November 2007 Edition

MACHINING RXs FROM DR. J

In the Cut

For machining, "cutting corners" means paying more attention to power, torque, and speed than in straight cuts

Cutting processes have undergone a great deal of change over the last decade. The development of new coatings and new cutting tool materials lets users operate at increasingly higher cutting speeds and rpm. Machine tool makers have matched the challenge by designing machines that can operate at higher rpm and with greater feed rates. But, it is important to adapt the cutting process to new machines or buy a machine that matches cutting processes.

In this regard, I am reminded of the success of Audi in the Le Mans racing series when the company changed from a high-revving gas engine to a high-torque diesel engine. The diesel engine let Audi drivers reduce gear shifting by 25 percent due to a larger and wider torque band.

In the machining arena, high productivity can be achieved with a large chip load and a relatively lower cutting speed, analogous to Audi's change to diesel, or with a smaller chip load with a high cutting speed, analogous to the use of the gasoline engine by Audi.

In either case, the type of machine must be matched to the type of cutting process to have the best results.

The Five Criteria

So what are the important aspects to consider as you develop a machining solution for your machine? I like to evaluate cutting processes on the basis of five criteria: Power, torque, and speed of the cutting process; bending moment at the tooling interface; radial and axial force applied on the spindle bearings; chatter behavior; and cutter and work piece deflection.

The power and torque impact the cutting process.

There are a number of simple ways to estimate the power and torque in a cutting process and a number of calculators that are available online for the same purpose. For a quick, back-of-the-envelope calculation, I'll use a specific horsepower for different material and multiply it with the material removal rate of the cutting process.

The Spindle Power graph shows estimated cutting power when cutting aluminum with a 1.5" four flute end mill with a feed of 0.011"/tooth. The two cases show the same material removal rate, but achieved with a different combination of axial depth of cut and radial engagement – skinny versus a stubby cut.

Both cuts nominally consume a little over 20 hp – as expected since the material removal rates are the same. But, the variation of power around the nominal for the skinny cut is 7.4 hp minimum, 23.5 hp average, and 30.7 hp maximum, while it is a much smaller – 17.1 hp minimum, 20.3 hp average, and 22.9 hp maximum – variation for the stubby cut. This is because of the different radial engagements.

Skinny cuts – long axial engagement with small radial engagement – where each tooth bangs in to the work piece, tends to have a larger variation than a stubby cut where multiple teeth are in the cut.

Cutting Corners

Once power estimation is made, compare it against the capabilities of the spindle. Pay particular attention to duty cycle ratings of the spindle. S1 spindle ratings have to be compared against the nominal power requirements while the S6 ratings should be compared with the peak horsepower requirements.

Most calculators produce nominal power and torque requirements. If the spindle is sized for the nominal power and there are large variations – as in a skinny cut – it is possible to overheat the spindle motor and drive and possibly damage the spindle bearings.

Be careful cutting a corner. The radial engagement dramatically increases in a corner. We've all heard the squeal of a cutter as it machines a corner.

Treat these cases by slowing down at the corner or using alternate tool paths to peck at the corner.

It is important to perform calculations and work within the parameters of the machine. In the future, I'll address other criteria and the importance of evaluating each against the capabilities of the machine.

Jairam Manjunathaiah, Ph.D., – Dr. J – serves as vice president of engineering for MAG Fadal, a division of MAG-IAS. He leads the new product development, product maintenance, and engineering team.

Have a machining question or dilemma?
The Doctor is in. If you have a topic you'd like the good doctor to address,
send it to Dr. J at pnofel@nelsonpub.com.

What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at www.ModernApplicationsNews.com or e-mail the editor at pnofel@nelsonpub.com.