920-229e-11.02

ReprintSD920-229e-11.02

Cutting ForceMeasurement

New Rotating Dynamometer for theAnalysis of high speed cutting Processes

J. Stirnimann, P. Tschanz,K. UnzickerKistler Instrumente AG,Switzerland

3

New Rotating Dynamometer for the Analysis of High Speed Cutting Processes

J. Stirnimann, P. Tschanz, K. Unzicker Kistler Instrumente AG, Switzerland

Abstract

Piezoelectric measuring technology has already been used successfully in conventional metal-cutting processes in order to investigate and further develop machining processes, tools and machine tools. In addition to the stationary small Dynamometer MiniDyn, there is now also a rotating high-speed Dynamometer available for clamping in machine tool spindles for research and investigation into applications in the new field of high speed cutting technology (HSC). This is the subject of the following article, which deals in depth with the Dynamometer construction, the type testing undertaken and the safety aspects.

1. Introduction High speed cutting (HSC) is a technology which today is already in widespread use and increasingly displacing conventional machining technologies. High metal removal rates, improved surface quality, machining of hardened components and the possibility of producing geometries it was previously impossible to manufacture (thin walls) within a short time, are the reasons for this success. These advantages are clearly depicted in Fig. 2.

It has been possible to counter to a great extent the reduction in tool life by appropriate expenditure on research and development. The following cutting materials have thus been developed which must be carefully matched to the workpiece material and the machining method: sintered carbide, extremely fine-grained sintered carbide, cermet, ceramic, CBN and CBN metal ceramic. All these tool materials are coated according to use. From the processing aspect, HSC is used for turning, milling, drilling, reaming, thread cutting (milling and drilling) and grinding. Fig. 3 gives an overview of the combined advantages of HSC.

Bild 1: HS-RCD

Fig. 2: Characterization of high-speed machi-ning [2]

Fig. 1: HS-RCD

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Increase ofQuality

- Better surface- Lower cutting forces- Vibration poor cutting

Decrease of costs- Shorter manufact. time- Shorter process chain- Dry machining

Shortening of manufacturing- Higher cutting velocity- Higher feed velocity- Higher cutting volume

Combined advantages of HSC

Fig. 3: Advantages of HSC

Figure 4 shows how the machining time can be reduced by reducing the manufacturing time and by shortening the process chain with HSC.

Fig. 4: Reduction of the manufacturing time with HSC [1]

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Fig. 5: S/N characteristic of force/vibration and AE-sensors [4]

In the past, cutting force measurements made it possible to assess conventional metal-removing machining methods safely and accurately, as well as to further develop them. For this purpose, rotating Dynamometers (built into machine tool spindles) were employed in research and development along with spatially fixed Dynamometers (screwed onto a machine table). AE-sensors (acoustic emission) were used for the acquisition of structure-borne sound signals – primarily for monitoring purposes. This measured variable is described in detail in [8].

The reasons why force measurements are so successful include the relatively simple interpretation of the signals. Furthermore, the signal-to-noise (S/N) ratio of force sensors for the chip thickness range of >0.1 μm is considerably higher than, for example, an acoustic emission (AE) signal (Fig. 5) [4]. Measuring devices for high speed machining need to have a natural frequency which is above the frequencies occurring during machining. Moreover, the measuring instruments must supply a reproducible measuring signal even with the smallest machining forces. The spatially fixed MiniDyn and the rotating HS-RCD described below meet these requirements thanks to their high rigidity and the piezoelectric measuring principle.

1.1 Rotating High Speed Dynamome-ter

A rotating cutting force Dynamometer is fitted in a machine tool spindle and measures the forces acting on the rotating tool. The data measured are thereby amplified directly in the Dynamometer and transferred by wireless transmission to a stationary receiver (Fig. 6).

Signal Conditioner Data acquisition PC

DynoWare – Data acquisition software

Machine spindle

Fig. 6: HS-RCD measuring chain

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The advantages of the rotating Dynamo-meter are: • Measurement at the rotating cutting

edge, i.e. measurement of the action force close to the cutting process during cutter engagement without measuring a reaction force.

• Constant mass of the measuring instrument during the cutting process and thus constant natural frequency, in contrast to the stationary Dynamometer, where the mass of the workpiece constantly reduces during metal removal.

