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Benchwork - Pedestal Grinder Procedures
 
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Grinding Wheels

Grinding wheels are made up of abrasive particles that are held together by a glue-like material called the bond (Figure 1).

Almost all grinding wheels made for pedestal grinders are made with artificial abrasives, namely silicon carbide and aluminum oxide. Artificial abrasives have replaced natural materials, such as emery and corundum, because of their superior hardness and durability.


Figure 1 Pedestal grinding wheel

Grain Size

Prior to molding into wheels, the abrasive particles are graded for size by passing over a series of vibrating screens. The grain size is determined by the mesh of the smallest screen through which the grains will pass. For example, if the mesh has 100 openings per lineal inch, the grit number will be 100.

The grain size determines the coarseness or fineness of the grinding wheel. The grain sizes of abrasive particles range from a very coarse crystal, to 240, a very fine particle. Special grinding wheels, though not those used on bench grinders, are made with grain sizes as fine as 600.


Coarse grit


Fine grit

Figure 2  Coarse grit grinding wheel and a fine grit grinding wheel

  Manufacture of Grinding Wheels

Grinding wheels are made by mixing the abrasive grains with the bond, and molding this mixture into forms of various shapes and sizes. The wheels are then baked in ovens or kilns to cause the bond to harden and cure. The grinding wheels are then bushed with lead, babbitt or plastic to reduce the hole size to the shaft . Lastly, the wheels are accurately trued, balanced, and inspected prior to shipment.

Grinding Wheel Selection

When purchasing a grinding wheel for the bench grinder, attention should be given to the size of the wheel (width and diameter), size of bushing, type and size of abrasive grains, grade, structure, bond, and maximum safe speed for operating the wheel.

Grinding wheels are fitted with paper washers glued to both sides of the wheel to act as cushions between the wheel and the flanges that hold and support the wheel. Wheel identification information, as well as the manufacturer's name and trademark, are generally printed on these washers (Figure 3).


Figure 3  Very important information is found on the paper washer of the grinding wheel.

NEVER operate a grinding wheel at speeds greater than those listed on the wheel.

The following are features, in list form, which must be specified when ordering a grinding wheel. (Not all will be used on every wheel.)

  1. Abrasive type (aluminum ode, silicon carbide, etc.)
  2. Grit size (8-600) (course to one)
  3. Grade (A-Z) (soft to hard) (the bond, not the abrasive.)
  4. Structure (0 or 1-12) (spacing of grains) (The larger the number, the wider the grain spacing.)
  5. Bond type (V Vitrified, S-Silicate, B-Resinoid, R-Rubber, and E-Shellac)
  6. Diameter
  7. Width
  8. Bushing size
  9. Shape

Grinding is the process of removing material by the cutting action of the countless hard and sharp abrasive particles of a revolving grinding wheel as they come in contact with the surface to be ground.

Grinding machines are made in a variety of types and sizes, depending upon the class of work for which they are to be used.

The grinding wheels are held between two flanged disks (Figure 6). A roughing or coarse grained wheel is usually mounted on one end of the spindle and a fine wheel on the other. A tool rest is provided for each wheel so that tools may be held or steadied while being ground (Figure 4).


Figure 4  Main parts of a pedestal grinder

The operator is protected against flying abrasive particles and ground material by the wheel guards, which are an integral part of a machine. Safety glass shields are also provided for additional protection.

These grinders are used for all kinds of general off-hand grinding and for the sharpening of drills, chisels, tool bits, and other small tools.


Procedure

Note: Grinders should be lubricated according to the manufacturer's instructions.

  1. Examine the grinder to see that the tool rest is set at the required height, is within 1/8 if an inch to the face of the wheel, and is securely fastened in this position (Figure 5).
  2. Adjust safety glass shields on the grinder to permit clear vision of the part to be ground and still protect the operator from flying particles.
    • CAUTION: Always wear safety glasses when using a grinder.


Figure 5  Position of the tool rest

  1. Start the grinder.
    CAUTION: Stand to one side of the wheel when operating the grinder.
  2. Hold the work in one hand, and steady it with the other. Place the work on the tool rest; then guide it against the face of the revolving wheel and apply enough pressure to grind, depending upon the hardness of the material and the wheel itself.

Note: Support the work on the tool rest to steady it when grinding, except in the case of the small tool bits which can be guided better by supporting them with the fingers or with a hand resting on the tool rest.

  1. Cool work in a water pot as it becomes heated from grinding, especially the small hardened tools which would lose their temper if overheated. Twist drills should not be cooled by dipping in water, as it may cause cracking.
  2. Grind the job to the required shape or size by moving the work back and forth across the face of the wheel. This will prevent wearing a groove into the wheel and will result in a flatter surface on the work.

CAUTION: Keep fingers away from the revolving wheel, especially when grinding small pieces. Also make sure that the tool rest is close enough to the wheel to prevent the work from slipping into the space between the two.

