Double Margin Drills

Is it desirable to use double margin drills?
What are the avantages and disadvantages?
When/where are they best used in RV construction?
Are they equivalent, in end result, to using a reamer?
Are they useful or desirable in the drilling of the prepunched holes in RV, preparatory to riveting?
Are they only desirable in certain special sections of the build?
Do they last well, or would you need to buy more than one of each size to do a QB (I'm thinking of Pan American brand)?
Are there other good brands?

John

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okay... what is a double margin drill?

Close tolerance holes

Hi John,

For high quality aircraft work I would always recommend using a good quality drill bit such as a double margin drill. Even just opening up rivet hole's to final size, a double margin drill would be ideal, poor quality drill bits may lead to dodgy holes.

For even better hole quality I'd recommend reaming up to final size for bolt installations, especially holes threw the wing spar.

"Are they equivalent in end result to using a reamer?"
A reamed hole is always a higher quality over a double margin drilled hole regardless of how well the hole was drilled.

Do they last well or would you need to buy more than one of each size to do a QB (I'm thinking of Pan American brand)?
Depending on what material your drilling, for 2024 T3 the drill bits should last for ages, if your drilling carbon steels or 7075 T6 using a cutting lube will help prolong the drill bit cutting edges.

I'm not sure what brands there are in the US, inspect the drill bits before you hand over your hard earned $$$, I'm sure you can pick a good quality drill just by looking at the grind finish, I did a quick search, http://www.tool-tool.com/avi_double_e.html this is similar to what I use at work, the flutes are straighter resulting in better hole.

Hope that helps,

Double Margin drills

This gives a good explanation of what a double margin drill is http://www.panamericantool.com/products.cfm

Cheers,

Double margin bits are as close to using a reamer as you can get-------as long as they are sharp.

For what we are doing, they should be fine unless the plans specify a reamer.

Mmmmm We have lots of Aircraft Drill Bits...

Jobber, 6", 12" and three lengths of threaded. They are all split point (NAS907J) and made of steel coated with Black Oxide or Cobalt that is un-coated. I have heard of double margin but only seen one years ago in Grampas tool box, and we have been in business for 25+ years.

Once you ream you will never drill unless necessary.

GAHco said:

Jobber, 6", 12" and three lengths of threaded. They are all split point (NAS907J) and made of steel coated with Black Oxide or Cobalt that is un-coated. I have heard of double margin but only seen one years ago in Grampas tool box, and we have been in business for 25+ years.

Click to expand...

I've seen them used in the production environment quite a bit. It's basically a less expensive version of a piloted reamer. We used the term "core drill", I never heard the term double margin either.