![High Speed Traverse Grinding of Shaft [GB23]](ris 10.jpg "Fig.1. High Speed Traverse Grinding of Shaft [GB23]")
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Abstract. The definition of high speed in the context of this report encompasses: high tool peripheral speed; high work-speed; and high material removal rate.
Introduction
High Wheel Speed [B12, CZ4, GB1, GB2, G5, G42, G48, G49, G50, J9, J23, R1, R2, S2, SL1, U16, U27, A10, A11, GB32, GB34, G77, J41, J42, J49, U47, CA5] High wheel (or belt) speed grinding requires a suitable machine tool, with appropriate spindle, grinding tool, subsystems and stiffness. Several high-speed grinders have been built since early generic explorations by WZL and Bremen. Guhring capitalized on the technology 20 years ago, with Landis, Giustina/RDC, Edgetek, Elb, Blohm, Tacchella, and others following suit. More recently, high speed grinding is being researched in universities with more specific commercial and environmental benefits in mind, especially those funded by the EC, which require end-users, OEMs and Universities to work together in such projects. Figure 23 shows wheel technology versus wheel speed.
Krajnik describes the recent funding received to develop a high speed cylindrical grinder
in Slovenia [SL1]. The machine will use Russian made wheels with high porosity [R1], with other European partners involved. A wheelspeed of 150 m/s has been discussed.
Also in Eastern Europe, the Czech Republic are investigating HEDG using experimental
and simulation methods (HEDG will be discussed in more detail in the following section).
Kuriyagawa [J41] developed an ultra-high speed grinding (UHSG) machine which is
capable of 400m/s wheel speed. It has reached the stage where it has demonstrated high
machining efficiency, even in comparison with a comparative cutting method. In
previous work, an ultra-high speed spindle unit (30,000 rpm, 22kW, 2*106
d.n-value)
was developed. In this latest work, a UHSG machine using the spindle unit was designed
and manufactured. Vibration, noise and fluid friction loss of the machine were measured
and compared with conventional grinding results. The UHSG tests were performed with
300m/s wheel speed and a vitrified bonded CBN wheel. The CBN wheel having a CFRP
core was specially designed. However, fluid supply was not enough in the grinding zone
of the UHSG, therefore the development of a new cooling and lubricating method is
needed.
The use of high wheel speeds for silicon nitride ceramic grinding was investigated by Kovac, up to 33,000 sfpm (176 m/s) [1], in the early 90’s. Huang [A1] is doing a similar study on zirconia, alumina and alumina-titania materials, using depths of cut of up to 0.08” (2mm). He found that the removal mechanism for the two alumina-based materials was dominated by grain dislodgement and lateral cracking along the grain boundaries, as compared to zirconia grinding, which was mainly by local micro-fracture and ductile cutting.
The UltraFlex [G42, GB24] project combines high speed grinding with MQL, to produce CVT gearbox shafts in a more efficient process chain. The approach uses high material removal rate peel grinding, with fluid flowrates less than 0.01 GPH. To achieve this level of performance, the entire machine and process has to be designed from the ground up. Also at WZL, they are exploring wheel speeds of 30,000sfpm (150m/s) in centerless grinding, again with MQL [G49]. With such high wheelspeeds the danger of spinners will be much greater than with conventional wheel speeds (a spinner occurs when the workpiece and regulating wheel loose traction and the work spins up to the peripheral speed of the wheel). An advantage of using MQL with the process, is the increased friction between the work and regulating wheel, as compared to flood or high velocity jets.
Using high wheels speeds to grind thermally sensitive TiAl intermetallic, is a recent approach by Uhlmann. The study showed CBN to be preferable to corundum, with the coolant setup critical. Much prior research has been carried out at more modest wheel speeds. Although superabrasives are dominant in high-speed grinding, conventional abrasives have also been used. Webster [2] reports specific material removal rates of 38 in3 /min.in (375mm3 /s.mm) on inconel 718, using a wheelspeed of 28,000sfpm (140m/s) Figure 24 shows the higher wear associated with CBN at such high rates. The grinding wheel used comprised of segments of seeded sol-gel alumina fibers bonded to a metal core, as shown in Figure 20 previously. Along the same lines, Klocke [G50] has used wheel speeds up to 33,000 sfpm (180 m/s) with grinding wheels containing micro-crystalline seeded sol-gel alumina, with specific material removal rates of 10 in3 /min.in (100mm3 /s.mm), on 52100 bearing steel.
High removal rate grinding is a relative term, since the absolute rates applicable to silicon wafer grinding are orders of magnitude lower than cast iron grinding. For a process to be called high removal rate it must be much higher than common practices, i.e. class leading. With this in mind, Zhang [C12] is using the ELID technology to HEDG ceramics. Although no removal rates are quoted in his one-page summary, it is unlikely that the performance is as high as in steel grinding, since he is focusing on ultra-precision grinding. Zhu [U47] also applies ELID at high wheel-speed and refers to the method as HELID. Zhu quotes a wheel-speed of only 6000sfpm (30 m/s) in his one-page summary, 48which is 50% higher than typical with ELID, since he has overcome some of the electrolyte turbulence issues between the electrode and the wheel.
References