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road and workshop
Page 75
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by Handyman
Benchwise: lathe sense (32)
ALTHOUGH there is no call on most vehicle fittings for long deep threads, the semi-trailer is an exception in that long threads are utilized for leg retraction; whether or not such threads are inside a leg tube, they will still be subject to the hazards of operating.
It's quite a common occurrence for a long screw-thread .to be damaged at its centre, and even should you straighten out this section there is no guarantee that the whole thread will be back in shape; even filling with weld and then picking up the thread in the .lathe does not always work, but it's worth trying if the area of damage is small.
If, however, the screwed spindle is bent or bowed over a large area, it is best to thread a new spindle. Where this is a heavy deep -V-form thread it usually takes a long time, but the process can be speeded up by an alteration in feed methods. Initially, I described how the tool point is fed straight into the work at exact dead-centre level, offering both sides of the arrow head to the work. The only variation is the one explained in the last article, whereby the tool is moved parallel with the lathe to widen the V in the early stages, using one side of the tool and then the other.
Instead of this system, the top-slide and cross-slide can both be used; to do this you first set the top-slide itself round to exactly 274-°, hold the thread angle of 550 and then slacken the tool post and again line up the tool straight into the work as
before. Now, on starting up, you will be using the top-slide instead of the cross-slide to advance the tool into the work. As this means that the tool is now entering at an angle of 274° to the lateral axis of the thread, it will, of course, only cut on the forward edge. Therefore only one side of the thread is cut by the tool profile, so the other side of the thread takes its angle from the advance of the top-slide.
Really heavy cuts can be taken by this method, and the semi-trailer's long leg thread which I mentioned, can be made much more quickly. The reason for using this method is to avoid the binding or wedging action, as the tool now cuts on one edge only, the same way as a normal turning operation. This may appear a simple operation but it is necessary to make certain checks and take precautions.
When starting up and before touching the top-slide, set the tool into the work by the cross-slide for a preliminary cut; where a dial index is fitted its reading should be noted, and if necessary chalked to identify the position.
On reaching the end of this trial cut, bring the tool out by the cross-slide, put the lathe into reverse and while this is running advance the tool for the next cut, this time by the top-slide feed. Once the thread starting-point is reached, wind the cross-slide forward to the chalk mark—its original setting—and the thread will be picked up for another cut.
Here the depth of cut relates to the amount of top-slide advance, so once the initial cut has been made the exercise is merely a repeat of advancing the tool by the top-slide and retracting it again by the cross-slide. Care must be taken to ensure that the cross-slide is always lined up with its original setting.
Once you have had a little practice with this system, you will find that the whole operation of cutting a V-form thread of large dimensions can be expedited, because you carry out the separate moves described without any step or pause; in other words with a little practice it becomes a continuous operation with even the reversing time being used to good advantage.
Very likely now that Continental and American trailers more frequently turn up in our transport workshops, the turner will find himself confronted with metric or American threads, probably in V-form but with one big difference in that they will most likely have an included thread angle of 60'. Therefore if the top-slide system is to be used, the top-slide must now be set round to 3CP. A mistake can be made here if by chance the angle is set from the longitudinal axis of the screw—the correct procedure is to set the top-slide from the axis of the cross-slide, otherwise an error of 24° will
OMIT.
While on the subject of metric and American threads in V-form, I must emphasize the need for thread gauges for all V-form work. These are, as a rule, in sets of blades not unlike a feeler gauge set, each blade being clearly marked with its tpi. The teeth of the correct blade match perfectly into the threads of the workpiece with no daylight visible, and any overriding can be seen at once, so this tool is indeed a valuable part of the turner's pocket kit Unlike the V-threads and their gauges, acme and square threads need a different approach and are best dealt with by a caliper and steel rule or a Vernier scale, although the steel rule on its own will do the job, as the exact number of threads within one inch can be noted. I must, however, point out one trap into which so many people have fallen (myself included!): this happened through taking only the number of threads within the inch or square or acme threads, when in fact the true tpi is the exact number of spaces also, thus a thread of 10 tpi must also have 10 spaces or grooves within that one inch. Should a strange thread be measured from a Continental machine or trailer and you are still left in some doubt, the measurement should, where possible, be taken over a longer length— several inches.