For small bender grooves, say up to 1/8" radius, I use a commercial radiused carbide grooving tool from Nikcole and trig out the cuts. I have a DRO on my lathe, so that helps. I use a spreadsheet to calculate the cuts at equal angle increments, say 5°. By using equal angle increments, the leftover cusps in the surface are equally spaced and easy to polish out if necessary. Incrementing by 5° means 36 cuts for a 180° groove. The raw coordinates are based on the center of the tool radius. For example, the Z (axial coordinate) for a cut is the groove radius minus the tool nose radius times the cosine of the angle and the raw R (radius coordinate) is the groove radius minus the tool nose radius times the sine of the angle. The radial (X) coordinates are transformed to DRO diameter readings by subtracting from the work radius and multiplying by two. I print the spreadsheet and step through it using the DRO. I have done this without a DRO, but it is not easy and I have to watch backlash. I used a long-travel dial indicator for Z.