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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES
FUNDAMENTALS OF
DRILLING & TAPPING
FUNDAMENTALS
OF THE LATHE
FUNDAMENTALS
OF MILLING
FUNDAMENTALS
OF SAWING
METALWORKING
SHOP TERMS |
SCREWCUTTING ON A LATHE The
mechanical generation of threads is essentially a very simple process - yet, at
the same time, a subject which has been subjected to the most intense research
and development. The following outline will explain the basic principles, but
not attempt to cover the detail as already published many times.
The book on screwcutting currently most suitable for the amateur (and
ideal to refresh the memory of the professional) is "Screwcutting in the
Lathe. This gives a complete breakdown of the process with simple-to-follow
instructions which enables even the complete beginner to cut threads
successfully. The
first use of the lathe for screwcutting was a revolutionary step. For thousands
of years the lathe had been, in essence, a potter's wheel turned on its side,
and capable, in engineering terms, of only the crudest work. By using a train of
gears to connect the lathe spindle to a long screw running along the length of
the bed - and the screw to the lathe carriage - the latter, together with
its cutting tool, could be forced to move a set distance for every revolution of
the spindle. If the spindle and its workpiece went round eight times, and the
cutting tool moved one inch, then a spiral would be cut with 8 turns per inch -
or 8 tpi. Threads were not new - cabinet makers had long been making their own
wood screws by eye and Hero of Alexander had devised a method of generating
larger threads - but what now became possible was the production of accurate
screw threads in any kind of variety and size needed - and for which there was
an increasing demand to translate the engineering ambitions of Victorian
engineers into working reality.
Once the lathe tool could be driven from a direct mechanical connection with the headstock (and so generate threads) it also became possible to produce, especially on longer work, a much smoother and more consistent finish - and at the same time greatly reduce the skill-tapping fatigue felt by the operator. This form of powered motion was originally called "self-acting" or "self-act" - and both terms were once widely used to distinguish between plain-turning and screwcutting lathes Different Threads: The first question that springs to the mind of the novice is, "Will my lathe be able to cut different types of thread?" (Whitworth, British Standard Fine, American National Coarse, British Standard Brass, American National Fine, British Standard Brass, Unified National Coarse, Unified National Fine, British Association, British Cycle Standard, etc.) The answer is yes. Providing the lathe has the changewheels necessary to gear the spindle to the headstock so that tool moves the right distance whilst the spindle revolves once - it can be done. The 'form' of the thread (which is what makes the difference between them) is entirely in the 'shape' of the cutting tool, which can be ground to replicate any thread shape and angle at will. If you wish, you could even invent your own …..…. A History Lesson: The two engineers closely associated with the development of mechanically-developed screw threads (although they did not "invent" the process) were Henry Maudsley, Machine Builder of London, England and Joseph Whitworth, Toolmaker, of Manchester, England. Maudslay was the first man able to make, and exploit, a very accurate screw thread. His masterpiece was a screw 5 feet long and 2 inches in diameter (1525mm by 51mm) with 50 turns per inch (50 per 25mm) on which ran a nut 12 inches (305mm) long with 600 threads. The apparatus was designed to average out pitch errors over small distances and was a vital element in the process of engraving the scale markings on astronomical and other critical measuring devices. Maudslay went on to manufacture a range of screwcutting lathes (using the principal of a "master thread" or "leading screw") examples of which can be seen in the London Science Museum, London and the Henry Ford Museum, Dearborn, Michigan, USA. Whitworth was a tool maker who brought a disciplined approach to engineering; his design and devilment skills ranged across almost the whole field of mechanics but, following the publication in 1841 of his "On a Universal System of Screw Threads" he is best remembered for his success in standardising what at the time was a chaotic system of hand-fitted, non-interchangeable threads. He collected together sample screws from a variety of workshops and, having examined their properties, proposed a system whereby the the ratio between the depth of the thread and its pitch was maintained over a range of sizes - and the angle of the thread be 55 degrees. The system was in use in his own workshops by 1858, and was quickly taken up by other engineers as its benefits of simplicity and interchangeability - to say nothing of its endorsement by the greatest living British engineer of the day - became obvious. Generating Threads in the Lathe: The reason that a screwcutting lathe can generate a thread is the inclusion of a "leadscrew" (originally called a "leading screw") in its construction. Any leadscrew needs to be very accurately made (they are often produced by specialist manufacturers, not the lathe makers) with an Acme, square or other thread optimized for the task - but never with a standard Whitworth or Metric form, as unfortunately found on many cheaper lathes. The leadscrew will reproduce its exact form (hence the need for accuracy) on the material to be threaded - providing it can be driven by a train of gears (or toothed belts in some cases) from the headstock spindle of the lathe. For daily use the changewheels are normally arranged to provide a very fine feed to the carriage; to set them for screwcutting means removing most or all of them and building up as fresh train - following the instructions on an engraved plate rivetted to the machine. At the end of the threading job the screwcutting train is removed and the fine-feed set of gears replaced. This time-wasting work can be largly avoided if a screwcutting gearbox is fitted -hence their popularity in industry - however, even a full "quick-change" screwcutting gearbox cannot generate every pitch of thread and it is sometimes necessary to substitute changewheels to extend the range of the box - or generate metric threads from an English gearbox, or visa versa. If the lathe changewheel chart is missing, all is not lost, the book, Screwcutting in the Lathe will help to calculate a fresh set. Hand
Forming Threads: It is possible to generate threads on a revolving
cylindrical surface without using mechanical assistance by employing a
"chaser". These look rather like wood-turning chisels with a thread
cut into their end face and are made from hard steel - tool steel for the
finest-quality ones - and vary in width and thickness according to the
particular thread they carry. They
are normally fitted to stout, wooden handles to give the necessary purchase
(which can be considerable) but using them successfully is not easy, and
beginners are well advised to avoid them completely - although they can have a
role to play in "cleaning up" a mechanically-cut thread. In use the chaser is rested against a suitable support - with
some lubricant between the two - and fed into the workpiece on center height
with a steady sliding motion. With luck the chaser will bite into the surface
and begin to form a spiral cut; as the other points on the chaser engage with
the spiral, the actions becomes, to an extent, self-stabilized and easier to
perform; many passes are normally required before the full depth of the thread
is generated.
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