Tyrolit finds best parameters for dressing grinding wheels

Citation

(2007), "Tyrolit finds best parameters for dressing grinding wheels", Aircraft Engineering and Aerospace Technology, Vol. 79 No. 3. https://doi.org/10.1108/aeat.2007.12779caf.002

Tyrolit finds best parameters for dressing grinding wheels

Tyrolit finds best parameters for dressing grinding wheels

For optimum results, a diamond dressing roll must rotate in the same direction as the grinding wheel at the point of contact, and at four-fifths its peripheral speed; and dwell revolutions must be zero.

Research carried out by Austrian grinding wheel manufacturer, Tyrolit, has identified the best parameters to use when dressing a vitrified aluminium oxide wheel using a rotating profile diamond roll. The theoretical work was tested by dressing a wheel prior to creep feed grinding of nickel alloy, a process widely used in the aerospace manufacturing industry.

The detail of the research is rather weighty, but boils down to an appreciation of how four process variables interact – the relative direction and the peripheral speed differential between rotation of wheel and dresser at the point of contact, the radial infeed speed of the dresser, and the dwell or sparking-out revolutions.

First consideration is relative direction of rotation. If one follows the trajectory of any single grain of diamond on the rotary dresser, it is easily demonstrated that if the wheel and dresser are moving in opposite directions at the point of contract, denoted by (2), the grain's trajectory into the wheel is low, touching almost tangentially, so the cutting action is soft. With same-direction (+) rotation, the diamond grain hacks into the wheel, generating a much rougher surface.

Key findings are presented in Figure 1. It shows that regardless of dresser infeed speed, the greatest theoretical roughness of the grinding wheel surface after dressing, and hence maximum grinding efficiency, is achieved when the peripheral speed of the dresser and wheel are identical and they move in the same tangential direction.

Figure 1 Key findings from Tyrolit research

The reason for this is that the diamond grain does not cut the wheel surface, but instead causes brittle fracture of the abrasive grain and vitrified bond through pressure, resulting in rough surface topography of the wheel. Unfortunately, this wears down the profile roller very quickly, so the +1 parameter cannot be used in practice.

Again from the graph, it can be seen that there is a forbidden zone centered on 1:1 speed ratio and extending either side from +1.2 to +0.8, the latter signifying that the dresser is driven at four-fifths the peripheral speed of that of the grinding wheel. It is this parameter that has been identified as optimal, imparting between 6 and 10m of mean surface roughness onto the wheel. At the same time, +0.8 preserves the sharpness of the dressing roll, extending the periods between reworking.

The 4-m spread in surface roughness of the grinding wheel is a result of raising the dresser infeed speed from 0.2 to 0.7m/rev, showing that the higher the final roughness (and efficiency) of the grinding wheel, the greater the amount of stock removed from it by dressing and hence the shorter the life of the wheel. So there is a trade-off between wheel efficiency and consumption that has to be determined for each application.

Allowing the grinding wheel and roll dresser to rotate together without any infeed force for a number of revolutions after dressing, equivalent to sparking out after a grinding cycle, has the effect of diminishing the roughness of the wheel and decreasing grinding efficiency. The effect is most noticeable with opposite- direction tangential motion of the wheel and dresser, but this is not recommended anyway. With same-direction tangential motion, a 10-m surface roughness is halved in little more than 100 dwell revolutions. The recommended number is zero.

Tyrolit then applied the theoretical parameters to dress one of its aluminium oxide wheels, selecting a medium infeed speed. Same-direction motion was used with the dresser running at +0.8 of the grinding wheel speed. Opposite-direction motion with the same speed differential, i.e. 20.8, was also tried. Further tests were carried out using a speed differential of +0.4 and 20.4. Dwell revolutions were set to zero and then 80 during all tests except the last, which was obviously going to produce a poor result.

Rather than try to measure the surface topography of the grinding wheel after dressing, which is impossible in practice, the wheel was used in creep feed mode to machine a nickel alloy test piece on a Blohm Profimat 412 profile grinder. The parameters of the grinding operation were measured as well as the surface of the ground nickel.

The practical results closely matched the theory in some important respects – see accompanying illustration (bottom). They were inconclusive regarding the surface finish of the ground nickel, which showed little variation over all the tests. The actual spread of mean roughness depth (R_z) on the nickel was 1.6m compared with the predicted variation of grinding wheel finish of 4.0m. Nevertheless, the greatest practical R_z of 3.88m was found, as predicted, for the wheel dressed using the best theoretical parameters.

What was demonstrated beyond reasonable doubt was that the predicted optimum settings of +0.8 with zero dwell revolutions was in fact correct. The clue came from measuring the normal force of the grinding wheel during machining, as well as the power drawn by the motor.

Only 1.037W/160N was recorded for the wheel dressed using the theoretically optimum settings, confirming that free- cutting grinding using a sharp wheel was taking place. Power drawn and normal force were around 50 per cent higher if 80 dwell revolutions were introduced, and double if in addition the wheel and dresser were counter-rotated at 20.8. Halving the differential speed to +0.4 and returning dwell rotations to zero increased power consumption and wheel force by over 40 per cent compared with the optimum.

Tyrolit's UK subsidiary has since 2004 operated a diamond roll dresser production facility in Crawley, West Sussex, offering a complete range of reverse electroplated dressing products.

Details available from Tyrolit Limited, Tel: +44 (0)1788 824500; Fax: +44 (0)1788 823089; E-mail: iohn.willis@tvrolit.com; web site: www.tvrolit.com