• Low and, during machining, constant moments of inertia

• The Dynamometer can be positioned anywhere during machining

As shown in Fig. 2, the cutting forces during high speed machining reduce with increasing speed. The new 2-component Dynamometer (Fz, Mz) can be used to quantify these forces in different materials and machining processes. High speed machining is defined by the cutting speed vc. At the maximum speed of 25’000 rpm, the HS- RCD equipped with a D = 10mm tool operates at vc = 785 m/min and therefore for many materials already operates within the range of high speed machining (see Fig. 7). When a tool with D = 20mm is used, the cutting speed is Vc = 1,570 m/min.

Fig. 7: Positioning the HS-RCD in the material-vc graph

vc = 785 m/min vc = 1'570 m/min

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1.2 Spatially Fixed 3-Component Dynamometer MiniDyn

Fig. 8: Measuring chain with 3-component Dynamometer MiniDyn, Type 9256A… [3]

Data Measuring range Fx,y,z –250 ... 250 N Threshold 1.8 mN Sensitivity Ex,z

Ey –11 pC/N –13 pC/N

Rigidity Cx Cy Cz

250 N/μm 300 N/μm 290 N/μm

Dimensions 80x75x25 mm Natural frequency fox,y,z >5 kHz Working piece clamping area 40x80 mm

Table I: Technical data of the 3-component Dynamometer 9256A1

MiniDyn 9256A1

Data acquisition PC

Charge Amplifier Fx

Fz Fy

DynoWare – Data acquisition software

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A spatially fixed Dynamometer is screwed onto the machine table of a machine tool. The workpiece to be machined is fixed to the cover plate. The machining forces are measured in the three spatial directions X, Y, and Z. As mentioned at the beginning, measuring instruments for high speed machining need to have a high natural frequency. The MiniDyn (Fig. 9) meets this requirement and can be characterized by the following main features: • Low thermal error: This was reduced in the preload direction

of the sensor compared with previous Dynamometers by a factor of up to 20.

• High natural frequencies:

Fig. 9: Natural frequency f0z in z-direction of

the Dynamometer 9256A1 When the natural frequency of 5,5 kHz with the frequencies occurring during high speed milling of for example fA = 1500 Hz (n = 30,000 rpm, 3-cutter tool), the Dynamometer natural frequencies are up to three times higher than the frequency produced by machining. With twin-cutter tools, the difference is accordingly 30% higher. Measurements can thus be made with small forces and high machine-produced frequencies with high measuring certainty.

• High rigidities, which even under overload conditions guarantee practically negligible deformations. The rigidity of the Dynamometer at 250 ... 300 N/μm is higher than the rigidity of a machine tool spindle at 200 N/μm.

• High resolution: Even small force levels of, for example,

10 mN can be reproducibly measured [5].

• Low height of 25 mm (see Table I),

by which the Dynamometer is very suitable for installation in the very confined spaces of an ultra-precision machine or a face grinder.

2. Structure of the New Rotating

High-Speed Dynamometer The design of the Dynamometer was undertaken with particular attention given to keeping its dimensions as compact as possible. Because of the very high speeds involved, its diameter was kept small, so that the centrifugal forces can be kept within limits:

To achieve high flexural rigidity, its structural length was also kept short (the HS-RCD has the same length as a standard tool), because the third power of the length l goes into the flexure f of a component:

g8502500025

137000254 22

'arpm'n

)(speedandmmRwhere:m/s'ωRa

z

z

Case

==

==⋅=

)2(,3

3forceRadialFwhere

IElF

f =⋅⋅

⋅=

Nat. frequency Fx,y,z = 5.5 kHz

50

0

Magnitude

kHz100 Hz

Freq Resp: Hyx 2:1

9

To achieve high concentric accuracy and rigidity, standard tool holders with HSK machine interface are primarily used. The most compact version of the HS-RCD is equipped with an HSK-A63. The A63 size was selected because it is the most widespread version found on the market. For adaptation to other tool spindles, the Dynamometer is provided with an interface to which other HSK or also steep-angle taper adapters can be screwed (Fig. 10). A draw-in collet chuck according to DIN 6499, Form B, size ER32 was selected as tool interface. Ultra-precision collets are used with a concentric running accuracy of 0.005 mm and balanced lock nuts, which are suitable for HSC.

Fig. 10: The new rotating High Speed Dynamo-

meter with HSK-A63 adapter The piezoelectric sensor is equipped with quartz crystals for measuring the axial force Fz and the torque Mz. The sensor case also contains a two-channel charge amplifier and an inductive data transmission for wireless transmission of the measured data. The charge amplifier can be switched by remote control between three measuring ranges, in which the signals to be transmitted always have a high output, so that the quantization noise of the A/D-converters in the data transmission system is negligible.

