Forces Involved in Milling and Optimizing Feeds

Hello all,
Some quick background, I have a shapeoko style machine that I assembled in my garage and I am currently milling aluminium on it for my robotics team. I am getting good results but taking it really slow which is bad for tools and of course takes forever. In order to speed this up, I am trying to better understand the forces involved in milling. I did some simple calculations with an equation that I found from this web page:

ForcesInMilling.pdf - Google Drive

From my understanding this force that I calculated relates to the amount of tangential force that the spindle must exert. What I don't think it shows is the amount of force with which the machine must push the cutter into the stock. So my question is this; is there any way to determine the force that the machine must push the tool through the stock with? Also, are calculations at this level of detail useful or should I just spend my time testing to optimize feeds and speeds (I feel like the actual result of an equation might not matter but it would allow me to see how different factors affect the forces in milling)?

Thanks
-Ben

Get a machinery's handbook. Lots of info in there. You don't need the newest edition, find a used one.
Also, don't be surprised if this thread is not locked. Home built CNC routers are not in the "industrial" grade of machines.
http://www.practicalmachinist.com/vb/general/machinery-discussion-guidelines-137724/
JR

Ben L said:

Hello all,
Some quick background, I have a shapeoko style machine that I assembled in my garage and I am currently milling aluminium on it for my robotics team. I am getting good results but taking it really slow which is bad for tools and of course takes forever. In order to speed this up, I am trying to better understand the forces involved in milling. I did some simple calculations with an equation that I found from this web page:

ForcesInMilling.pdf - Google Drive

From my understanding this force that I calculated relates to the amount of tangential force that the spindle must exert. What I don't think it shows is the amount of force with which the machine must push the cutter into the stock. So my question is this; is there any way to determine the force that the machine must push the tool through the stock with? Also, are calculations at this level of detail useful or should I just spend my time testing to optimize feeds and speeds (I feel like the actual result of an equation might not matter but it would allow me to see how different factors affect the forces in milling)?

Thanks
-Ben

Click to expand...

.
i use excel spreadsheet with math formulas to do the math. i dont calculate thrust as well as side forces i just calculate total hp required and the approx forces so i dont know exactly what levels are being pushed on the part in particular directions. i use a machability rating
.
1018 steel 1hp will remove 0.8 to 1.6 cubic inches per minute. there is no exact number as it depends on type of cutter and the feed rate or how thick the chips are
.
aluminum about 2 to 6 cubic inches per hp per minute. aluminum can vary in properties depending on alloy and condition
.
many machines have a hp limit if you try to mill more than using 1hp they will chatter and vibrate. of course bigger machines the limit might be 30hp

While the question as phrased does not fit within the forum guidelines I am curious about the answer. I can see it being helpful when trying to determine (without simply relying on experience) just how well you have to hang on to something. As I'm sure many places do we end up with new folks using equipment and it would be nice to have a way to help them visualize these forces other than "You'll know when one flies out that it wasn't enough" and "You must secure everything as if it was supporting a train".

jccaclimber said:

While the question as phrased does not fit within the forum guidelines I am curious about the answer. I can see it being helpful when trying to determine (without simply relying on experience) just how well you have to hang on to something. As I'm sure many places do we end up with new folks using equipment and it would be nice to have a way to help them visualize these forces other than "You'll know when one flies out that it wasn't enough" and "You must secure everything as if it was supporting a train".

Click to expand...

.
where i work i use excel to calculate many things. takes seconds to enter a math formula and can save the file for reuse of course. i have a tool file of every tool number made up to exact specs and tolerances like amount tool sticking out of tool holder amount. i record when i have sudden tool failures and record what i was doing when i got excess chatter and vibration. thus can calculate the limits of each tool based of material being removed per minute.
.
then using conditional formatting i can enter width and depth of cut machinability rating and other data and have cells go yellow or red depending if near limits or in the red or above recommended max. each tool has a different limit for example if i got a .250 dia drill bit at 3" sticking out length i can do say 3.0 ipm feed but if drill is sticking out 18" i have to reduce the feed or it will bend and break. this is what i mean by tool limits. you have to standardize tool setups to tolerances to know what they can normally take
.
smaller tools you have to worry about tool limits. bigger tools like a 6" dia facemill you got to worry about machine hp limits
.
not everybody does the calculations it is a more advanced way of doing things

Since you're on a robotics team I'm assuming you're in FIRST or in university. Either way you're looking for a more engineering-oriented answer.

The most basic answer I can give you is to look up "Merchant's Circle". This is basically a static (sum of forces equal zero) equilibrium look at the forces involved in machining.

If you access to online journals you can probably search that term and get some better modern answers. There has been lots of research done into machine tool dynamics and vibrations which all come back to cutting forces.

