For DRIVERS, MECHANICS & FOREMEN.
Page 39
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A PRIZE OF TEN SHILLINGS is awarded each week to the sender of the best letter which we publish on this page: all others are paid for at the rate of a penny a lute, with an allowance for photographs, All notes are edited before being published. Mention your employer's name, in conftdence, as evidence of good faith, Address, .0., M. and .P.„ "The Commercial Motor," 7-15, Bosebery Avenue, London, E.C. 1,
Lamps Alight.
On Saturday, November 29th, light your lamps. at 4.25 in London, 4.16 in Edinburgh, 4.20 in 'Newcastle, 4.30 in Liverpool, 4.29 in Birmingham, 4.35 in Briltol, and 5.13 in Dublin.
When Ordering Replacements.
The sender of the following oommunieation has been awarded the 10s. prize this week.
120331 " H.M." (West Bromwich) writes : —"A good many otherwise capable mechanics find themselves at a loss when they have to make sketches from which replacement parts have to be ordered, Of course, in most cases the needlf.or sketching at all may be obviated, and the part ordered with greater certainty of being correct if the damaged or worn component be sent along with the order. In man, • cases, however,time would. be saved by quickly sketching the part and sending off the order while the particular portion of the chassis effected is being dismantled. In others, it may be possible to carry en a little while longer with the old piece, while the new one is on its way. It is then that the advantage of being able to make useful and readable sketches comes in handy.
"I happen to stand, so to speak, at the receipt of custom, and it is to me that.-orders for spares frequently come. I can therefore testify, with some feel g, to the inadequacy of the information which is generally provided. Very often the sketches only remotely., if at all, resemble the part which is wanted. Even if the sketch is good; one or two important dimensions are usually missing, and dissatisfaction is frequently the upshot. Even when the old 'part is sent as a pattern all is not necessarily well. Such parts are naturally, as a rule, somewhat worn, and indication should be given, with the order, of the extent of the wear and of its precise locality. "No attempt should be made to draw freehand. A ruler or straight-edge of some kind is advisable, and also a pair of cheap compasses. If an exercise book containing leaves of squared paper be procured i will be found to be an immense help ; it may he had ruled either in eighths of an inch or in tenths. The former is better. The smallest parts should be drawn ;u11 size, some, on account of their size, will have to be sketched half size; rarely will it be necessary to come down to a quarter scale, then only for the largest pieces.
"At least two views of the part should he given. In some cases where it is rather complicated, more than two may fie required. The accompanyingesa.mple sketch [which we have had redrawn.—E.1 is of a double gearwheel from the gearbox of a touring car. I have shown two views, an end view, looking down the bore of the wheel, and a side view. Commence on the end view. Lay the gear on the bench with the bigger wheel below the other. Look down upon it, and make your first sketch to represent the gear as you see it. Using your compasses, draw a circle for the outside diameter of the smaller wheel, a second will serve to indicate similarly the top of the teeth of the larger wheel. Measure the diameter of the neck between the two wheels and to indicate that draw a broken line with your compasses. It only remains, in this first view, to draw in the shape of the hole in the wheel, which happened, in my case, to be a square one with rounded corners. • ' "The other view is obtained by laying the gear down on its side. Plain lines, drawn over those of the squared paper, using the ruler or straight edge,
may represent the tops of the teeth of the wheels, and their sides: The'roots of the teeth should be indicated by broken lines, and so should the hole in the wheel. Give dimensions far the outside diameter of each wheel; measuring caiefull5 to the tops of the teeth; state the number of teeth in the, wheel and the width of ;face. The overall 'length of the part Should be-given. in every case, as well as a string of dimenslops whieh should themselves add up to the same as the overall length. A useful cheek is thus afforded. The hole in the gear must be carefully measured, both across the flats and also from corner to corner.. Check by measuring...in both directions before eonsigniry your measurement to paper. Any square corners should be carefully shown so on the sketch and fillets should also be correctly indicated. In order to avoid the risk of error, it is better to write4on the sketch 'square corner' or rad.,',orwhatever it may be. The size of the hole is best indicated bviinaking a wire gauge. The ends of the wire should be filed square, and should fit the hole tightly or not acording to the amount of wear which has taken place in shaft or wheel. When allowing for wear in shaft•take care that if the bole in the new wheel is made small it will pass over the remainder of the shaft, which may not have worn it all. A good tip, in this connection, and particularly useful when dealing with a square hole, iis to slip the wheel on its shaft, and. fit, between wheel and shaft, a set of shims, or thin packing pieces, until the wear is properly taken up. Remove the wheel and make the wire gauge to fit, the hole with the shims in place."