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While waiting for a few engine mount components to get finished, and
knowing that as soon as the mount is completed I'll have to install it on
the firewall in order to work on the nose gear, _and_ having decided long
ago that I was going to engine turn the SS firewall sheet, I'm just a few
rows from having it completed and thought I'd share a couple pointers with
those of you who are contemplating a similar project someday.....
There were instructions in an old issue of Sport Aviation about how to go
about this, but the only things I remembered were that Scotchbrite pads
applied to the turning mandrel would do a good job, and that I'd need a
locator with a hole in it as a guide to help position the mandrel as I
progressed along my pattern.
First the mandrel... I used a 2" hole saw to cut out 2 disks 1-7/8 OD,
drilled the centers of them out to accept a 7/16 bolt, then cut an
additional circle out of a piece of 1/4" plexi to use as a cover over the
bolt head. (I used a 1" forstner bit to sink the bolt head into the lower
piece of plywood.) Ran a nut down to clamp the plywood parts to the bolt,
then glued and screwed the plexi over the front. The bolt shaft is what I
chuck into a 1/2" drill for the turning operation. Use contact cement to
attach the Scotchbrite to the plexi, trim it off flush with the
circumference of the mandrel and you're ready to start.
I ran some test patterns on the back of the SS sheet to see what looked
good and found that if I moved the turner (for lack of a better term) in
7/8" increments I'd get coverage over the preceeding centers and wind up
with a really pretty fish-scale looking pattern. I used a piece of 1/2"
flakeboard with marks every 7/8" along its length to help me position the
guide horizontally, and started with a piece of board almost as wide as my
workpiece to provide the spacing vertically. As each row was completed I
just sawed 7/8" off that big board, located my measured guideboard up
against it, then used the lines on the guideboard to indicate the end of
the locator board and went on with the next row. It actually was pretty
simple, although very time consuming, I'll bet I have 10 hours in it so far
and I'm still not quite done.
One thing I would recommend is that when you start applying the pattern and
using the Scotchbrite disk to mark the sheet, it really helps if you
position the drill in such a way that the pressure is brought to bear only
on the quadrant that is going to be showing. When I first started I was
being very careful to bet a good pattern all the way around each circle...
after a couple rows were completed I realized that there was actually only
1/4 of each circle left showing as the pattern progressed, so I started
concentrating on that corner and found that successive passes went much
quicker as I hadn't already surfaced the material where the exposed
portions were going to be, and the pressure around the edge was more than
adequate to erase the little centerpoint that appeared with each circle. I
also started with a clamp to steady my locator board, but found that I
could apply enough pressure with my free hand as long as I ran the drill in
the direction which would pull the locator board toward the guide board. I
can now make 4 circles in the time it took me to make one originally, and
they're more uniform to boot.
Anyway, if you feel the urge to do this, go for it, it's really an
interesting diversion, not too horribly difficult, and makes for a really
pretty piece of metal.
<Marv>
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