Milling Feeds & Speeds

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