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View Full Version : Chipload and how can I tell I got it right?



Your Old Dog
12-31-2008, 08:13 AM
How can you define a good chip load? I thought it was more art then science but then I read this:

20. Chipload from IPM, RPM and Number of Teeth in Cutter

from this ad in ebay:

http://cgi.ebay.com/Machinist-Machine-Shop-Calculator-68-Programs-loaded_W0QQitemZ350147142899QQcmdZViewItemQQptZLH_ DefaultDomain_0?hash=item350147142899&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A10%7C39%3A1%7C240%3A1318

So, is proper chipload determination more of an art or a science? I note sometimes my chips are thinner then at other times and I'd really like to know when I've got it right as I expect it will effect cutter wear. Is it a relationship of thickness of chip to it's width and if so, what would be a reasonable relationship?

SVS
12-31-2008, 09:32 AM
That looks like a handy unit.

The chipload calculation using IPM RPM and tooth count really gives average chip thickness. I say average because chips taper. Thin at the start of the cut, thick at the end when milling conventionally, and opposite for climb milling.
To get "total chipload" you have to factor in DOC, and length through the cut. That is when art and experience start to take over for one offs. You have to juggle your machine, endmill, surface finish, feed, stepover, and depth per pass.

I visualize Feed, WOC, and DOC as a three dimensional graph. I try to set feed by the book, and then vary WOC and DOC inversely to suit the situation.

Does that make a lick of sense?

lazlo
12-31-2008, 11:18 AM
Every material has an ideal feed rate, measured in inches per minute. This is translates into a chip load per tooth -- .002" to .005" is typical for a HSS endmill, and the ideal feed rate for aluminum is twice the ideal feed rate for mild steel, since aluminum shears a lot easier than steel.

Since feed rate is a function of tooth load, you also have to take into account the number of teeth -- a 4-flute endmill run at the same feed rate will have half the chip load of a 2-flute endmill, so you have to run the 4-flute at twice the linear speed to get the same chipload.

That may sound overly complicated, but it boils down to the fact that if you run the feed too fast (i.e., crank the table too fast), each flute on your endmill is taking too big of a bite, and you get lousy surface finish (tearing), and you'll kill the cutter in short time. If you run the feed rate too slow (i.e., crank the table too slow), each flute on your endmill is taking too little of a cut, and you're re-cutting the workpiece over and over, cutting swarf, and basically dulling the cutter without doing a lot of work.

I keep a little chart taped to my milling machine, and I just look up the ideal feed by diameter of the endmill and number of teeth, but if you have a computer in the shop, this is a great calculator:

http://www.custompartnet.com/calculator/milling-speed-and-feed

lazlo
12-31-2008, 11:22 AM
By the way, the concept of ideal speeds and feeds apply to any machine tool, and the ideal feed rate for a material is the same whether the tool is a mill, lathe, drill press, or shaper.

Speeds and feeds calculations are explained in every machinist text, but here's a short but sweet online tutorial from the University of Wisconsin:

Calculating Speeds and Feeds for Twist Drills (http://www.wisc-online.com/objects/index_tj.asp?objID=FAB1202)

J Tiers
12-31-2008, 07:09 PM
That may sound overly complicated, but it boils down to the fact that if you run the feed too fast (i.e., crank the table too fast), each flute on your endmill is taking too big of a bite, and you get lousy surface finish (tearing), and you'll kill the cutter in short time. If you run the feed rate too slow (i.e., crank the table too slow), each flute on your endmill is taking too little of a cut, and you're re-cutting the workpiece over and over, cutting swarf, and basically dulling the cutter without doing a lot of work.


Seems like too little feed is less about "re-cutting", and more about "rubbing" and not cutting. At a certain chipload, the "chip" is too small for the edge to cut. So several edges rub, then one takes a cut, and it repeats over and over. The edges that "rub" get heated, since no chip exists to take away heat. Edges get dulled, dubbed-over, and shiny, especially if the low chipload is from excess surface speed.

The "re-cutting" happens at any feed, it has only to do with direction of cut, since conventional milling pushes out chips ahead of the cut, ready to fall in and be cut again. But you need to cut, and not rub, and the bigger the chip, the more damaging it would be. With light feed, you rub, get thin chips, and not very many of them.

I find that most cutters "like to cut", meaning that an honest cut on the work rarely dulls them. Mostly it is a light cut rubbing off the edge, or a heavy cut breaking teeth, that causes damage, in my experience.

