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Drilling?

S

Steve Sampson

Well Known Member
I never understand instructions like these in the plans. It is embarrassing to be building my second RV and still not fully understand this!

In the plan it says to drill "D". This is slightly less than .25", the size of an AN4 that will go in there.

Is the idea:
a) You drill "D" then hit the bolt with a mallet? Ouch!
b) You drill "D" then ream to .25"? Normally VANS call for a reamed hole if thats what they want.

Why not just say drill to .25"?

If its not clear what I am talking about there are pictures here.

Thanks.
 
Steve,

The nominal diameter of AN hardware is often slightly smaller than you think. I've just measured an AN-4 bolt, it is 0.246", or the same size as a letter D drill. If a "light drive fit" were required the maximum permissible is only a few tenths of a thou, and must be reamed (0.0006" interference on a 5/8" dia bolt is the definition). So an AN-4 interference should be 0.0002", or there abouts, if that were required - and you have to make sure the hole is perpendicular.

I would drill D and see if an AN-4 will go in, if it doesn't then open up to 6.3mm or 1/4 = E.

Pete
 
Drilling , holes and fit.

Pete - when you say "..see if a AN4 will go in", with how much force? Pushing with my fingers or gentle tapping. It is these subtlties that none of the docs tell you.

Is the reason they define the hole as "D" not 1/4" to try to get a tighter fit? I guess the question is really is that the reason why "D" is less than .25?

Thanks,
 
Hole size issues

Steve,
In the world of large production aircraft, or anything with a high fatigue loading, the most common permanent jointery uses an interference fit. I know it sounds bizzare, but the fasteners used actually get "driven" in prior to the nut or self torqueing shear collar installation. In laymens terms, this does not allow for any fastener/hole "wiggle" and infact allows some stretching of the hole under load. In the world of light aircraft, this is not as common,but ulimately a 1/4 (.250) hole is not Tight for a -4 AN bolt. The "D" being .246-.247 will be tighter for the bolt....I'm not sure which holes you are drilling, but I never use a 1/4...always a "D"...can always ream to .250 if needed....

Bill E.
 
Majority of the time, a twist drill will drill a hole slightly bigger than itself. Most bolts are a thou or so undersize, and a "D" drill will usualy drill a bit oversize...it sounds like Vans is trying its best to get you a snug hole. If you got to hammer it in with a sledge, I doubt that kind of fit Vans was going for. A few taps with a mallet, that sounds acceptable.
 
Drilling

Thanks for all the replies guys. It was really useful to me and makes lots of sense. (Judging by the number of hits my web site took I cant be alone....unless people are just chuckling at my lack of knowledge!)

I think we need a bit more of this 'basic' info when we are building these aircraft. Pity VANS dont do a bit more in the introductory chapters in the build manual. the aircraft handbook also dont properly cover just what has been said here. The complex stuff is better covered.

Unless I have 101% confidence I start worrying as the ASI hits 200mph.

Thanks again.
 
Steve,

If you can't push it in by hand before you paint the hole is too small. If Van's called out 0.250, and supplied 0.246 bolts the fit would be too loose. As there is a tolerance on AN bolts (the one I had to hand was 0.246), a D size hole seems correct. The earlier plans/build manuals do leave a fair bit to "interpretation", its just how they are.

Pete
 
I prefer a reamer....

JonathanCook said:
Here is a good article on how and why to drill holes that will be used in bolted applications.

http://www3.sympatico.ca/fhofmann/art_round_hole.html

Jonathan
Click to expand...

My only addition to this article would be to use a reamer for the final hole. The only reason that the D drill works is because a hand held drilled hole is probably a bit oversize.

The AN-4 hardware is 0.246 to 0.249 inches diameter.

Specifications are here...

http://www.gen-aircraft-hardware.com/images/pdf/an3_thru_an20.pdf

Avery even sells a 0.249 reamer, which will make the best fit hole for all (as opposed to specific) AN-4 bolts.

https://www.cart.averytools.com/pc-535-29-chucking-reamers--high-speed-steel.aspx

We amateurs can make much more accurate, and rounder, holes with a reamer.... :)
For the low cost involved, I prefer the accuracy of a reamer. Most of the RVs can use just the 0.249 and a 0.189 sizes.

gil in Tucson
 
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chucking reamers

All,

Can one run a chucking reamer through with a hand-drill (carefully)? Appears inaccurate. I can setup the bench press, but that may be fun.

-Bruce
 
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