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 anddiamond coated honing tool lives can be increased up to 10 to 20times, while grinding performance can be increased up to fourtimes. It is no surprise that the tool grinding time and costs -and quality of finish - is directly dependable on the quality ofthe grinding wheel. Distribution of CBN or diamond grains on awheel surface is random - which influences grindingcharacteristics.

Dr Gregor Burkhard of the Institute of MachineTools and Manufacturing, the Swiss Federal Institute ofTechnology, Zurich, has developed a precise grain-placing methodbased on medical practices.

The result is significant shorteningof tool grinding time and grinding wheel life.

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

With random spacingof CBN or diamond grains, the spaces in between the grains canclog, resulting in 'burning' of the tool or workpiecesurfaces, said Dr Burkhard.

Usually CBN and diamond coatedgrinding wheels consist of a steel or aluminium wheel body thecircumferential work surface of which is electroplated withnickel or nickel alloy to secure the abrasive grains.

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

The adhesive holds them in place duringthe electroplating process.

'The grains are randomlyarranged,' said Dr Burkhard, 'which means that thedistance between the individual grains - which serves totransport cooling lubricant into, and chips out of, the machiningzone, is also random.

This represents a major disadvantage ofsuch tools, as individual spaces can easily become clogged duringmachining, which then eventually leads to abrasive burning on thetool and, therefore, to spoilage.' Another point made by DrBurkhard was that with existing grinding wheel electro-platingmethods, the abrasive grains are only 'mechanicallyclamped' in place and grain projection is only 40 - 50% ofthe mean grain diameter.

With the new grain dispensing method, itis possible to determine the size and location of the vacantspaces.

Then, if an active brazing alloy bond is used, grainprojection can amount to 80-100% of the mean grain diameter.'This permits good supply of the cooling lubricant into, andgood removal of the chips from, the machining zone, ' saidDr Burkhard, 'thus enabling higher metal removal rates andcutting times.' Dr Burkhard's team has adapted the useof micro-dosing from biomedical engineering processes.

Normallyused for precision pipetting purposes, micro-dosing producesuniform drops with a maximum diameter of about 40 microns at anemission frequency of up to 2000Hz.

Such a dosing head consistsof a glass capillary tube connected to a reservoir via a feedpipe.

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

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

A heater is alsoincluded to set the optimum viscosity of the fluid.

So DrBurkhard's team built a prototype device to sequentiallydose the work surface of a grinding wheel body with liquidadhesive drops in specific patterns and distribution.

The wheelis then rolled through a thin layer of abrasive grains, the sizeof the drop determining whether a single, or clusters of, a CBNor diamond grain are picked up.

Brazing paste is then applied tothe wheel work surface.

In practice, the prototype'micro-dosed' structures were applied to mandrel-typehoning tools (or honing sleeves) to hone the interrupted bore ofa hardened gear wheel to the finished size of 12F7 diameter.Before, the process was carried out in three stages usingelectroplated honing sleeves with settings of 0.04, 0.04 and acalibrating pass of 0.01mm (on diameter).

The mean service lifeof the electroplated tools was 5000 strokes at the 0.04mmsetting.

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

The micro-dosedand brazed honing sleeve lasted for 6000 bores.

Yet themicro-dosed tools had not yet been fully optimised in terms ofgrain size and grain spacing! Further development resulted in themicro-dosed honing sleeves producing a service life of 10 to 20times greater than the traditional electro-plated honing sleeves.Development went on to produce internal grinding CBN and diamondcoated pinwheels and a disc-type grinding wheel.

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

'This is four times the valuefor conventional tools,' said Dr Burkhard.

Bear in mind thatthe increased metal removal rate was achieved without optimisingcoolant to suit the new wheel.