Sunday, June 14, 2009

Bolting the inner skins to the aluminum frame

Today I worked on attaching the inner skins to the frame.

After seeing how another builder had done it, I thought I would try. I like the idea of being able to remove the skins at anytime. My previous set of skins are glued together using 3M #77. They didn't glue together perfectly aligned, which has always been a sore spot for me.

I also wanted to make sure that I can removed the skins with the legs on. My previous set attached above and below the shoulder, so unless the legs are off, I can not remove them.

So after giving this little project some thought all week, I printed off a photo of what another builder had done....and Jerry's tutorial on attaching the skins for good measure.

I pondered several ways to line up where I wanted to drill the holes. The best method seemed to be to use a pencil behind the skins and mark the top and bottom of where the skins met the 1/4" inch frame plates.

Since we are dealing with some small tolerances, I couldn't take any chances with the drill bit wandering when I drilled the hole. I used a center punch to notch (dent) the site of the hole.

I then used a 1/8th inch drill bit to drill through the skins. I placed the skins on a thick piece of plywood for this. Some of the holes may require some filing to smooth out.

The moment of truth...clamping the skins back into place and seeing if they line up with frame plates! They do! The ones near the top came out OK. This one in the photo is good, while the other hole near the top was a smidge too high.

Using a 3/32 drill bit, I drilled through the skin holes and into the frame rings.
( HINDSIGHT THOUGHT...I wish I had used a few more clamps for stability. I have one hole that is a little off, the vibration of drilling moved the skins slightly. As a result, I will have to stretch one hole a little to line up with the frame hole.)

Once the holes were drilled, it was time to tap the holes. Using a 4-40 tap and some liquid soap, it was time to SLOWLY create the threading. I would turn until it got snug, go back a half turn, then slowly advance another. I removed the tap, cleaned the debris off the bit, re-applied some dish soap to the tip, then continued. Do not hurry this step at all!

After wiping down the debris and using a can of compressed air, the threaded hole is done.

Now it is time to countersink the skin holes so that the screws sit flush inside the skins. This way the outer skins will fit on top smoothly.

The countersink bit will take some tweaking to get each hole just right. Just take your time, stop frequently to check if the screw head is flush or needs more work.

Below are the 4-40 screws I used for mounting the skins.

After all that work, here's the result we are looking for...

And here is the end result. I have two holes on the top of the skins on each side of the shoulders. One on each corner near the bottom edge and two in the middle area to keep the skins snug against the body.
Is it pefect? Of course and any skilled builder can point out I'm off a bit here or there. But is it functional? Yes. I've saved myself a day of dealing with adhesives on a large surface like the skins. (Last time, I had several days of super gluing spots that didn't adhere well)
Next up....the outer skins, once they are dry from painting!