On 2010-06-18, Ignoramus13320 wrote:
Finally I need to make something to good precision, which is to make a
mounting plate for a US Digital encoder. The spec says shaft
misalignment should be under 0.01", which means that the individual
mounting holes need to be even more precisely placed, so as the
cumulative error will stay within spec.
Is it possible to mount it in such a way that you can adjust the
location? 0.010" is not that tight a spec anyway. If it were 0.001"
that would be a bit more trouble.
Anyway, step number 1 is to cut a 4x1/4"x12" aluminum flat into three
pieces. I would like to do this on a Bridgeport
Don't you have a bandsaw? You could use a bandsaw to make it
just a little oversized, and then the mill to clean up the finish on the
edges.
and to use a thin
endmill, so as to keep as much material as possible. Should I use
carbide or HSS and what speed would you recommend for, say, a 1/8"
endmill?
No need for carbide, and it is a lot more brittle.
You probably want to add only a 1/16" depth of cut for each pass
(half the diameter of the end mill). It should be a two-flute one so it
is center cutting, and so cutting forces are less likely to deflect the
sides of the mill into the sides of the cut. (The greatest deflection
force on a 2-flute end mill is when a flute is passing across the
direction of travel -- and with a two-flute, the other will be behind
the mill where the already cut space is.) If you get deep enough, you
will have part of the flute twist around to the sides.
The Brits call a two-flute center cutting end mill a "slot
drill" because it is so good for this task.
You'll want to have compressed air blowing chips away from the
groove so they don't keep getting re-cut. (Or a shop vac nozzle right
beside the cutting location.)
Perhaps use something like WD-40 as a cutting lube if you can't
clear the chips as you go.
Second question, I will need to tap #4-40 holes in same aluminum. How
risky is this as far as tap is concerned and should I use lubrication?
Depends on the tap -- and the grade of the aluminum. If it is
something like 6061-T6 it should tap and mill nicely. If it is a plain
mild aluminum it will gum up both the end mill and the tap with likely
disaster to both.
For mild aluminum, you probably want a thread forming tap
(displaces metal instead of cutting it out) and a different size tap
hole than with a cutting tap. (_Machinery's Handbook_ has a table which
gives the right size for that.)
For the 6061-T6, use WD-40 as a threading lubricant or the
TapMatic marked explicitly for Aluminum (absolutely *not* the original
formula -- if you can find it -- which attacks the aluminum).
And Ideally, you want to use a "gun" tap (spiral point tap) in a
tapping head in the drill press or the mill for tapping through holes.
That is designed to chase the chips ahead of the tap, so you don't have
to keep backing up the tap to break the chips. Regular (plug) taps are
more of a problem -- especially in soft aluminum. If you don't have the
tapping head, a hand wrench and a guide to keep the tap perpendicular to
the surface of the workpiece.
Good Luck,
DoN.
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