Improved Cutting Performance of Diamond Beads by Means of Innovative Shape
Improved Cutting Performance of Diamond Beads by
Means of Innovative Shape
L. Risso, B. Vicenzi
MIMITALIA srl, Via alla Costa 24, 17047 Vado Ligure, SV
Diamond 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.
The 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 . 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 ,
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 technology
A totally new system based on Metal Injection Moulding (MIM) technology  has been
developed and patented , and a trade mark has been deposited .
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 .
After debinding the component is ready to be processed in a conventional sintering
The use of a metal powder containing a consistent fraction of diamond grits has not
modified this general flow-chart.
3. Fabrication of the beads
A 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
Table 1. Feedstock composition
Cobalt UF WC Binder
Weight % 77.2 9.0 2.6 11.2
Table 2. Binder composition
Wax 1 Wax 2 Polyethylene Stearic acid
Weight % 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.
The 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.
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
B ead D iam eter, mm
Feed Rate, m/h
0 200 400 600 800 1000 1200 1400
S quare m eters of cut
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 discussion
The 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.
Cutting pressure (A.U.)
New Geometry 10
11.5 11 10.5 10 9.5 9 8.5 8 7.5
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
C=(Co+Sm+Cm)Tt + Ctnt + (Co+Sm)Ttcnt (1)
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) :
VF x D y T k = constant (2)
where x, y, and k are other constants. So, if all other variables are kept constant, the
equations reduces to
FT n = constant (3)
there is a relationship between nt, F, and Tt:
nt = ∝ Tt F ∝ F n
Now, under normal cutting conditions > 1 , so nt increases with increasing F, and thus the
second and the third term of (1) increase with the reduction of Tt. 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.
Cost per part, A.U.
0 10 20 30 40 50 60 70 80 90 100
Feed rate, A.U.
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 Bulge
Total surface of one cut (m2) 5000 5000
Number of wires for one cut 4 5
Time necessary for one cut (h) 1666 1351
Average feed rate (m/h) 1 1.23
Relative 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.
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.
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L. 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.