[GeorgeRace]

I am not sure how to describe what I want to do. Here is a go at it!

I make a lot of aluminum access openings for experimental airplanes. They mainly consist of a backing plate, and a front plate with a hinged door and appropriate fasteners. When installed they "sandwich" the airplane fuselage skin between my front section with the door and the backing plate. Rivets are put in about every inch all around and we now have a hinged access opening into the area. This works just great for flat surfaces, as I can CNC drill the holes in the front plate and back plate in separate operations, from basically the same drawing. So, the holes in the front and back plate match precisely.

Now the real question. Lets say that I want to make a curved front and back plate, with a radius curvature of 3 feet. I think that if I make it as described above, when the two plates are bent to fit the curvature, the inner and outer plate holes will be out of alignment, if I start with a flat drawing. But, I think that if I were to draw it with a 3 foot radius, and then place the holes, about an inch apart, around the perimeter, the inner and outer surfaces and holes would now be parallel with each other and the rivets would easily slip into place. The outer skin being sandwiched between the front and back plates is only .032 thick. Am I making a fuss over something that will not even be a problem.

I guess I need to take one of my kits and bend it around a coffee can, use the outer plate as a template to drill holes in the coffee can, and then see if the inner plate would allow clecos to be put into place and hold the pieces in alignment.

I hope this makes sense! In the end, maybe it will really not even matter!
Your thoughts and comments would be appreciated.

George

[lancut]

How I always done when working on aircraft skin, access panels etc, layout the hole pattern in one piece, drill #40 or so pilot holes. Lineup and backdriil the holes in the other piece, stick couple cleckos in and open the holes to size by either using the coredrill reamer etc..to mach

[bushwakka]

I would expect that there will not be much difference.

I worked it out in metric and converted, but 8" hatch would have about 8thou difference when the sandwiched sheet is 40thou with a 3 foot radius.

Not sure if it should be from the front to front or the front to back measurement though on your 2 sheets (probably front to front, so the difference might be more when the sandwich is is larger).

++++++working+++++++++
work out the length of the arc for the inside piece of the arc segment = 2x pi x radius x angle of segment. Then work out the difference in length between the 2 when the radius is changed by a small amount.
The end result will be

**difference = height between hole centres x (1 - inside radius/outside radius).**

I'm pretty sure the maths is right, but you should check before putting on any experimental planes.

Paul

[lancut]

Just drill by hand , all you drilling is 2024 or so, .032" thick aluminum.

[GeorgeRace]

Thanks for all your comments and suggestions. As I kind of expected, the difference is going to be miniscule. It really helps to have the formula and be able to do the calculations.

George

[Kiwi]

Here is a VB program which does the calculations.
I think I have the maths correct?

[GeorgeRace]

Hi Kiwi:
Thanks for the little program. I will see if it works for me.
George