3. Requirements for the HS-RCD and Specifications

The rotating HS-Dynamometer has been completely newly developed. High speeds require well balanced components [6], precision concentric running and a safe design with respect to the bursting behavior of the measuring instrument. Some aspects of the HS-RCD development are described below.

3.1 Requirements Concerning High Centrifugal Acceleration:

The high speed of 25,000 rpm produces a centrifugal acceleration of 25,850 g on the radius of the outer wall of the case. This produces high centrifugal forces at the electronic components in the sensor as well as at the telemetry antenna. How the sensor-integrated electronics have to be arranged in order to withstand the high centrifugal forces was investigated right at the beginning of the sensor development. In exhaustive tests, printed circuit boards equipped with components were exposed to centrifugal accelerations of up to 100,000g, the results of which were used to decide the mounting locations for the charge amplifier board and the data transmission system. The charge amplifier board was positioned perpendicular to the machine axis and the telemetry designed as a flexible print inserted in the inner wall of the antenna case. An aspect which is important for safety is the bursting resistance of the antenna. The standard “Milling tools for high speed machining” [6] was used as a basis for designing the speed limit. This requires a test speed of n = 40,000 rpm. The antenna case was tested up to n = 56,000 rpm, corresponding to acentrifugal = 130,000 g. The antenna enclosed in glass fibers withstood this load undamaged.

106

HSK-A63

Sensor

Telemetry & Electronic

Collet ∅1...16 mm

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3.2 Balancing Requirements High speed tools require careful balancing. This not only concerns the tool holder, but also the clamped tool, adapter sleeves and lock nuts. A balancing accuracy of G1 to G6.3 can be interpreted from the standard [7], in which many customers demand a balancing accuracy of G2.5. This demand is, however, not feasible in practice, since even a tool change worsens the balance. This can be illustrated with the HS-RCD: • Balanced to G2.5 • Rotor mass M = 1.5 kg • Residual unbalance e = 1.25 g mm/kg at

n = 25'000 rpm → this signifies an unbalance on the

antenna of:

The extreme demands, which can be deduced from ISO 1940-1 [7], and the limited centering capability of the machine and tool interfaces prompted the VDMA (Precision Tool Association) and the PTW of Darmstadt Polytechnic to carry out a study. Accordingly a feasible balance requirement of G16 was found for all tool sizes and the associated relevant cutting speed range. G39 is recommended for the size of the HS-RCD. On the HS-RCD, preliminary balancing of the antenna case is carried out and the sensor is balanced to G6.3. After the sensor and tool holder are assembled, the complete Dynamometer is balanced to ≤G6.3. The increased balancing accuracy is required so that measurements will not be impaired.

)(.][ 30510 gmmkg

kgmmgRMe

m =⋅

⋅⋅⋅=

11

3.3 Technical Data • Measuring range Fz N –3000 ... 3000 Mz Nm –50 ... 50 • Overload +20% Fz N –3600/3600 Mz Nm –60/60 • Threshold Fz mN ≤30 Mz Ncm ≤0.1 • Sensitivity range I Fz mV / N ≈3 Mz mV / Nm ≈140 • Linearity Fz, Mz ±% / FSO ≤±0.8 • Hysteresis Fz % / FSO ≤1.0 Mz % / FSO ≤1.0 • Crosstalk

(defined as 100 % range) Fz → Mz Ncm/N ≤±0.05

Mz → Fz N/Nm <±0.8

• Natural frequency (Dynamometer fixed by the HSK adapter)

fOFz

Hz

≈5000

fOMz Hz ≈2500 • Material HSK Adapter Steel DIN1.7131 • Maximum speed rpm 25’000 • Weight without collet adapter & with

lead-through adapter for cutting fluid kg 1.51

• Collet type ER 32, Form B

DIN 6499 • Tool diameter D mm 1 ... 20 • Degree of protection DIN40050 IP 67 • Balance grade G 6.3 • Max. cutting fluid pressure at the entry

of the HS-RCD p bar 70

• Operating temperature range T °C 60

Table II: Technical data of HS-RCD

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4. Application Results The following measurements were all carried out on a HERMLE – machining center 800U with 5 driven axes. The machine is equipped with a motor spindle with nmax = 15,000 rpm, HSK-A63 interface and internal cooling.