For instance, one I just recently came across titled "Modeling of cutting forces in end milling based on oblique cutting analysis" (DOI 10.1007/s00170-015-7724-8) has some good information on end mill cutting forces. A mathematical model is created and backed up using real world testing.

Kistler makes some pretty cool instruments that researchers use to figure this kind of stuff out (they are VERY expensive), but with some strain gauges, creativity, and a oscilloscope you can do some of this yourself for much cheaper: https://www.kistler.com/fileadmin/files/divisions/sensor-technology/cutting-force/960-002e-05.14.pdf

Hope this helps.

- Matt

there are many that will sell you software or calculators to do stuff rather than just use excel. many run on smart phone but many shops got a computer next to each cnc with a much bigger screen
.
you can do a lot with microsoft office excel and powerpoint. many schools teach how to use it for class work now a days

Ben L said:

Hello all,
From my understanding this force that I calculated relates to the amount of tangential force that the spindle must exert. What I don't think it shows is the amount of force with which the machine must push the cutter into the stock. So my question is this; is there any way to determine the force that the machine must push the tool through the stock with? Also, are calculations at this level of detail useful or should I just spend my time testing to optimize feeds and speeds (I feel like the actual result of an equation might not matter but it would allow me to see how different factors affect the forces in milling)?

Thanks
-Ben

Click to expand...

You should just concern yourself with trying to run the cutting tools at recommended speeds & feeds. And adjusting to taste…

Later on you may want to calculate hp & torque requirements (or not). Force calculations are different. Once you have your machine & the cutting tool figuring forces is out of your hands (except for chatter).

A machine designer gets into forces with “cutting stiffness” (Kc for material) and the “dynamic stiffness” (Km for machine tool) and the forces are quite high. Mostly it would be used by a shop person (industrial engineer) to work away from chatter prone operation. Basically if Kc/Km < ½ then a chatter free operation is assured.

For calculating the numbers do get quite high (attaching material values for Kc/w (lb./in.squared)) depending on what it is… It doesn’t hurt for a machinist to be aware of these things though.

Good luck,
Matt

DMF_TomB said:

.
where i work i use excel to calculate many things. takes seconds to enter a math formula and can save the file for reuse of course. i have a tool file of every tool number made up to exact specs and tolerances like amount tool sticking out of tool holder amount. i record when i have sudden tool failures and record what i was doing when i got excess chatter and vibration. thus can calculate the limits of each tool based of material being removed per minute.
.
then using conditional formatting i can enter width and depth of cut machinability rating and other data and have cells go yellow or red depending if near limits or in the red or above recommended max. each tool has a different limit for example if i got a .250 dia drill bit at 3" sticking out length i can do say 3.0 ipm feed but if drill is sticking out 18" i have to reduce the feed or it will bend and break. this is what i mean by tool limits. you have to standardize tool setups to tolerances to know what they can normally take
.
smaller tools you have to worry about tool limits. bigger tools like a 6" dia facemill you got to worry about machine hp limits
.
not everybody does the calculations it is a more advanced way of doing things

Click to expand...

I can see that working well for a CNC system. In my case we're talking a widget thrown in a Kurt on a Bridgeport, and generally never the same thing twice. I have basic guidelines for "with this tool and material turn X RPM with Y feed (albeit by hand) and Z depth of cut". I'm looking for a way for them to understand the forces involved. Driving a cut too hard and then letting the cutter dwell so that there is a sideways step actually helped a lot (yes guys, the machine/part/too/fixture really did bend that far), but it would be nice to have actual forces imposed on the screws.

I liked the old Teenut approach to this.

(probably nobody remembers him here)

1) set cutter speed using standard SFPM tables (workpiece, cutter type)

2) feed the axis by hand faster and faster until the machine starts jumping around
on the floor. Back off slightly.

3) set the auto feed rate at that value.

jccaclimber said:

I can see that working well for a CNC system. In my case we're talking a widget thrown in a Kurt on a Bridgeport, and generally never the same thing twice. I have basic guidelines for "with this tool and material turn X RPM with Y feed (albeit by hand) and Z depth of cut". I'm looking for a way for them to understand the forces involved. Driving a cut too hard and then letting the cutter dwell so that there is a sideways step actually helped a lot (yes guys, the machine/part/too/fixture really did bend that far), but it would be nice to have actual forces imposed on the screws.

Click to expand...

