This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
By the use of the device shown in Fig, 10, it is plain that a different "master screw" was needed for each different pitch of thread to be cut, although the diameter of the work might be anything within the range of the lathe to hold and drive, so that provision was made for supporting the inner ends of the piece to be cut and the "master screw," and for driving the latter by the former. The idea of driving the "master screw " or lead screw at a different speed from that of the piece to be threaded had not yet been thought of, and it was years before this development took place.
But before proceeding to this phase of the development of thread cutting, and consequently with the further development of the lathe, let us look a little farther into the methods of generating threads. That is, of producing the "master screw," from which other screws might be made.
The author well remembers during his boyhood an old curiosity shop out in the country in which various kinds of hand machines were made and repaired. Among other things made were various appliances and devices for spinning woolen yarn and reeling it up into skeins of forty threads to a "knot," as it was called. To furnish an automatic counter for this reel a worm-gear of forty teeth was used which engaged with a single threaded worm on the reel-shaft. Both the shaft having the worm formed upon it and the worm-wheel were of wood, usually oak or maple, and the thread was formed by wrapping a piece of paper around the turned shaft and cutting through this with a knife so as to make its length equal to the circumference of the shaft, its width representing the longitudinal distance on the shaft. This piece of paper was then divided into equal parts at each end and inclined lines drawn upon it as shown in Fig. 11, the divisions being equal to the pitch of the thread, found by spacing the circumference of the worm-gear blank for the forty teeth. The paper was then glued around the shaft and the diagonal lines gave the correct development of the screw thread, which was worked out with a fine saw, a chisel, or knife, and a triangular file. The teeth of the worm-gear were similarly cut to the proper V-shape, and the result was a perfectly practical and really workmanlike piece of mechanism that answered the purpose remarkably well.
This same method of laying off screw threads was in practical use many years ago and was used by one Anthony Robinson in England as early as the year 1783, at which time it is recorded of him that he made a triple-threaded screw 6 inches in diameter and 7 feet 6 inches in length. It is said that he first laid off one thread by the method above described, leaving a sufficient space between the convolutions for the other two threads. This first thread was then worked out by hand with the time-honored hammer, chisel, and file, and he afterwards used this thread as a guide for making the other two by the same primitive means.
In the light of the present facilities for cutting threads this process seems most tedious and laborious, and yet much of the machinist's work of that time was equally slow and must have sorely taxed the patience of the workman, whose principal and often only machine was a lathe of very crude design and workmanship, and in which he managed to do not only turning and boring but slotting, splining, milling, gear-cutting, and an endless variety of similar jobs, and in lieu of a planer having recourse to his ever ready cold chisel, hammer, and file, which with a straight-edge enabled him to make many a flat surface of remarkable nicety considering his limited facilities. And from these pioneer machinist's came the American machinist of to-day, the most thorough, best educated and expert mechanic the world has ever seen,
Fig. 11.-Thread Developed on Paper.
It will doubtless have been noticed that in the earlier examples of the lathe, as in most of the machines in use, the framework of the machine in the lathe, the bed, and legs, were made of wood with the various metal parts secured to them. A good example of this method of construction, as well as the general construction of the lathes of the date when this one was built, is shown in front end elevation in Fig. 12, and in front elevation in Fig. 13. The history of this lathe is well known to the author, who was well acquainted with the old Scotchman, one John Rea, who had a small machine shop, wood shop, iron foundry, and sawmill in East Beekmantown, Clinton County, New York State, during and for many years prior to the civil war.
This lathe had, as will be seen by an inspection of the drawings, a bed composed of two timbers, placed at the proper distance apart and supported upon wooden legs, which in turn rested upon a cross timber supported by the floor. The timbers were of hard maple, those forming the bed being about 5 inches thick and 12 inches deep and were about 15 feet long. The lathe would swing about 32 inches over the bed. The patterns were made by Mr. Rea, the castings made in his foundry, and the machine work done in the nearby village of Plattsburgh.
The "ways" or V's of the lathe were of wrought iron about 5/8 x3 inches let into a "rabbit" cut on the inside edges of the timbers forming the bed, and fastened by large wood screws. The top edges of these iron strips were chipped and filed to an angle of about 45 degrees to the sides, thus making the V an angle of about 90 degrees. The head-stock had cast in it square pockets in which the boxes for the main spindle were fitted by filing, and were held down by a rough wrought iron cap through which passed two threaded iron studs which had been cast into the metal. Upon these were two nuts as shown. The main spindle was of wrought iron and carried a wooden cone pulley built up on cast iron flanges keyed to the spindle. There were no back gears.
Fig. 12. - End Elevation of "Chain Lathe."