Planing Machine Driving Mechanism 1 |
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An extract from:-Text Book of Applied Mechanics, by Thomas Cryer 1903
66.The Planing Machine is a machine which is used for the production of plane surfaces, and it does this by the so called copying principle, the bed of the machine being formed with two or more plane surfaces on which the table (which is provided with similar plane surfaces) slides backward and forward, carrying the work with it, which is subjected to the action of the cutting tool. It will thus be seen that the surface obtained by the cutting action of the tool will be a counterpart of the plane surfaces on which the table slides. The work is secured to the table by screws, clamps, or other suitable means. The tool is moved on a cross slide placed parallel to the face of the table and the slides; a cross traverse of the tool is arranged by hand or by a self-acting motion, as well as a vertical and angular traverse for vertical and angular surfaces. Fig.128 is a perspective drawing of a machine in which the table is driven by a rack and pinion (method 3, Fig.132) 67. Methods Of Obtaining Backward and Forward Motion of The Table. Method 1. - The motion of the table in both directions being of the same speed, and the driving being by a screw. Fig.129 shows one end of the bed with a small portion of the table,
the driving pulleys and the bevil gear for reversing the motion. The screw S passes under the table T, engaging with the threads of a nut which is secured to the table by studs or bolts; the screw is provided with suitable bearings in the bed of the machine. In order that the motion of the table may take place in both directions, the screw must be driven first in, and then in the opposite direction. The driving is by one belt; there are three pulleys, of which the middle one L is a loose pulley, the outer one F is secured to the shaft, which passes through the pulley A, the bevil pinion D being also secured to the shaft; the pulley A is either made in one piece with the pinion C, or it is let on to the boss of the pinion, forming practically one piece; this pulley and pinion revolve independently of the shaft; the wheel W is secured to the screw S. If the belt be on the loose pulley L, the machine will be stopped; if it be moved on to the pulley F, the shaft will be driven, and with it the pinion D , driving the wheel W and screw S in the direction shown by the arrow marked 1; if the belt be moved on to the pulley A, the driving will be to the pinion C, which gearing on the opposite side of W to D will drive the screw S in the reverse direction, or in that shown by belt forks B, and a bar which receives its motion
by an arrangementof bell crank levers and connecting rods secured to the table, which can be adjusted to suit the length of the cut required. This arrangement is shown in the figure, and a bell crank lever is shown seperately in Fig.130. In using them a small amount of motion in one direction is to be changed into another direction; the motion in the two directions may be the same amount, or not, as required. The figure shows an arrangement whereby the motion of A is changed into that of B, in a direction at 120 degrees with the first, the relative motion being as 3 to 2, or A's mo. : B's mo. :: CA : CB In practice, the angle moved through by the levers should not exceed 60 degrees. The method of finding the position of the centre of motion C is as follows :- The directions are set out at the required angle DFE, perpendiculars DG and EK to these directions, and of length proportional to the amounts of motion, are set out on the sides at which the centre is to be placed, where the parallels to the directions or motion passing through extremities of these perpendiculars meet, gives the centre C required. If the motions are in different planes, the lever would be made in two seperate parts, but the principle would be the same. Method 2.- Fig.131 shows an arrangement whereby the speed of the screw is quicker in one direction than in the other; by
this means time is saved during which the cut is not being made. There are two wheels W of different sizes keyed to the screw; the driving is through each of them in succession. The pulleys and wheels are arranged on the shaft as in Fig.129, the moving of the belt being the same also. Method 3. - In fig.132 the table is driven by a rack pinion P, gearing with a rack R secured to the underside of the table. The driving is by one belt, which acts on any of the pulleys F,L, or A. L is a loose pulley. Pulley F and the pinion B are secured to the shaft. Pulley A is practically one piece with the pinion C, which gears with wheel D; wheel D and pinion E run together on a sepperate shaft.
The pinions B and E each gear with the wheel G, and on the same shaft as G the rack pinion P is placed. If the belt be on pulley L the machine is stopped; if it be placed on F, the driving is to be the pinion B, then from B to wheel G, and so to the rack pinion P in the direction of the arrow, if it be placed on A, the driving is to the pinion C thence to wheel D and pinion E, which drives G as before, but in a reverse direction (as shown by a dotted arrow) and at a reduced speed. Numbers of teeth have been assigned to the various wheels and pinions in order that the speed in each direction may be obtained. Suppose the reolutions of the pulleys to be 78 per minute, then the slow rate of motion of the table would be obtained thus:- Number of revolutions of rack pinion per minute = 78/1 x 12/30 x 15/45 = 10.4 but the rack pinion has 12 teeth 1.25 pitch. therefore it moves the table 12 x 1.25 = 15 ins. per rev., or (15 x 10.4) / 12 ft. in 10.4 revs. = 13ft per minute. For the quick motion the driving is direct from pinion B to wheel G, therefore number of revs. of rack pinion per minute = 78/1 x 15/45 = 26. therefore Motion of table = (15 x 26) / 12 = 32.5 ft per minute. Or the return speed of the table is 2.5 times that of of the cutting speed, whereby time is economised. The belt is moved automatically by bell crank levers as before. ____________________________________
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