Sawing Machines

Circular Sawing Machines.

These comprise a large group, from little ones to saw bone, fibre, aluminium, brass and copper, to the more powerful types that cut iron and steel. The speeds of rotation of the saws for the soft materials are high, and the feeding can often be effected by hand instead of by slides or levers such as are necessary in steel and iron sawing. Sometimes it is the saw that is fed along by a slide, sometimes the work on a slide instead. The machines which cut girders have a special yielding motion to the feed device : this causes the saw to feed more slowly through the thick sections than through the thin webs, which can naturally be penetrated at a faster rate. The saw is clamped on a spindle with nut and washer for ordinary cutting, but the flush-side machines have the saw attached with sunken bolts, so that one side of it is perfectly flat. This provision is essential for iron and steel foundries, to part off the superfluous runners or gates (which feed the molten metal into the mould) flush with the surface.

Ordinary saw blades are formed with the teeth cut in the same disc, but the larger blades, especially for steel forging sawing, have inserted teeth, each held in with a wedge and screw fasten ing, so that any breakages can be made good cheaply and quickly. The largest blades cut armour-plate and ingots, also pieces out of big crankshaft forgings to form the webs. Duplex blades, run ning side by side at the appropriate distance apart halve the time of cutting out such slabs.

In the iron and steel works hot metal during forging or rolling operations is parted by a special sort of machine, the hot saw, that cuts at a fast rate. The large machines act on the pendulum princi ple, the blade being run in bearings at the bottom end of a deep swing frame which is pushed across by hydraulic or pneumatic cylinder to pass the saw through the hot metal, resting on a slide way beneath. The speed of revolution is high, and large horse power is consumed.

Friction Sawing Machines.

These are peculiar inasmuch as the circular blade has no cutting teeth, but is driven at so high a speed that the friction generates such heat in the immediate vicinity that the metal is burned through. For instance a railway

rail of 90 lb. per yard can be parted off in 7 seconds, or a chan nel section of 12 in. by 6 in. with a in. thick metal in 14 seconds, leaving a clean smooth finish. Some assistance is given to the action if the blade is very slightly notched around the periphery, though not like the true teeth of a saw. (F. H.) strictly, a mill in which logs are "broken down" into balks, deals, flitches, battens, planks and boards for sale or further treatment. But often the word is applied to a mill the plant of which includes planing, moulding, tenoning and other machines for finishing processes. The biggest mills are usually situated near a timber supply, brought by river or rail, and the design of the mill is in some degree affected by the mode of transit. More space is necessary for storage in the rail-borne example. In the water-borne system the logs float right into the mill and arc dragged out in turn by a winch. An overhead crane serves the stock-yard in the rail system, and carries the logs on to the machines.

The cutting is performed on various kinds of big machines, a preliminary operation often being that of cross-cutting to obtain convenient lengths. Cutting up into the various thicknesses is done by either reciprocating or band-saws, or circular saws, the log be ing held with dogs on a table which feeds it past the saw. Some band-sawing machines are of horizontal design, some vertical, the latter taking up less floor-space. The log-frame is a machine with a set of vertically reciprocating blades, suitably spaced apart, and it divides a log into boards at one pass of the table. The number of blades may be few, not exceeding four in some cases for cutting thick pieces, or as many as fifty for thin boards. Re-sawing ma chines are those for further dealing with material partly broken up, such as flitches and deals. The great quantity of sawdust and chips from the machines is neatly disposed of by pneumatic ducts ending in the boiler house, on the system mentioned in