In what condition is the 4140... That means everything, though what you were
doing would result in a shitty finish no matter what.. I'm not being that big
of dick.. I'll explain why. Why you will get a shitty finish, not why I'm not
being that big of a dick.
You've already had a bunch of people talk about insert height.. Fly cutters
are shit, but when you *REALLY* need that mirror finish, one insert in a
face mill is hard to beat..
DOC(depth of cut).. A thou, a single tiny itty bitty little thou? If you bought a cheap
facemill, you've probably got more variation than that in the heights of
your inserts, in which case you would most likely accidentally be running an
effective fly cutter with the wrong feed rate.
We're still on DOC.. A THOU!!! What is the edge hone on your insert? If you are
trying to take a thou, you had better have some ground and polished, super up-sharp
shiny inserts meant for aluminum (but work great on other stuff, in the right conditions).
Lets go back to condition of the material. 4140 annealed/normilized acts a whole heck
of a lot like a low carbon steel. 1018, A36 which shouldn't actually be called A36, the
spec should be "Unpredictable structural GUMMY steel". Up until close to a 30C (above PH
condition) 4140 still acts like a "Gummy Steel". It rips and it tears.
How do you get away from the ripping and tearing? There are a couple of ways.. SHARP,
and I mean SHARP. Ground and up-sharp inserts, the kind meant for aluminum, but that's
more of a lathe thing.. When you can't get the revs to get the surface speed, and we're
getting to that..
SPEED!!!!!!! Something to do with crystallization and blah blah blah, (that's above my
pay grade). F'n fast.. you are running 120sfm.. I've face milled 4140 at over 1400 SFM
and got good tool life, and it was nice and shiny..
DOC. For some reason, on a gummy steel, you NEED a decent DOC to get a good surface finish.
.001 is not going to do it, on almost any material.. .010 to .030" at least..
Feed, you *NEED* a proper feed rate..
moving the table very slow
So what
was your feed rate?? Just because its a manual machine does not mean that you can't
set a proper feed rate.. One Mississippi Two Mississippi Three Mississippi ETC....
DO THE MATH!!! The rules don't change just because you are in your garage, the metal doesn't
care.. The cutter doesn't care.. The RULES are the same weather you are using a
$500 POS Grizzly Mill, or a million and a half dollar high end Japanese horizontal.. The
material doesn't care if its in a Top Secret shop at Sandia labs, or in Joe Blows Garage
out in the suburbs, it acts exactly the same, the rules don't change.. If its a million$$$ part
made from a block the size of a suburban, or a bracket you are making to fix your lawnmower,
the RULES DON'T CHANGE...
There is a lot of imagination involved in machining. Its a very creative trade.. But Feeds and
speeds, those aren't very creative, they are pretty well written in stone. They do however get
creative when running long stickouts on tricky setups... But facing some 4140, nothing tricky
there..
Like the lady that wanted to make a really really good roast, instead of 1 hour at 400 degrees,
she decided 4 hours at 100 degrees would be better.. It didn't work out. Slower is not
always better.
I'm sorry if that still sounds like I'm a dick.