GLAZING

GLAZING AND LOADING, THEIR EFFECTS, CAUSES AND REMEDIES

Glazing
When the surface of a grinding wheel develops a smooth and shining appearance, it is said to be glazed. (Fig 1) This indicates the abrasive particles on the wheel face are not sharp. These are worked down to bond level. Fig1.

Loading
When soft materials like aluminium, copper, lead, etc. are ground the metal particles get clogged between the abrasive particles. This condition is called loading. (Fig 2)

The effects of a glazed or a loaded grinding wheel are almost the same. They are:
❖ Excessive cutting pressure between wheel and work
❖ More heat generation
❖ Burning of the ground surface
❖ Poor surface finish
❖ Inaccuracies in the size and shape of the work piece
❖ Wheel breakage (sometimes).

A dull or glazed wheel should be dressed for the following reasons.
❖ To reduce heat generated between the work surfaces and the grinding wheel
❖ To reduce the strain on the grinding wheel and the machine.
❖ To improve the surface finish and accuracy of the work.
❖ To increase the rate of metal removal.

CAUSES AND REMEDIES OF GLAZING
Wrong selection of grade and size
Set the wheel to the recommended speed.
Feed too fine
Set the feed rate correctly.
Dirty coolant
Change the coolant.
A glazed or a loaded grinding wheel can be reused after removing the glazed or loaded particles from the grinding wheel face

MOUNTING THE GRINDING WHEELS
Great care must taken in mounting the grinding wheels on the spindle because of the high cutting speeds of the grinding wheel. Fig. illustrates mounting the grinding wheel. The following points are important in connection with mounting the wheel

1. Spindle,
2. Flange,
3. Intermediate layer,
4. Wheel,
5. Screwed joint

1. All wheels should be closely inspected just before mounting to make sure that they have not been damaged in transit, storage, or otherwise. The wheel must first be subjected to the ringing test. For this purpose, the grinding wheel is put on an arbor while it is subjected to slight hammer blows. A clear, ringing, vibrating sound must be heard. If a grinding wheel contains fine cracks, discordant sound that fail to vibrate will be emitted. This test is applicable to vitrified and silicate wheels. Shellac, resinoid or rubber loaded wheels will not ring distinctly.
2. The abrasive wheels should have an easy fit on their spindles or locating spigots. They should not be forced on.
3. The hole of grinding wheels mostly is lined with lead. The lead liner bushes should not project beyond the side of wheels.
4. There must be a flange on each side of the wheel. The mounting flanges must be large enough to hold the wheel properly, at least the flange diameter must be equal to the half of the grinding wheel diameter. Both the flanges should be of the same diameter, other-wise the wheel is under a bending stress which is liable to cause fracture.
5. The sides of the wheel and the flanges which clamp them should be flat and bear evenly all round.
6. All flanges must be relieved in the center so that the flanges contact the wheel only with the annular clamping area. If they are not properly relieved, the pressure of the flanges is concentrated on the sides of the wheel near the hole, a condition which should be avoided.
7. Washers of compressible materials such as card board, leather, rubber, etc. not over 1.5 mm thick should be fitted between the wheel and its flanges. In this way any unevenness of the wheel surface is balanced and a tight joint is obtained. The diameter of washers may be normally equal to the diameter of the flanges.
8. The inner fixed flange should be keyed or otherwise fastened to the spindle, whereas the outer flange should have an easy sliding fit on the spindle so that it can adjust itself slightly to give a uniform bearing on the wheel and the compressible washers.
9. The nut should be tightened to hold the wheel firmly. Undue tightness is unnecessary and undesirable as excessive clamping strain is liable to damage the wheel.
10. The wheel guard should be placed and tightened before the machine is started for work.After mounting the wheel, the grinding machine is started. The grinding wheel should be allowed to idle for a period of about 10 to 15 minutes. Grinding wheels must be dressed and trued which are out of true, before any work can be started.

DRESSING AND TRUING GRINDING WHEELS
Dressing removes loading and breaks away the glazed surface so that sharp abrasive particles are again presented to the work. This is done with various type of dressers. A common type of wheel dresser, known as the star-dresser,For precision and high finish grinding, small industrial diamonds, known in the trade as bors, are used. The diamond or group of diamonds is mounted in a holder. The diamond should be kept pointed, since only the point can be used for cutting. This is done by the holder down at a 15° angle and using a new surface each time the wheel is dressed. A good supply of coolant should be used when dressing with a diamond, as overheating can cause the diamond to fracture or drop out of its setting. Very light cuts only may be taken with diamond tools.

Truing :  

Truing is the process of changing the shape of the grinding wheel as it becomes worn from an  original shape, owing to the breaking  away  of  the abrasive and bond. This is done to make the wheel true and concentric with the bore, or to change the face contour for form grinding. Truing and dressing are done with the same tools, but not for the same purpose.The only satisfactory method of truing a wheel is by the use of a diamond tool in a similar manner as explained before. In turning a wheel with a diamond, the feed rate must not exceed 0.02 mm, otherwise grooves may be cut into the wheel

1.Wheel,  2.Dresser, 3.Work rest

BALANCING GRINDING WHEELS:
If wheels become out of balance through wear and cannot be balanced by truing or dressing, they should be removed from the machine and discarded. Wheels should be tested for balance occasionally and rebalanced if necessary-Wheels that are out of balance not only produce poor work but may put undue strains on the machine. Small wheels may be balanced by milling a short recess on the inside of the flanges and filling with lead. Large wheels should be placed on a balancing stand and balanced by moving weights around a recessed flange. Now-a-days, grinding wheel mounts are provided with devices to enable balancing to be done whilst the wheel is running and between grinding operations