Hollow Corundum Microspheres in Diamond and CBN Tools

The use of hollow corundum microspheres (hereinafter, HCM) of a specified size in diamond tools enables us to deal with
several tasks at a time, i.e. to:

Mechanical strength, sphericity, and size of HCM

The spherical shape and relatively high strength of corundum microspheres (40-120 MPa) ensures their wonderful mobility in
the process of moulding in a hydraulic press, and the pressing force reduces 1.5 to 2 times due to the reduction of internal friction
of ceramic mixture components when redistributing and binding in the mould.
The extended surface and size of hollow corundum microspheres (5 to 125 microns) enables us to achieve unique cutting
properties for artificial porosity diamond tools. Together with bonding (composition of the bonding: copper: 30%, tin: 20%, and
formaldehyde resin: 50%), the mixture which contains diamond ACP 100/80 grain (25%) and hollow corundum microspheres
sized 70 to 100 microns (15%) enables us to achieve reduced specific consumption of diamonds from 1.75 mg/g to 0.73 mg/g and
burrs on the processed surface from 0.5 to 0.12 microns (Patent 1355470).

Volumetric deformation and coefficient of thermal expansion of HCM

When diamond tools with HCM are baked, their volumetric deformation reduces. This happens due to the low coefficient of thermal expansion of corundum microspheres when they
are heated or cooled. The reduced volumetric deformation of the final diamond segment enables us to reduce the amount of raw mixture which is taken a little more than enough
for its further machining to the size provided in the drawing. In the case of extensive manufacturing of diamond tools, the annual savings of raw mixture will amount to tons.

Effect of self-sharpening HCM

A hollow corundum microsphere has an internal closed cavity of a certain size (the size of this cavity depends on the size of the
microsphere and thickness of its walls). When added to the mixture containing diamond grains, HCM finds itself among sharp
faceted diamond particles and creates a pore around itself. The diamond is much stronger than HCM. Therefore, metal is mainly
cut by diamond grain, while the opened microsphere "mildly" trims small burrs. Protruding particles of the microsphere disintegrate
under mechanical stress forming new sharp edges. Thus, the self-sharpening effect appears.
Due to this self-sharpening effect, the surface of the processed part obtains special characteristics as follows:
  1. less burrs once processed by the diamond tool;
  2. less smeared because the cavity of corundum microspheres and neighbouring pores are flushed with coolant stream and self-sharpened; and
  3. as a result of better cooling of diamond tools, overheating of the diamond tools and processed material reduces, which enables us to reduce the amount of defective articles (e.g. burns on vital parts).

Chemical stability of HCM

HCM are chemically stable because they are made from aluminium oxide and are stable to chemically aggressive additives (bonding) in diamond tools.

High baking temperature and environmental friendliness of HCM in use

In the process of high-temperature processing – annealing and baking – of abrasive tools with HCM additive, no harmful substances evolve in the environment, and the microsphere
does not disintegrate due to its spherical shape and high softening point (1600-1800°C).