Countersinking technique... I must be doing it wrong.

[danielhv]

Im working on the wing spar... Im using a #30 pilot counter sink... I've pre-drilled the holes out with a #21 bit, and have a piece of aluminum angle clecoed underneath to help keep the pilot centered. Problem is, that on some of the holes, the bit seems to vibrate or something and makes a pretty jagged finish. Doesnt matter if I use light pressure or hard pressure... I am making sure the cage is flush, etc... any ideas on what I'm doing wrong?

-Daniel

[captainron]

You may want to try to countersink before reaming the holes, try Bolube, or try a countersink with the one-flute, hole-style bit.
www.aircraft-tool.com

search: 'zero flute' ATS part # 4100-30

[danielhv]

Originally Posted by captainron

You may want to try to countersink before reaming the holes, try Bolube, or try a countersink with the one-flute, hole-style bit.
www.aircraft-tool.com

search: 'zero flute' ATS part # 4100-30

I have neither of those... and to make my day even better... I already reamed all the holes to make the holes in the aluminum angle.

[vlittle]

Rivet on the nutplates first, then countersink using a #30 countersink. The nutplate will keep the bit centered.

Not my idea, I think it was in the instructions for my 9A.

V

[danielhv]

Originally Posted by vlittle

Rivet on the nutplates first, then countersink using a #30 countersink. The nutplate will keep the bit centered.

Not my idea, I think it was in the instructions for my 9A.

V

Yea, I saw that too, but wasnt sure how I was going to debur the backside with the nutplate there. I dunno... Hindsight is 20/20.

[Sparky]

Originally Posted by vlittle

Rivet on the nutplates first, then countersink using a #30 countersink. The nutplate will keep the bit centered.

I second Vern's suggestion. That's what I did and it turned out perfectly -- and it's the easiest method I've seen. The countersink bit may chatter a bit at first until the pilot enters the nutplate - then it's as smooth as cream. I used a two flute countersink in a microstop cage.

[bullojm1]

Daniel-

Are you using an air drill? I found I have terrible results when countersinking using an air drill. I use a $10.00 Harbor Freight electric drill now and turn the countersink bit at a slower rate (< 1000 RPM). I get a perfect countersink every time with no chatter. Play around and see if that doesn't work as a quick fix!

[Michael Brown]

I had the same problem. I just slowed down the drill and that seemed to resolve the issue. I did not like the idea of putting on the nutplate first either. Good Luck!

[Webb]

My 2 cents worth.

I noticed you said #21 bit and a #30 countersink.

The two should match. If the hole is bigger than the "point" of the countersink, it is going to drift and make an ugly hole. Even if you have an extra aluminum strip for a guide, if the hole is larger than a 30, its gonna drift.

Buy a proper size countersink for the hole. 12 bucks from ACS. While you're at it, buy and extra countersink for a #40. The point will eventually get snapped off and the bit will eventually get dull when you are working on the fiberglass parts. You will need it later on.

If the hole size matches the point diameter of the countersink, it's not going to drift.

Also, don't waste your time deburring the hole you are countersinking. The countersink will do that for you. Do debur the backside of the hole

[airguy]

Originally Posted by bullojm1

Daniel-

Are you using an air drill? I found I have terrible results when countersinking using an air drill. I use a $10.00 Harbor Freight electric drill now and turn the countersink bit at a slower rate (< 1000 RPM). I get a perfect countersink every time with no chatter. Play around and see if that doesn't work as a quick fix!

I find that very interesting - it must be a matter of technique, because I had horrible trouble with chatter at a slower RPM using an electric drill. I now use a countersink cage (with the nylon foot) on an air die grinder feeding full pressure for max rpm, and give it fairly hard feed pressure for the cut. I don't have ANY problems doing it that way. Maybe it's a combination of speed and feed rate?

As for the nutplates - yes it works very well to C/S the rivet holes first for the nutplates, deburr, rivet the nutplates on, then C/S the screw holes using the nutplate as a guide for the C/S bit. You don't need to deburr under the center hole after you C/S the top - no metal gets cut under there after you put the nutplate on. Just use an airgun to blow all the chips out.