Improved Cutting Performance of Diamond Beads by Means of Innovative Shape

L. Risso, B. VicenziMIMITALIA srl, Via alla Costa 24, 17047 Vado Ligure, SV

S. BernieriDiamond Pauber srl, Via Aprilia 5, 54100 Massa, MS

The geometry of the profile of a diamond bead can strongly influence the cutting performance of a wire if operating in a controlled environment. The use of Diamond Injection Moulding (DIM) technology allowed to produce a bead and consequently a diamond wire with a double bulge geometry. A comparison with a wire made with conventional beads of the same composition in terms of diamond, metal matrix and sintering conditions, has been made in the same operating conditions. The resulting cutting speed and duration of the tool have been compared.

1. BackgroundThe fabrication of the beads for the assembling of a diamond wire for stone cutting is

commonly performed by means of hot pressing, both uniaxially and isostatically [1-2], but more and more often by means of the press-and-sinter route [3]. In the former processes the densification of the powders is aided by a pressure applied by a graphite ram or by a inert gas; on the contrary, in the press-and-sinter process densification occurs at low pressure (or vacuum) only because of the effect of temperature. In all cases a preforming of the green bead by means of uniaxial cold pressing is required. In the last years, the pressing of the green bead has very much improved because of the increasing popularity of granulating techniques [4], that determined a real benefit in the duration of the mould, the productivity, and homogeneity of the metallurgical structure of the bead. The limit of uniaxial pressing lays in the impossibility of moulding shapes more complicate than a simple cylinder. In the present work we describe how it is possible to overtake this limitation and obtain intricate geometries for a diamond bead. A new diamond bead with a double bulge profile as been fabricated and tried on a full scale test.

2. Description of DIM technologyA totally new system based on Metal Injection Moulding (MIM) technology [5] has been

developed and patented [6], and a trade mark has been deposited [7].With the MIM process a metal powder is intimately mixed with a polymer mixture, in an

amount suitable to make the mixture (the so called feedstock) able to flow above a certain temperature, and fill a mould with a common press for plastic injection moulding. In figure 1 a commercial feedstock is shown in the usual granulated form.

Fig 1. Metal powder and polymer mixed in a commercial feedstock

Usually the moulding takes place as shown in figure 2. A rotating screw moves forward the feedstock coming from the hopper loading the material in front of the screw for the injection. During this transfer the material gradually melts. When the space between the screw and the nozzle is filled, the screw moves ahead as a ram injecting the melted feedstock into a steel mould where a cavity reproduces the shape to fabricate. The mould is usually divided in two halves and its temperature is maintained stable at a value well below the feedstock melting point. Therefore after injection the material solidifies into the mould, that can be opened to eject the solid green part.

Fig 2. Description of the injection moulding stage

Fig 3. Debinding of the green parts

The polymer (binder) necessary to allow the powders to flow, must be extracted from the parts before sintering. In fact a sintering furnace is not designed to process a high amount of organic material. The extraction of the binder is named debinding, and can be performed in

special furnaces (thermal debinding, see figure 3), with the aid of a solvent (acetone, exane, water), or exploiting the catalytic effect of nitric acid vapours [8].

After debinding the component is ready to be processed in a conventional sintering furnace.

The use of a metal powder containing a consistent fraction of diamond grits has not modified this general flow-chart.

3. Fabrication of the beadsA feedstock with the composition shown in table 1 and table2 has been prepared. The

mixture has been processed in a Battaggion Z-blade mixer at the temperature of 140 °C for 2 hours.

Table 1. Feedstock composition

Cobalt UF WCDiamond 40/50

meshBinder

Weight % 77.2 9.0 2.6 11.2

Table 2. Binder composition

Wax 1 Wax 2 Polyethylene Stearic acidWeight % 10 69 20 1

The feedstock has been moulded in a modified commercial injection moulding machine at a temperature of 120 °C and at a hydraulic pressure of 50 Bar. A four-cavities mould reproducing a double bulge bead has been fabricated.

Fig 4. As moulded bead

In figure 4 is shown the bead in the green stage immediately after moulding. The debinding has been done via a thermal route, in a special oven at the maximum

temperature of 300 °C and under a nitrogen flow.Sintering as been performed in hydrogen at the temperature of 940 °C for 120 minutes. A

density 8.5 g/cm3 (96% theoretical) has been reached. In figure 5 some beads are shown after sintering around the steel support.

Fig 5. Double bulge beads after sintering

After sintering the beads have been brazed onto the steel supports and assembled on a 19.6 meters long wire, with a linear density of 30 beads/meter.

4. ExperimentalThe experimental wire has been used to cut marble blocks (see figure 6) until the beads

were completely worn. One half of the same block was cut with a conventional wire with the same linear density of beads of standard cylindrical geometry. The dimension of the blocks were approximately 2.5x2x2 m.

Fig 6. Marble slab under experimental cutting

The diameter of the beads, initially of 11 mm, has been progressively measured after each cut. The linear speed of the wire was kept constant at 30 m/sec. In the cutting machine it was also possible to maintain the absorbed electrical power at the constant level of 11 KW, by varying the feed rate F.

F

In these experimental conditions, the traditional wire began cutting with a feed rate of 0.7 m/h and was completely worn when the feed rate was 1.2 m/h. The double bulge bead started at the much higher feed rate of 1.65 m/h and reduced progressively its speed to the final value of 1.2 m/h. The traditional wire was able to produce 1300 m2 of marble cut surface, whilst the double bulge one stopped at 1000 m2.

Fig 7. Double bulge bead after some cuts

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0 200 400 600 800 1000 1200 1400

Square meters of cut

Bea

d D

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2.20

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Rat

e, m

/h

Diameter, Traditional Diameter, Double Bulge Feed Rate, Traditional Feed Rate, Double Bulge

Fig 8. Experimental behaviour of the wires

In figure 8 it is possible to see the plot of the bead diameter and of the feed rate F of the experimental wires versus the marble cut surface.

