The Wood Turner's Handybook - A Practical Manual for Workers at the Lathe: Embracing Information on the Tools, Appliances and Processes Employed in Wood Turning. Paul N. Hasluck

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      Fig. 25. AUTOMATIC ROUNDING MACHINE.

      The hollow mandrel is mounted in bearings on a head-stock. The speed at which this should be driven is determined by the diameter of the work produced. For three-inch rods five to seven thousand revolutions per minute would be a fair rate, and the smaller the work the higher should be the speed. The lower rollers in the pairs are adjusted to a suitable height, according to the diameter of the work to be turned, by means of the hand-wheels, shown at both ends, below the bearings of the tangent screw shaft. The weights above bring the upper rollers in contact. The pressure is regulated by adjusting the weights along the lever arms to which they are fixed with set screws.

      The necessity of driving revolving cutters at a high speed in wood-working machinery is now fully recognised. Not only does increased speed of the cutters allow the wood to be fed at a correspondingly faster rate, but increased velocity of the cutting edge produces a cleaner and better surface. It is only during recent times that wood-working machinery has been driven at the very high speeds named. Formerly it was considered to be impossible to manufacture bearings that would work satisfactorily under these high speeds. It has, however, now been ascertained that cutters will do more work of a better quality when driven at high velocities, and though the machines which carry them have to be constructed with extra caution, yet the consequent extra initial cost is inconsiderable when compared with the increased productive capacity.

      Great attention must be paid to the correct balancing of cutter-heads which are driven at a high speed. Any deviation in the equipoise will impart a tremor, or spring, to the tool, and bad work will result. The bearings must be kept plentifully supplied with a suitable lubricant. A channel should be cut the whole length of the plummer-block bearing for the oil to flow in, and thus ensure a constant supply.

      Spindles which have to be driven very fast should have their bearings as small in diameter as possible, consistent with the duty the spindle has to perform. The bearings are generally made very long in proportion to their diameters: twelve diameters long is perhaps a safe average. The greater the amount of bearing surface the longer it will wear. It is not superfluous to remind readers that to increase the length of a bearing does not increase the friction, and whether a spindle runs in bearings one inch or twelve inches long the same power will drive it, providing always that the diameters of the bearings are the same. An increase in the diameter will necessitate an increase in the power required to drive, and thus the necessity of keeping the diameters of bearings as small as possible is manifest.

      The speed at which cylindrical work can be turned in a rounding machine of the construction illustrated may be judged by the figures given above. With cutter-heads running at over 5,000 revolutions per minute, the material may be turned at the rate of from thirty to forty feet length per minute. The enormous quantity of rod that can be turned out by a machine in constant work is surprising.