Precision dosing ups CBN/diamond wheel life

Using a biomedical engineering micro-dosing technique, CBN and diamond coated honing tool lives can be increased up to 10 to 20 times, while grinding performance can be quadrupled.

Using a biomedical engineering micro-dosing technique, CBN and diamond coated honing tool lives can be increased up to 10 to 20 times, while grinding performance can be increased up to four times.

It is no surprise that the tool grinding time and costs - and quality of finish - is directly dependable on the quality of the grinding wheel.

Distribution of CBN or diamond grains on a wheel surface is random - which influences grinding characteristics.

Dr Gregor Burkhard of the Institute of Machine Tools and Manufacturing, the Swiss Federal Institute of Technology, Zurich, has developed a precise grain-placing method based on medical practices.

The result is significant shortening of tool grinding time and grinding wheel life.

Dr Burkhard presented his results at the Fritz Studer Grinding Symposium, held in Thun, Switzerland, earlier in 2002.

With random spacing of CBN or diamond grains, the spaces in between the grains can clog, resulting in 'burning' of the tool or workpiece surfaces, said Dr Burkhard.

Usually CBN and diamond coated grinding wheels consist of a steel or aluminium wheel body the circumferential work surface of which is electroplated with nickel or nickel alloy to secure the abrasive grains.

Usually, the surface is coated with an adhesive, and the wheel rolled in a layer of abrasive grains.

The adhesive holds them in place during the electroplating process.

"The grains are randomly arranged," said Dr Burkhard, "which means that the distance between the individual grains - which serves to transport cooling lubricant into, and chips out of, the machining zone, is also random.

This represents a major disadvantage of such tools, as individual spaces can easily become clogged during machining, which then eventually leads to abrasive burning on the tool and, therefore, to spoilage." Another point made by Dr Burkhard was that with existing grinding wheel electro-plating methods, the abrasive grains are only 'mechanically clamped' in place and grain projection is only 40 - 50% of the mean grain diameter.

With the new grain dispensing method, it is possible to determine the size and location of the vacant spaces.

Then, if an active brazing alloy bond is used, grain projection can amount to 80-100% of the mean grain diameter.

"This permits good supply of the cooling lubricant into, and good removal of the chips from, the machining zone, " said Dr Burkhard, "thus enabling higher metal removal rates and cutting times." Dr Burkhard's team has adapted the use of micro-dosing from biomedical engineering processes.

Normally used for precision pipetting purposes, micro-dosing produces uniform drops with a maximum diameter of about 40 microns at an emission frequency of up to 2000Hz.

Such a dosing head consists of a glass capillary tube connected to a reservoir via a feed pipe.

It forms a jet with its mid-section enclosed by a piezoactor.

The piezoactor contracts when a voltage pulse is applied, so causing a drop to be emitted.

A heater is also included to set the optimum viscosity of the fluid.

So Dr Burkhard's team built a prototype device to sequentially dose the work surface of a grinding wheel body with liquid adhesive drops in specific patterns and distribution.

The wheel is then rolled through a thin layer of abrasive grains, the size of the drop determining whether a single, or clusters of, a CBN or diamond grain are picked up.

Brazing paste is then applied to the wheel work surface.

In practice, the prototype 'micro-dosed' structures were applied to mandrel-type honing tools (or honing sleeves) to hone the interrupted bore of a hardened gear wheel to the finished size of 12F7 diameter.

Before, the process was carried out in three stages using electroplated honing sleeves with settings of 0.04, 0.04 and a calibrating pass of 0.01mm (on diameter).

The mean service life of the electroplated tools was 5000 strokes at the 0.04mm setting.

In tests, with setting at 0.08mm on diameter, life of the electroplated honing sleeve was 1000 bores.

The micro-dosed and brazed honing sleeve lasted for 6000 bores.

Yet the micro-dosed tools had not yet been fully optimised in terms of grain size and grain spacing! Further development resulted in the micro-dosed honing sleeves producing a service life of 10 to 20 times greater than the traditional electro-plated honing sleeves.

Development went on to produce internal grinding CBN and diamond coated pinwheels and a disc-type grinding wheel.

In tests with the latter, a wheel of 300mm diameter and 20mm wide was used in tests in hardened tool steel in a Maegerle surface grinder, producing a related time/machining volume of Qw = 20mm3/mms at a cutting speed of Vc = 40m/s.

"This is four times the value for conventional tools," said Dr Burkhard.

Bear in mind that the increased metal removal rate was achieved without optimising coolant to suit the new wheel.

Dr Burkhard is currently seeking new project partners.

Future work will involve studying grain patterns and densities to further increase metal removal rates and observe the effects on surface quality and surface texturing.

Dr Burkhard can be contacted through Fritz Studer in Thun.

(This was Manufacturingtalk's Top Story on 16 May 2002).

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