automating the mold-material grinding process

Sawa, T.

Paper:

Automating the Mold-Material Grinding Process

Takekazu Sawa†

Department of Design and Engineering, Shibaura Institute of Technology3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, Japan

†Corresponding author, E-mail: [email protected][Received March 15, 2019; accepted September 11, 2019]

Grinding is difficult to control because abrasive grainsare scattered randomly on the surface of the grindingwheel, and the quality of the grinding work is stronglydependent on the skill of the operator. Therefore, au-tomation and optimization technologies should be es-tablished immediately for grinding, along with othermachining work. From this perspective, we observedthe bending vibrations of a diamond wheel during agrinding project and developed a technique to iden-tify the grinding condition by using a microphone tomeasure the small noises from the vibration (calledbending-vibration noise in this paper). In this paper,we report the application of the technique to an ordi-nary grinding wheel, and our attempt to automate thegrinding work of STAVAX and SKD11 metal materi-als.

Keywords: grinding, automation, mold material, moldmanufacturing, STAVAX

1. Introduction

The Japanese manufacturing industry has not only ex-cellent research and development abilities, but also highlyskilled workers in the machining fields. However, thenumber of sufficiently skilled workers is decreasing yearby year because the low birth rate reduces the possiblenumber of successors with the needed skills. One solu-tion to this problem is to study the skills (tacit knowledge)scientifically and formalize them in order to automate themachining technologies, allowing for stable quality andhighly reliable machining. It would also be useful to de-velop a navigation system that judges the quality of themachining work to enable ordinary workers to work at ahigher level.

Many studies have been conducted on technologies forrecognizing machining phenomena to enable grinding au-tomation and optimization, including studies on grind-ing resistance [1, 2], workpiece-surface roughness [3], ob-serving the grinding-wheel working surface [4], grind-ing power [5], acoustic emission sensors [6–8], magneticbearings [9], various types of monitoring [10–14], accel-eration sensors, and underwater microphones. All of thesereported technologies can be used to recognize grinding

conditions.However, a fundamental problem is that it is difficult

to quantify experimental results, even for the same work-piece material, because of varying experimental environ-ments and machining conditions, including the precisionand rigidity of the grinding machines.

From this perspective, we detected the bending vibra-tions of a diamond wheel during a grinding project anddeveloped a technique to identify the grinding conditionby using a microphone to measure the small noises fromthe vibration (called bending-vibration noise in this pa-per) [15, 16]. This technique does not depend on the ex-perimental environment or the machining conditions, e.g.,the precision and rigidity of the grinding machine.

In the present study, we apply the technique to the wetgrinding of plane surfaces of STAVAX and SKD11 steel,which are used as mold materials, and examine whetherthe technique can be used to recognize a change in thegrinding form.

2. Relation Between the Grinding-WheelFrequency Characteristics and theBending-Vibration Noise

A grinding wheel has various types of free vibration:bending, twisting, and up-down-right-left. Among these,the natural frequency of the bending vibration is deter-mined by the shape and material of the grinding wheeland flange.

Generally, the sound pressure of a vibration noise de-pends on the vibration direction and location, and islargest when the noise comes from the largest-area sur-face of a vibrating body. Therefore, it could be expectedthat a noise coming from the side surface of a grindingwheel would have the largest sound pressure and wouldbe detected most easily. From this viewpoint, we usethe bending-vibration noise from a grinding wheel as therecognition signal for grinding phenomena.

First, we studied the relation between the frequencycharacteristics and the bending-vibration noise for thebending vibration of a grinding wheel attached to a flange.The frequency characteristics of the bending vibrationwere measured with an acceleration sensor, mounted80 mm from the center on the side surface of the grind-ing wheel, using a double-sided tape.

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Fig. 1. Microphone position.

Fig. 2. Impulse response of the grinding wheel, measuredby microphone and acceleration pickup (before attachmentto the spindle head).

The bending-vibration noise was measured by a micro-phone fixed 80 mm from the center of the grinding wheeland 1 mm from the side surface. The microphone wasvertically mounted to the side surface, and its directionwas adjusted so that it coincided with the direction ofthe bending-vibration displacement of the grinding wheel.Fig. 1 illustrates the position of the microphone.

