Repairing Your Own Machine Components

Many industrial concerns have workshops of their own. For the

repair of worn shafts, the lathe machine is excellent. Keyway

slots can be machined by using a milling machine, while a

shaping machine can do machining of large flat areas. A

drilling machine does drilling of holes.

A skilled Maintenance Engineer should know how to use all these

machines in order to make his own repairs in a safe manner. Very

often he has to supervise machinists. The information below

should be useful for that purpose.

Lathe Machine

The lathe machine uses a single-point-cutting tool for a variety

of turning, facing, and drilling jobs. Excess metal is removed

by rotating the work piece over the fixed cutting tool to form

straight or tapered cylindrical shapes, grooves, shoulders and

screw threads. It can also be used for facing flat surfaces on

the ends of cylindrical parts.

The work piece is clamped onto a horizontal rotating shaft by a

3-jaw or 4-jaw chuck. The latter chuck can be used to cut

off-centered cylinders. The rotating horizontal spindle to which

the chuck is attached is usually driven at speeds that can be

varied.

The cutting tool is fixed onto a tool rest and manipulated by

hand. It can also be power driven on straight paths parallel or

perpendicular to the work axis. This is useful for screw cutting.

Internal turning known as boring results in the enlargement of

an already existing hole. The holes are more accurate in

roundness, concentricity, and parallelism than drilled holes. A

hole is bored with a single-point-cutting tool that feeds along

the inside of the work piece.

Shaping Machine

The shaping machine is used to machine flat surfaces, grooves,

shoulders, T-slots, and angular surfaces with single-point tools.

The cutting tool on the shaper oscillates, cutting on the forward

stroke, with the work piece feeding automatically toward the tool

during each return stroke.

Drilling Machine

The drilling machine is used to cut holes in metal with a twist

drill. By changing the cutting tool, they can be used to do

reaming, boring, counter boring, countersinking, and threading.

Milling Machine

The milling machine uses a rotating cutting tool to cut flat

surfaces, grooves, and shoulders, inclined surfaces, dovetails,

and T-slots. Cutters of many shapes are changed to cut different

grooves.

Cutting Tools

Metal-cutting tools are classified as single point or multiple

point. The lathe and shaping machine use single point cutting tool

while the milling and drilling machines use multiple-point-cutting

tools.

Metal is cut either by moving the work piece like in the lathe or

by moving the tool like in the shaping machine, drilling or

milling machine. Clearance angles must be provided to prevent the

tool surface below the cutting edge from rubbing against the work

piece. Rake angles are often provided on cutting tools to cause a

wedging action in the formation of chips and to reduce friction and

heat.

Tool Materials

In order to remove chips from a work piece, a cutting tool must be

harder than the work piece and must maintain a cutting edge at the

temperature produced by the friction of the cutting action.

Carbon Steel

Carbon steel tools even though comparatively inexpensive tend to

lose cutting ability at temperatures around 400 degree F (205

degree C).

High-Speed Steel

High-speed steel, containing 18 percent tungsten, 4 percent chromium,

1 percent vanadium, and only 0.5 to 0.8 percent carbon, permits the

operation of tools twice or three times the speeds allowable with

carbon steel

Cast Alloys

Cast-alloy cutting-tool materials containing cobalt, chromium, and

tungsten are effective in cutting cast iron and retaining their

cutting ability even when red hot.

Cemented Tungsten Carbide

The hardness of Tungsten Carbide approaches that of a diamond.

Tungsten carbide tools can be operated at cutting speeds many times

higher than those used with high-speed steel.

Oxides

Ceramic, or oxide, tool tips consist primarily of fine aluminum oxide

grains, which are bonded together. These are very hard.

Cutting fluids

An overheated tool can become blunt and soft very fast. Therefore

very often, cooling fluids cools the cutting points of the tool. This

serves to lubricate and cool.

Water is an excellent cooling medium, but it corrodes ferrous

materials. Sulfurized mineral oil is one of the most popular coolants

as it can both cool as well as lubricate. The sulfur prevents chips

from the work from melting on to the tip of the tool.