Getting well-centered holes with the unibit

One technique I've had difficulty with, after having got through the wings and much of the fuselage skeleton already, is how to accurately center large holes drilled with the unibit. It seems like I can never get the hole EXACTLY centered where I want it. It some cases this doesn't matter much, but in other cases Van's leaves us with very little margin for error regarding edge distance. Some of the bulkhead holes come to mind here where, if your hole is any more than 1/32 off center, you might be left 1/32 shy of the usual 1/4" needed for edge distance by the time you get a 5/8" hole. Thus my question is about the best technique to drill up to size...

In most cases I've started with Van's predrilled hole, if it exists, and drilled up to 1/4" using a couple of intermediate sized bits. Once it's at 1/4" then the pilot cutter of the unibit should be centered. Even doing it this way, though, does not always seem to provide perfect accuracy. Sometimes I'm right on, and sometimes I'm not by the time the hole is drilled up to full size. Again, being 1/32 or sometimes 1/16 off by the time I'm done doesn't really matter, but sometimes it does.

Any suggestions for tried and true methods to getting big holes EXACTLY where you want them? This is one of those cases where I, as a relatively experienced builder now, find myself somewhat humbled.

Happy Super Bowl weekend! I've been waiting a long time for those Seahawks!

First, draw the outlines of the hole exactly where you want it to end up with a very fine tipped marker (Avery sells the best ones I've found) and a plastic hole template. It even helps to draw it just a tiny "skidge" oversize. Then, as you take the hole out in the small increments with the unibit, keep checking "by eyeball" to see that you're staying concentric. If the drilled hole starts drifting a little off concentric, you can force it back where you want it by holding lateral pressure on the drill. This tecnique worked well for me.

Another tip I learned was to turn the unibit pretty slowly. Your battery drill will produce a nicer hole than your high speed air drill.

Wish I'd learned these tricks early in the project instead of towards the end!

John Miller

I'm not up to anything on my kit that requires accurately drilled large holes yet, but in general if I need an accurate big hole in metal, I get it close with a regular bit and then ream it. YMMV, but that's just how I've always done it 'cause I have a wobbly hand and the same problem as you with the unibit. If I don't care about 1/32" one way of the other, I unibit it. I know people that can get their holes dead on accurate (not match drilling, either) with a unibit and it's always impressed me.

I don't know if this has anything to do with it or not but some uni-bits have two cutting edges and others have only one. I find that the single bit cutter does a nicer job than the double cutter.
Just for what it's worth.
51HS RV9A

holes

John C is right about the reamers. If there is already a hole that you need larger, the reamers work fantastic. I read here on this forum over and over about how great reamers are and until I tried one I wasn't a believer. Now I have a LARGE assortment of reamers. I now use drill bits to start the holes and reamers to enlarge existing holes. They just don?t drift. I don't know if your question was answered but if you are enlarging a predrilled hole, use a reamer.

Reamers

If you are going to use reamers in place of the unibit, for all sizes that the unibits support, get ready to plunk down some serious cash. I am a huge fan of reamers, but my current assortment is somewhat limited.