When cutting discs on a lathe, one important factor is the stylus rake angle, the angle at which the cutting stylus makes contact with the disc surface. We’re focusing specifically on cutting into plastic, not lacquer. Lacquer is a softer material and can tolerate a much steeper rake angle.
When cutting into PETG (the material all of our discs are made from), a rake angle of 8 degrees is typically required.
Measuring Rake Angle Accurately
Getting an accurate measurement of your stylus rake angle is very straight forward. Our good friend Joel over at Leadscrew Grooves shared a useful free tool that can simplify the process.
The tool is a Google Chrome extension called ‘Protractor’. Here’s how to use it:
1. Lower your cutterhead onto the blank disc.
2. Take a clear side-profile photo of your stylus in contact with the disc.
3. Open the image in Google Chrome.
4. Use the Protractor extension to measure the exact angle of the stylus against the disc. Make sure the disc is sitting nicely on 90 degrees on the protractor, set the green arm to 0 degrees, and the blue arm following the angle of your stylus. See the attached image for our example.
This method allows you to achieve a much higher degree of precision than just relying on visual alignment. Once you have set your rake angle, you must fully recalibrate. Make sure that the stylus tip travels directly to the centre of the spindle, reset weights and finally measure groove depth. With PETG a depth of 50µm to 70µm is a good place to be for playback and stylus wear.
