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I've read and studied a lot of different methods to drill out the final size of the attach holes, including some creative modification of drill bits to help prevent ovaling of the hole (such as grinding down the tip of a larger bit to pilot the bit through the hole). I believe I just discovered a drill bit that can be purchased and do exactly what most of these drill bit modifications attempt to do.

http://www.panamericantool.com/double-m ... s-937.html

Item #13-250 is a 3/8" double-margin bit with a 1/4" pilot. I haven't purchased one yet and my only question would be how sharp is the pilot portion, as the idea is for it not to do any cutting and only follow the existing hole. The double-margin feature basically helps make the hole have a near-reamer finish.

Right now this is the front-runner for how I believe I'll drill the attach holes, I'm curious what others think of it.

I've heard of people using this bit: http://www.mcmaster.com/#29525a53/=qjrex3

It seems like the same thing you posted except yours is a lot less expensive. I wasn't particularly excited about spending $40 on the McMaster one. The Sonex Foundation has a tool loan program that has most (all?) of what you need for wing rigging including one of these bits: http://sonexfoundation.com/Tool_Loan_Program.php

From what I've read, a longer bit would be even better. Both of these seem to be 5".

I hope to drill these holes in a couple weeks, so I'm curious to see what others post.

I will probably end up getting one, even if the 1/4" pilot is sharp, it will be a lot easier to dull it than to modify a normal bit.

Pan American tools has a $50 minimum order. I've bought quite a lot of supplies from them though, including all my clecoes (cheapest I could find anywhere that weren't used), most of my drill bits and reamers (including threaded bits and shanks for them), a pneumatic cleco tool and one of their Nova drills (the best quality tool in my whole shop.)

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Or if you're brave enough to trust you're wing attach block measurements. Clamp the completed spars together and updrill them to 3/8" in a drill press prior to wing assembly. The risk of ovaling the holes whilst rigging can be all but eliminated by using a long 3/8" OD x 1/4"ID bush and long 1/4" and 3/8" bits. Need only to hand drill the forward and rear angles whilst rigging. Only 1/8" of material 4 times.

The ovaling a lot of people experience is caused purely by the combination of the drill not being held consistently straight whilst hand drilling the attach blocks, and drilling a large hole through about 3" of soft aluminium takes quite some effort. Making it likely the drill operator will have trouble keeping things straight. The somewhat awkward position the drilling needs to be done in, also doesn't help.

Just need to plan well ahead. The actual rigging process only took about 3 or 4 hours the 3/8 bolts are tight and need to be turned and tapped to remove or install.

Here's a photo of the tools needed and a couple of drawings








Steve
Sonex 892 3300TD 140 hrs

Fastcapy wrote:I looked at those ones from Pan American and you had to buy a minimum of 6 of those bits at $10 ea.

I completely missed that, what a bummer. I might try to negotiate if I get a larger order of other items together.

One way to avoid ovaling the holes is to drill in increments. When I bought my plane the holes were so ovaled that the left wing tip moved up and down about an inch. Rather than rebuild the wing attach fittings I went to a larger size bolt. I drilled from 3/8" to 7/16" using drills that were each 1/64" bigger than the previous one....25/64"; 13/32"; 27/64". For the final 7/16" hole I used a reamer rather than a drill. I also used lots of WD-40 as a drill lube. The new holes are perfectly round and the bolts have to be tapped into place.

I attached my wings this weekend.

I used a 6" 1/4" black oxide bit then switched to this one:

http://www.fastenal.com//web/products/details/0350457



Here's a picture I took while doing a practice run that better highlights the stability provided by this drill bit. The contacts with the hole wall are a lot wider than I was expecting.



The bit was very stable in the hole due to the double margins. I used an electric drill and stopped about 5 times to remove chips. The last time I removed chips I was almost bottomed out in the hole, so I unchucked the bit and slid it about 1/4" out and was able to make it all the way through without having to switch sides. Both holes seem great.

The biggest "gotcha" I had was my rear spar attachments were only pilot drilled when I started. I thought I was supposed to drill to 1/4" with the wings attached to the fuselage, but was unable to get the drill positioned without interfering with some riveted angles on the fuselage walls. I then decided to remove the wings and attach them resting on sawhorses. This allowed me to attach the main spars solidly together with bolts in the 3 alignment holes. I was then able to attach the rear spar channel and drill up to 1/4" without interference. This is probably a step I skipped somewhere.

That looks like the best solution yet...