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A carriage dead-stop is a very useful accessory on an all-purpose lathe, which the model engineer's lathe certainly is. It finds uses in facing and parting to dead length, repeat turning of steps and chamfers, boring to a fixed depth, and cutting graduations using some form of headstock dividing attachment. It can be used for internal threading too provided a hand-operated mandrel handle is used (it's clearly not suitable for threading using self-act). Myford offer a multi-stop as an adjunct to their turret attachment, usable independently, but this stop is mounted on the rear of the lathe which is hardly convenient for day to day use (it's not cheap either). The device described here is suitable for the Myford Super 7B. I'm not sure whether the design is suitable for some older Super 7's. Whilst the design of the gearbox casing has been modified I don't think, having looked at one, that this will cause a problem, but owners of those machines with inset halfnut engagement levers (and this includes the ML7R with gearbox fitted) would have difficulty mounting the carriage stop rod. If you can work around this problem then the device may be fitted.
The specifications for my own design were that it should be mounted on the front of the machine where it can be conveniently adjusted, and any adjustment should be easily accomplished and rapid. The stop should not interfere with normal lathe operations so it can remain in place until required. It was important to me that the minimum of modifications would be required to the lathe itself (preferably none) as, with the high cost of my new machine, I have an aversion to drilling holes in it and chopping bits off! The dead stop offers 4 pre-set lengths, and the use of interchangeable plain stop rods mounted on the carriage enables various working distances from the chuck jaws. With the carriage positioned as near the headstock as possible, and with topslide in mid position, the maximum adjustment range (i.e., shortest to longest stop whilst still allowing the indexing body to rotate) will be about 1 3/4". It is possible to extend this range by working with the top slide and using longer screwed and plain stop rods. The effective working range is also increased when working further from the chuck. I believe the device presented here fulfils most of the requirements for common usage.
Concerning mounting the device, the only modification needed is the provision of 2 blind-ended 4 b.a. tapped holes in the gearbox casing to secure the stop body shaft and base, the contact point on the carriage can make use of an existing 5/16" B.S.F. tapped hole which houses a grub screw locating the half-nut engagement lever. Into this hole is screwed either short or long stop rods to contact the screw-adjustable rods in the stop body. With the dimensions given this should allow allignment of screwed and plain stop rods - but check the position of this hole on your own machine. It may be possible to mount a separate base plate on the carriage to carry the plain stop rods on lathes other than the Super 7B.
The real difficulty was in finding a suitable mounting point for the stop body and sliding shaft at the front of the lathe between gearbox and carriage. The ideal place was the machined face of the gearbox casing but it was not obvious how the protruding gearbox shaft end collars could be avoided. Eventually I elected to make a larger collar to fit over the end of one of the shafts, where there was just room for it to be held by small cap-head bolts set into it's periphery, the stop shaft could then be screwed into this. Careful positioning of the components would allow the rotating stop body to clear the leadscrew guard.
The base for the stop shaft is a simple turning made from 1 3/4" dia free-cutting b.m.s. A slice about 5/8" long would be suitable. In the 3-jaw, face the end and using drills and boring tool produce a recess 1.125" dia and 0.150" deep. Turn round in the chuck and face the other side (interpose a parallel about 1/4" thick between chuck body and the previously machined face so you can machine right across), finish size is 1/2" thick though this is not critical. Two 'bites' need to be taken out of the base for it to lie flush on the machined end of the gearbox and to clear moving parts. One is to clear the end of the second gearbox shaft, and the other to allow passage of the leadscrew cover. The latter could be cut shorter but I prefer to make the part fit the machine rather than the other way round. The semi-circular recess, of 1.010" radius, nn" deep, is machined first either by clamping the part to the faceplate, flycutting in the lathe, or as I did - using a boring head in the milling machine. Leave cutting the clearance for the leadscrew cover until after the part is fitted, the necessary amount to be removed can then be easily marked off.
Locate the centres for the two 4 b.a. mounting screws from the diagram, centre pop and drill No. 31, counter bore the holes on the outer face to a depth of 1/8" using a 0.170" dia cutter to sink the screw heads (or just use a No.18 drill and chamfer the screw heads to suit). It would perhaps be better if there were 3 screws spaced equidistant around the base periphery, but difficulty would be encountered drilling and tapping for the screw nearest the leadscrew. This would necessitate removal of the leadscrew and probably also the gearbox from the lathe. In practice, the two cap-head screws are quite sufficient to hold the base firmly in place as most of the stresses are axial (compressive) with little radial stress to cause deflection.