A capacitor-type microphone was used for the exper-iment. It had 1.5–2.2-kΩ output impedance and −62 ±3-dB sensitivity. The grinding wheel (WA120K7V) hada round plane shape with a 205-mm outer diameter,31.75-mm inner diameter, and 13-mm width. The flangewas a standard accessory with a 78-mm outer diameterand 57-mm width and was made by Kuroda Precision In-dustries.

Figure 2 shows the frequency characteristics (impulseresponse) of the bending vibration of the grinding wheel.The wheel was suspended by a string with the flange at-tached and was hit once by a steel rod in the perpendiculardirection on the grinding surface, for comparison with thefrequency-analysis result. The excitation by the steel rodwas made 180◦ away from the position where the acceler-ation sensor was attached. This experiment was repeatedfive times and the reproducibility was confirmed.

One can see from Fig. 2 that the frequencies at whichthe acceleration amplitude increases and the frequenciesat which the sound pressure of the bending-vibrationnoise increases perfectly coincide with each other. The

Fig. 3. Impulse response of the grinding wheel, measuredby microphone and acceleration pickup (after attachment tothe spindle head).

result indicates that the natural frequencies of the grind-ing wheel attached to the flange are 3.7, 5.8, and 8.5 kHz,and the sound pressure of the bending-vibration noiseincreases at specific frequencies (natural frequencies),which are determined by the specifications of the flangeand the grinding wheel.

It was also found that an impact perpendicular to thewheel’s grinding surface caused the bending vibration andthe bending-vibration noise. In other words, we clarifiedthe possibility that observing the bending-vibration noisegenerated by the grinding wheel could be used to measurethe normal grinding force.

Figure 3 shows the frequency characteristics of thegrinding wheel’s bending vibration. The wheel wasmounted on the spindle of the grinding machine attachedto the flange and was hit once by a steel rod on the grind-ing surface, for comparison with the frequency-analysisresult. The acceleration sensor and microphone were at-tached in the same positions and in the same manner as inthe experiment shown in Fig. 2.

Similarly to Fig. 2, one can see from Fig. 3 that thefrequencies at which the acceleration amplitude increasesand the frequencies at which the sound pressure of thebending-vibration noise increases perfectly coincide witheach other, and the frequencies are 3.7, 5.8, and 8.5 kHz.

This result indicates that the bending-vibration noisearises from the grinding wheel itself and that, since thewaveforms of the acceleration amplitude and the bending-vibration noise are maintained, the spindle of the grind-ing machine hardly affects the bending vibration and thebending-vibration noise of the grinding wheel. In otherwords, the bending-vibration noise is not influenced bythe rigidity of the machine spindle. Hence, the grind-ing phenomenon recognition based on noise observationsis more versatile and useful than the conventional tech-niques reported thus far.

We performed a finite-element method-based simula-tion analysis to find the vibration mode of the grindingwheel at the natural frequency of 3.7 kHz. The analysisresult is shown in Fig. 4.

The analysis model was designed on the assumptionthat a grinding wheel was attached to a flange, and the

Int. J. of Automation Technology Vol.13 No.6, 2019 723

Sawa, T.

Fig. 4. Mode analysis of the grinding wheel using the FEM.

outer periphery was free, while the inner periphery wasfixed. The 3D model that we analyzed reproduces a grind-ing wheel with a flange; the wheel’s dimensions are thesame as those of the one used in the experiment. Namely,the outer diameter was 205 mm, the inner diameter was31.75 mm, and the thickness was 13 mm.

It is difficult to calculate a grinding wheel’s proper-ties since it is made of composite materials, consistingof abrasive grains, a binding agent, and pores. It was as-sumed for the analysis that the wheel was an aluminum-alloy-based wheel with super abrasive grains. Therefore,in the simulation, we used the physical quantities of thealuminum alloy (Young’s modulus of 69.3 GPa, Poisson’sratio of 0.30, and density of 2.68 g/cm3) for the grindingwheel and the physical quantities of steel (Young’s mod-ulus of 206 GPa, Poisson’s ratio of 0.30, and density of7.86 g/cm3) for the flange. Fig. 4 illustrates a second-order bending mode, as in the mode analysis reported pre-viously [15].

