I thought I was finished with the new instrument panel last weekend, but I discovered several minor issues that needed sorting out when I took out the microscope. I had to rework two nutplates that were hitting the mounting angle on the fixed part of the panel, and I needed to clean up quite a few rough edges. I finally dropped the fixed and removable panels off at the paint shop on Wednesday. I'll pick them up on the week of 17 Jan.
I started working on the rear seat riser on Wednesday night. The rear seat riser is a platform that lifts the rear seat cushion about three inches off the floor at the front, and slants back to be about 1.25 inches high at the rear (rear seat riser plans). I made a trial plywood rear seat riser in Oct 2003 to confirm the dimensions, and learned that I needed to make it a bit lower if I wanted to take Jim Manton (one of the other test pilots at work) in the back seat. I made it a half inch lower than the plans called for. If that doesn't give him enough head room, I'll pull it out when I fly with him. I'll probably also insert a one-inch balsa spacer between the seat riser and the seat cushion when Terry rides in back.
Wednesday I got the various parts laid out on a sheet of aluminum, cut them out and deburred them. Thursday I pulled out my cheap sheet metal brake, played with some scrap aluminum to figure out how it worked, then bent up the first rib. Yesterday I bent up the other three ribs, and got three of five bends done on the big piece of sheet that makes the top. It really made my head hurt to sort out the bend allowances and setbacks (www.tpub.com), to be sure to get the ribs the size I wanted after making the bends in the sheet metal, but it all worked out very well in the end.
I'm recording how I did this here, so I don't have to strain my brain so much the next time I have to bend some sheet metal parts to a desired size.
The lines on the rib blank mark the desired location of the outside edges of the part, after all bends are complete.
The sheet metal brake is clamped to the workbench, with the movable part facing away. The first bend is made with the flange facing away. The line on the blank is lined up with the break between the two parts of the sheet metal brake. The metal bar is placed back from the line, with the distance between the line and the edge of the metal bar equal to the material thickness plus the desired bend radius. In this case I used 0.032" thick material, and I wanted a 0.063 bend radius, so the bar was clamped 0.095" from the line, which equals 3/32".
The second bend was a lot more complicated to sort out, as I had to turn the part the other way, or the first bend would hit the clamping bar. I had to dig into the Standard Aircraft Handbook to find a chart on bend allowances. It really made my head hurt, but I eventually decided that I needed to offset the line on the part from the brake pivot line by a distance equal to the bend allowance (from the chart in the Standard Aircraft Handbook), less the total of bend radius plus material thickness. The clamping bar had to be set the same distance from the brake pivot as before, as the desired bend radius was the same.
The third bend was the larger end. I had to slip the clamping bar as far to one side as possible, to make room for the other flange to go during the bending process. The last bend was the small end, and was done with hand tools.
Things went quite well, until the fourth bend on the large piece of sheet that forms the top of the seat riser. I'm not sure exactly what happened, but that bend ended up a bit skewed. It is OK at one side, but the other side is out by about 0.10". Grrr. :( I stopped for the day, frustrated.
I was tempted to chuck it on the scrap pile, and start over. But I don't have a spare piece of aluminum laying around, and I am pushing this cheap brake pretty hard to do these bends. It takes a lot of force to bend a piece of 0.032 along a 17" bend. There is so much force that the clamped bar tends to slide, which is perhaps how I screwed up that bend. I can't be sure to get a better job on all five bends next time. The rear seat riser can't really be seen unless you lift the seat cushion, the error is not that big, and it won't affect the structural integrity, or how the rear seat riser functions. So, I'll use it as is, with some shims, I think.
I've got a 6 AM flight tomorrow morning to Chicago, and then Wichita. I'll be gone for the week, so I'll get back at this sometime next weekend.