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.

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

Slicing up plate with a small endmill is a lousy way to go. Better to
cut with a bandsaw or even hacksaw and then finish mill the edge with a
decent sized end mill (.5"+).

Tapping small holes is very risky. Do the complete process for each hole
without moving the mill X/Y. Load up your chuck in the spindle, put a
small center drill in the chuck, lock the X and Y and spot the hole.
Change the center drill out for the appropriate tap drill and drill the
hole, using a squirt or two of Kroil (or other suitable lube) as you
peck drill. Change out the drill bit for a spring loaded tap center
(ENCO, MSC, etc.) with a point that will rest in the center hole at the
back of your tap handle. Load the tap in the tap handle, rest the tip of
the tap in the hole and bring the spindle with the spring center down to
rest in the back of the tap handle, compressing through most of the
spring travel. Apply a few more squirts if Kroil and begin tapping, 1/2
turn in, 1/4 turn back, gradually progressing. If the hole is deep,
periodically back the tap most of the way out and blast if clean with
air, relube with Kroil and continue. Doing all of this in one sequence
with the X and Y locked ensures you remain on center and avoid side
strain that would break the tap. When one hole is complete, unlock the X
and Y, locate the next hole position and start the process over.