.
sure excel can calculate hp and cutting forces i use it all the time to get a rough ideal so i know whether machine has enough hp and i got a secure enough setup (vise/fixture)
.
vibration though do not under estimate. i have seen parts loosen up and move before that were more than strong enough. i often have had to add jam nuts to fixtures so all the pushers and clamps dont loosen up from vibration.
.
same with a Bridgeport turret knee mill. i have often seen the turret get pushed or rotated out of tram or alignment from cutting vibration. think of a impact wrench often you can hold the socket from turning with your hand but the impact wrenches impacting or pounding can add considerable force or tightness to the bolts. do not under estimate vibration and chatter
.
bridgeport often damaged from running a big flycutter the whack whack whack is pounding action and turret can move and setscrew key in collet can get damaged. seen it many times over the decades

jccaclimber said:

In my case we're talking a widget thrown in a Kurt on a Bridgeport, and generally never the same thing twice. I have basic guidelines for "with this tool and material turn X RPM with Y feed (albeit by hand) and Z depth of cut". I'm looking for a way for them to understand the forces involved. Driving a cut too hard and then letting the cutter dwell so that there is a sideways step actually helped a lot (yes guys, the machine/part/too/fixture really did bend that far), but it would be nice to have actual forces imposed on the screws.

Click to expand...

Seems to me that you did figure a clever way with the feed stop leaving a clear mark (proves you loaded the machine tool & it relaxed when the load was relieved).

Tom uses unit power (horsepower) to figure his stuff (normal for shop use). It is related but doesn’t contemplate the shear angles modifying power requirements and it’s not directly a force value. The unit stiffness values I attached in post #8 are a direct force value. So D6AC @ 57RC does require a FORCE of 1,329,000 pounds per inch squared to shear while 6061-T6 aluminum requires only 76,000 pounds per inch squared to shear at the posted tool angles.

I’ll attach a generic unit horsepower listing as it’s generally more useful for machinists.

Good luck,
Matt

Learn to determine and set up for chipload per tooth on the cutter. Formula is desired chipload times number of teeth, times rpm equals feed rate in " (or mm, depending on how you wish to measure) per minute. Don't get too carried away with taking the absolute maximum chipload your hp will provide, as your cutters and fixtures will usually reach the limit long before the machine. Better to take four hours to make a part than to wreck the part 3 hrs in, because you fed it too heavy and it loosened in the clamps, and have to make another one.

Some good information here from Matt and AzDrake (nice first post!), but we should note that the OP is trying to machine aluminum with a noodle machine: Shapeoko XL/XXL Kit – Carbide 3D , and while it can be done, it's really not ideal. I suspect our engineer-in-training would be better off spending time analyzing the stiffness of the machine assembly he's using and looking for simple ways to stiffen it if at all possible.

it is interesting to note, that spindle deflection forces will be different for conventional and climb milling as well as dependent on % of engagement.

basics aluminum machinability with 1 hp you can machine 2 to 6 cubic inches per min
.
hp = 33,000 ft/lbs per min
.
say you are machining 2 cubic inch aluminum per min so say using 1 hp to keep it simple
.
if cutter going 330 sfpm thats 33000/330= 100 pounds of force
.
BUT say using a flycutter with one tooth that only cuts 1/2 of the revolution the force is 200 lbs cause cutting is only 1/2 the revolution
.
say you are cutting something small narrow with a flycutter thats cutting a large diameter so only cutting 10% of the revolution then the force is 10x100= 1000lbs
.
there are fluctuation or pulsations in the forces they are not even. even if you have a 6 flute cutter if you are only cutting 10% of the cutter revolution its 10 times the force than if it was cutting 100% of the cutter revolution (when milling)
.
the whack whack whack is why a part can move especially when all the cutting is occurring on a small part of the revolution, machine the heavier ones act as a fly wheel all the weight spinning helps to not stall the motor BUT forces pushing on part you should realize are like a hammer hitting the part. for example a 8 lb hammer hitting a 200 lb part quickly can often move the 200 lb part.
.
machinability rating is different with different cutters at different ipt feed. basically if i got a 1.5 hp motor and figure i got 1 hp at spindle by the time it goes through belt and gear drive than all i got to do is test what maximum cubic inches per minute i can cut to figure what 1hp how many cubic inches can it cut. some cutters like a corncob roughing end mill cut more efficiently probably cause wavy teeth there is smoother forces less whack whack whack

For those interested, I tracked down robert bastow's discussion, which I present forthwith:

=========================================================================
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 ********* 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
============================================================

A relatively complete list of his post on the old rec.crafts.metalworking are here, made possible
by scott logan:

http://www.loganact.com/teenut/teenut.txt

jim rozen said:

For those interested, I tracked down robert bastow's discussion, which I present forthwith:

Click to expand...

I remember Teenut, I remember the post, and I have that post squirreled away on my computer. I also have post on freehand drill sharpening using Teenuts "Bionic Darex"

CarlBoyd