High-Speed Milling of Hard Metals

8/12/2019 High-Speed Milling of Hard Metals

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High-Speed Milling of Hard Metals

It can be done, but there's more involved than buying a new spindle

By Craig McQueen

Application Team Leader

Makino Inc.

Mason, OH

[email protected]

Until recently, metals above R!" were considered too hard to machine at high speed, so the

soft-steel part would be roughed and semifinished, and when it came bac# from hardening the

details would be finished off by a capable machine tool or by hand$ %hese operations ran at slow

speeds to #eep the machine from crashing, and to avoid brea#ing the tool$

%o machine hardened steels at high speed, you must have a machine tool, control, and toolingthat are up to the tas#$ &lso, you must be able to write a program that gives appropriate

consideration to the stresses a tool undergoes at high speeds when milling hardened material$

&nd finally, you must rethin# how your people are trained, because a different mindset isreuired to successfully machine hardened steels at high speed$

(iven the reuirements inherent to high-speed machining )HSM* of hard materials, and the

challenges summari+ed above, why is this process worth pursuing

uite simply, once mastered, it's a great way to improve productivity$ .n average, we've

been able to reduce cycle times from traditional hard-milling processes by half to two-thirds, and

improve surface finishes to the point that no hand polishing or bench time is reuired$ In fact,we're consistently seeing surface finishes of / 0m Rain highspeed, hard-milling applications of

steels hardened to R12$ 3e are successfully removing hardened metal at 4222 fpm )52" m6min*for steels hardened to R!", as much as 122 fpm )475 m6min* for steels to R!"8"7, and as high

as !22 fpm )499 m6min* for R12: material$

%esting continues, and we're finding that we can push our spindles faster everyday, removing

even more hardened metal at higher speeds$

So what's involved in successful high-speed hard milling

&s a first reuirement, it's pretty obvious that the spindles had to speed up for HSM;from92,222 rpm on average to around !2,222 rpm;to permit faster feeds without increasing cutting

force$ %he speed isn't the most important thing to note, however, because higher speeds don't

necessarily mean shorter cycle times$ If you can't hold the same accuracies in HSM as in

conventional machining, and have a reliable system, you'll


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%o handle a spindle operating at high speed, > processing speed has had to improve$ %oday's

high-speed processors are significantly faster than those used years ago, and the control software

is more advanced$ .ur company's latest control software pac#age, for e=ample, which isdesignated S(I$!, has been optimi+ed specifically for HSM$ In addition, modern bus architecture

and connection speeds have enabled controllers to communicate with the spindle much more

efficiently and reliably$?unctional parts, such as the servomotors and bearings, must be able to handle HSM$ %heservomotors must be designed to get ma=imum torue without overloading the motors, to

increase acceleration of the a=es, while maintaining smooth motion that preserves mechanical

components$ Many of the new digital servosystems provide 41 million pulses6ballscrewrevolution to assist in this tas#$ Right now, the best bearings for HSM are hybrid steel race

ceramic balls, which permit less torue at higher speed with finer finishes, and are electrically

inert$

@on't forget to consider the chip management system$ It's important when doing HSM that chipsare evacuated from the wor#space uic#ly and efficiently, given the increased metal

removal rate$!en you"re cutting into materialsthat are over R!", the process generates vibration

and heat$ Aoth can lead to inaccuracies, chatter, tooling failures, growth and other problems$

%he first step to ta#e in dealing with vibration and heat is to employ a stable machine tool andtoolholder$ Stable machine tools have rigid, heavy castings$ ast-iron construction is especially

important to limit deflection and provide thermal stability$

More massive machine tools will fight the forces you're applying to hardened material$ >ot that

you should buy a machine tool based solely on its weight, but mass is something to be #ept in

mind if you'll be hard milling$

&nother important feature is thermal control$ &s a part is machined over long periods of time, themachine e=periences thermally driven changes caused by ambient temperatures$ & stable

machine will not permit those temperatures to negatively influence part-cutting$

Many molds are machined for days, especially large molds, and thermal control becomes

important to maintain polish-free surface finishes and tool-to-tool blending$ & well-cooledspindle that provides repeatable, predictable thermal growth helps maintain these characteristics$

Spindle core cooling is a process patented by our company that can reduce spindle growth in

high-speed spindles$ ore cooling limits the amount of thermally driven growth, and enables the

spindle to stabili+e very uic#ly$ ooling oil temperature is accurately controlled, and the sameoil is pumped into the center of the spindle$ So the oil that lubricates the spindle also cools it$

