How to Calibrate DRO Scales

DRO scale calibration is an essential step that ensures maximum accuracy from a digital readout. In a traditional DRO, calibration is used to remove scale error. This error can be caused by two factors. First, the encoders used in the scales have manufacturing tolerances that can lead to slight deviations from nominal resolution. Second, the scale might be mounted at a slight angle in relation to the axis of travel, resulting in a cosine error. For instance, for each 1 degree of misalignment the encoder will have 0.0018" error over 12" of travel.

Fortunately, a linear error is very easy to eliminate by following a simple calibration procedure that uses the tools you likely already have in your shop. The process consists of moving each axis by a known distance and then telling TouchDRO what the distance was. The application will perform the necessary calculations and set the resolution accordingly.

Since TouchDRO can work with many different scale and encoder types, calibration serves an additional purpose of letting the application know the native resolution of each scale. Out of the box, every input is set to 25400 counts per inch, which is right for a 1-micron glass or magnetic scale and wrong for everything else. Once the scales are mounted to the machine, the Calibration Wizard sets the correct resolution and counting direction for each input, and compensates for any linear error at the same time.

Tools and Materials

Calibration Block

You will need a stable block of known dimensions. For a small machine, a standard 1-2-3 block (or a metric equivalent) is sufficient, but a larger block would provide better accuracy since any edge indication error would be spread over a longer distance. The wizard will warn about a block shorter than 50 mm (about 2"), but will let you continue.

Dial Indicator or Edge Finder

During the calibration process, you will need to accurately indicate the edges of the calibration block. For the axes that are perpendicular to the spindle axis of rotation you can use an edge finder or even a touch probe, but for the Z axis a plunger-type dial indicator is more appropriate.

For best results, it's recommended to use a dial indicator for all axes since it can indicate the edge more accurately than an edge finder. The machine's lead screw is likely not accurate enough for this purpose, so simply moving the axis and counting the revolutions of the dial is not advisable.

Calibration Procedure

Before starting the calibration, ensure that the scales are mounted correctly to the machine and you have performed the steps outlined in the "TouchDRO Initial Configuration" guide. The application has to be connected to the adapter, and the input you are calibrating has to be sending data.

Each adapter input is calibrated individually, using the Calibration Wizard. Where the wizard is opened from is covered in the next section; the procedures below all start once it is open.

Please note that TouchDRO uses the conventional "Right Hand" rule for the coordinate system where the Z axis is parallel to the spindle's axis of rotation. While the application will not prevent you from using a different convention, many of the functions will not work correctly.

Opening the Calibration Wizard

The wizard is opened from the adapter, because an input's resolution belongs to the adapter rather than to the machine profile. Calibrating an input therefore calibrates it for every machine profile that reads it.

  1. Connect to the adapter and wait for the device-status button next to the connect button to turn green
  2. Press that button to open the "DRO Adapter Details" dialog
  3. On the block for the input you are calibrating, press the gear button to open its "Encoder Settings (Input X)" dialog
  4. Press the ruler button on the "Calibration" row, under "Resolution"
Encoder Settings Input X dialog with Linear Scale selected, resolution 25400 counts per inch, and the Calibration row reading Not calibrated beside an amber ruler button
Fig. 1: The "Encoder Settings (Input X)" dialog. The ruler button on the "Calibration" row opens the Calibration Wizard

The "Calibration" row reads "Not calibrated", "Calibrated" with a date, or "Entered manually", and the ruler button is amber until the input has been calibrated and green afterwards, so an amber ruler marks an input that still needs doing. The wizard runs only on the adapter you are connected to; the same dialog opened from "Adapter Profiles" in the application menu shows the stored values but can't start a calibration.

Which procedure the wizard follows depends on the input's "Measurement mode". A "Linear" input is calibrated against a block of known size, as described next. An "Angular" input is calibrated by counting revolutions, as described under Angular Axis Calibration.

Linear Axis Calibration

For the linear axes (X, Y, and Z) the process is as follows:

Step 1

Mount the calibration block to the machine. The mounting method will vary with the machine type but the key point is to make sure that the block is held securely and the edges that you will be indicating are strictly perpendicular to the axis of travel. If using a dial indicator, you can use two 1-2-3 blocks, as shown in the photo below.

class: center alt: 1-2-3 blocks set up for linear scale calibration caption: Fig. 2: 1-2-3 blocks set up for scale calibration
class: center alt: 1-2-3 blocks set up for linear scale calibration caption: Fig. 2: 1-2-3 blocks set up for scale calibration

Step 2

Open the Calibration Wizard for the input you are calibrating, as described above. The "Calibration Block Size" dialog will open, asking for the size of your calibration block (or the distance the axis is expected to travel). Enter the dimension of your calibration block, taking into account the diameter of the edge finder if applicable, select the units, and press "Continue".

