SECTION 7.5

General. History proves that grinding wheel breakage can result from many causes. However, for precision grinding wheels, one factor stands out. Careful analysis of many precision wheels broken during use shows that improper mounting is involved in more than 50% of all breakage's. For this reason, each of several methods of mounting precision wheels are individually discussed in the following sections.

In all cases, metal machine components must not press directly on abrasive surfaces. A blotter is to be used between the abrasive and any type of clamping flange or device. Similarly, when a spacer is used beside or between wheels, a blotter must be placed between each spacer and its mating abrasive surface. The spacer's clamping surface(s) must conform to the same geometric requirements that the safety code places on that machine's flanges. For example, for multi-screw flanged machines, any spacer used adjacent to a grinding wheel should be relieved near the bore, be the same OD as the flanges, and meet their flatness requirement.

To detect possible damage during in-plant storage, handling, or transport it is important that all wheels be thoroughly inspected immediately before mounting and at the mounting site. This means they should be inspected visually all over, despite any earlier visual inspections, and ring tested. Exceptions are segments, plate-mounted and nut-inserted wheels, mounted points, and some small (less than 4") wheels; these are difficult or impossible to ring test.

To ring test a wheel that is small enough to hold in one hand, the wheel should be suspended from the hole on a finger. If the wheel is too heavy, it can be supported by a hoist. If it is not possible to suspend the wheel freely in the air, it can be rung while in a vertical position on a clean hard floor. The wheel should be dry and free from sawdust or other packing material.

Tap the side of the wheel gently with a nonmetallic instrument (the handle of a screwdriver or a wooden hammer) about 45 degrees to one side of the vertical centerline and 1 or 2 inches inboard from the OD. Rotate the wheel about 45 degrees in its suspension and tap again. A good vitrified wheel will have a clear metallic ring. A resinoid wheel may not give a clear ring and must be visually inspected for cracks. Large, thick wheels may be ring tested by striking the wheel on the periphery rather than the wheel side.

If there is any reason whatsoever to suspect that a wheel has been damaged, do not mount it. But also do not use the ring test to "disprove" the suspected damage. The ring test should be used to detect unknown cracks but not to discount other evidence of damage--it is not infallible. When damage is suspected, the only safe options are to destroy the wheel beyond further use, or return it to its manufacturer for re-inspection.

Arbor Mounted. New, clean blotters should be used on both sides of the wheel. They assure even pressure on the flanges around the arbor hole. Be careful not to wedge the blotter within the wheel bore.

Flanges and spacers should be of equal diameter (inspect with a straightedge), flat, and free of nicks. Nicks and other protrusions should be carefully removed with a file and stone. The diameter of the flanges should be at least one-third the diameter of the wheel. See the safety code for exceptions, flange dimensions, and spindle diameters.

The wheel should slide easily onto the arbor. If it doesn't, do not force the wheel. Try to use a larger, wider, or narrower wheel than the size for which the machine was designed.

If the wheel has the word "TOP" on it, rotate the wheel so that "TOP" is at the 12 o'clock position when the clamping nut is tightened. This will keep the hole clearance on the bottom, as it was when the wheel was manufactured. This provides better concentricity and balance, and means less initial truing.

The nut should be uniformly tightened only enough to keep the wheel from slipping; do not use a hammer because over-tightening can warp the flanges, putting high bore stresses in the wheel. The nut should tighten in the opposite direction from wheel rotation!

Collet and Flange. Follow all general instructions (see Mounting Grinding Wheels-General and Mounting Grinding Wheels-Arbor Mounted) down to tightening the nut.

First, tighten the screws with slight finger pressure only. Then tighten to 15 ft. lbs. with a torque wrench. Follow a criss-cross sequence of tightening the screws. Repeat this tightening procedure at least three (3) times or more until all screws are pulled to the prescribed ft. lbs. figure. Wider wheels may require more; the torque recommended by the machine manufacturer should be followed if it is other than 15 ft. lbs.

Excessive tightening may spring the flange and collet. Warpage will reduce the contact area between the flanges and the wheel, and produce dangerously high bore stresses.

Additional precautions are necessary for mounting large wheels on multi-screw flanges. These are covered in the next section.

Large Wheels (Including Centerless). No large, heavy wheel should be mounted manually. The lifting effort required precludes accuracy of alignment and smooth mating of the wheel bore and spindle. Both of these are essential to maintain the safety of the wheel. The initial contact between the spindle and the bore is likely to be jerky and can cause chipping or other damage to the wheel. Some cocking is almost unavoidable. A power lift should be used to avoid these problems.

Some large centerless grinders require removal of the spindle from the machine to change wheels. The machine manufacturer usually provides special fixtures to do this job. Power lifts for these fixtures are recommended. Chainfalls do no usually provide small enough steps of vertical movement to allow accurate alignment, so cocking of the spindle is likely and this endangers the wheel bore. Also, both spindle and wheel bore should be horizontal during mating. This allows the person mounting the wheel to feel any resistance to entry of the spindle into the wheel bore, so he can avoid forcing or cocking. He does not have these factors under his control if mounting is done vertically: the weight above the spindle or wheel is usually enough to chip or crack the wheel, and this is likely unless alignment is absolutely perfect.

Mounted Wheels. The length of overhang of the wheel beyond the chuck has a bearing on the maximum safe speed. The safety code or the Grinding Wheel Institute's "Mounted Wheels-Safe and Efficient Operation", latest edition, should be consulted to be certain maximum safe speed is not exceeded.