Offer up the base and spot through the upper bolt hole onto the gearbox casing. Make sure you make a good countersink to locate for the tapping drill - you don't want the drill to wander from position. Remove the base and drill to a depth of 7/32" using a No. 31 tapping drill. You want to avoid drilling right through the casing. While it would probably do no harm for the upper screw as it's above the oil level, the lower one is below it. In any case, you want to avoid getting swarf into the gearbox. Use a bush with a locking grub screw as a depth stop (useful to keep a stock of these in various sizes). I used a cordless drill for this job and encountered no problems keeping the drill square enough for practical purposes. Carefully tap to full depth finishing with a plug tap, there's not much depth of thread but it's sufficient for our needs.
Open out the upper No.31 hole in the base using a No.29 clearance drill, and cut a 4 b.a. cap-head screw to length so it protrudes just less than 7/32" when seated. Use this screw to bolt the base lightly to the gearbox, and adjust by 'feel' such that the clearance allowed for in the base recess is roughly central. Nip up the screw to hold it firmly in place. Now you can spot through for the second bolt. Remove the base again and repeat the process of drilling and tapping. You should find that the base can now be bolted securely to the gearbox using both screws, if the drill has wandered a little the 4 b.a. clearance holes can be opened up with the next size drill (the No.29 was a close fit, No.24 is often quoted as clearance size). Check that there is no friction imposed on the gearbox by turning the handwheel whilst in gear. If there is, then one of the shaft ends is binding - either the inner recess is not central to the shaft or the semi-circular cut-out is not placed quite right. Adjust as necessary.
Run the carriage and leadscrew cover towards the headstock such that the cover contacts the base. Scribe on the base around the cover to indicate where to cut the clearance. Remove the base and mill a slot using a 1/4" cutter. The less aesthetic might elect to simply cut off the edge of the base to give the clearance, but it doesn't look so good. Re-assemble the base on the gearbox.
At this point you could conveniently make the longer of the two 3/8" dia plain stop rods for mounting on the carriage. This is about 2" long threaded 5/16" B.S.F. for a length of 1/4" at one end. On my machine the latter dimension was the depth of thread clear above the head of the grub screw, again, this is probably quite sufficient for our needs but another 1/16" or so can be obtained (if yours is less) by shortening the grub screw by that amount. Again, you need a close-fitting thread so open up the die a little. As this surface on the carriage is unmachined it would be better to provide a seating by spot-facing 1/2" dia around the 5/16" threaded hole. You can achieve this in-situ by screwing in a short (about 1" long) 5/16" rod to act as a pilot, and making from silver steel a matching facing cutter with a 5/16" reamed bore. In any case, on my machine the hole itself is tapped true to the lathe axis - and this is the important thing. If you make the plain stop rod a little longer - say an extra 1/2" - a point can be machined on the end (to be turned off later), and this can be used to locate the height of the horizontal datum line on the base. From this point scribe a horizontal line, and at 3/8" from the left hand edge of the base make a centre pop. This is the position of the 5/16" tapped hole for the stop body shaft.
Unscrew the base and set up in the 4-jaw so that the centre pop is running truly. Make sure the base is flat against the chuck face (tap it with a small hammer) and then centre deeply and drill right through with a letter H drill (or 17/64"). Either screwcut (preferably) or use a 5/16" x 26 tpi tap held in the tailstock to thread the hole, then remove from the chuck and clean off any burrs.
Part (nn), the stop body shaft can be made next. This is preferably turned from a 3 3/8" length of 1/2" dia silver steel, though precision ground m.s. could be used. The exact length is governed by allowing 1/8" clearance between the end of the shaft and the carriage when the latter is at it's far left-most position. Screwcut one end 5/16" x 26 tpi for a length of 3/8", use the mating hole in the base as a gauge - you want a firm fitting thread here which is not going to easily unscrew and it also needs to be square to the base. Mount the base on the gearbox and, with the shaft held in a drill chuck screw it firmly home by hand.
The next job is to machine a slot in the shaft to locate the locking screw. You really want this at the front so mark on the shaft the approximate centre line for where the slot should go. Remove the shaft and set up either in the lathe or milling machine, and mill a slot 3/32" deep the length of the shaft using a 1/8" cutter. Remove any burrs thrown up by the cutter with a fine file and emery cloth. Replace the shaft in the base.
Part (nn) the stop body can be made next. This is made from a 11/8" length of 1" dia material (either silver steel or ground m.s.). In the 3-jaw face the end, centre and drill right through 1/4", followed in stages to 31/64" (or bore to 10 thou under 1/2"). Finally ream 1/2" dia. Turn the end down to 0.750" dia for a length of 3/8" leaving a sharp shoulder to provide a seating for the rotating stop guide. Turn in the chuck and face the other end. Remove to the drilling machine and whilst set up on a V-block drill and tap for the 1/4" BSF locking hex grub screw (or 6 mm if you can no longer obtain BSF, they are quite difficult to get hold of now). Try the body on the shaft - it should slide smoothly without any shake. Turn a 1/8" shoulder on the end of the locking screw to engage the slot, only moderate tightening will be required to rigidly lock the stop body.
