This is a simple Arduino project using a 7-segment LED backpack and MPU6050 sensor to add digital angle reading to any faceting machine stylus.
Required components include:
- Faceting stylus (duh)
- Adafruit 7-segment LED and backpack
- Adafruit MPU6050 accelerometer and gyro
- Lots of high temperature hot glue
AliExpress vendors sell faceting arms that take 6mm brass dops and cost as little as $50. With this, you can bring one of the bells and whistles of a $5,000 UltraTec V5 to any ancient faceting machine, or even one of those cheap Chinese machines that cost $300.
I strongly recommend soldering all connections. In an earlier version of the project I attempted to have a removable connector on the MPU6050. This is a terrible idea because the stylus is always in motion and the connector is a weak point. During serious work, it would wobble and break communications between the Arduino and the sensor. This froze the sensor and required me to reboot and recalibrate.
Additionally, because of the length of the run between the sensor and the board and proximity to spinning motors, a shielded cable is ideal. In my setup, I use a MIDI cable cut in half and run to a DIN-5 connector on the project box. The connector is then soldered directly to the Arduino. MIDI cables are designed for long runs and have just enough wires to support the sensor.
After you solder your wire onto the MPU6050, you should cover any exposed wire and the connection points in hot glue.
This is important. The stylus will constantly be moving up and down and side to side and putting pressure on these wires. Even if you have the best soldering skills in the world, the wires will eventually strain and snap off above the solder points. A casing of hot glue will distribute the force while locking all the wires in place.
When the glue is cooled, clean the back of your sensor and the mount point of the stylus with rubbing alcohol.
This program is designed for the MPU6050 to be mounted with the back side flush against the side of the stylus nearest to the user. It should be mounted flush with the edge of the stylus and as flat as possible.
To ensure a good bond, I let the stylus and sensor sit on a heater before applying the glue. If you are a lapidarist, your dop wax pot works fine for this.
In faceting, we don't care about having the sensor level with the earth: we need it level with the spinning lap.
I am going to assume your faceting post is unbent and at a right angle to the lap. If it is not, then you have a serious problem no Arduino is going to be able to solve.
To calibrate, compile and upload this sketch with float offset = 0;
Next, use a square dowel or other flat device alongside your stylus and move it all the way down until it is square with the post.
Check what is displayed on the 7-segment display: this is your offset. Change the value of offet to this number and upload the sketch again. Your display should now show a number between 0.1 and -0.1.
To confirm calibration, rotate the stylus up to 90 degrees. Again, use your flat tool to square it against post. You should see a reading between 89.8 and 90.1.
Do not be concerned if the digital 90 does not match the protractor's 90. A lot of cheap faceting machines come with protractors that are not properly zeroed, and they drift considerably. Even a $6 MPU6050 board is probably more accurate.
If you want to test further, because you don't believe me, grab anything soft like a chunk of malachite or fluorite and quickly cut a 96-24-48-72 square girdle, then pick any side and cut a 45 degree angle. Remove it from the dop and verify the results with a protractor. You should realistically be able to achieve within 0.4 degrees of precision if you are using a true lap.