3. Experimental Equipment and Conditions

To determine whether grinding phenomena could bedetected by observing the bending-vibration noise, weperformed a grinding experiment using a machine with aplane-grinding wheel (GS-45FL II, Kuroda Precision In-dustries). Fig. 5 illustrates the grinding experiment.

The bending-vibration noise was measured with amicrophone mounted 80 mm from the center of the wheeland 1 mm from the side. A dynamometer was mounted onthe machine table to simultaneously measure the grindingresistance force and the bending-vibration noise duringthe grinding work.

Table 1 details the grinding conditions.

Fig. 5. Schematic diagram of the grinding experiment.

Table 1. Grinding conditions.

WA120K7VGrinding wheel (205 mm × 13 mm × 31.75 mm)

(Wheel size) RZ46J8V(180 mm × 13 mm × 31.75 mm)

Wheel speed: Vg 20–25 m/sTable speed 12–18 m/minDepth of cut 2–5 μmWork piece STAVAX (HRC52), SKD11 (HRC58)

Work piece size 50 mm × 100 mm × 5 mmWet grinding

Grinding fluid Wet grinding (soluble type 1/20)3.6 L/min

Fig. 6. Sound pressure and grinding force on removal vol-ume (in the case of STAVAX: HRC52).

4. Experimental Results

Figure 6 shows the relation between the removal vol-ume and the bending-vibration noise (sound pressure),or the normal and tangential grinding resistance forces,for the wet-type plate grinding of STAVAX steel. TheSTAVAX plate was thermally processed, with a hardnessof HRC52. The experiment was performed twice and thereproducibility was confirmed. Since the removal volumeis given per unit width of the grinding wheel in Fig. 5, theunit of the total removal volume is mm3/mm.



(a) Removal volume 400 mm3/mm (b) Removal volume 450 mm3/mm (c) Removal volume 500 mm3/mm (d) Removal volume 550 mm3/mm

Fig. 7. Workpiece surface at change point of sound pressure.

Figure 6 also shows that, as the removal volume in-creases, the bending-vibration noise and the normal andtangential grinding forces increase. However, when thetotal removal volume is 450 mm3/mm, the bending-vibration noise and the normal grinding force suddenlydecrease and the tangential grinding force also decreasesslightly. When the total removal volume is 500 mm3/mm,the bending-vibration noise and normal and tangentialgrinding forces start increasing again. After the totalremoval volume exceeds 550 mm3/mm, the bending-vibration noise and the normal and tangential grindingforces increase and decrease repeatedly.

This result indicates that the bending-vibration noisedoes not have a clear correspondence with the tangentialgrinding resistance force, but has a good correspondencewith the normal grinding resistance force. With con-ventional technologies, it is difficult to measure the nor-mal grinding resistance force during plate-grinding work.However, with the proposed technique, the force could bedetermined by observing the bending-vibration noise.

With a total removal volume of 550 mm3/mm or higher,the sound pressure of the bending-vibration noise remainslow. Since there is a correlation between the sound pres-sure of the bending-vibration noise and the normal grind-ing force, the sound pressure should return to the levelwhere the total removal volume is less than or equal to450 mm3/mm. This behavior could be caused by unsta-ble grinding, which causes grinding burning, and couldbe attributed to a change in the grinding-wheel excitation(double hitting) or a change in the vibration mode. An ad-ditional experiment would be needed for a detailed analy-sis.

Figure 7 shows photographs of the workpiece with thetotal removal volume at around 450 mm3/mm, where thebending-vibration noise and the normal grinding resis-tance force suddenly changed. Fig. 7 indicates that theworkpiece surface is nicely finished with a total removalvolume of 400 mm3/mm, while the grinding burning oc-curs on the surface when the total removal volume is450 mm3/mm. Significant grinding burning also occurswhen the total removal volume is 500 mm3/mm, and dis-appears when it is 550 mm3/mm.

