'toad and workshop
Page 51
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Handyman
3enchwise: lathe sense (29)
C THIS STAGE let's spend a moment on at has been learned:— Le speed of the lathe lead-screw can be -ied where the lathe is equipped with tachable wheels; with the aid of -npound gears all the required speed anges can be arranged; wheels are usually irked with their teeth numbers, therefore should not be a difficult•matter to work t the driven speed from any combination. While there are many charts and lists ailable that cover all the combinations, !se can confuse the trainee until he has come quite familiar with just what is lied for, so he should not try to fathom t from a list, or from the plate on the he, just what he needs—it is a matter of flutes to mark driver and final-drive wheel th chalk and do a free-wheel ratio check, volving the mandrel by hand.
Having come this far in simple steps, let progress. Most textboolA tend to insist exact formulae and these, unfortunately, ay appear complicated to the learner. ierefore, this week. I hope to clarify atters with a few words on screw-thread laracteristics, which consist basically of re: diameter, length, thread form, pitch id lead. Pitch and lead can be misleading though on a single-start thread they are le and the same thing, and indicate the [reads per inch of length, or the amount in ches which a screw will advance per single volution.
Many lead-screws are themselves cut at eight threads per inch of shaft, thus one turn of the screw will advance the thread one -kin., and the meaning of the word "pitch" can be understood as referring to the distance advanced each revolution. A thread of *in. pitch is in fact eight threads per inch, but remember I am referring to a single-start thread and that in this instance pitch and lead are the same thing. However, there is a different meaning when multiple-start threads are dealt with, so for the moment I will stick to cutting a single -start thread.
Let me make doubly sure that the point about mandrel and lead-screw speeds is made quite clear—for instance if the mandrel is geared to revolve at double the speed of the lead screw, and the lead screw is eight threads per inch, then a thread will be cut at 16 tpi. It follows that if you further increase the ratio, that is increase the mandrel speed to, say, three turns to one, of the lead screw that is three to one, then you will produce a thread of 24 tpi. If on the other hand you lower the mandrel speed and arrange that the lead screw is running at double mandrel speed, then you will produce a thread at only 4 tpi.
A wide range of threads can be cut by using the same pitch of lead screw for all, simply altering its speed in relation to the speed of the workpiece in the lathe. As I have mentioned earlier, most screw-cutting lathes have their gear ratios or speed-change data printed on a plate fixed to the lathe, br lists and charts are easily obtainable.
However. I would not ask any trainee to try and fill his head with all the figures for this work. Instead, the important thing is for him to fix in his mind the basic principle for cutting a thread, how to set up for a given lead screw speed, and then keep by him for reference as required the various changes. In other words, never force all the data on him first, let him get on with setting up and cutting a thread and his memory bank will build up as he goes along.
Obviously, on a lathe where loose wheels are used to change speed, some calculations will be necessary, particularly where a compound train has to be set up. but a good point to remember is that most change wheels progress in steps of five teeth, so this permits a fairly convenient multiple of five.
Now for a look at threads in common use, and by this I mean the form or shape of the tooth and its purpose in engineering. While in vehicle repair work we meet mainly two forms of thread so far as screw-cutting is concerned, there are in fact four main thread patterns. vis for ordinary bolts and nuts, they are Whitworth, BSF, BA and Unified which is the V-thread and is the most commonly used.
This is not a true straight-sided V, however, as with the British style the root and top of the threads are rounded, whereas American. Continental or metric forms are flattened top and bottom. Of the various measurements, only the angle and depth need concern the garage turner, and these two figures are 55 degrees for the angle and the depth is usually .64 of the pitch.
The second thread, seen to a considerable extent on trailer legs and other long threads such as two-way jacks or pullers, bench vice, etc, is the square thread; it can also be found on the lathe cross-slide. This is a simple thread to work and its depth is equal to half its lead.
The third pattern is known as the Acme thread and it is a little like the square thread in that it is flat-topped but the teeth slope outwards to the roots at 29 deg, plus a .010in. allowance or clearance top and bottom. This type of thread will be found on a large number of lathe lead and feed screws, where the split-nut principle is used as a means of engaging and releasing drive, as the taper is an advantage in assuring easy engagement and is also a neat mating of the two parts.
The fourth thread—the Buttress—is designed to take load mainly one way: each tooth has a vertical loading face and a back face sloping out to the root at 45deg. This thread is ideal for lift jacks or a press where the one way load can be difficult to calculate.