On 2009-10-05, Michael Koblic wrote:
"DoN. Nichols" wrote in message
...
I interpret this that doing the full thickness slot is a bad thing,
Yes -- and doing it with a four-flute end mill is another bad
thing.
Hm. when I started I was told "two flutes good, four flutes better (for
steel)"
That depends. If you are using just one side to face the edge
of a workpiece, then four flutes doubles the number of "teeth", so the
feed per tooth can be doubled for a given RPM and workpiece
material/cutter material combination.
presumably the mill is conventionally milling one side of the slot and
climb
milling the other with resulting sideways movement.
While you are cutting, the leading flute in the direction of
travel (which is doing most of the work) deflects the end mill to the
side opposite of its travel, allowing the next flute back to cut out the
one side of the slot. This causes the kind of thing you see -- with the
amount you see depending on the rigidity of the entire setup and the
length of exposed end-mill. (A longer mill has more flex than a shorter
one.
I have not seen this
documented elsewhere so my question is: Do those with the Big Beasts see
the
same thing? Is this a reflection on poor rigidity of my mini or is this a
universally bad procedure to do the full thickness at once?
Bad to do more than about half the diameter of the end mill in
thickness at a pass.
It may be even less than that. I have not shown all I have done but when I
did 0.100" passes there was a definite tendency for the slot to wander.
O.K. And as someone else mentioned, are you locking the axis
which you don't want to move? Vibration can cause the handwheel to
rotate, especially if it is not a balanced handwheel (extra mass on the
side opposite the crank handle, so it will balance in any position).
Even worse to use 4-flute end mills. What you want to use is
two-flute ones (often called "slot drills" in the UK I believe because
they are so much better for slotting).
The other question is the mill bit: I think I milled less than 10 slots
with
this particular one. Am I unreasonable expecting it to last longer? Was
it
my technique or the provenance of the bit? Are there manufacturers I
should
look to preferentially for replacement?
What coolant?
Jus Rapid Tap.
Hmm ... I would use something like Rigid's high sulfur threading
oil in preference to that for that particular task.
And an alternative possibility is simply a jet of compressed air
directed right at the cut to blow chips clear so they can't build up on
the cutter and bind it in the cut.
What RPM?
I could BS and say 550 rpm but the truth is somewhere between "vroom-vroom"
and low pitched "brrr". The regulator is supposed to go from zero to 1100
rpm but I strongly suspect it is not linear. How it holds rpm under load is
anyone's guess.
O.K. With a 1/4" end mill, your top speed puts you at 71.99
SFM, which is probably not bad with a mild steel and a *good* HSS milling
cutter.
What feed per tooth?
That is even harder to quantify.
Yes -- when your RPM is an unknown, yes, you don't know what you
have even if you know your IPM (inches per minute) based on how fast you
are cranking the leadscrew. (Of course, in the machines where it really
matters, the spindle is geared to the table feed, so it is truly a feed
per tooth, even if the spindle loads down a bit -- so does the feed.)
And in CNC, it is computer determined.
I follow the Machinery's book advice (see
below).
And the
galvanized key stock is probably a rather gummy steel. Go for 12L14 (as
for turning) to find out how much better things can be.
But yes -- the Chinese origin of the end mill suggests that it
could be very poor quality steel.
Buy some *two*-flute end cutting mills by a maker such as
Cleveland, check the RPM against the diameter (calculate the SFM
(Surface Feet per Minute) and compare it to the tables in books like
_Machinery's Handbook_ (pretty much any old edition will do for this).
look up the feed per tooth (for a two-flute end mill here), and working
with the RPM calculate how many inches per minute you want to feed. You
don't want to go faster, but you don't want to go too much slower,
either.
There seems to be a lot of room for interpretation vis-a-vis the steel
hardness.
And with key stock, it is more the gummyness of the steel, not
the hardness. I believe that key stock is low enough carbon so you
can't harden it.
But yes -- if you had hardened steel, the speeds have to be
reduced.
I usually take it that I should be OK with 60-90 SFM. At the high
end the rpm for 1/4" mill are appx. 1500. I have never got as high as 1100
(nominal) as the machine starts shaking.
And the shaking could cause the other axis to feed if it is not
locked. Does your machine have locks for each axis?
As for rough milling the feed should be "all that the machine, work, fixture
and tool will withstand". IMHO they should also include operator's nerves.
:-) Read the posting by Teenut (archived somewhere that someone will
surely point out) discussing how a real machinist sets the feed on a
machine in production. (The basic idea is to start cranking until you
feel uncomfortable about the machine's behavior, back off a little, and
then bring the power feed up until you feel it catch up with your
cranking. :-)
Note that some Chinese boxed sets have both two-flute and
four-flute in each size, and some of them can be pretty good. It
depends on where you get them, among other things. I have a set which I
got from MSC, and they are useful for a lot of things, but for serious
work I use Cleveland or other US-made mills bought for the task.
Right, two flutes it is!
For slot milling, at least. For a lot of other things, four
flutes is a better choice. Slots are a special case.
Good Luck,
DoN.
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