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The Ames chase-type screwcutting mechanism
Like all other genuine "Precision Bench Lathes", the Ames could employ both the "Chase" and "Top Slide-with-Changewheels" methods of thread generation, the former system devised by Joseph Nason, of New York, who obtained US Patent No. 10,383 on January 3rd, 1854 for an "arrangement for cutting screws in lathes." In the Ames "Chase" method a T slot, which ran down the back face of the bed, held brackets that carried the long "transmission rod" on which the cutting-tool slide pivoted and slid. A separate casting carried in the T slot held, in two vertical arms immediately behind the headstock, the Master Thread. The Master Thread was also known as a hob, or leader, and was available in a wide range of standard and special threads.
A "half-nut", held in the end of an arm connected to the "transmission rod", pressed on the Master Thread and transmitted its pitch, via an adjustable toolholder, to the workpiece. The interconnection of the cutter holder and the half nut allowed the nut to be lifted out of engagement and the cutting tool returned by hand to the start without stopping or reversing the lathe. A little additional depth of cut was then applied by adjusting the rest "stop", the half-nut rested back on the Master Thread - and the cut restarted. Unlike the tool slide fitted to the Wade lathe, which was an especially well-designed unit with the tool carried on a compound slide rest (which enabled both lateral and vertical adjustments of the tool position to be made), that on the Ames was a very simple affair with the tool held in an ordinary compression clamp. Whilst the chase system produced threads of an absolutely accurate pitch, and was especially suited to delicate operations on thin-wall tubes used to construct such items as microscopes and telescopes, the length of thread that could be cut, and the number of threads per inch or mm, depended upon the availability of the appropriate thread master - although in the case of the Wade additional gearing was provided to extend the threading range of each Master Thread by a multiple of 1 to 10. For instance, a 10-pitch master would cut 10, 20, 30, 40, 50, 60, 70, 80, 80 and 100 TPI. To use the attachment, the lathe was run in reverse for right-hand threads - with the toolholder moving from left to right. For left-hand threads the master thread and its nut were reversed, and the toolholder moved from right to left.
Instead of a "V-shaped" thread from, the threading "masters" were of the buttressed type with side of the thread vertical, this face usually being set to face the headstock. The thread design - it can be found on older metal-working vices - has the benefit of not producing any upwards thrust so, unlike a V thread, does not to throw the bronze follower out of mesh with the master. Also, interestingly, the thread being cut does not vary in accuracy if follower and master are not in full contact for, should a careless operator fail to move the operating lever a sufficient distance, or with less force than necessary, the thread would still be generated and the tool not tend to "skip over" the partially-finished job as might happen with a V or Acme form.
Even inexpensive lathes could be equipped with form of screwcutting mechanism - an example can be seen on the Goodell-Pratt Pages.
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