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VFD - Spindle Calibration

These instructions apply to all VFDs where the spindle is controlled through the Mach4 Control Spindle tab

Overspeed a Motor

If you want to command your spindle motor or grinding wheel to go faster than the base frequency, command the spindle to go max RPM and use a tachometer to measure the speed. Then use the following formula to calculate your new Max Output Frequency (Hb105):

Desired Max RPM * Current Max Frequency / Current Max RPM = New Max Frequency 

For example, if your Current Max Frequency is 60 and you measure the spindle runs 1030 RPM but you actually want it to go 1515 RPM, calculate your new Max Frequency as follows: 

1515 * 60 / 1030=88 Hz. 

Make sure to recalibrate your RPM feedback and your spindle speed as shown below. 

Calibrate Spindle Speed

Does your CNC control need it? Yes - it needs to be accurate enough to meet your needs. 

  • The rated speeds and feeds for your tools and materials should perform as expected.
  • You should have comparable performance across all your machines when running the same spindle speed. 
  • Accurate spindle calibration enables you to make cleaner and more accurate cuts – improving the quality of your final product.
  • Scheduled preventative maintenance should include verification of the accuracy of the system, including the spindle speeds. 

This documentation focuses on knee mills as a common example and is adaptable to other machines. 

Spindle control methods

The specific spindle control method is configured during the installation. This document assumes that the spindle motor ID plate parameters for your VFD have been entered into the software, and the VFD and motor are operating correctly.  The last step in the spindle setup will be calibration.

  • Analog 0-10v from the Apollo III is managed through the face of the drive with Yaskawa Drive Wizard software. The wiring configuration is documented here in the 2000 Series Apollo III Operating Manual.
  • Ethercat VFDs are configured through RapidPath. RapidPath sends a speed command to the VFD – often an RPM command. If that implementation is correct, you should have accurate spindle speeds. Small calibration adjustments can be made in the Mach configuration spindle tab, and that is documented below. If your spindle speeds are significantly off, you need to contact MachMotion support for assistance. 
  • Modbus VFD configuration is described here: Yaskawa V1000 / A1000 / GA500 / GA800 TCP Modbus Communication Setup


Machines often have both gears and speed controls.

The control needs to know which physical gear is engaged to control the spindle motor for the correct speed. If you use multiple gears on your machine, there will be gear change buttons on the screen, and you can also use M-code commands to change gears. When you command a gear change, the software will prompt you to make the physical gear change, then it will change the software parameters to match.

The speed control needs to be set to the maximum and left there. Going forward the VFD will provide the full speed range within each gear. 

Set a spindle to always run at maximum RPM

In some cases, a machine was originally configured to run the spindle motor at full speed, and the gears and speed controls are used to bring the RPM down to the required value. A motor running this way will always generate its maximum torque, which is very helpful when using large tools.

Pull down Configure | Control | Spindle tab

Set your MinRPM and MaxRPM for each gear to the same value.
Set your Max Spindle Motor RPM to the rated value listed on the spindle motor plate.

Leave the FeedBack Ratio values alone as they are no longer meaningful.

The spindle section of you screen can now be ignored. You will adjust your spindle speed entirely through the gears and speed control.


The rest of the spindle configuration below is not needed when it is configured to run maximum RPM.

Set MaxRPM values for each gear

Note:  The MaxRPM is where you adjust the maximum tool RPM. To calibrate the TSpeed adjust the feedback ratios. You can overspeed a motor using these instructions.

Pull down Configure | Control | Spindle tab


If you have a tachometer, you can set the MaxRPM to the RPM reported by the tachometer.

This is not to destroy your machine. For calibration purposes this forces the VFD to the maximum hertz output - therefore maximum tool RPM. We use that to calibrate accurate tool RPM for normal production.

  1. Temporarily set all MaxRPM values to twice the labeled RPM on the speed control.
  2. Verify that the VFD is sending the maximum Hz to the spindle (see next section)
  3. Run your machine in each gear range with the temporary MaxRPM values you used above. Verify that the VFD is putting out maximum Hz. Record the actual MaxRPM values from the tachometer for each gear range.
  4. Enter the actual MaxRPM values that the tachometer reported in the spindle tab.


Verify that the VFD is sending maximum speed commands to the spindle

You need to know that the VFD is telling the spindle motor to run at full speed. Assuming the motor is configured to run at 60 Hz, then if the VFD is sending 60 Hz, the motor is running at full speed. You just entered the MaxRPM values into the spindle config. Set your machine for one of the gears and run the spindle at the MaxRPM. You can check the VFD output two ways, and both are equally accurate:



With the enclosure door open, check the LED on the VFD. This VFD is running at P44.59% of 60 Hz = 26.754 Hz. The "P" prefix on the number indicates the number is a percentage.

  • Use the up or down buttons to cycle through the display.
  • A V1000 will show the frequency with an "F" prefix.
  • A GA500 will show the frequency with a "-" prefix.

Open a web browser and enter this IP address in the search bar It will open a welcome page for the VFD with two buttons at the bottom. Click the Monitor button and look for the Output Frequency line. This is the same machine running at the same time, and it also reports 44.59% = 26.754 Hz.

Adjust feedback ratios for TSpeed

The formula is MaxRPM  / Maximum Motor RPM = feedback ratio

Pull down Configure | Control | Spindle tab


Check the ID plate on the spindle motor for RPM values. In this image we are running at 60 hertz; so we use the second value: 1735


MaxRPM  / Maximum Motor RPM = feedback ratio

4485 / 1735 = 2.5850144092219020172910662824207

You can paste the the full value into the spindle FeedBack Ratio field for this range, or just 2.58501

Note: the feedback ratio only adjusts the value in the TSpeed field. It does not have any effect on the actual RPM of the spindle. If the spindle speed reported by a tach does not match commanded speed, then go here: Set MaxRPM values for each gear

Auxiliary Spindle MaxRPM

If your VFD is enabled as an Auxiliary spindle (under MachMotion Parameters), set the Auxiliary Control Max RPM to the max RPM of the motor.