15 Grinding and Polishing
GRINDING
The type of machine used for off-hand grinding in school may be of the bench type (fig. 1) or pedestal type: or it may be a combined buff and grinder, either pedestal or bench type with a polishing mop on one end of the spindle and a grinding wheel on the other.
Grinding wheels consist of particles of abrasive material, such as aluminium oxide or silicon carbide, which are held together by a bonding agent. Figure 2 shows a magnified portion of a wheel showing the abrasive particles held in the bonding material. The grade of the wheel refers to the strength of this bonding agent.
The abrasive particles are known as the grit, and these are graded by passing through a screen, e.g. 46 grit will pass through a screen having 46 meshes to the linear inch.
We must therefore consider the grit and the grade of the wheel. Generally speaking when a piece of metal is being ground the abrasive particles become blunt after a time. If the wheel is right for the material being ground, the grade (i.e. the bonding material) of the wheel should be soft enough to allow the blunt particles to break away thus exposing fresh sharp particles. It follows then that when grinding a hard metal the particles will become blunt more quickly so they must be allowed to break away more quickly.
This means that the harder the metal being ground, the softer the grade must be and vice versa.
When ordering a wheel at least five points must be mentioned: diameter, thickness, size of hole, grit and grade. A maker's catalogue should be consulted.
Wheel Dressing
After long use the wheel goes out of true and the edges become radiused and if the wrong material has been ground the wheel also becomes glazed (the equivalent of pinning on files). A wheel dresser of the kind shown in figure 3, known as the Huntington type wheel dresser, is held firmly on the rest with the star wheels against the periphery of the wheel. As the wheel rotates the abrasive particles on the periphery are removed. The dresser should be held firmly and moved slightly from side to side to cover the width of the wheel. After truing the wheel , the tool rest should be adjusted so that the gap is about 1/16".
Goggles or an eye shield must always be used when grinding.
Cutter Grinding
Milling cutters, when they become blunt, are ground on a cutter-grinding machine. The cutter to be ground is pushed on to a tapered mandrel which is then set up on the machine between centres (fig. 4). Each tooth of the cutter is rested on the finger which is stationary and the cutter is drawn across the periphery of the grinding wheel. Each tooth in turn is taken across the grinding wheel in the same manner. Many complex grinding operations with a high degree of accuracy can be done on these small machines. The makers supply full instructions with each machine.
POLISHING
Polishing done on the buffing machine is similar in principle to that done by hand. The polishing materials are used in order of their fineness finishing with the finest one.
A buffing machine is shown in figure 5. The mops screw on to the tapered point shown at figure 5A. Various grades of mops arc available from coarse stitched mops to soft "swans-down". Similarly abrasive compounds in 1 lb blocks (fig. 6) are in various grades. The block is pressed against the spinning mop so that the friction and heat causes some of the abrasive compound to adhere to the mop. This is now ready to polish the work which should be pressed against the mop as shown in figure 7.
Each grade of mop should be used for one grade of abrasive only, so that when a finer abrasive is used, a softer mop is used with it. Polishing blocks consist of wax charged with abrasive powder. The following list shows the abrasives in order of coarseness and the type of mop recommended for each one:
Polishing Compound Mop Carbrax Stapol Tripoli Stitched calico Crocus Unstitched calico Rouge Swansdown
Clean the mop from time time to remove the old abrasive. This can be done by firmly holding a piece of wood, from which nails are protruding (fig. 8) against the mop whilst it is spinning. Recharge the mop with the correct abrasive when it is clean.
Safety
Never hold work in rag. It might get caught up. Never polish lengths of chain—these are dangerous if they become entangled. Polish thin pieces of metal by holding them flat on a suitable piece of wood.
