"Ignoramus13320" wrote in message
...
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.

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 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?

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?

i

OK, the task is to cut a 12 inch long chunk of aluminum into three pieces
that need to be square and as large as possible. (not quite what you said
but this is what I think you mean.

The problem with using an end mill to make a really thin cut is that the
smaller the end mill the easier it is to overload the cutter and have it
flex.

What I would do is to use a skill saw or a band saw or a jig saw to make the
cuts and then square them up with the mill. If the saw kerf of the skill
saw is 1/8 you will loose 1/4 to 3/8 over all. that means you end up with
square plates 3 7/8 finished size. If that is not good enough then go buy
some more aluminum.

The skill saw technique will be to attach the plate to a chunk of wood that
has a stop affixed to it. Now to this you clamp a saw guide that you can
hold the guide of the skill saw against and keep everything as square as you
can. make one cut from each side after marking lines to delineate the outer
limits of what you are willing to go.

Now once you have the three blanks you can then square them up and size them
on the mill.

If you don't have a skill saw (or a table saw) the band saw or the jig saw
can be used if you mark the lines with a caliper and then carefully cut
between them.

The 4-40 tap will work fine just drill carefully (center drill, then tap
drill) and then use aluma-tap and go easy.

When you size and square the plates, be sure to tram the head of the mill
and then indicate the vise. Doing that will get you as close to dead nuts
as you are going to get.

Good luck.

--
Roger Shoaf
If you are not part of the solution, you are not dissolved in the solvent.