On Sat, 16 Jan 2016 13:40:12 -0500, Ned Simmons
wrote:

On Sat, 16 Jan 2016 10:20:28 -0500, 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????

Or do I just farm it out to a CNC shop??

If I had access to a horizontal mill it would make life a lot easier,
but nobody has them any more.

I'm going to assume the slot depth is 3/8", not 1-3/8". While the 3/16
end mill may cut close to .188, as measured with a block or pin,
unless you have flood coolant with plenty of velocity available
(unlikely on an un-enclosed mill) the sides of the slot will be quite
rough. I'd use a 5/32 2-flute cutter. Bring the slot to within .010 of
depth with a couple passes with the cutter on center line. Widen the
slot by offsetting the cutter about .005 per side and measure the
result. Set your final offsets taking into account the result of the
previous cut and bring the slot to final depth at the same time. You
can climb mill the final passes if you find you get a better finish
that way. Brush on some alum-tap or kero or WD40 before each pass.

Don't bother with the drilled holes unless the slots have blind ends.
If they do, a hole to depth at each end gives the end mill a place to
dwell at the ends without rubbing, and acts as a visual cue.

Not knowing the condition of the mill it's hard to predict how close
you'll get, but with a bit of care I'd expect to get within 3-5 tenths
on my DRO equipped pretty-tight mill. Within a few thousandths without
paying a lot of attention.
The slot depth is the full 1 inch thickness of the stock, and the
length of the slot is 1 3/8"
The mill doesn't have a DRO. It is old but lightly used and well
maintained. Being a 3/8" mill holder, it is almost impossible to buy
an end-mill smaller than 3/16 with a length of 1/2" or more, which is
required to cut the full slot, cutting from both sides. Our local
machine tool supplier doesn't carry them, but an old machinist friend
has a 5/8" long cutter from a box of cutters he bought at a close-out
auction over 10 years ago - brand new never been used, that he has
given me.