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Analysis of Siemens PLM White Paper "Uncover peak performance in HSM"

Last Updated: Jul 27, 2011 10:17AM EDT

The July 20, 2010 White Paper from Siemens PLM entitled, "Uncover peak performance in HSM" (a copy is attached below)  recognizes the science of machining dynamics and its stability lobe diagrams, even showing an example on page 4:

Its conclusions are spot-on, "Operating the system at peak MRR, and within safe and stable limits, ensures optimum utilization of machining equipment".

and this,

"In many cases, when chatter is encountered, machinists tend to reduce spindle speed to eliminate chatter. While this technique will result in a stable cutting condition, it may not be the most efficient. Instead, it is often possible to increase the spindle speed, which will eliminate chatter, while also improving cutting efficiency."

However, we do not see the cost effectiveness of what they describe as a "practical no-cost implementation". Requiring at least 84 test cuts for each tool in a machining center takes away from valuable production time, ties up an operator while consuming both material (including pre-machining of the test block) and tooling. Not exactly no-cost.

The example described in this paper required 84 separate test cuts. It yielded a resolution of only 500 RPM on spindle speed and .5mm on stepover. The depth of cut was fixed at 4mm. Chip load was unchanged at 0.072 mm/tooth. If you wanted to increase the depth of cut or feed per tooth, you would have to take more test cuts.
If you wanted to further optimize the speed, even more tests would be required. For example the test showed that at 9500 RPM was stable at a 3.5 mm stepover, but could only reach between 1.5 and 2.0 mm at 10,000 RPM, a very large drop off. Additional test cuts would detrmine at what speed this large drop-off would occur. Is it a 9501 or 9999 RPM? Finally, an arbitrary test range of 5000 to 11,500 RPM was selected. 

With the availability of the low cost Dashboard (a 3D graphical representation of the stability lobe diagram) produced with the data collected from less than 3 minute tap-test, we believe there is a faster and lower cost alternative to the technique described in this paper while still achieving its goal to "identify optimum spindle speeds and cutter loads for any given tool, holder, machine and workpiece material combination".

The paper acknowledges this by the following sentence:
"A global view of the stability lobe diagram, either computed with chatter 
prediction hardware tools or by the method prescribed 
in this white paper, helps users achieve high 
MRRs at certain stable RPMs."

We welcome them to our side. A few years ago UGS, the company that became Siemens PLM, stated in a brochure that their software provided "chatter-free" machining. Today they endorse the need for stability lobe diagrams.



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