The Calibration Block Size dialog with a 2 inch dimension entered and inches selected
Fig. 3: Enter the size of your calibration block

In the example shown in the photo of the blocks you would enter "3" (without quotes) and select inches, since the X axis is measuring the long side of the 1-2-3 block. If we were using an edge finder instead of the indicator, we would enter 3.2 instead, accounting for the diameter of the edge finder.

Step 3

The "Indicate First Edge" dialog will appear, naming the edge of the block to indicate first. The edge depends on the machine type and the axis, and it is always the one closest to the axis origin: on a vertical mill the left edge for X, the front edge for Y, and the bottom edge for Z; on a lathe the front edge for X and the left edge for Z; on a horizontal mill the left edge for X, the bottom edge for Y, and the front edge for Z.

The Indicate First Edge dialog asking to locate the left edge of the calibration block
Fig. 4: Indicate the first edge

Once you have indicated the edge, press "Continue".

The Sampling Axis Position dialog with a progress bar
Fig. 5: TouchDRO samples the position for a few seconds

At this point, the "Sampling Axis Position" dialog will collect a sample of readings from the scale. Keep the axis still. This usually takes a few seconds, and longer with an adapter that reports slowly. The sample lets the application confirm that the reading is stable and average out any jitter, which is common with iGaging EZ-View and DigiMag scales. Once the sample has been processed, the wizard will proceed to the next step on its own.

Step 4

The "Indicate Second Edge" dialog will ask for the opposite edge, the one farthest from the origin: the right edge for X on a mill, the back edge for Y on a vertical mill, and so on.

The Indicate Second Edge dialog asking to locate the right edge of the calibration block
Fig. 6: Indicate the second edge

Once the edge is indicated, press "Continue".

TouchDRO will again sample the scale output, as in Step 3, and proceed to the next step. If the two positions are nearly the same, the wizard will stop with "Readings Too Close" and send you back to indicate the second edge.

Step 5

The "Calibration Completed" dialog will show the calculated resolution, in encoder steps per inch and microns per step. Press "Apply" to store it, or "Discard" to leave the input as it was.

The Calibration Completed dialog showing the calculated resolution for the X axis
Fig. 7: The final dialog displays the calibration results

"Apply" writes two things to the adapter's encoder settings for that input: the resolution, and "Reverse counting direction", which the wizard turns on if the count ran backward between the first and the second edge. The axis's own "Invert Readout" setting in the machine profile is left alone. The "Encoder Settings" dialog will then reopen with the new values already stored, the ruler button green, and the "Calibration" row reading "Calibrated" with today's date.

The Encoder Settings dialog for input X after calibration, with the resolution at 25380 counts per inch, the Calibration row reading Calibrated Oct 23, 2026, and the ruler button green
Fig. 8: The "Encoder Settings" dialog after "Apply", with the measured resolution stored

This concludes the calibration for the first axis input; now you can repeat this process for any remaining inputs.

Angular Axis Calibration

Angular axis calibration differs from the linear axes. It doesn't require any calibration block and instead relies on counting full revolutions of the calibrated device. The wizard follows this procedure for any input whose "Measurement mode" is "Angular".

Step 1

Open the Calibration Wizard for the angular input, as described above. The "Indicate First Position" dialog will open. Rotate the axis to its 0 degree mark and press "Continue".

The Indicate First Position dialog asking to rotate the axis to the 0 degree position
Fig. 9: The "Indicate First Position" dialog

TouchDRO will sample the encoder readings for a few seconds and, if successful, proceed to the next step.

Step 2

The "Indicate Second Position" dialog will ask you to rotate the axis clockwise exactly 5 full revolutions. To eliminate any backlash, rotate in the indicated direction only and stop at the 0 degree mark. When done, press "Continue".

The Indicate Second Position dialog asking to rotate the axis clockwise exactly 5 full revolutions
Fig. 10: The "Indicate Second Position" dialog

As in the previous step, the application will sample the encoder for a few seconds and proceed to the next step.

Step 3

The "Calibration Completed" dialog will show the calculated resolution in encoder steps per revolution and degrees per step.

The Calibration Completed dialog showing the calculated resolution for an angular axis in encoder steps per revolution
Fig. 11: The final dialog shows the calculated resolution

If the results look right, press "Apply". This stores the counts per revolution for the input and, if the encoder counted the other way, turns on its "Reverse counting direction" setting.

Tachometer Calibration

The tachometer is not calibrated with the wizard. Its one setting is the number of pulses the sensor produces per turn of the spindle, entered in the "Set Pulses per Revolution" dialog, which opens from the gear button on the tachometer input in the "DRO Adapter Details" dialog. See "Tachometer Settings". When the sensor's pulse count is not known, an external tachometer can be used as a reference.

Step 1

Start the spindle and measure the RPM using a reference tachometer.

Step 2

Compare the reading with the TouchDRO tachometer reading. If the readings differ, adjust the pulses per revolution and re-check. The value can be adjusted by trial and error: if TouchDRO reads high, increase the pulses per revolution; if it reads low, decrease it. Alternatively, use the following formula:

Actual pulses per revolution = Current pulses per revolution × (TouchDRO RPM / Reference RPM)