I have no doubt that re-cutting has an effect, it can double or triple the effective depth of cut from the perspective of the edge....

lazlo
12-31-2008, 07:38 PM
Seems like too little feed is less about "re-cutting", and more about "rubbing" and not cutting. At a certain chipload, the "chip" is too small for the edge to cut. So several edges rub, then one takes a cut, and it repeats over and over. The edges that "rub" get heated, since no chip exists to take away heat. Edges get dulled, dubbed-over, and shiny, especially if the low chipload is from excess surface speed.

Sure, that sounds right Jerry -- better analogy than re-cutting.

And like most of us have found empirically, it's better to err on the side of too little feed than too much feed.

J Tiers
12-31-2008, 09:11 PM
Actually, I usually slow the cutter, and crank up the feed, at least with a horizontal mill.

I have burned the edge off more cutters than I have broken, so obviously I have not had the feed high enough.

lazlo
12-31-2008, 09:52 PM
I have burned the edge off more cutters than I have broken, so obviously I have not had the feed high enough.

You need to use a speeds and feeds table :D

(Donning my asbestos suit and ducking all the tomatoes:) )...

J Tiers
12-31-2008, 10:00 PM
You need to use a speeds and feeds table :D

(Donning my asbestos suit and ducking all the tomatoes:) )...

Actually, it has been due to mystery metal, which work hardened....... not what I thought it was! And some time ago, at that......

Besides, I have found that the tables are pretty useless, because I am NOT trying to get max chips per cutter, and I don't have the type machines the tables are for....

So the feeds don't work out like that... and horizontal mills have many factors that affect how the feed works out..... like getting the cutter started into the cut..... would sound like a jackhammer if done per the chart..... you gotta ease in, and then go faster when 2 or 3 teeth are cutting.

Your Old Dog
01-01-2009, 08:39 AM
Don't want you to think I posted and then ran off. I made a reply after the 3d or 4th post but it doesn't show so.....


Thanks guys, the explanation is very easy to understand. I'll continue to listen to the sound of the mill and my auto X feed groaning to tell when I've got it right. I haven't burnt any cutters but have snapped a 2 and for a full blown, balls to the wall machinist rookie of only 2 years I guess I got something right :D

philbur
01-01-2009, 09:38 AM
Below is the definitive answer, even worth a second read for those that have seen it before. :

...................................
Newsgroups: rec.crafts.metalworking
Subject: Re: Speeds and feeds horizontal milling
From: Robert Bastow <Tubal_cain@hotmail.com>
Date: Tue, 02 Feb 1999 23:33:41 GMT

Brian Evans wrote:

> Read with interest the discussion on feeds and speeds
>
> Any advice from anyone who's had some horizontal milling experience?
>
> Thanks, Brian


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 behaviour under load. the part itself, its rigidity and
clamping etc etc.

Heck a Kray Mainfrain 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 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.

Whats 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 rehersal.

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
horspower 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 instict 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 shuuder 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 start 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 realise 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
workpiece.

You turn, and without a backward glance, you stride confidently toward the
canteen for a welcome "cuppa" On the way we happen to meet.."Hey Robert" you
ask "what feed rate are you using on that job?" "Haven't a clue" say I "go
check the dials..I'm off for a cuppa!!"

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 Bastow
.................................................. ..

Your Old Dog
01-01-2009, 10:04 AM
Thanks a million Pfilbur for taking the time to run that down and post it here. It's confirmation for me of the way I've been doing it but wondered if I was right.

lazlo
01-01-2009, 11:09 AM
Below is the definitive answer, even worth a second read for those that have seen it before. :

That's exactly my point Philbur -- that's Teenut (Robert Bastow)'s famous post to rec.crafts.metalworking. It was a lengthy (and polite) discussion about calculating speeds and feeds, and most of the pro machinists were advocating calculating (or looking up) the correct speeds and feeds, when Teenut went into his poetic missive.

Unless you've got 50 years in the business like Teenut or Lane, you're not going to be able to "feel" the correct feeds and speeds like a master Tool and Die maker can...

But more to the point, with all that experience and art, a Master Tool and Die Maker is empirically finding the same values that have been calculated for you in the Machinery's Handbook. Yes, you need to tweak those values according to the power and rigidity of your machine, but mild steel will always cut best at 100 SFPM, regardless of what machine, and what cutter, and aluminum will always cut best at 200 SFPM...

