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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>