The indexing stop guide is made from a 3/8" thick slice of 2" dia f.c.m.s. This diameter just clears the leadscrew cover on my machine (it would be as well to check from the actual job). Drill, bore and ream the centre hole 3/4" dia, or if you don't have a reamer that size bore to .750" plus half to one thou to give a smooth fit on the stop body with no discernable play (use the body as a gauge). If you have a dividing head (I had previously made the Versatile Dividing Head according to G.H. Thomas' design, a very worthwhile addition to the workshop) then the next step is straight forward. If not, you are faced with the only other option and that is to mark out and centre pop first (though it won't make too much difference if you are slightly out). You need to drill and tap the 4 holes for the threaded stop rods 5/16" x 26 tpi on a 1 9/16" PCD. You can check with the pointed stop rod that this PCD dimension is correct on your own machine. Drill and ream the 3/16" diameter radial hole which projects into the 3/4" bore, follow this with a No. 3 drill to a depth of 1/4" and tap 1/4" BSF for a grub screw. This hole will house a spring and 3/16" ball bearing to provide for indexing.
Now would be a good time to make the 4 screwed stop rods. This can be done at 1 fell swoop by screwcutting an 8 inch length of 5/16" bms 26 tpi. Use a travelling steady. The tapped holes in the rotating stop guide can be used as a gauge to obtain a nice fit completely free of any shake. These screws must not be a sloppy fit or they won't hold their depth setting. I had tried to use a tailstock mounted die but just couldn't seem to adjust it open sufficiently to obtain the required fit. Part off 1 3/4" lengths and grind the ends flat and square,. Mount the stop guide on the stop body with one of the 4 screwed rods in place. Mount your pointed plain stub stop on the carriage and use this to align the first index position of the rotating stop body. Mark across both parts of the stop body and then remove them from the shaft. You are now going to form the 4 indentations on the 3/8" dia end of the stop body for the 4 index positions. Clamp the two parts together with the previously made marks aligned, and set the 3/16" reamed hole vertical under the drill. Pass a 3/16" drill down through the radial hole to make an indent on the stop body. If using a dividing head you need simply remove the rotating stop body and proceed, indexing round in 90 degree steps. Otherwise, you will need to mark off on the stop body the positions for the other three indents first. Follow the drill indent with a 3/16" ball-ended mill to form the seating about 1/16" deep (use a home-made cutter from silver steel if you don't have the mill - easily made in the form of a round-ended D-bit). Repeat the process 3 times to form all the seats. Wind a spring from 24 swg wire 13/4" free length and an easy fit in the 3/16" bore. When assembled you will find the indexing is very positive with no play at each stop position. As these indentations and bearing surface are subject to some wear it would be worthwhile hardening the stop body, tempered to dark straw if made from silver steel, or case-harden if made from p.g.m.s.
The retaining ring (part nn) holds the rotating stop guide on the sliding stop body. I had considered using a threaded ring but elected instead to use a thin steel washer (1" dia by 1/16" thick), held on to the end of the stop body by three 8 b.a. screws. The threaded ring might perhaps have been better as there is not much room for the screws. Clearance between the retaining washer and the rotating guide should be no more than a thou - just enough to let it rotate smoothly. Skim in the lathe if necessary.
All that remains to be done now is to finish off the 2 plain stops and either add knurled knobs to, or cut screw slots in the ends of the 4 screwed stop rods to facilitate easy adjustment.
The use of the accessory requires little comment. The illustration shows a typical use cutting graduations using a headstock dividing attachment. My initial worries about rigidity have proven unfounded and the stop is very positive in action. I have yet to test my other reservation - that of having the stop set and accidentally activating the self-feed. I rather hope the stop body would be forced back in these circumstances (I tend not to clamp it very tightly). I am considering replacing the lock screw arrangement with a combination screw and bronze key which might be better in this situation. I had considered using a 40 tpi thread for the adjusting screws (1 full turn would then equal 25 thou), but to offset this adjustment would be slower. Also, there is actually room for 6 stops (as I think the Myford multi-stop has), but I can't think of a use for more than 3 or 4. One thing that is pleasing is that the device looks as though it belongs there rather than appearing like an obvious home-made 'thingummy'. My own humble opinion, of course!
(c) Chris Heapy 1996.
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