in A&P
school I was taught to use up to 300fpm for aluminum with hss bits,
depending on the alloy used.

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Dear Del (and the Group),

I got most of my basic aviation metalsmith training from my uncle Sam between
1956 & 1960. You'll find the figures I've cited in any of the manuals from
that era for any of the services. I happened to be Navy but the basic stuff is
the same in the Air Force manuals (AF Manual 52-11, etc.) which were reprinted
by the EAA.

The figures are for carbon-steel drill bits having an included angle of 90
degrees (ie, standard issue 'aviation' drill bits, at that time) and used dry
(ie, no lubricant).

Drawing from the same sources (ie, military training manuals) you will find 300
SFM listed for lathe & milling operations using HSS bits and cutting fluid.

Other cutting tools (and drill bits) such as 5% Cobolt, carbide or steels
treated with titanium-nitride vapor depostion may be operated at speeds higher
than those listed but their use departs from the issue at hand about which I
think there are three points that need to be made.

The first point is that the typical homebuilder has no idea in the blue-eyed
world as to the relationship between tool-speed and producing an accurately
drilled hole. They are using tools inappropriate to the task and no one
bothers to explain why their eighth-inch drill bit turning only 1200rpm in
their K-mart 1/4" drill-motor keeps skittering across the surface, other than
to tell them they need an air-drill.

Secondly, offering them an equation or a printed table that links rpm to drill
diameter should give them some idea that the two are related, especially if
some stress is placed on the fact that skinny drills must spin faster than
fatter drills. The hope here is they'll pick up the idea that they are dealing
with a CUTTING TOOL rather than some kind of rotary punch, where
one-speed-suits-all and you simply have to PUSH HARDER when making a bigger
hole.

The third point has to do with the Conventional Wisdomites who, having learned
that Faster is Better, run around repeating the highest number they've heard
with regard to drilling, confusing airplanes with dentistry. (If 3000 rpm is
good then 30,000 rpm must be GREAT, right?)

The truth is, most homebuilders are using dull drills, having the wrong angle,
chucked into a K-Mart Blue-Light special turning maybe 1200rpm. I've no idea
if they are using plain carbon steel or nitrided cobalt -- and neither do they.
But I do know that if they are making bad holes the SFM formula I cited will
PROBABLY help them make better ones.

As for the rest of it, as in who has the biggest SFM equation or the longest
drill -- it's difficult to address the matter in a cogent fashion without
getting into the specifics of alloy, temper, drill-type production rate and so
forth. Fortunately, I don't think we have to. The SFM equation I cited may be
the lowest common denominator when it comes to riveting but it will at least
allow the builder to produce good holes, usually with better accuracy and less
effort than they're presently getting with their 1100 rpm Makita and a packet
of 1/8" double-enders from Harbor Freight. There's nothing in the rules that
sez you can't ADD to that by offering other equations specific to particular
alloys or drill bits.

-R.S.Hoover