Grinding parameters
Usage parameters of grinding wheels for grinding machines depend on many factors, such as the type of machine, dressing, operating conditions, the finish required and so on. Here we offer a rapid summary of the most common usage variables and parameters and how the behaviour of the grinding wheel is affected either by changing them or by particular operating conditions.
TYPE OF MACHINE
If the grinding machine is rigid, harder grinding wheels should be fitted than on less rigid machines or with a spindle that has play. The more rigid the machine, the more the grinding wheel tends to lose its grains.PERIPHERICAL SPEED OF GRINDING WHEEL
When the rotation speed decreases, the grinding wheel behaves as if it were softer, thus improving its grinding capacity. As a general guide, with every 5-8 metre/second reduction in peripheral speed the grinding
wheel loses approximately 1 degree of hardness. In contrast, if we need the grinding wheel to retain its profile or to produce a better finish (i.e. behave as if it were harder) the peripheral speed should be increased. Within reason, however: a grinding wheel that turns too slowly tends to lose its grains before the abrasive granules have the chance to work. Conversely, when increasing the speed, the limit is the maximum allowable speed indicated on the label of the grinding wheel, which absolutely must not be exceeded to avoid the risk of the tool exploding.Normally the recommended peripheral speed is slightly lower than the maximum indicated on the grinding wheel.
SPEED OF WORKPIECE
When the speed of the workpiece is reduced, each granule has to remove a larger quantity of material and is therefore subjected to greater pressure, meaning that the grinding wheel self-dresses more easily, and so behaving as if it were softer. The desired effect is not always achieved, however. Often on grinding wheels of medium-to-high hardness the greater stress to which they are subjected cancels out the self-dressing action of this expedient. As a rule, the speed of the workpiece can range between 10 and 20 metres per minute. For surfaces grinding this value corresponds to the table speed.FEED RATE
The feed rate may vary between 1/3 and 1/4 of the thickness of the grinding wheel for each turn of the workpiece. For tangenziale grinding the same concept applies to
the lateral excursion of the table.
DEPTH OF CUT PER PASS
The depth of cut depends of the grade of finish desired and the grain size of the grinding wheel used, as well as on the type of machining process.There are no set parameters.
SURFACE CONTACT AREA
The surface contact area between grinding wheel and workpiece is very important. The smaller it is, the more the grinding wheel cuts and does not become clogged. If greater grinding capacity is required, reducing the surface contact area is strongly recommended, especially when grinding using vertical-axis wheels, for example cup or segmented wheels. In this case simply executing a chamfer on the edge of the cup wheel of reducing the number of segments fitted on the head of the grinding machine solves a lot of problems.The following table summarises the effect of changing operating parameters on the behaviour of the grinding wheel.
INFLUENCE OF OPERATING PARAMETERS ON GRINDING WHEEL BEHAVIOUR
| PARAMETERS | HARDNESS | CAP.ASP. | FINISH | |
| peripheral speed of wheel | > < |
> < |
< > |
> < |
| feed rate | > < |
< > |
> < |
< > |
| depth of cut per pass | > < |
< < |
> < |
< > |
It is clear that by varying the operating parameters appropriately it is possible to use the same grinding wheel for more than one kind of machining process.
Of course, when large numbers of pieces need to be produced it is always advisable to use a dedicated grinding wheel.