My first lathe was a Cowell ME 90, which was later followed by a Drummond flat bed in a search for greater capacity. Both of these lathes feature a dog clutch on the lead screw to enable the power traverse to be disengaged automatically at a preset stop. This was found to be a most useful feature.
I then went to a modern machine and replaced the Drummond with a Prazimat DLZ, which has an adjustable carriage stop, but no means of automatically releasing the clutch. I use the stop all the time and to make life easier, I have replaced one of the securing Allen screws with a handle. It is now a simple matter to position the stop without the use of tools.
Having been inspired by the article in MEW issue 96 page 30 by Peter Foyle, and the description of a similar system by the late J Radford in his book, I decided it was time to try and resolve the problem with the Prazimat (after 18 years!). What follows is the device I came up with. As can be seen in photo 1, it is quite simple and requires little work or modification to the lathe. I made a start with the most complicated bit.
The pivot block
This was made from a stub of 25mm diameter FCMS. I used round stock because I didn’t have any square big enough. As it turned out, I am pleased with the final appearance and would use round stock again.
The stub was mounted sideways in the four-jaw chuck and a facing cut was taken to a depth of 5mm. The piece was then turned around with the machined face held square against one of the chuck jaws. Another facing cut was taken to a depth of about 3mm so this face was just level with the previously machined face. The location of the 2mm Dia. pivot hole was marked out and the hole drilled.
The piece was then mounted in the milling machine vice, seated on the larger face and the 10mm slot was cut to 7mm depth with an end-mill. The piece was marked out for the two holes in the centre of the slot and transferred to the drilling machine where a 3mm hole was drilled right through the centre position and then opened up to 5mm for a depth of 6mm. The second 5mm hole was then drilled to the same depth. It was then back to the mill where the piece was again mounted on the large face but this time a 1.5mm drill was pushed just under the rounded end before finally tightening the vice. Light cuts were taken with the same end-mill until the new surface ended just past the 2mm cross-holes. This extra cut is to provide clearance for the trigger to operate.
This is made from a piece of 10mm x 4mm BMS flat, cut to length and squared off to 65mm long. In the centre of the bar, 57 mm from one end, mark out and drill the 5mm hole. When complete, file a 30deg. bevel on the bar leaving a small flat portion on the end. Turn the piece over and scribe a line across the width 12.5mm from the opposite end and mill a shallow V groove to lock against the pivot pin, which is just a piece of 2mm Dia. Stock, cut to length and the ends rounded off.
The Pivot Block is now placed on the ledge above the clutch cam where it will probably be found that unevenness in the casting prevents both flat faces seating. Simply file a chamfer on the corner of the block until both faces seat fully. With the block level with the end of the ledge, spot through the 3mm hole to mark the end of the apron and drill tapping size for the M3 thread to a depth of 6mm. Although access is restricted, it is quite easy to drill the alloy casting with a hand drill, keeping everything square by sighting against the lead-screw. A ‘T’ handle tap wrench will sort out the tapping of the thread.
The block may now be mounted with an M3 Allen screw. With two 5mm Dia. x 10mm long compression springs placed in the holes, the trigger is pressed down on to the springs to allow the pivot pin to be pushed through, securing everything. The constant pressure of the springs should hold everything in place.
The bracket
Take a piece of 16mm x 4mm BMS, cut and square off to 45mm long and file a 2mm chamfer on one end which will become the top outer edge. Mark out and drill the four 3mm clearance holes, then align the chamfered end of the bracket square to the left hand outer edge of the lower portion of the carriage stop block, chamfer outwards and spot through the two 3mm holes. Drill the holes in the stop tapping size to a depth of 6mm and tap M3. This is then fitted to the carriage stop with 2 off M3 Allen screws.
This is a 16mm x 16mm cube of BMS squared up on all faces. Clamp the square end of the bracket to the bottom of the block, with the chamfer side of the bracket towards the block. Spot through the two holes and drill tapping size to a depth of 6mm and tap M3. On the face at right angles to the M3 holes, mark out and drill and tap through for the M4 thread fine adjustment screw. This screw is an M4 socket head cap screw 20mm long with the top of the head rounded slightly. The screw is fitted into the stop with a locknut. The profile shape is for appearance sake and is left to the user’s choice. The Stop is secured with two M3 Allen screws.