Note: Remove as much metal by rough grinding as is possible; then use the finer wheel for finishing. (Figure 2)

Do not grind on the side of the wheel except when absolutely necessary, and then with only light pressure.

  1. Check work with a gage or other measuring tool.
  2. Stop grinder.

Mounting a Grinding Wheel

The grinder spindle has a right-hand thread on the right end and a left-hand thread on the other as a safeguard against the wheel loosening. Each wheel is mounted directly on the spindle of the bench or floor grinder and is held between a pair of flanged collars by either the right- or left-hand spindle nut.

Great care must be used when mounting wheels so that no undue strains are set up, which might cause the wheel to break. A wheel that fits tightly should never be forced on the spindle. Instead, the lead-bushed hole should be scraped to make it fit freely.


Figure 6  Spindle parts for a pedestal grinder

Blotting paper washers or pads of some soft compressible material are always placed between the side of the wheel and the flanged collars. This lessens the danger of setting up strains in the wheel, causing it to crack. The clamping nut should be drawn only tight enough against the flanged collar to prevent the wheel from turning on the spindle. After mounting, the wheel should be checked for trueness and balance.

CAUTION: Replace all guards before starting the grinder.

When starting the grinder with a newly mounted wheel for the first time, always stand to the side of the wheel and allow it to run for one minute before starting to grind. This will guard against any injury in the event the wheel is faulty and should break apart.


Dressing a Grinding Wheel

Dressing is the process of restoring the sharpness of the grinding wheel by breaking away the dulled abrasive crystals or by removing the glazed or loaded surface of the wheel, thus presenting new sharp cutting edges of the abrasive grains. This breaking away is caused by the pressure of the dresser crushing the bond and releasing the dull abrasive.

This process should not be confused with trueing, which refers to the shaping of any part of the wheel to run true or to alter it to some desired shape.

The tools used for dressing are made in a variety of types and are called dressers. The more commonly used off-hand dressers are: the star type and the diamond stick (Figure 7).

 
Figure 7  Grinding Wheel Dressers

To dress the wheel, support the dresser on the tool rest so that the point of contact is slightly above the center, and with the handle tilted upward at an angle as shown in Figure 8.


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Figure 8  Dressing face of a grinding wheel

Slowly press the dresser against the face of the revolving wheel until it "bites." Then move it back and forth to obtain a straight surface, and at the same time, hold the dresser rigidly enough on the tool rest to maintain trueness while dressing.

Loading is caused by: 
1) metal too soft and/or 
2) wheel bond too strong (Figure 9).
  When self-sharpening the wheel, the pressure of grinding will either fracture the grain or pull it out of the bond when it becomes dull. This action exposes new, sharp-cutting edges.

Figure 9  Loaded grinding wheel

Grinding is a safe operation if a few basic safety rules are followed. Use the rules listed in Figure 10 as a guide for safe grinding.  These rules are based on material contained in ANSI B7.1 safety code for safe use and operation of abrasive wheels.

WARNING
IMPROPER USE MAY CAUSE BREAKAGE AND SERIOUS INJURY.

DO and DON'T
DO ALWAYS HANDLE AND STORE WHEELS IN A CAREFUL manner. DON'T USE A CRACKED WHEEL OR ONE THAT HAS BEEN DROPPED or has become damaged.
DO VISUALLY INSPECT all wheels before mounting for possible damage and ring test vitrified wheels. DON'T FORCE A WHEEL ONTO THE MACHINE OR ALTER the size of the mounting hole - if the wheel won't fit the machine, get one that will.
DO CHECK MACHINE SPEED against the established maximum safe operating speed marked on the wheel. DON'T EVER EXCEED MAXIMUM OPERATING SPEED established for the wheel.
DO CHECK MOUNTING FLANGES for equal and correct diameter. DON'T USE MOUNTING FLANGES ON WHICH THE BEARING SURFACES ARE NOT CLEAN, FLAT, AND FREE OF BURRS. 
DO USE MOUNTING BLOTTERS when supplied with wheels. DON'T TIGHTEN THE MOUNTING NUT EXCESSIVELY.
DO BE SURE WORK REST is properly adjusted.  (Center of wheel or above; no more than 1/8" away from the wheel.) DON'T GRIND ON THE SIDE OF THE WHEEL. (see Safety Code B7.1 for exception.)
DO ALWAYS USE A SAFETY GUARD covering at least one-half of the grinding wheel. DON'T START THE MACHINE UNTIL THE WHEEL GUARD IS IN PLACE.
DO ALLOW NEWLY MOUNTED WHEELS to run at operating speed, with guard in place, for at least one minute before grinding. DON'T JAM work into the wheel.
DO ALWAYS WEAR SAFETY GLASSES or some type of eye protection when grinding. DON'T STAND DIRECTLY IN FRONT of a grinding wheel whenever a grinder is started.
DO TURN OFF COOLANT before stopping wheel to avoid creating an out-of-balance condition. DON'T FORCE GRINDING so that the motor slows noticeably or the work gets hot. 

Figure 10. Safety Rules for Grinding wheels and abrasives.

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