Procedure for setting chatter-free cutting conditions using CutPRO and Process Damping

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Kelley York
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Setting rough cutting conditions Two methods are possible; selecting relatively high speed

1 Procedure for setting chatter-free cutting conditions using CutPRO and Process Damping Hoshi Technical Research, 25 January 29, 2017 (Revised 1 February, 2017) 1. Setting rough cutting conditions Two methods are possible; selecting relatively high speed range using stability pocket, and relatively low speed range using process damping principle. [Procedure 1] Identify tool tip dynamics FRF by impulse testing of the end of the tool mounted on the machine spindle. The frequency of chatter to happen is identified. In this example, 1010Hz. 1

2 1.1 Setting rough cutting conditions by Stability Pocket. [Procedure 2] Compute stability chart using the impulse test result in the above. [Procedure 3] Confirm cutting conditions by Miling Process Simulation 2

Sas(rpm)=120(Natural Frequency,Hz)γx(Cs x h))/(tool Diameter, mm) γ: Feed to tangential ratio of the cutting force components. Cs: Ratio pf average thickness of cut to feed per tooth h.

3 Amount of spindle power required is estimated by Milling Process Simulation as below. 1.2 Setting rough cutting conditions by Process Daming. [Procedure 4] Use following equation to obtain the Asymptotic spindle Cutting Speed Sas by Das Tobias ( ) model. Sas(rpm)=120(Natural Frequency,Hz)γx(Cs x h))/(tool Diameter, mm) γ: Feed to tangential ratio of the cutting force components. Cs: Ratio pf average thickness of cut to feed per tooth h. h: Feed per tooth mm/t Value of Sas is for example: =120x1000x2x(0.63x0.15)/12=1909rpm This is the spindle speed below which chatter does not occur no matter how large Radial or Axial depths of cut are selected. Smaller than the computed Sas=1909rpm, selected is 1900rpm (Cutting speed V=72m/min) for rough cutting by process damping. But it is subject to a limit due to the horsepower available from the spindle drive at the speed. 3

In the example machine, spindle speed set at S1900rpm, and assuming to take 10mm axial depth of cut, amount of spindle power may be calculated by running Milling

But in this case, to guarantee chatter-free calculation, the FRF of the end of the tool has to be set to a very rigid value, for example 10 to the 10 th power N/μm.

As noted in the figure above, estimated power requirement for rough cutting has been 58.5kW. TTrue value will be ten times as small, but is 5.85kW.

4 In the example machine, spindle speed set at S1900rpm, and assuming to take 10mm axial depth of cut, amount of spindle power may be calculated by running Milling Process Simulation of CutPRO. But in this case, to guarantee chatter-free calculation, the FRF of the end of the tool has to be set to a very rigid value, for example 10 to the 10 th power N/μm. Also, it is needed to temporarily set the spindle speed 10 times higher for sake of the computation speed. With the original speed, computation time takes too long. As noted in the figure above, estimated power requirement for rough cutting has been 58.5kW. TTrue value will be ten times as small, but is 5.85kW. This is not possible by the machine tool under consideration. Therefore, rough cutting condition by process damping is not used in this example. Considering the condition of the available machine tool to use, decision has to be made which condition to take either obtained by 1.1 Stability Pocket or by 1.2 Process Damping. 4

Rough cutting condition set by the stability pocket has been selected in the current example.

Setting Finish Cutting Conditions [Procedure 5] Identify cutting condition for finish cutting straight part of the

However, in order to avoid chatter at the sharp corner where direction of the tool advance suddenly changes by 90deree

5 Rough cutting condition set by the stability pocket has been selected in the current example. Photograph of the rough machined surface is exhibited below. 2. Setting Finish Cutting Conditions [Procedure 5] Identify cutting condition for finish cutting straight part of the profile using Process Damping computation of CutPRO. However, in order to avoid chatter at the sharp corner where direction of the tool advance suddenly changes by 90deree after the straight profile, spindle speed needs to be set below Sas value obtained in the procedure 4. Also it is recommended to take a cutter having only one flute. The reason is: (1) Chatter is less likely to occur when number of flutes of the cutter is small. 5

(2) Even in using a tool having multiple flutes, the surface geometry is generated by a cutting edge

One flute, thus one cutting edge has minimum amount of the runout, therefore giving the best surface

6 (2) Even in using a tool having multiple flutes, the surface geometry is generated by a cutting edge having maximum runout. One flute, thus one cutting edge has minimum amount of the runout, therefore giving the best surface quality. Cutting condition selected for finishing straight part of the profile: S1800rpm, number of flutes of the end mill 1, radial depth of cut 0.1mm (Down Cut), axial depth of cut 20mm, feed per revolution f0.15mm/rev, Feed Speed F180mm/min. Milling Process Simulation indicates that good quality surface should be generated by the finish cutting condition set in the above. 6

7 Slight vibration mark is visible at the sharp corner. From the analysis of the vibration captured during the finish cutting test, the vibration mark has been confirmed not to be chatter mark, but the mark was generated by the torsional natural vibration of the spindle. 7

8 It is known, on the other hand, effect of the torsional natural vibration of the spindle can be suppressed by chucking the tool via. vibration absorbing metal as illustrated below. End of text 8

Flowchart of setting Chatter-free Cutting Condition

Flowchart of setting Chatter-free Cutting Condition 1 Procedures for setting Chatter-free Cutting Condition -Illustration by case study- Hoshi Technical Research, 12 March 2017 revise 30 March Part 1 Chatter-free