Pillars
The pillar replaces the stop pin fitted to the bottom of the clutch cam. It is a simple turning job from 8mm FCMS hexagon bar as per the sketch . The flat on the top should be horizontal when fitted and should contact the bottom of the apron as the detent ball engages with the cam in the disengaged position. It is secured with a nut and a spring washer.
The second Pillar is another turning from 8mm hexagon. This pillar replaces the front left outer M4 screw securing the bottom cover to the apron. After fitting the pillars, a small 21SWG tension spring, 11mm long is hooked over the pillars, keeping the cam in the disengaged position.
Pin
This is turned from 5mm dia. Silver steel and hardened. The pin is located by placing the cam in its engaged position and spotting through the 5mm hole in the trigger to make a mark in the blackening of the cam. I thought this would be the easy bit, wrong, the cam is casehardened. I found that with perseverance, a TC drill, (masonry drill) used at a slow speed with no lubricant, the drill could penetrate to its full diameter for about half a millimetre, I then changed to a HSS drill which made short work of drilling to a depth of 8mm. The pin is pushed into the cam with a touch of adhesive.
We are now ready for a test. The stop adjuster screw is run out so that the trigger is released just before the carriage hits the stop and the lock nut tightened. Using the calibrated collar on the carriage handwheel set to zero when hard against the stop, a dry run with the lathe switched off saw the clutch disengage when the saddle was half a millimetre from hitting the stop.
Another test under power using the fastest traverse speed; saw the feed disengage a lot closer to the stop. I presume this difference is due to friction in the drive components of the clutch when under power. Nevertheless, the system provides a reliable means of stopping the feedscrew drive prior to contacting the carriage stop. I just wish I had bothered to pursue the matter sooner.
One final thing, when everything is seen to be working, the trigger is case-hardened at both ends, in particular the 5mm hole and the face which the stop screw contacts. You could of course make the trigger from gauge plate and then harden and temper it.
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About the Author
Hi There I have spent most of my working life in engineering machine shops.
I started in engineering at age 15. I became a power press setter setting up manual and roll feed power presses up to 150 tons. One of the main components made was the top ring of shock absorbers for the Armstrong Patents Company in Hull. Another large customer was Vauxhall motors. We produced components for Rickman motor cycles, British Sagull outboard motors and many others.
l moved on to a company called Willcox and Gibbs. They were an American company who had a factory in High Wycombe and another factory in Poole. I went there as a drilling machine operator. This was at the time of the 3 day week. We ran out of work and all the men asked to be made redundant. I was asked to stay on and became factory foreman at age 17. I taught myself to machine industrial sewing machine components to very tight limits, often less than 1 thou. This included milling, drilling, turning and surface grinding. I also kept cutters sharp on a Clarkson grinder.
When they shut the factory down after several years, the manager bougt the machines and took over he premisies an we started looking for sub contract work. We produced some items for a lock company in Bournemouth. These made up into a lock controlled cartrdge insertion machine to bolt onto the top of safes. This was mainly sold to garages so staff could put money into the safe without having access to it.
l also also managed to get British Seagull to give us a chance. They gave us a drawing and a pair of sand castings to machine a crankcase for the new 170 outboard. We were a standby as the Seagull buyer had given 5 sets of castings to Villiers. Villiers installed a new CNC machining centre to produce these crank cases and we used a knife and fork. We produce a perfect crankcase and Villiers produced 5 crankcases covered with 4 jaw chuck marks. We received the contract for the new crankcase and went on to get the casing tube line, the gearbox line, the cylinder hads,the casing tube (drive shaft tube) and many other components. The casing tube line included producton silver soldering so I have problay done more silver soldering than most of the people reading this. We probably used about 50 sticks of silver solder a day. When Seagull went bust, the boss closed the factory down due to ill health.
i found a job as a self employed miller on a bridgeport turret mill. This helped me to mill very fast and very accurately. I remember scrapping two components in 3 years. I remember because it cost me money.
I moved to Aylesbury about 3 years later and found a job as a CNC miller. I had not done CNC milling before but had always been interested in computers. I taught myself to CNC mill using the manual. I was supposed to have been taught but the chap eaching me was off sick for six weeks.
I have had many jobs programing and operating CNC mills over the remainng years producing such varied item as military and aircraft components. The only aircraft I would have liked to have produced bits for was Concorde but apart from special purpose jacks for the wings, I never got to do any bits. So basically I started of producing very small sewing machine components and every size up to 4 metre long beams for the Airbus super guppy wing fixture and also many weird ad wonderfull parts of machines.
I am now sort of retired due to ill health and now do next to nothing.