In the tests listed, aluminum AlMgSi1 was the material machined throughout. However, measurements were also made in CrNi steel (1.4301) as well as hardened steel (HV 58).

Fig. 11: Drilling test with vf = 6000 mm/min

Cutting parameters: Tool: Material: Remarks

vc = 400 m/min

Solid carbide twist drill D = 8.5 mm

Anticorodal AC100 (AlMgSi1)

Drilling with internal cooling

n = 15‘000 rpm f = 0.4 mm/rev vf = 6000 mm/min ap = 20 mm

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2-1.00

0

1.00

2.00

3.00

4.00

5.00

Time [s]

Drilling D8.5-08Zoom on

Cycle No.: 1

Smoothing on

Mz HS-RCD [Nm]

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2-200.00

0

200.00

400.00

600.00

800.00

1000.00

1200.00

Time [s]

Drilling D8.5-08Zoom on

Cycle No.: 1

Smoothing on

Fz HS- RCD [N]

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Fig. 12: Milling test at vc = 470 m/min


vc = 470 m/min n = 15‘000 rpm f = 0.2 mm/rev

Solid carbide milling cutter D= 10 mm, 4 cutters


Milling with external cooling

vf = 3000 mm/min ap = 3 mm ae = 5 mm Fig. 13: 5-axes milling in aluminium


vc = 200m/min n = 4‘000 rpm

Solid carbide milling cutter D= 16 mm


Milling with external cooling

f = 0.2 mm/rev vf = 800 mm/min ap = 32 mm ae = 0.2 mm

2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2

-36.00

-28.00

-20.00

-12.00

-4.000

4.00

Time [s]

Milling test

Cycle No.: 1

Zoom on Fz [N]

2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2

-0.20

0

0.20

0.40

0.60

0.80

Time [s]

Milling testZoom on Mz [Nm]

4 8 12 16 20 24

-100.00

-80.00

-60.00

-40.00

-20.00

0

20.00

40.00

Time [s]

5-axis milling Fz [N]

4 8 12 16 20 24

-0.40

-0.20

0

0.20

0.40

0.60

0.80

1.00

Time [s]

5-axis milling Mz [Nm]

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Fig. 14: M12 – Thread cutting in aluminium


vc = 18.5 m/min Thread cutter M12 Anticorodal AC100 (AlMgSi1)

n = 500 rpm ap = 10 mm

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

-300.00

-200.00

-100.00

0

100.00

200.00

300.00

400.00

Time [s]

Thread cutting Fz [N]

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

-4.00

-2.00

0

2.00

4.00

6.00

8.00

10.00

Time [s]

Thread cutting Mz [Nm]

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5. Summary This article introduces piezoelectric sensors which, because of their high natural frequency and small construction, are particularly suitable for high speed machining. The rotating High Speed Dynamometer enables measurement of the torque Mz and axial force Fz on a rotating tool up to n = 25’000 rpm. The data are transferred by induction from the rotating sensor to a spatially fixed receiver. High rigidity is obtained as a result of the short structure and the HSK-A63 spindle adapter, thereby allowing cutting parameters as with a normal tool.

6. Literature [1] Kuster F., Requirements of modern

metal cutting technology for the machine tool of tomorrow, 4th Swiss-mem Metal-Cutting Seminar, 2001

[2] Schulz H.: Hochgeschwindigkeits-

bearbeitung / High speed machining, Munich Vienna, Carl Hanser Verlag, 1996

[3] Stirnimann J., Kirchheim A.: New

cutting force Dynamometers for high-precision machining, Industrial Too-ling Conference, Southampton, 1997

[4] Dornfeld D.A.: Monitoring of ultra-

precision machining processses, 8th International Machine Tool Engineers Conference (IMEC), Osaka, 1998

[5] Klocke, F.; Koch, K.-F.; Zamel, S.;

Technology of high and ultra-preci-sion machining, ZWF, 90 (1995)5, pp. 217-221

[6] Milling tools for high speed machi-

ning, safety requirements, ISO 15641, 1998

[7] Assessment scales for the balancing of

rotating rigid bodies, VDI 2060 and ISO 1940-1, 1986 respectively

[8] Cavalloni, C., Kirchheim, A.: New

Acoustic Emission Sensors for In-Process Monitoring, Progress in Acoustic Emission VII, eds. T. Kishi, Y. Mori, and M. Enoki, The Japanese Society of Nondestructive Inspection, Tokyo, Japan, 1994, pp. 91-97.

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