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Removing Serious
Metal On A Serious Mill
By the late Robert Bastow AKA "teenut"
Milling speeds and feeds are a real
can of worms... not because there aren't readily available
GUIDELINES... but simply because different people have different
(honest) opinions based upon a whole range of different experiences.
THE ONE CONSTANT FACTOR IS CUTTING
SPEED IN FEET PER MINUTE... It doesn't matter whether I am using a
1" diameter slitting saw on my lathe cross slide, or you are
running a 10" diameter x 12" long slab mill on a 60 HP
Cincinnati mill... if we are both using HSS cutters on hot rolled
steel, we are both limited to the 80 to 100 surface feet per minute.
You may find it hard to believe that,
during a six year apprenticeship, during which I ran SCORES of
different mills... from the teensiest Instrument Mill to 48 foot
Planer Mills... I never was taught, nor did I find it necessary to
apply any "magic formulae". But the reason for that is
simple... THERE AREN'T ANY!!
The objective is to remove metal as
quickly (therefore economically) as possible. In the early days of
(particularly horizontal) Mills it was common practice for
manufacturers to rate and compare them in terms of "CUBIC
INCHES OF METAL REMOVED PER MINUTE" And, believe me, some of
those old slabmillers could shift IMPRESSIVE amounts of metal.
But there are so many other
variables... some you have control over... width and depth of cut,
feed per tooth, coolant, tool geometry, SHARPNESS!! etc. And there
are a whole HOST more that you, generally, do NOT have control
over... Age and CONDITION of the machine. Size of the machine,
rigidity of its design, its dynamic behavior under load, the part
itself, its rigidity and clamping etc.
Heck a Cray Mainfraim couldn't
calculate all the "BEST" parameters for all the jobs and
all the machines in a large shop. So how DID we do it? As do it we
did... most Jobs were "on ticket"... piecework!
Commensurate with meeting specs. on fit and finish... we were paid
to shift metal as fast as possible.
In reality it was nowhere as
complicated as one might imagine! Get a job ticket, go to machine...
never seen it before! Clamp down job, install cutters. Quick
reference to Starrett Chart pinned in lid of tool box (No-body
figured it out in their head... the chart was quicker, especially on
a Monday morning!!) X" dia. at 90 ft/min = Y rev/min. Crank the
speed change dial (on most CINCI's, Kearney & Trecker, Herbert's
etc.). The speed and feed changes were through crank handles on
large dials.
Now set the depth and width of cut.
HMMmm! In MOST cases the fastest way to shift metal was to engage as
much of the cutter as possible and get as many teeth cutting as wide
and deep as possible at the same time... "Bury the Bugger"
the saying went. That way you removed more metal per tooth, per rev
and were less likely to wear the cutter out before the job was done
Limiting factors... HP... got MORE
than enough. Machine rigidity... slide conditions etc... NO IDEA...
never seen the bugger before... only one way to find out though!!
What's next... the work piece... this one is sturdy enough to take
some "elbow"... Set Up... NO PROBLEM... we soon learned to
fasten things down so Dynamite wouldn't shift 'em... before tickling
them with fifty plus Horsepower.
What's left?... the FEED rate... you
know, how many thou per tooth per rev... I have absolutely NO IDEA
until all the other factors start inter-acting and the whole stage
play gets into the dress rehearsal. Curtain up time, light a fresh
fag and take a last swig of cold tea. Lights, curtain... One last
check around... spindle clear, feed disengaged, SAFETY CHECK...
these machines are NOT fitted with an "OUCH" switch. No
"oily rags" about (apprentices rubbernecking) No laborer
shoveling chips out of the back of the machine. Bootlaces tied,
floor clean and dry... two or three clear escape routes... nothing
to trip or fall over. Did I mention safety glasses? Safety WHAT??
Deep breath. Concentrate. Start
spindle. Coolant, GENTLY feed cutter into job BY HAND. Feel the cut,
feel the whole set up shuddering and settling into equilibrium as
the cutter digs deeper and the motor starts to push some serious
horsepower into its job. Slowly build up the hand feed rate until
the cutter is "Buried"... In full depth and width.
Continue to gradually increase the feed rate, as every sense and instinct
strains for any sign of trouble. So far so good... you
figured the right starting points... now you and the machine begin
to understand each other and trust starts to grow... We are NOT
going to hurt each other or let each other down are we!!!... Still
gradually increasing the hand feed pressure... the machine, now all
the slack is taken up, all the castings have bent and bedded into
unity... is READY!! Quick check of the chips, cutter seems happy,
coolant flow good... You're READY.
NOW!! Lets show them what we REALLY
CAN DO!! Start to crank the feed faster and faster until you feel
that first shudder of discomfort... back off a bit... engage power
feed and crank the selector handle fast until you start to feel the
power feed catch up with and overtake your hand feed. Ease off on
the crank handle... let the machine take over... But don't let go
yet... Every sense organ is tuned in as the machine settles down to
a steady pace after its quick acceleration... everything feels,
sounds, smells, good... turn up the feed another notch... settles
down... happy... turn it another... settles happy... turn it
another... machine grunts... unhappy... turn it back... happy.
You just arrived at the CORRECT FEED
PER TOOTH !!!!
Slowly you relax, letting your hands
creep away from the knobs and handles... the machine munches on...
in equilibrium... chips and coolant sound like frying bacon...
machine is bunched into and happy with its job. You turn to find the
cigarette... after that first puff... has burned away. Light
another, wipe your hands... gradually your senses retreat from the
machine as it does what it does best... shifting metal.
You have a bond of trust now. You and
that Machine. It will let you know in good time... in your secret
language... if something starts to go amiss. It trusts you, to hear
and respond, before any harm befalls it. You are a team now... both
doing the job you do best. Now you realize your throat is dry!! No
cold tea left, check the clock, check the job... ten minutes left
"in cut", before you need to stop and replace the work piece.
You turn, and without a backward
glance, you stride confidently toward the canteen for a welcome
"cupper". On the way we happen to meet... "Hey
Robert" he asks "what feed rate are you using on that
job?" "Haven't a clue" say I "go check the
dials... I'm off for a cupper!!"
It may not be the answer you want
Brian... But I'm afraid it's the only one I can give you!
Happy milling,
Robert Bestow
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