On Jun 18, 3:40*pm, Ignoramus13320 ignoramus13...@NOSPAM.
13320.invalid 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

1/8" HSS at the highest speed your Bridgeport is comfortable with
should be fine. The last one I used was loud enough at 2000 RPM that I
could no longer hear the end mill cut. I run 1/8" HSS at 3200 RPM on
the Clausing. That's a skosh high but not excessive even for steel.

One advantage of milling vs bandsawing is that you don't have to
square up the rhomboidal center piece afterwards.

If the ends are square I think I would set a vise stop to put the cut
line just past the end of the jaws and then make four passes, both
sides of both ends, so you only have to mill 1/8" deep. If they aren't
square a rod work stop clamped in a tee slot and touching the work at
its center should get you close enough to clean and square them up
easily.

Since my mill doesn't have a DRO or enough quill travel to accommodate
the fancy gizmos I start a tap straight in the drilled hole by guiding
the tap shank with the untightened drill chuck jaws and turning the
tap with one of these:
https://www.hardwareworld.com/files/pi/mD/U/TVE3.jpg
clamped onto the flutes.

Once it's started straight I back off the quill and switch to another
tap in a more convenient holder.

My well-used Clausing mill has a smooth and sensitive enough feel to
use it as a manual tapper
http://www.phase2plus.com/pics/265-110.jpg
but I wouldn't try that on a Bridgeport, and usually on the Clausing
only to start the tap.

You don't need to tap #4-40 the full 1/4" depth. If you drill part way
through from the back at one size smaller than the OD of the tap
threads you can hand-tap the holes by using the larger hole as a
starting guide. The very shallow cut is enough to force the tap to run
straight but not enough to break it. A hole slightly larger than the
tap saves time but doesn't force alignment quite as well. Use a stub
drill bit bottomed in the chuck so you can swap it with a jobbers
length tap size drill without losing your depth stop position.

Some people like this method:
http://www.finelinehair.com/home/tai...iage_mod_2.jpg
The next time I tram the head of my mill really square and maybe
surface-grind the vise base I might make one. It could be bored on a
lathe too, if you have the small boring bars.

Which one of those I use varies with diameter and material. Sometimes
I even power-tap.

jsw