My technique, FWIW
- Adjust the cage depth by countersinking some scrap before you do any real parts.
- Re-check the adjustment after the first few holes because the countersink bit can tend to tighten and settle a little deeper into the cage.
- Secure the part you'll be countersinking in a vise or using clamps. A secure part reduces the chance of bit chatter.
- Make sure that the holes are drilled (or preferably reamed) to the correct size for the countersink pilot, and are free of burrs.
- Make sure that the part is thick enough for your countersink depth. Material that's too thin will lead to a razor edge hole and/or chattering of the countersink bit, both of which are not good. If the material is too thin, then dimple instead of machine-countersinking.
- When countersinking, hold down the cage with your left hand to ensure that it is firmly seated level on the part, and is not rotating (letting it rotate can scuff the part).
- For an aluminum part, use medium speed (~600RPM is good) and moderate pressure to reduce the chance of chattering.
- When it feels like the cage has bottomed out and the countersink bit is no longer cutting, increase the pressure for a second just to make sure, then release. I would advise against "wiggling" as this could enlarge the hole or alter the geometry of the countersink. Better to keep everything concentric and square.
- This technique works great with a drill press, air drill (at much less than full RPM), or cordless drill. The drill press is actually the best option where possible, because it inherently ensures that everything stays concentric and square and also has the least chance of chattering. (miyu1975- what problems have you had?)
- Clean out the chips from the countersink bit & cage between every hole.
YMMV, but this has worked well for me.