An automatic carriage stop for a Prazimat lathe.

Jim Whetren comes to a stop.

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.
 
The completed stop.
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 didnít 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.
 
Drawing 1.
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.
 
The trigger

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.
 
Drawing 1 a
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 ĎTí 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

Drawing 1 b
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.
 
The stop

Drawing 1 c
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 userís 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.
 
The carriage feed engaged.
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.
 
 
The cariage feed disengaged.
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.