E-MAIL   Tony@lathes.co.uk
Home    Machine Tool Archive    Machine-tools for Sale & Wanted
Machine Tool Manuals   Machine Tool Catalogues   
Belts   Books   Accessories


Chase Screwcutting - Thread Chasing

Simple in principle and easy to use, the method of "chase screwcutting" has been used on lathes since the middle of the 19th century with a different form, of a sliding headstock spindle, being widely used on earlier ornamental turning lathes. Frequently used by optical instrument makers, who needed a short but very accurate thread, a disadvantage of the "chase" system was the need to keep in stock a range of master threads and their matching star-shaped bronze followers. However, with the relatively limited range of thread pitches required, this was not unduly expensive and, usually included in the set to allow the manufacture of chuck backplates, etc., would have been one coarse-pitch pair with the same thread as used on the lathe's spindle nose..
In its simplest form a
master thread (or hob or leader) is attached to or made to rotate with headstock spindle and a guide, held in contact with it, transmits the motion to an adjustable cutting tool engineered so that it can slide along the bedway and impart a copy of the thread to a workpiece held in a chuck or between centres. The system as used in modern times may have been 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." More complex arrangements of the same type have also been used, with trains of gears to vary the ratio between the master and generated threads - that fitted to the American precision Waltham lathe being a good example. Based on ornamental lathe practice, a further development was to arrange for the headstock spindle (or even the whole headstock) to slide, a system seen here in use on a Kärger lathe, and a system that was also a feature of most "Swiss-Auto" mass-production lathes used for the manufacture of tiny parts. Still in use today on this type of lathe these are now (of course) fitted with CNC controls and capable of prodigious rates of output. The last conventional lathe on which it appears to have been available was a version of the Schaublin 102 marketed in in the early years of the 21st century.
Similar chase-screwcutting arrangements can be seen on the pages devoted to, amongst others, the American makers Goodell-PrattPratt & WhitneyAmesWaltham Machine Works and Wade.
Sliding headstock screwcutting, a variation on the theme, can be seen on the Karger, Auerbach, Lorch and other pages..


The Potter chase Screwcutting Attachment being used to cut the thread on a collet

Potter precision bench lathe
Like most other genuine "Precision Bench Lathes" manufacturers, Potter offered the option of a "Chase" screwcutting attachment. The method involved a
Master Thread (also variously known as a hob or leader) mounted behind the headstock and driven through a set of changewheels. A "half-nut" pressed onto the Master Thread and conveyed its pitch, via a sliding bar connected to an adjustable toolholder, to the cutting tool which was in contact with the workpiece. The interconnection of the cutter holder and the half nut allowed the nut to be lifted out of engagement (by a handle attached to the toolpost upright) and the cutting tool returned by hand to the start without stopping or reversing the lathe. A little additional depth of cut could then be applied by the tool slide, the half-nut rested back on the Master Thread - and the cut restarted.
Whilst this system produced absolutely accurate threads (and was especially suited to delicate operations on thin-wall tubes used to construct such items as microscopes) the length and pitch of thread that could be cut depended upon the availability of the appropriate Master - although in the case of the Potter the thread length of five inches would have been sufficient for most jobs and the eight additional changewheels supplied as standard multiplied by eight the number of threads each Master could generate. Each hardened Master Thread - they were available in both inch and metric forms - was provided with a fluted section at one end so that it could be used to "hob out" its own replacement half-nut.
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.
Unusually, unlike many of its competitors, the Potter needed neither another slot in the back of the bed, nor a special headstock casting to use the attachment, instead the whole unit was mounted on brackets which clamped to the top of the bed. A neat touch was the provision of a pad which fitted on the bed and protected it from accidental blows by the rather heavy hand lever.

Goodell-Pratt Bench Lathes Nos. 125 & 494.
The Screwcutting Attachment, (No. 166) could only be supplied if it was built into the lathe when new.  Although just a  24 t.p.i master was supplied as standard, any pitch could be ordered. 


Waltham lathe fitted with "Chase" Master-Thread screwcutting. 
This picture shows clearly the well-proportioned items used on the Waltham to perform this early type of screwcutting. The bed was turned so that the T slot ran down the back of the bed; in the slot were three adjustable supports two of which carried the Master Thread (also known as a
hob or leader) - whilst at the headstock end the third bracket supported an arm that carried additional gearing to extend the threading range.
A "half-nut", held in the base of an adjustable tool-slide, pressed on the thread and transmitted its form 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 of the thread without stopping or reversing the lathe; a little additional depth of cut could then be applied by the tool slide, the half-nut rested back on the master thread - and the cut restarted.
Whilst this system produced absolutely accurate threads, and was especially suited to delicate operations on thin-wall tubes used to construct such items as microscopes, 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.
In order to reduce the spindle speed for screwcutting, or larger faceplate work, Waltham offered a special headstock with a completely enclosed 4 : 1 ratio epicyclic-reduction gear.
Alternative and sometimes more highly developed arrangements of the same system can be seen. on the StarkPratt & Whitney, AmesPotter and Wade pages

The Stark lathe, like many in its class, could be fitted with traditional "chase screwcutting" where a T slot, which ran down the back of the bed, held supports which carried a sliding shaft to which was attached a toolholder. Above the shaft, at the headstock end of the lathe, was a "master thread" driven by a selection of gears from the end of the main spindle in such a way that its threading range was extended by a multiple of six.
A follower (with an insert carrying a few threads of the same pitch) pressed against the master thread and transmitted its form to the workpiece via the sliding shaft and a threading tool held in an adjustable slide rest.
Whilst this system produced absolutely accurate threads, and was especially suited to delicate operations on thin-wall tubes used to construct such items as microscopes, the length of thread that could be cut, and the number of threads per inch or mm, depended upon the availability of the appropriate master.


E-MAIL   Tony@lathes.co.uk
Home    Machine Tool Archive    Machine-tools for Sale & Wanted
Machine Tool Manuals   Machine Tool Catalogues   
Belts   Books   Accessories


Chase Screwcutting - Thread Chasing