Wed Apr 15 2009

Drilling engine mount to firewall

We started the day by drilling out the rivets that attach these spacer blocks on one half of the carry through F-704 spar. The rivets which Van's supplies (AN426AD4-14) are really too short to do the job. Quite some time ago I ordered .1lb of AN426AD4-22 rivets from Van's, but after a half hour or so of looking around, we couldn't find them so we moved on to something else. If they don't show, I'll have to order some more.

Since I now have the motor mount, I figured we'd drill the mount to the firewall before disassembly and priming. The trick is to get the holes drilled in exactly the right place.

After thinking about it some, we came up with the idea of getting a couple bushings that would center a small starter hole (#28 drill) in the hole. I would've really preferred two steel bushings, but all I could find in a smaller bushing was this nylon one.

It fit perfectly in the motor mount bolt hole. We then used the bushings as a pilot and drilled the bottom right corner.

After drilling a scrap piece of 1/8 angle aluminum with a #28 hole, we flipped the firewall over and used the #28 drill bit to hold the scrap aluminum (hidden below the firewall in this picture) in position relative to the firewall and transfer drilled several #30 holes so we could rigidly cleco the scrap piece in place.

Now with the scrap angle in place, we can hopefully prevent the stainless firewall from pulling up when we drill.

These concentric circles were traced around the two bushings so that we could observe if the hole was drifting out of position.

We gradually stepped up the hole size in 1/32 increments. Each time the drill bit engaged the stainless the bit would really grab. I was actually unable to chuck the bit tight enough to keep it from slipping in the chuck, so we switched to a different drill with a nice Jacobs chuck.

In order to prevent the drill from grabbing, I was using high speed with very low pressure, but when this 9/32 drill hit the stainless, it snapped it right off. Dang! This was a brand new cobalt bit and it never made it through the first hole. Crap.

After talking to a couple people, we ran out to (cough...cough...) Harbor Freight and picked up this...errr...high quality three piece unibit set for $10. The idea was to modify the bit on the left (which goes up to 1/2 inch final size) by grinding down the last two steps so that 3/8 would be the final size. Then we could just use the engine mount as a pilot.

On the way back from Harbor Freight, we stopped at work and talked to a couple of the guys in the machine shop. I borrowed a series of reamers so that we could step the hole up with reamers which hopefully wouldn't grab.

I also borrowed several 3/8 inch diameter reground (undersize) end mills. One of the machinists thought this would work well for machining through the stainless.

Upon returning home, we tried the reamers. They worked great and soon we had our first hole finished.

The bolt went right through. A nice tight fit just like it's supposed to be. With the bottom right hole complete, we moved on the the top left hole.

After pilot drilling the top left hole, then leaving the bushings in place with the #28 drill through the bushings and the hole, I decided to try using one of the undersized end mills to bore the bottom left hole. It was about 1/32 inch undersized so I figured that would leave enough material to ream to final size. Unfortunately the motor mount shifted around too much during the milling, and I ended up with a slot instead of a hole. Argh! I was pretty frustrated at this point, so I went upstairs and ordered some new parts.