, and the error is repeatable and proportional to the distance traveled – then you are experiencing a calibration problem and that is my subject in this section.


Let's try the move a couple of times and over different distances so you're sure of what's going on. Make sure you correctly understand how absolute distance is working. That is, if you issue a [MX] 2 Command your tool will go to where it thinks the 2.000 location is for the X-axis. If you issue the Command again, this time the tool won't move; it's already at 2.00. You need to give it the Command [MX] 4 to go another 2 inches up the X axis. At this point also check to make sure the speed has not been inadvertently set too high. For a PRSstandard tool the XY Move Speed should probably be in the range of 2-4 in/sec and the XY Jog Speed at 4-6 in/sec. Try the Z Move Speed at 1.5 and the Jog Speed at 3 in/sec. Stalling will occur if the speed is set too high. For a PRSalpha the XY Move Speed should me 4 in/sec and the XY Jog Speed 12 in/sec. These values can be found on the left side of the green console or by typing VS.

So, assuming that we've got the idea of the Commands right and the speed is not set too high, does the tool seem to be running relatively smoothly and consistently making the same error -- for example, going a little too far or a little short. I assume so … and that is why you find yourself here. We now simply need to get the ‘Unit Values’ set right in the software to correspond to the mechanical gearing on your tool. When you first installed the ShopBot Control Software you had a lot of options for the type of tool that you have, and it's easy to make the wrong selection. Type UV in the ShopBot software and write down the unit values there.

Setting Unit Values:

Setting the Unit Values in your ShopBot with the [VU] Command is how you accurately define distance in relation to the gearing of your motors and pinions. The Unit Value for each axis is: Number of Steps per Distance Unit. So if you are working in inches, the value is Steps-per-Inch, and if you are working in mm, the value will be Steps-per-mm.

For any basic PRSstandard ShopBot, or PRTstandard that has been upgraded with a Version 4g Control board:

  • Unit Values for X and Y Axes = 1833.465
  • Unit Values for Z Axis = 2291.831

All PRSstandard and PRTstandard Tools use a Unit Multiplier of 1.

Some early PRT ShopBots have different gearboxes or pinion sizes, and custom ShopBots may also have different gear ratios. If your tool does not move the correct distances with the values above, check the gear ratio of the motor and count the teeth on your pinions. Then find the correct Unit Values for your tool in the chart below, using the gear ratio of your motors and the number of teeth on the pinion gears. The X and Y Axis will be the same but the Z axis will probably have a different pinion size and possibly a different motor gear ratio.

Once you've found the correct Unit Values, enter them in the ShopBot Control Software using the [VU] command. If you are running your tool in Inch Mode, enter the Inch Unit value. If you are running your tool in mm Mode, then enter the Metric Value. Once the Unit Value had been entered in one or the other Unit system, you can switch back and forth between inches and mm and the Unit Values will automatically be managed.

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