Fri Jul 17 2020

Center arm rest

This morning, I found some nice, flat pieces of particle board and fixtured up the lid and the recent layup which will form the wedge at the aft end of the lid. I mixed up a bit of epoxy and clamped it all together.

While that was glueing, I worked on the rest of the arm rest compartment. I started by fitting up the additional aluminum pieces which go around the perimeter of the compartment. They will need some trimming as well as some bends.

I've learned that the key to doing precision sheet metal work is to get everything setup using test pieces. First, add the appropriate bending guides to achieve the correct bend radius. Then, adjust the position of the bending edge and adjust the clamp for whatever thickness material (plus the bending guides), then adjust the bend stop to get the right bend angle (after springback). Once all that is done, some test bends are in order to ensure the bend is positioned correctly. Here's a little test part I'm using to check the final bend position relative to the line drawn. I'll be bending the left side of this part up. The desired goal is to have the outside surface of the upbent tab be exactly even after bending with the left side of the black line. So, since this part is 1.3455 long, and the left side of the line was drawn .493 from the left edge of the part, once bent, the part should be .8525 long (i.e. 1.3455 - .493 = .8525).

I positioned the test piece carefully. Since this part is .025 thick, I used another scrap of .025 to check the offset of the line from the nose of the bending guide.

After bending this is the result. Target: .8525. Actual: .8490. Deviation: .0035. Not bad!

So, now that the brake was all setup, it was time to make bends in the actual part. I realized while setting this up that after bending, the part would be trapped in the brake. No big deal...it just requires removing the top section of the brake to extract the part.

I completed both bends and trimmed the recently bent tabs to the correct length. Wow, it fits just right in the top of the center arm rest compartment.

Since I'm getting close to riveting this together, I decided to make a test piece for trying out some different riveting approaches. You can see the slot between layers of aluminum, into which will fit the tab of the lid.

Since it's desirable to have a flush surface on the outside of the center arm rest compartment as well as around the inside, my first thought was to use a double-dimpled rivet arrangement similar to what was done on the horizontal stabilizer. On that HS, I noticed (after the fact) that the dimples from the opposing sheets contacted each other (I still need to fix that). In this case, I didn't want the same thing to happen, so I measured the height of a dimple (including the thickness of the sheet metal). Since the sheet is .025 thick and the center layer is .040 thick (half of which is available for this dimple), the overall dimple needs to be .045 or less. I'm a bit over here. No worries, I'll just file it down a bit.

Here's the dimple after filing.

I countersunk the center layer on both sides. Unfortunately, in order to let the dimples recess in these (opposing) countersinks, the hole size opened up. Not a problem in and of itself, but it means I can't rely on the pilot of the countersink.

I first attempted to form the manufactured head with the squeezer. It wouldn't form down enough into the dimple. So, I broke out the rivet gun, backrivet plate, and backrivet set. I was able to get the manufactured head to form down into the dimple to the point where it was flush, but it puckered the heck outta my test parts. I'm going to have to try something else. Maybe I'll get some soft rivets and try those.