Wing Attach Fitting Hole Size

[wltrmtty]

I'm creating CAD drawings of my wing attach fittings to have them cut out by CNC waterjet. Should I undersize the holes common to the wing spar so I can ream them with the spar holes?

[cub builder]

Waterjets are accurate, but cut circles typically will leave a tiny edge that's a bit uneven where they start and stop the cut. (at least they do from the shop at work) I would under size the hole in the drawing with a pointer and note on the drawing to drill and ream to size when fitting. Depending on the design and how well it's fitted, you may only want to ream to size.

-Cub Builder

[CraigCantwell]

I'd just do a pilot hole in them. Probably nothing bigger than a #20, so that you can get the fittings aligned easily when attaching them to the fuselage. Then, once the wings are correctly positioned, drill and ream them with the spar in place. The reason for a small hole, is that it's much easier to move a #20 hole than a .750" or whatever your near final sized hole is. Been there and done it a number of times in the production world.

[ekimneirbo]

I'm creating CAD drawings of my wing attach fittings to have them cut out by CNC waterjet. Should I undersize the holes common to the wing spar so I can ream them with the spar holes?

There are two or maybe more ways to look at this. First, if you are sure that the location of the attachment hole will give you exactly what you want when assembled, then I would have the waterjet cut the hole slightly undersize and ream it to finish size. The reason I say this is that if you are using a common two flute drill bit, you will need to restrain the wing attach fittings, use a dial indicator to zero your location, and use a drill press to enlarge the holes. Two flute drills often don't cut round holes even when done this way. Thats why in industrial situations "core drills" with more flute are often used. Take a 2 flute 3/4 drill and start drilling thru a piece of 1/4 aluminum that has a 3/8 pilot hole. Stop drilling before you get completely thru the aluminum and look at the pattern the two flute made. Even if the hole ends up reasonably round, the location often isn't exactly the same as the original pilot hole. So, my suggestion if you go this route is to have the waterjet cut the hole about .010 undersize and then ream it. Less chance of moving your location. This scenario portends that all your measurements and calculations are very accurate. Often, when assembling precison sized and located holes...things don't fit perfectly. That leads me to option number two which I think is the best way.

Have the waterjet cut a small 1/8-1/4 inch hole in the attach fitting. Install the wing and locate it properly. Double check that both wings are located correctly. Triple check that both wings are located properly. Check dihedral and fuselage level with measurements from a digital level. Then use a water level to check the ends of the wingtips. See if all the measurements jive. Then when you are absolutely sure.....drill thru the pilot hole so the spar has a mating hole.Then slowly enlarge the size of the hole with multiple drill bits until you get close enough to finish ream the hole. I suggest using an adjustable reamer so that you can add a few thousandths(.003) for ease of alignment unless this is final assembly , then just add maybe .0015. Precision holes drilled at separate times often do not line up for assembly when performed in amatuer conditions. Professional CNC machining of precision parts usually allows for variation by incorporating tolerances. Everything fits together, but they are not all machined to the perfect median dimension. Ergo, a 1/2 inch hole will have a -.000/+.005 tolerance. This means the hole can be up to .005 larger, but no smaller than 1/2 (.500) inch. The mating bolt will have a tolerance of -.005/+.000 meaning it can be .005 smaller, but no larger than 1/2 (.500). The bolts will always fit in the hole that way, but a worst case scenario has .010 play. The best way to insure that in a one off build that your holes will be in exactly the right place and size is to match drill the two components while they are aligned in the correct location.

[Matt Gonitzke]

Holes cut by a waterjet also have a rough surface finish and taper slightly, especially if the material is thick. Be sure to leave enough material to ream the hole to final size.

[wltrmtty]

Thanks!