by Graham Howe
THE SCREW CUTTING clutch mechanism originated, I think, with
the Hardinge lathe and was used to simplify the cutting
of machine screw threads without having continually to stop and start the motor for reversing.  The type of lead
 screw used (metric or imperial) determines the way in which threads are cut, and the following article assumes
the lathe has an imperial lead screw.  A metric lead screw lathe conversely presents the same conditions with a bias
 towards metric threads.

Using the thread Indicator on imperial threads
With a lathe designed specifically for imperial thread ranges there is usually a thread engagement indicator
 mechanism positioned at the side of the saddle.  Imperial range machines have a lead screw which has a prime
 number of threads such as 4, 6 or 8 threads per inch, the latter most common with smaller lathes such as Myford
The screw cutting procedure basically includes the following steps: set the cutter to start depth, engage the
 screw cutting lever on a known indicator position, machine the thread to its full length and then disengage the lever,
withdraw the cutting tool, and manually wind back the saddle to the start of thread, reset the depth of cut and repeat the process until full depth of thread had
 been reached.  During this time the motor would usually remain running.

Metric threads on an imperial lead screw

Using the same lathe with an imperial lead screw presents a few problems cutting metric threads because the indicator is not an exact multiple of the metric
 thread pitch.  The upshot of all this is that the indicator is of no value so when the screw cutting lever is engaged it has to remain engaged until completion of
 the full depth thread. 
Although the basic steps are the same as above the only way to ensure the lever remains engaged requires the motor be stopped at
 the end of cut and reversed to the start position.  This constant need to stop and start the motor is not only very time consuming but also wearing on the
 motor and control switching.  The solution to this problem is the adoption of a clutch to automate the reversing of the lead screw while the motor  is running
 and remains in one direction during the complete process.

Screw cutting clutch concept
The above description identifies the need for a clutch mechanism especially for metric threads (in the case of an imperial lead screw).  However,
 once a clutch is fitted then the whole process of machining screw threads is very much simplified since the lever after having initially been engaged can
 remain so and the clutch automatically provides the ability to reverse to the start of a thread and importantly, remain in exact synchronization of the thread
 form being cut.

Screw cutting clutch for the BH600G lathe
A mutual contact with whom I had discussed the design of my Sunderland Gear Machine,
 introduced me to Gray Meek an engineer of much experience who among other projects had previously
designed a screw cutting clutch similar to that used on the Hardinge, a lathe Gray had used in the past.  I
think his original design was for the Myford S7 as he made two prototypes for customers some 20 years
ago but despite these prototypes being very successful and actively used the further promotion of the
 design remained hidden. 
That was until Gray resurrected it for an article in Engineering In Miniature
magazine and a book he was about to publish and to be reviews son this website next time.  Gray was keen to widen the scope of his clutch design to
 include a variety of lathes and I provided him some dimensional information relevant to the Myford ML7. Later I then bought a Warco BH600G lathe (also known as the G9249 in the USA) and so asked Gray if he
 would further expand his design to include my new acquisition, which he readily agreed. The BH600G is a
substantially larger lathe than the Myford but, with a conventional gear train and Norton style gearbox plus
 the lead screw was 8tpi, to cut metric
 threads demanded his clutch design.
Over a period of a few months the design
appeared and although the BH600G looks
 like a larger version of the Myford ML7, the design of the clutch was very different, though
 based on the same working approach.  Gray told me that every lathe presented new problems,
 design-wise, but so far he had managed to find a solution. 
To make matters worse for him I
 requested that he incorporated a facility to manually disengage the clutch from use by a simple
 lever.  The reason for this request was due to the noisy gear train especially at high speed
 running of the tumbler gears.  With the clutch design both tumbler gears operate at the same
 time and I guessed it might get even noisier!  Thread cutting, as I see it, is done at low speeds
 and in the case of the BH600G probably using back-gear.  The lowest ‘normal’ speed is 300rpm
 which is I think too fast for comfort especially on short thread lengths so my request was, for
 me, a crucial requirement. 
A few weeks passed by an Gray casually mentioned that was now
sorted using an eccentric to rotate the driving tumbler gear out of engagement with the spindle
 gear.  It seemed this was not his main problem as the BH600G had several obstacles (spindle
bearing cover) to avoid and eventually, an ingenuously, he devised a method for the main
 control shaft of the clutch mechanism to pass through the centre of the main gear shaft.  With
 this solved he passed over the basic drawings to me which left out some of the later parts such
 as the auto link mechanism for stopping and starting as this was, for him, difficult to design with
 a lathe he had never seen!
At this stage I want to pay tribute to his design and engineering skills however, on completing the clutch he casually remarked that he had some worries
 about it and hoped it would work out!  Well it did thankfully.

