Hi Jim,
I'm not a machinist but will be happy to pass on to you what I think I know :)
There is a short paragraph on using reamers on Murphy's Technical Support
page (
http://www.murphyair.com/SUPPORT/Support.htm ) in the document titled
Standard Aircraft Assembly Procedures, Section 1.38, Page 33.
Generally whenever you make a hole for a fastener, you want it to fit that
fastener as closely as possible. The tighter things fit together, the less
movement. Movement of the fastened parts will enlarge the hole and that is
not a good thing. Reaming a bolt hole gives you the best fit.
Reamers, for our purposes anyway, are designed to remove only small amounts
of metal. Something in the neighborhood of .007 inches if I recall correctly.
They cut much less aggresively than a standard drill bit and leave a much
smoother finish to the hole. As well, I believe they are machined to closer
tolerances than a drill bit.
If you haven't already, you should get yourself a table or chart of drill
sizes such as this one at
http://www.engineersedge.com/drill_sizes.htm. This
lists the decimal sizes of all the common metric, inch, letter, and number
drills. I find this handy because when I want to drill a hole for an AN3 bolt
that will be reamed to final size, I first drill a #40 hole, then drill up
from there to just under 3/16ths inch. I can see from the drill size chart
that the next smallest hole size below 3/16ths inch is made by a #13 drill.
So I drill my #40 pilot hole with succesively larger drill bits until I've
drilled it to #13, or .1850 inches diameter. Then I use a 3/16ths inch reamer
and ream the hole to final size .1875 inch. In this example, the reamer only
removes .0025 inch of metal. You could drill the #40 hole up to a size #14
and the reamer would still only be removing .0055 inch of metal. So when I
buy a reamer, I buy or make sure I have a drill bit on hand that is of the
next smaller size.
Whenever possible I always drill/ream holes in the aircraft fittings using
the drill press. It is much more accurate because you can't hold a drill
perfectly steady.
Building my Rebel, I've only ever had a need for straight flute, high speed
steel, 'chucking' reamers which I've purchased at McMaster Carr
(
http://www.mcmaster.com/). They aren't too expensive. One each of 3/16ths,
5/16ths, 1/4 inch, and 3/8ths inch sizes has been all I've needed and when
you are using them to drill aluminum, they last forever.
Hope this helps.
Roger Hoffman #687R
Eugene, OR USA!
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