road and workshop
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by Handyman ,
Benchwise: lathe sense (9)
EARLIER in this series, writing of the need to reclaim long spindles; trailer leg shafts, brake camshafts and suchlike, I pointed out that the lathe can now play a major part in cutting both downtime and cost. It is, however, essential that the turner or lathe operator fully understands his machine, or can at least recognize a wear condition or maladjustment, and is able to guarantee a true and accurate-job.
The average garage lathe stands for long periods neither used nor cared for, until suddenly it is called upon for a number of rush tasks and the results are not always considered worth while. It may be that something has not worked out right or that a final measurement is wrong for no good reason; perhaps both the machine and operator are to blame. This sounds a bit rough but it is quite often true, and arises entirely from the fact that the lathe is at the mercy of everyone.
Essential checks Essential checks and servicing tasks have to be made if the finished job is to be of value. When speaking of reclaiming and truing up a long shaft or spindle, the first essential is to check that the lathe tool will travel exactly parallel to the longitudinal axis of the work, which means quite simply • that the tail-stock must offer its centre in a spot-on true line with the headstock spindle.
Tail-stocks lead a hard life, constantly on the move, subjected to load and vibration and can move out of alignment enough to throw a 2ft to 3ft job many "thous" out. Tail-stocks have their main casting separate from the base, and this enables adjustment to be made across the bed of the lathe; this adjustment can be used, of course, to deliberately set the tail-stock across for a special job such as a shallow taper turning operation, but its main purpose is to provide a means of resetting the tail-stock truly parallel with the ways. But many part-time turners have no knowledge of this adjustment, or how to go about checking and setting up the tail-stock for a long and true parallel cut.
I would like to explain this essential task, and so remove any mystery that may appear to surround this operation. As I have indicated, any lathe tail-stock can be a few "thous" out of line after a spell of general work, and while this may not show up too seriously on short jobs, it can make a real mess of that• long shaft or spindle when absolute accuracy is needed at each end, and this particularly applies with long gearbox, shafts, steering spindles and those prop-shaft jobs that are now rearing their heads much more than when vehicles were slower.
Normally, in a well-equipped workshop with a full-time turner, the lathe kit will include a test bar. This bar normally takes in almost the full distance between head and tail centres, and is of sufficient diameter and stiffness not to sag or bow under its own weight. Where such a test bar exists it is almost certain that there will also be a dial or clock gauge, thus in these good circumstances the task is not difficult.
The test bar is fitted between centres and firmly preloaded by the tail-stock hand wheel, the clock gauge is set up in the tool post with its plunger set against the test bar, the carriage is then wound along the lathe bed from tail to headstock, and any variation on the dial is noted. Where variation is indicated between either end the adjustment at the tail-stock base is used to remove this offset, and a state of neither plus nor minus is aimed for over the full length of the test bar.
This, of course, is easy as the tackle is there to do the job, but often no test bar is available and probably there is no chance to borrow one, thus the turner has in front of him a long job to be finished true, but no Means of ensuring that this is going to happen when down to size. Therefore he has to set up the reclaimed shaft between centres and tackle the job another way, viz. prepare the lathe for a first cut at full length, but set for a fine cut to avoid loading or springing the workpiece, as this would throw out test readings.
After this first fine cut, readings should be taken with a micrometer, the reading for each end carefully noted down, and then the tail-stock should be set over exactly half the amount of the difference in the correct direction for truly parallel results. To simplify this, if the job measures smaller at the tail-stock end, it indicates that the tail-stock is too near to the tool and must be set over away from it; if the test reading is larger then the tail-stock must be brought across towards the tool, and, of course, the whole job can be speeded up if a clock gauge can be obtained.
However, the same result can be obtained by either a micrometer or vernier, but it is possible that a second cut may be necessary before it is satisfactorily proved that the cut is truly parallel with the lathe. If there is no clock gauge, micrometer or vernier available, then a rather old-fashioned test method will be needed. Although this calls for a test bar or a known true shaft, centre up and then with the aid of a pointed tool in the holder, just touch the shaft end by moving the carriage along; the variation can be noted between ends, and the adjustment made by the earlier system of moving half the noted variation. Here it helps to place a white card under the tool or on the lathe carriage—this shows the gap between tool and work quite clearly.
I should emphasize that if any serious work is to be tackled with the garage lathe, this aspect of lathe care must become a regular part of the operator's duties if quick and accurate results are to be achieved with long jobs; it is well known that full-length tasks of this nature are probably one of the most exacting jobs the turner will face.