On Sat, 16 Jan 2016 23:39:01 -0600, Jon Elson
wrote:

wrote:

I've got a little job I need to do/get done. I am working on the wing
strut fittings for the bush-plane. There are 4 struts - so 8 strut end
fittings. They are made of 1X1 6061T6 - 4 abot 4 1/4 inches long, and
4 about 5 1/2 inches long.
2 need 1/4 inch slots, 4 need .195" slots, and 2 need 3/16 " slots -
all 1 3/8" deep.

I may have the use of a bridgeport style mill that used 3/8" shank
cutters, and I have located a 3/16" end mill that has 5/8" cutting
depth (end mill)

How close to 3/16 will I likely be able to get? (thinking the cutter
is going to want to "walk" a bit in the cut) Would it help a lot if I
drilled a row of 1/8" or 5/32 holes down the center of the slot first
to reduce the chip load????

Don't cut the slot full-width in one pass. Use an end mill smaller than the
desired slot width. Start right down the middle of the slot, and take cuts
running back and forth the length of the slot, going deeper by about 1/2 the
diameter of the cutter each pass, until you break through the bottom. Then,
offset the part a bit, and take one side up to the desired slot dimension.
Then, offset the other way and do the other side. if the machine is tight,
do these cuts in the climb-milling direction, where the feed is with the
rotation of the cutting edge. This gives dramatically better finish in
aluminum. (If the machine is loose, this cutting direction can cause the
work to jump into the cut, so not a good idea.)

This is all MUCH easier with a CNC machine, of course.

Jon
It will be done on the CNC with a small bit. For the old manual mill
there was no bit available smaller than 3/16 long enough to do the
job.