As #or t!e tool!older, you need to decide what #ind of holder fits your application best$ &

mill chuc#, for instance, is great for roughing in hardened steels$ It provides e=cellent

vibration damping, good runout, and rigidity$ Mill chuc#s aren't very accurate, though, so a colletchuc# might be a better option$


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%he biggest thing to consider when pic#ing a toolholder is how much damping you need$ %he

toolholder system should not allow energy to transfer into the spindle interface, the critical

contact point between the tool and the spindle$ Minimi+ing the damage that roughing will do byabsorbing energy with a toolholder damping system will e=tend tool life, and improve bearing

life and runout$

Runout for roughing tools will typically range from 2$22" to 2$2224B )2$49/82$225 mm* whilefinishing tools held in shrin#-fit holders can achieve runout of about 2$229 to 2$2222!B )2$2"82$224 mm*$

& well-made collet chuc# will absorb vibration, has good runout, accepts many tool si+es, and

can be used for the finish wor# needed in most applications$ If you need to get really accurate, a

shrin#-fit holder becomes your best option$ In most roughing applications, however, it won'tabsorb the vibration created during high-speed roughing of hard materials$ Aasically, shrin#-fit

holders should be used for semi-finish and finish routines$

>o matter what toolholder you use, when you're running high speeds in hardened steels it

becomes especially important to balance them$ 3e balance all holders to g9$" at ma=imum

system cutting rpm$

Cou need to use a tool that is specifically designed for cutting hardened steels$ @on't assume

normal tooling will wor#$ Several manufacturers offer tools that are rated to cut "2 R, even

R12, materials$ Ae sure to do some tests with your tool, machine tool and toolholder to ma#esure all the ingredients add up$

Make sure your programming so#t$areallows the use of many techniues that can be

valuable when tac#ling hardened steels at high speeds$ %rochidal roughing, effective lead-in andlead-out control, arc-fitting corners, high-tolerance toolpaths, and gouge-chec#ing can influence

your ability to hard mill$ If the program on the > won't allow control of these factors or

permit employment of these techniues, loo# for new software$ Usually, software capable ofhandling these techniues is designed specifically for hard milling$

&nother important programming factor involves understanding how to handle stepovers and

stepdowns relative to material hardness$ %his often-overloo#ed point can ma#e a big difference

in how well you can hard mill$ %able 4 shows our company's basic guidelines$ 3e suggestthat chip load be #ept at about 4D of cutting-tool diam$ &nd when cutting hardened

materials at high speeds, we recommend leaving "D of the semifinish cutting tool diameter as

plus stoc# )the amount of material left after the roughing routine*$

%uring t!e roug!ing operation, you'll need to determine the effective diameter of the cuttingtool to engage to ma=imi+e feed rate and decrease cycle times according to the appropriate

formula$Aasically, be sure that your program and programming ability can handle high speeds in

hardened steels$ 3ithout the proper techniues, you'll burn up tools and scrap parts left and right$

If you've never programmed for high speed machining before and6or for hardened steels, oddsare you won't be able to do both without some training$

Hard milling at !ig! speedis very different from hard milling at slow speeds, and is very

different than high-speed soft milling$ %he same principles don't necessarily apply, and the


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margin of error is much slimmer$ (iven these facts, we believe it's essential that shops who ta#e

on high-speed hard milling have a staff prepared to handle this change$

& good e=ample of the difference between soft milling and hard milling at high speeds is the

tendency to thin# that it's .E to leave mista#es to be machined, benched, or polished later$ Ifyour goal is to reduce cycle time by s#ipping the soft-roughing step, there's no room for

benching after the piece is machined$ Instead, each step of the manufacturing process must bescrutini+ed, and the part must be e=amined and approved for the ne=t step$ %his Berror-freeBprocess must be instituted at every step, from receiving the blan# to the final e=amination of the

part$ If a bad part is left unchec#ed or passed onto the ne=t step, the whole process collapses,

lead times increase, and you might as well go bac# to the old way of doing things$

Mac!inists and mac!ine operatorsmust be trained on how hard to push the machines andtooling$ It's easy, once you see progress in high-speed hard milling, to push the process until

something brea#s$ Aut crashing a machine tool, ruining a spindle, or destroying specialty tooling

can become e=pensive and time-consuming$ Ma#e sure your people are trained on thecapabilities of the euipment and tooling used for high-speed hard milling, not

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High-Speed Milling of Hard Metals