5. Results and discussionThe improved cutting speed of the double bulge bead can be explained with the different

contact pressure at the interface with the stone. In fact the new geometry allows the pressure to be applied only at the top of the bulges. As soon as the bulges are worn, the pressure decreases because of the progressive flattening of the contact surface, as can be easily seen in figure 7. The theoretical behaviour is shown in figure 9, where the contact pressure of the new bead is compared with the traditional one. With the traditional bead the contact pressure progressively increases as the diameter of the bead is reduced by the wear. As far as the double bulge geometry is concerned, in the initial stage the cutting pressure decreases because of the flattening of the bulge top, and then increases as soon as the bulges are completely worn, and the geometry becomes the same as the traditional bead.

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7.588.599.51010.51111.5

Bead diameter

Cu

ttin

g p

ress

ure

(A

.U.)

Traditional BeadNew Geometry

Fig 9. Cutting pressure on the beads vs. bead diameter

It is interesting to notice that the theoretical behaviour of the cutting pressure plotted in figure 9 is in accordance with the experimental feed rate as shown in figure 8.

The improved cutting feed rate is very important in the calculation of the cutting costs. In fact, it can be easily shown that in any cutting operation the costs follow the general expression [9]:

C=(Co+Sm+Cm)Tt + Ctnt + (Co+Sm)Ttcnt (1)Where:

Co Cost of labour (€/min)Tt Cutting time for one cut (min)Ttc Down time for substituting the wire (min)Sm Depreciation cost of the equipment (€/min)Cm Operating cost of the equipment (€/min)Ct Cost of the wire (€)nt Number of wires necessary for one cut

The first term of (1) decreases proportionally to the reduction of cutting time T t; nevertheless, the cutting time decreases when the feed rate F increases and thus this is usually causing a faster wear of the bead.

For simple tools there is a relationship between tool (wire) lifetime T, cutting speed V, feed F and depth of cut D (extended Taylor's relationship) [10]:

constant (2)

where x, y, and k are other constants. So, if all other variables are kept constant, the equations reduces to

constant (3)

there is a relationship between nt, F, and Tt:

(4)

Now, under normal cutting conditions , so nt increases with increasing F, and thus the

second and the third term of (1) increase with the reduction of T t. This is clearly shown in figure 10, where it is possible to see that a optimal cutting speed can be found which minimizes the total costs.

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1.5

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0 10 20 30 40 50 60 70 80 90 100

Feed rate, A.U.

Cos

t pe

r pa

rt, A

.U.

Machine cost

Tool cost

Total cost

Fig 8. Behaviour of the cutting costs in a general cutting operation

We did not do a complete Taylor analysis testing several wires at different feed rate to find the most economical cutting parameters, but by means of (1) we compared the cost coming from the use of the traditional and innovative wire as shown in table 3.

Table 3. Cost comparison

Conventional Double BulgeTotal surface of one cut (m2) 5000 5000Number of wires for one cut 4 5Time necessary for one cut (h) 1666 1351Average feed rate (m/h) 1 1.23Relative cost of the cut 1 0.84

A cost saving higher than 15% is possible in spite of the lower marble cut surface produced by a single wire with double bulge beads, simply because of the higher average cutting speed.

6. Conclusions A new process based on Diamond Injection Moulding has been developed and

patented. The process allows to produce tools with intricate geometries not possible to make so far.

With the new process a innovative diamond bead has been produced and tested in a full scale plant. The new double bulge geometry allows the wire to cut with a higher speed, that determines a cost saving higher than 15% in spite of the slightly reduced wire lifetime.

References[1] J. Konstanty, A.Bunsch. Hot Pressing of Cobalt Powders. Powder Metallurgy 1991 No

3, p.195[2] N. Hung. Cumulative Creep and Hot Isostatic Pressing of Particle-Reinforced Metal

Matrix Composites. Journal of Materials Processing Technology 2000, Volume 101, Issue 1-3, p.104

[3] M. Del Villar, P. Muro, J. M. Sanchez, I. Iturriza, F. Castro. Consolidation of diamond tools using Cu-Co-Fe based alloys as metallic binders. Powder metallurgy 2001 No1, p.82

[4] G. Weber. Granulating: A New Process for Diamond Tool Producers. Euro PM International Workshop on Diamond Tool Production Proceedings, Turin, 1999, p.73

[5] R. M. German. Powder Injection Moulding. Metal Powder Industries Federation eds, Princeton, 1990

[6] L. Risso. Metodo di Produzione di Elementi Diamantati. Italian Patent 2006 TO2004A000328

[7] EU Trade Mark DIM. Deposit Number 5455613. Classes 6, 8, 40. November 2006[8] M. Blomacher. Acetyl Based Feedstock for Injection Moulding and Catalytic

Debinding. Metal Powder Report 1998, No1, Volume 53, p. 40[9] R. Chiara, G. E. D’Errico, F. Rabezzana. Development and Applications of PVD

Processes to Cutting Tools. Surface Engineering, Datta and Gray eds., Cambridge, 1993, p. 245

[10] F. W. Taylor. On the Art of Cutting Metals. Transaction of American Society of Mechanical Engineers 1907 No 28, p.31

Personal InformationL. Risso is currently working at MIMITALIA srl, as quality manager. He has covered this

position since 2002, when he founded the company together with other four partners. He was previously working at Centro Sviluppo Materiali S.p.A. where he managed the Technical Ceramics and the Industrial Diamond laboratories. He holds a degree in Physics (1988) and a

specialization degree in Materials Science and Technology (1991), both from the University of Genova, Italy.