Counter-sink problems

[Crossbow]

My approach is a bit labor intensive, but I like the results.

For thick material, use the cage with a slow cordless drill to make a shallow counter-sink, then use the hand tool to get the counter-sink to the correct depth.

And in the case of thin material to be counter-sunk, only use the hand tool.

Like I said, labor intensive, but it prevents me from making counter-sinks that are too big.

[roee]

- 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.

[Paul B]

I will try out some of the suggestions tonight (on some scrap to start with). Specifically: 1. Use a slower turning cordless drill. 2. Clean the shavings out of the tool before every hole (if needed). 3. Take my time on each hole, make sure the tool is flat and check depth often with a rivet.

I have not decided if my spar is ruined or not. If I can figure out how to post pictures, I might do that and ask for some more advice. Thanks again to everyone who responded.

[Bernie RV7]

I'm going to buy a new cordless drill tomorrow specifically for countersinking..
the one I have in mind is a Bosch Lithium powered 10.8 volt 410 RPM and 25 NM Torque. Will it do the job or do I need more speed ?

[longranger]

I sprang for Avery's countersink tool with the ball bearing cage and polished metal (non-nylon) foot.

http://www.averytools.com/p-452-poli...sink-cage.aspx

It works great in my air drill. It's a bit more expensive, but worth it, IMHO. It's my belief that the ball bearings and solid metal foot provide for a much more stable tool. I've NEVER had it to "chatter", and it doesn't heat up while doing successive holes.

[David Paule]

I've found that at least in .040 material, two things help chattering of a three-flute countersink.

1. Use a slow speed, as people have mentioned.

2. Make a back-up piece of aluminum. I'm using a piece of scrap from the spar bars on the tail that's about 3/16" thick - definitely more than necessary.

Together I get good countersinks.

Dave