There is frequent necessity for drilling and other cutting on work which cannot be moved to a machine. There have been devised many types of small portable machines, usually designated as portable tools, which can be readily transported to large work for effective use in drilling, chipping, grinding away rough places, and cutting-off.

For machine-shop use the list of portable tools generally consists of the following viz.:

(1) Various types of portable drills driven by electric or pneumatic power or by hand. The ratchet drill for hand drilling is the simplest and lightest of these types.

(2) Portable boring bar, motor driven, for boring the stern bearings of ships before launching.

(3) Valve re-seating machine, hand or electrically driven, for truing up the seats of valves which have become leaky in use. This is clone without removing the valve body from its pipe line.

(4) Pneumatic hammers for driving a cold chisel for many kinds of surface or edge chipping and for cutting a narrow path across a piece of metal in cutting it off.

(5) Pneumatic and electric grinders. These are essentially small grinding wheels mounted on a suitable shaft so supported that the machine can be held in the hands and the revolving wheel pressed against the spot to be ground.

(6) The oxy-acetylene blowpipe. This is used to direct its flame along a path on a metal plate, burning its way through the plate. It is used for many cutting operations in bridge, boiler, ship and other work, and is very useful as a portable cutting apparatus, though it of course leaves rough edges at the sides of the path burned through by the flame.

(7) Very useful also in various shops are lifting jacks and the differential pulley.

Jacks are made for lifting either by screw or hydraulic power. They exert great force and can be operated usually by one man. A hydraulic jack which will lift 50 tons a distance of 18 inches weighs about 325 pounds and can be carried by two men. Dilute alcohol is much used in the hydraulic jack because it does not freeze, but it lacks the desired quality as a lubricant.

The differential pulley, shown in Fig. 215, may be operated by one man to lift weights ranging considerably beyond a ton, according to its capacity. It consists of a double-sheaved wheel, or block, at the top and a single-sheaved wheel below. An endless chain is rove over these sheaves. The distance between the two wheels is increased or decreased by pulling one side or the other of the loose part, or bight, B of the chain. The lifting power is due to the fact that the two sheaves of the upper wheel are of slightly different diameters.

Fig. 215.

Differential

Pulley.