From this result, one can conclude that a change in thebending-vibration noise has a clear correspondence withthe occurrence of grinding burning. Hence, observing thebending-vibration noise could be used to determine the

Fig. 8. Sound pressure and grinding force on removal vol-ume (in the case of SKD11: HRC58).

grinding condition.Figure 8 shows the relation between the removal vol-

ume and the bending-vibration noise for the wet-typeplate grinding of SKD11 steel. The SKD11 plate usedin the experiment was thermally processed and can beused as mold material; its hardness is HRC58. The exper-iment was performed twice and the reproducibility wasconfirmed.

In the experiment, we used a grinding wheel(RZ46J8V) suitable for the workpiece material. Itwas confirmed that the specific frequency at which thebending-vibration noise increases depended on macro-scopic factors, e.g., the flange shape and the grindingwheel’s outer diameter and thickness, but not microscopicfactors, e.g., the type, size, and concentration of the abra-sive grains and the type of bonding agent. Namely, achange in the grinding-wheel specifications does not af-fect the relation between the bending-vibration noise andthe grinding force.

In the experiments, we measured the sound pressurefor the specific frequency of 6.9 kHz; however, the spe-cific frequency of 5.8 kHz was used for the observation inFig. 7. This is because of the difference in the outer di-ameter of the grinding wheel and, hence, the natural fre-quency.

Figure 8 shows that the bending-vibration noise growsin proportion to the total removal volume. The bending-vibration noise increases suddenly when the total removalvolume is 600 mm3/mm and decreases suddenly when it


Sawa, T.

(a) Removal volume 550 mm3/mm (b) Removal volume 600 mm3/mm (c) Removal volume 650 mm3/mm (d) Removal volume 700 mm3/mm

Fig. 9. Workpiece surface at change point of sound pressure.

is 650 mm3/mm.Figure 9 shows photographs of the workpiece with the

total removal volume at around 600 mm3/mm, where thebending-vibration noise suddenly changed. One can seefrom Fig. 9 that the workpiece surface is nicely finishedwhen the total removal volume is 550 mm3/mm, while theslight grinding burning occurs on the surface when thetotal removal volume is 600 mm3/mm. In addition, thegrinding burning decreases when the total removal vol-ume is 650 mm3/mm and disappears when the total re-moval volume is 700 mm3/mm.

The sudden increase of the bending-vibration noise inthe experiment could be due to the crushing and cloggingof the abrasive grains. In other words, when the total re-moval volume exceeds around 600 mm3/mm, the productlife, measured on the basis of the crushing and clogging,reaches the limit. Then, the abrasive grains are repro-duced by spontaneous edge sharpening for further grind-ing. Therefore, with the total removal volume larger than650 mm3/mm, the bending-vibration noise fluctuates andthe grinding burning occurs repeatedly.

5. Conclusions

In this study, for the practical use of automated grind-ing technology based on our developed observation of thebending-vibration noise of a grinding wheel, we appliedthe technique to the grinding of a mold-material plate. Wedrew the following conclusions from the results.

(1) An impact perpendicular to the grinding surface ofthe grinding wheel caused a bending vibration inthe wheel and produced a vibration noise. Thus,the vibration noise from the bending vibration couldbe used to measure the normal grinding resistanceforce.

(2) The bending-vibration noise was not significantly in-fluenced by the spindle of the grinding machine orthe rigidity of the spindle head.

(3) Although a future detailed study is necessary, thebending-vibration noise had a certain relation to thenormal grinding force and could be measured insteadof the force. Our experiments at least indicated thatthe bending-vibration noise could be used to recog-

nize a grinding phenomenon, as the normal grindingforce can be.

(4) Our observation experiments showed that thebending-vibration noise could correspond to achange in the grinding situation (in our case, grind-ing burning).

AcknowledgementsWe would like to thank Kuroda Precision Industries for providingthe grinding machine, grinding wheels, and workpieces.

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Name:Takekazu Sawa

Affiliation:Associate Professor, Shibaura Institute of Tech-nology

Address:3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, JapanBrief Biographical History:2010- Associate Professor, Tokyo Denki University2013- Associate Professor, Shibaura Institute of TechnologyMain Works:• Mechanical processingMembership in Academic Societies:• Japan Society for Precision Engineering (JSPE)• Japan Society for Abrasive Technology (JSAT)


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automating the mold-material grinding process

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