So you can spend a lifetime trying to learn the correct speeds and feeds by feel, or you can just look it up in a table, and get most of the benefits of their extensive experience in 3 minutes :)

That's not saying that after looking up the correct cutter speed you're going to start making parts like Lane, but at least you have taken care of one source of errors...

loose nut
01-01-2009, 11:33 AM
This might be oversimplifying but the way I was shown was

On steel with HSS tools as an example, watch the chips, if they look right "nice sixes and nines (depending on the material of course) and they aren't smoking blue then thats a good indicator that you have it right.

alanganes
01-01-2009, 11:59 AM
For those of us lacking the years of experience and tool & die maker training (that's me!!), this was something that I found helpful. It is from a Popular Science magazine article that I clipped and saved years ago. I have no idea how accurate the results this gives are, I would guess they are pretty conservative. I've used this little formula for years to get me in the ballpark, and get decent results. It is from the October 1972 issue of Popular Science magazine, page 106 if the link does not work for some reason:

LINK!! (http://books.google.com/books?id=psqW_3WpLaMC&pg=PA106&dq=%22cut+metal+like+a+pro%22#PPA106,M1)

I wrote the formula and chart up on a laminated 3x5 and keep it by my machines. I don't refer to it as much as I used to, but find it handy from time to time.

SVS
01-01-2009, 12:06 PM
Thanks for that rebuttal Lazlo. May be my hangover talking, but I found that romantic little feel good story ultimately nearly devoid of hard information though I did enjoy it-No offense intended to any one. He makes one concrete point-set sfpm by the book, half of another one-bury the cutter, and then devolves into mysticism, missing the point that feed is also easy to set by the book. I know what he meant but that sort of rubbish encourages the notion that flying by the seat of your pants is the manly way to machine.

Furthermore, on the near dozen different heavy mills I own, used to own, or have run the feed CAN NOT be changed on the fly, and It is physically impossible to maintain the feed by hand till the power feed "catches up". I'm not calling him a liar and he really may be an old mill hand, but my suspicious side would tell me he's run a Bport with DC powerfeed, read some old time machine shop books, and is a talented writer.

philbur
01-01-2009, 12:17 PM
This is contrary to the point made by Robert Bastow.

Quote:
.........................
"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"."
.........................

How long does it take to learn to ride a bicycle, no formulas envolved. If you don't listen to your machine you may never learn its' language.

Just a thought from a novice.
Phil




Unless you've got 50 years in the business like Teenut or Lane, you're not going to be able to "feel" the correct feeds and speeds like a master Tool and Die maker can....

lazlo
01-01-2009, 12:42 PM
This is contrary to the point made by Robert Bastow.
.........................
"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 during that 6 year apprenticeship, when he was working 40 - 60 hour shifts, how long did it take for him to learn the proper speeds and feeds? I don't know about you, but I don't have thousands of hours, and hundreds of dollars worth of endmills to spend experimentally learning speeds and feeds.



How long does it take to learn to ride a bicycle, no formulas envolved. If you don't listen to your machine you may never learn its' language.

I think a better analogy is learning to ski, which is much more technical than learning to ride a bike. You can learn to ski without instruction, but it takes a lot longer than if you get even basic starter lessons, and you end up with a really nasty skiing style and a lot of bad habits...

Same deal with welding classes -- the first thing the instructor asks is "who here think's they're a welder?" Anyone who raises their hand, the instructor says "OK, you're going to need a lot of hands-on to unlearn all your bad habits."

By the way, the speeds and feeds tables give you a good starting point -- you still have to learn the "language" of the machine to get the best finish and cutter life.

philbur
01-01-2009, 12:50 PM
You are probably right. I may be overstating my case.

Phil


But during that 6 year apprenticeship, when he was working 40 - 60 hour shifts, how long did it take for him to learn the proper speeds and feeds? I don't know about you, but I don't have thousands of hours, and hundreds of dollars worth of endmills to spend experimentally learning speeds and feeds.




I think a better analogy is learning to ski, which is much more technical than learning to ride a bike. You can learn to ski without instruction, but it takes a lot longer than if you get even basic starter lessons, and you end up with a really nasty skiing style and a lot of bad habits...

Same deal with welding classes -- the first thing the instructor asks is "who here think's they're a welder?" Anyone who raises their hand, the instructor says "OK, you're going to need a lot of hands-on to unlearn all your bad habits."

By the way, the speeds and feeds tables give you a good starting point -- you still have to learn the "language" of the machine to get the best finish and cutter life.