Design and manufacture

The design for the main clutch mechanism belongs to Gray Meek and as such I am unable to provide any drawings but I know
 he will provide these or give me permission for me to help on this front if needed.
(we are happy to pass on requests - click here) On receiving Gray’s drawings I made my own set of drawings by modelling. This helps me understand the task in hand and
 also proofs the dimensional accuracy of the parts. All dimensions provided by Gray proved to be accurate but a few minor
modifications had to be incorporated on assembly. These modifications should be updated into the source drawings. The
linkage and application of this to the BH600G is of my design and thankfully a very simple task.
The starting point for this design is to make the gears. Having my own Sunderland-based gear cutting machine I was able to make these using this machine
 despite the gears being at the limit of its scope. 
The gears are all 1.5 Mod, 20 degree PA. I decided to make the two ‘tumbler’ gears (25t & 30t) from Delrin
 as this has shown to be remarkably strong and quiet running.  As an aside, when making these gears I was able to see my gear cutter working in an
 unstressed condition since Delrin is a soft plastic material and this led me to identify a solution to a problem on the machine which had been bugging me for
 over six months! With this new-found knowledge, work stopped on the clutch and the Sunderland was stripped and a new module added which has proved
 to be very effective.
The other two gears are from steel and 40t.  These have a phosphor bronze bush pressed in and a recess on one side to house
 the clutch ring.  The main gear shaft is then made, I used a hard alloy steel because I had been given some and the
 Myford did not like cutting it but the bigger lathe managed well.  Hopefully, the alloy steel will provide good wear
 resistance but is not essential.  Fitted to the main shaft is an
outer rotating shaft which is keyed for the clutch ring and 
 output gear to the main gear train.  This was made from steel but, and a change to Gray’s design, I included a
phosphor bronze bush.  This was actually easy
to incorporate by initially force-pressing a solid phosphor bronze rod into the
 rough turned work and then boring/reaming the bronze to shaft size.
Next the clutch ring which is double sided with a part recess and an outer groove for the actuating
lever-plate.  This requires some care in manufacture and takes time to machine using the rotary table on the milling machine to machine the part-
At this stage a parcel arrived from Gray which was
the aluminium billet needed to be carved up to house the
 mechanism. I was very thankful to Gray for providing this as I think, getting this locally would have been impossible. The modelled drawings I had made helped me understand how best to machine this block especially so as there was
 no second chance should a mistake be made.  Essentially, the block is first machined on the rear side, then the front
 side and lastly milling off the redundant metal.  I planned the process to be done using my DRO on the milling
 machine but it decided mid-way to stop working and it is only then that the virtue of a reliable DRO becomes evident! 
Despite this set back, the machining was finished and offered up to the lathe where it became evident it was a very
tight fit due to the spindle bearing housing. This required re-machining the recess to very slightly enlarge it. 
 With some of the smaller ancillary parts made a full trial assembly can be made. 
I think anyone contemplating making this clutch unit must accept that it will
require time and patience fitting the parts.  The clutch ring is operated by a lever-plate which in turn is operated from a shaft (or shafts).  The movement is
 controlled by a small shaft which fits to the main aluminium body and again I added a phosphor bronze bush for this to slide on though Gray is of the opinion it
 is ok without a bush.  This shaft has a V-ring machined into it which acts as an indexing point using a spring and ball to locate the centre position.  It was
 Gray’s view that while the ‘V’ used on the other designs it might not be best placed and some alternative position for controlling the centre and left/right
 positions of the clutch would have to be remote from the main block. 
This proved to be partially correct and in practice I designed a set of two opposing
 collars fixed to the operator handle shaft with three spring loaded balls to locate (index) into their appropriate ‘V’ set in the opposing collar.  All this was to
 some extent trial and error but eventually a solution was found that worked but it also needed the original ‘V’, ball and spring on the inner part to prevent
 localised movement. Fitting these collars and setting the precise location of the ‘V’s for the spring loaded balls is not simple and takes time. However, a
 process has now been devised should any builder experience problems on this front.
The other thing Gray understandably omitted from the drawings was the linkage
 mechanism.  This linkage fits to the rear of the operating shaft such that when
 moved it will push the clutch out of engagement for both directions of saddle travel. 
The mechanism I designed comprised a rear-end lever, push rod, two control stops
 and saddle stop. The rear end lever was initially a problem as there was little room
 to fit any lever as is would foul the gear cover. Eventually a fabricated design was
 arrived at which only required a very minor fold to an inner edge of the gear cover to
allow the cover to open fully as before.  The push rod was a length of 0.5” steel bar
 with a flat milled on one side and this runs the length of the lathe bed at the rear. 
The extreme end, near the tailstock, needed some sort of support for the push rod
 and this was done utilising the already fitted support bar on which the splash-back is
 fixed. The two adjustable stops slide along the push rod and are fixed as appropriate with a cap screw  that clamps a
 bent U-shaped spacer against the rod flat thus eliminating any possibility of raising burrs.  Finally,  the main stop
 control arm was fabricated from steel plate and this fits to the top of the cross slide in the T-slot with adjustment to cater for different sizes of thread outer

This is not a dead easy project but worth the effort because from now on machining metric or any other threads is dead easy.

See here for Part 2 - using the new clutch.