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road and workshop
Page 65
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by Handyman
Benchwise: lathe sense (14)
IN vehicle manufacture the taper has lost none of its value as the simplest and safest form or means of securing two moving parts together, whether a semi-rotary steering spindle and arm, or a gear or diff shaft, clutch, primary shaft, etc. However, as with all hard-worked items, from time to time some slackness or freedom can set up, and in a short while excessive clearance and noise draw attention to the joint in trouble: quite often first sight of the ribbed, scarred. or even chipped taper is enough to justify an order for a new part.
While sound engineering shops in most large towns can do this work, they may not be able to do it immediately, giving the garage with its own lathe a distinct advantage, provided that the turning skill is there to go with it. This is fair enough comment, but all too often the part-time garage turner will admit that he has not dealt with tapers, at least not successfully, as this exercise eems to have several built-in snags and appears to need some special skill.
Mated parts
In fact the snags are really tricks of the trade, the skill called for is to finish with two perfectly mated parts. Nevertheless, there are situations which can defeat all but the professional—for instance, the gearbox mainshaft, which has a reasonably easyto-recover male taper but a mated female taper in the companion flange which calls for care and some practice.
However, let common sense prevail, as a gearbox mainshaft costing more than £35 but in good condition except for a scored taper. is well worth a new companion flange costing £8 lOs which can have the reclaimed shaft taper fitted to it; also, the flange is probably far easier to obtain than the shaft
For purposes of explanation at this juncture I will assume that the male taper has had skilled welding attention, is undamaged and machineable. There are at least four methods whereby the lathe can be made to cut a taper. but only two are in general use: (1) swivelling the top slide to travel the tool at the required angle to the lathe bed; (2) setting over the tailstock at the point of adjustment at the tailstock base described in a previous article. There is, of course, an extra fitting. the taper turning attachment which is specially designed for the job, and most useful for quantities. However methods (I) and (2) will meet vehicle needs, as in the case of most shaft tapers the somewhat limited top slide travel will be adequate.
Let me start with the top slide, where the need is to reclaim the fairly steep taper of a gearbox mainshaft that has suffered a loose flange for many miles. is scarred and has the keyway area well and truly upset. There are two jobs to be faced: one, to have the taper surface given a layer of metal that will enable it to be machined back to dead size, and the other to re-establish the keyway. If this happens to be a half-moon keyway we may have to convert tp a fiat key, unless we can obtain the use of a milling cutter—however. I will deal with that problem when we come to it.
Meanwhile, the main problem facing the trainee turner is first to determine the taper measurement for setting the top slide—this means swivelling it the exact amount needed to finish at the angle called for by the mated part, not an easy task at first, and a micrometer may not be of much use, the thin-bladed Vernier gauge being the tool I prefer. Given distances can be measured off on the taper, and lines scribed on the work by revolving it against a tool; these distances must be exact, of course, and are mainly required where we are dealing with a new or odd taper.
Test tapers
By far the best method for the range of vehicle tapers is for the turner to make up his own test pieces covering the normal range he will expect to meet; this really simplifies the whole task for th future. It is no use handing a spare-time turner a roughly reclaimed shaft and then expecting him to sort it out, but if he is set the job of making up test tapers in readiness, by chucking or between centres, with the aid of new shafts as patterns whenever available, and then setting his top slide by these new tapers, he can turn up a test piece that will serve all future occasions at the particular measurement.
Once a male test piece is in being, this halves the task facing the turner given the rough workpiece to true up. The test piece should be of sufficient length to go between centres, and its reverse end trued up for chuck location, in order that a new flange can be offered to the test piece for taper comparison. Once the test piece is truly centred, and having checked that the tailstock centre is dead in line with the lathe, the dial or clock gauge can be mounted on the tool post. and the clock plunger set against the test piece taper at zero. The top slide will be correctly set round when the dial reads zero over the full length of the test piece taper.
One important tip—it is vital that the clock gauge plunger is positioned at the exact dead-centre height, otherwise if above or below the true dead-centre line, the top slide can be incorrectly adjusted and the work spoiled. Where there is no dial gauge, the lathe tool can be put in contact with the taper at nil load, and moved along its length, the faint scribed line indicating the accuracy of the setting. I will give further details of this particular exercise next week.