Digital Readout (DRO) for mini lathe using cheap digital calipers

Digital calipers can be used as an inexpensive alternative to a commercially available DRO for home workshop machine tools.

Many hobbyists attach the calipers to the axis of their machines, and use the screen and buttons directly on the calipers as a DRO. This has several disadvantages:

• Calipers are not shielded from cutting fluids and chips
• While you can zero the caliper, there is no way to set the position to a value.
• For a lathe, the X-axis dimension will be a radius, not a diameter.

The PCB of commonly available digital calipers has traces for serial data output which can be read by a microcontroller. Use of a microcontrolled enables the connection of a display, and calculation of relative position from a user input entered by way of a keypad.

Example: Use of DRO with lathe turning operation

1. Round stock is placed in the lathe chuck.
2. Stock is faced, and the Z-axis is zeroed.
3. Stock is turned lightly to remove any eccentricity, and the diameter is measured. This X-axis diameter is entered into the DRO using the keypad.
4. The part can then be turned to the correct length as displayed by the DRO Z value, and to the correct diameter as displayed by the DRO X value.

In this project, I have connected two sets of digital calipers to an Arduino Nano board to display the X (cross slide) axis diameter and Z (carriage) axis position of my 7x14 mini lathe. The DRO is accurate to 0.01mm in the Z-axis, and 0.02mm in the X-axis diameter (this is because the cut of the diameter is 2 x the travel of the caliper scale).

I have also written code and included connections in the schematic for a 3 axis version for a milling machine.

Modification of calipers

Calipers were disassembled, and connections soldered for the serial line.

I used CAT5 cable as it was what I had available.

Cable secured to case of calipers.

The jaws were cut to size with an angle grinder, and slots cut for M4 screws to fix the calipers to the mini lathe. Caliper jaws are made of hard steel, so drilling is not advisable. Slots also allow for some adjustment.

Modified calipers were reassembled and tested.

Schematic

Calipers are ordinarily powered from a 1.5V battery. A circuit was designed to supply the calipers with 1.5V, and to convert the serial line to the 5V logic level required by the Arduino nano microcontroller.

An I2C display is used for the screen, and a matrix keypad for user input.

PCB design

The PCB was drawn to the dimensions of the lid of an off the shelf plastic enclosure, and all through hole components were hidden behind the keypad.

The PCB was printed by JLC PCB.
Components were soldered in place.
The PCB mounts nicely to the enclosure.

I have as yet neglected to print a sticker for the keypad.

User interface

Mounting calipers to the mini lathe

Holes were drilled and tapped in cross slide for M4 screws to mount the X-axis caliper.

An M4 nut was soldered to the sheet metal chip shield to form an integral bracket.

Chip shield installed and X-axis caliper mounted.

A small aluminium clamp was milled to fix the Z-axis calipers to the lathe bed ways. The clamp allows you to adjust position of the bracket, as the calipers only permit ~140mm of travel.

Chip shield installed over Z-axis calipers.

DRO mount

The DRO is attached to the mini lathe using a cheap "magic arm" camera mount for easy adjustment.

The plastic packaging that the enclosure was sold in protects the DRO from cutting fluid and chips.

Project Files

https://github.com/bigredlevy/Digital-Caliper-DRO