Match drilling

[Michael Burbidge]

How do you prevent the bit from drifting when match drilling harder material to softer material. I.e.nThe original hole is in the softer material.

I'm currently try to match drill the rear bulkhead bar stock to the rear bulkhead.

[Transporter]

This is definitely a case where I prefer reamers vs drill bits. If you buy them in increments you can enlarge the hole in the thick material gradually without turning the hole in the thin stock into an oval.

Not cheap tools, but worth it IMHO. I use them with Boelube paste (the blue stuff in the tub) when reaming thick material; it's a good lube with easy cleanup afterward.

Check Avery, Brown, Cleaveland, etc.

Good luck,
Mike

[apkp777]

A "gun barrel" reamer (drill bit with no flutes) is the way to go. It's unlikely that you have one. I would just take care and with a new bit (regular) you will do just fine.

A set of gun barrel bits is a worthwhile addition to your tool kit. You can probably find them at Aircraft Tool Supply or the Yard, Avery or other.

[rzbill]

I don't remember this being a problem if you are speaking of the 711 (911 in your case?) aft bulkhead assy.
If memory serves, these are punched approx #40 from the factory and need to be drilled out to #30 for an AD4 rivet.

In that case, I would have drilled #40 first and accepted the walking that occured (trying to keep to a minimum, of course) and then drill #30 to finish them up nice and concentric.

#1. Change bits. Dull bit and too much presure = walking and ovalized hole. Sharp bit, light pressure to get a center started in the barstock.

If you are in the tough position of needing to drill final diameter on the first shot because the thin part is already #30 from previous drilling, here is a way to do it with stuff you may have in the shop. It's tedious but will save your bacon.

Get a thick (1/8 or more) piece of aluminum (steel is better) and drill it to final size. Use this as a drill guide. Align this guide to the finished hole in the thin sheet by inserting a drill bit backwards into the hole. This presents the full diameter of the bit, rather than the tapered tip for alignment. You may have to grind the back of the bit flat because some bits have a miniscule step on the back.

With the bit inserted through the guide, through the hole in the sheet and bumped against the raw material you want to drill....CLAMP THE GUIDE TIGHT.

Remove alignment bit and proceed to drill that hole.

REPEAT until you get out trouble in the problem area.

Avoid problem in future (the above is a good penance way to remember this)

Good luck and I hope this helps

[JonJay]

It is difficult to use a bushing in a small hole, but for larger holes a steel bushing allows you to drill the pilot hole on center elliminating drift on the final drill. Not sure what you are drilling to what.