Oblong Holes

[ToadF16]

I would like to get a discussion started about tolerances in holes. I have noticed that despite my best efforts, I will occasionally have a hole that is drilled slightly oblong. I had one such hole on a rudder part. I took a picture and sent it to Van's. I received a very general response that everything should be OK.

Rather than e-mail Van's on a case by case basis, I would really like to understand, from a technical perspective, what is acceptable, and what is not when it comes to drilled holes. I now face the following problem. This is a rib on the elevator, where the elevator horn attaches:



It appears to me that 3 out of the 6 holes are oblong.

Would you accept this part and rivet as is? Does anyone have a link for the technical specification (IE mil spec) that is pertinent in this case?

This forum is a real life-saver for the amateur builder. Thanks for your responses.

Toad

[bsacks05]

Some things that come to mind which may help are:

Use high rpm for aluminum
Be sure the drill bit is sharp/new/good quality
Be sure the bit is not bent
Use moderate pressure on the drill motor
Use a drill guide to keep the bit perpendicular
Hold parts in a vise
Use a drill lubricant
Pilot drill then step up to the proper size

There are tolerances for all fastener holes usually called out by the aircraft manufacturer.

[robertahegy]

I concurr with everything that Bruce stated. You can never have too many sharp, new bits. Unless you are very good at sharpening bits, buy a large quantity of #30 and #40 bits and discard them often. Incorrect sharpening can lead to a poor quality bit that will not center and will wander, making poor quality holes. A high speed air drill (3600 rpm) is the best tool for drilling these aircraft parts. A good drill and a good rivet gun should be the primary tools. Supplement these with good bits, dimpler dies and countersinks. I also recommend a pneumatic squeezer with an adjustable set holder and several yokes like the longeron, and 1 1/2-2", and a 1'" and 4" no hole.

Best of luck,

Roberta

[jsharkey]

All of the above is good advice.

I have found that Cobalt Aircraft drills work best on aluminium - 135 deg included angle I believe. A pack of 10 each #40 and #30 from McMaster-Carr have lasted me the whole project. They suck for steel though - use carbide tipped drills for that.

Also for larger holes on sheet metal (>3/16") a stepped hole cutter works better than a twist drill that can leave a lobed hole. Hole cutters can wander though so a drill press and/or cross hairs marked on the part can help to keep it centered.

Remember also that rivets swell when set so there some decent leeway on the hole.

Jim Sharkey
RV6 tip-up - Connected the battery today - It lives!

[asav8tor]

Did you drill that in assembly or use at Al as a guide for the steel horn?

Try not to do that if that's what you did.




You can put a few extra rivets on the horn to ease your mind.