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This lathe attachment is based on a design sketched by Geo. H. Thomas, which in turn was adapted from that of an earlier design by J. A. Radford. GHT's diagrams can be seen in his book "The Model Engineers Workshop Manual" (TEE Publishing, ISBN 1-85761-000-8), though I don't know if J. A. Radford's original design was ever published. GHT is justifiably remembered as a master in the area of small machine tool design - the large number of Model Engineers making tools to his designs bear testimony to this fact. So, far be it from me to try to improve on his ideas. What I have done however, is to offer a fully dimensioned drawing to suit the Myford S7 lathe (GHT purposefully left his plans vague because of the large number of different lathes the attachment may be used with), and to add my own experiences as a much less-skilled model engineer in my attempt to build the tool.
The chuck backstop's function is two-fold, to act as a depth stop for machining to dead length, and to provide a datum face square to the lathe axis so that thin work (disks, washers etc.,) can be accurately mounted in the chuck for boring and facing. The first function is really an aid to mass-production, such as cutting screws or pins to size. The second is probably not something you're going to want to do every day, but when you need to there's little else that will do it as accurately. How effective the tool is depends critically on the accuracy of it's machining. To be at all successful a high degree of concentricity must be maintained throughout, and fitted parts need to made to very close tollerances. Overall then, it's an exercise in good machining technique. Although fairly quick to set up it's very important that the mandrel nose taper be thoroughly cleaned before mounting the tool on the lathe. Failure to do this will result in small bits of swarf being embeded into the sides of the taper bore seriously damaging it's accuracy.
Make a start by machining the MT shank. Chuck a length of F/C 3/4" mild steel about 4-1/2" long, and set the top-slide for turning the taper (use one of the various methods mentioned here in other project pages - I used the 'sighting' method with good success). The narrow end of the taper should be furthest from the chuck for machining, and with the taper turned to size the exposed end should be faced and centered with a 5/16" center drill before drilling right through 5/16". Bore out the end to 29/64" dia., for a depth of 11/16" and tap 1/2" x 32 TPI to full depth. I used a tap for this job rather than screw-cutting, which perhaps would have been strictly correct. Remove the chuck from the mandrel nose and insert the MT shank in the socket (remember to clean the bore first). Tap it into place with a soft-faced mallet to make sure it doesn't shift whilst being machined. Use a parting tool to cut to length (about 3/8" longer than the front of the mandrel nose will be about right). Drill right through 11/32" and bore to .005" under 3/8" before finally reaming to size. Boring is essential - I made the mistake of relying on drilling with a stiff 23/64" stub drill followed by a 3/8" machine reamer. I thought this would give me a true bore but the result was a hole eccentric by .002" which was useless. I had to bore the job out again to 1/2", plug the hole with a length of 1/2" silver steel (Loctited in place), and then do the job again - properly this time. When examined later the run-out of the carrier when inserted in the bore was smaller than I could easily measure using my DTI. Incidentally, it's useful to know the exact size hole your reamer cuts - I know mine produces a hole 0.376" diameter, probably because it's new and has seen little use (it's common for new reamers to cut oversize but it's more usually about +0.0005").
Next job is to chuck a 10-1/2" length of 1/2" OD x 5/16" bore tube. The only material I had in this size was of some indeterminate type of stainless steel - which proved to be horrible stuff to thread. A sharp tool. slow speed, and high-pressure cutting paste sorted it in the end. One end of the tube needs to be screw-cut 1/2" x 32 TPI for a length of 11/16", and made a good fit to match the threaded end of the MT shank. The other end should be treaded the same but for a length of 1-1/2", again, use the threaded MT shank as a gauge for the thread cut by your 1/2" x 32 tap. It was noticed that the 1/2" x 32 die that I had would not even begin to screw onto the threaded tube - had I cut the thread with that it would certainly have cut undersize for the 'matching' tap.
The push rod was made from 5/16" silver steel 13" inches long. Screw-cut one end 1/4" x 40 for a length of 1/2", use a parting tool to make a groove for the thread to run out into, and also to provide a nice square shoulder. A longitudinal slot 1/8" wide x 1/16" deep x 3-1/2" long is first machined on the opposite end (use the lathe + vertical slide or miller), then screw-cut for the length of the slot 5/16" x 32 to match the adjusting nut. The rear clamp nut with it's locking screw are simple turning jobs, as is the push-rod adjusting nut. I screw-cut the internal 5/16" x 32 thread on the latter (mainly because I didn't have a tap that size!).
The carrier for the thin plate backstops can be made next. I turned this component from a 4-1/2" length of 1/2" silver steel, held in the chuck one end, and supported by a dead half-centre (carbide tipped) in the other. As all diameters were turned at one setting they had to be concentric. The longer 3/8" diameter part was furthest from the chuck so that the 1/4" x 40 tapped hole could be machined at the same setting. As I know my reamer cuts 0.376" I carefully turned the diameter to exactly this size with a sharply honed tool, and then polished off a few 10ths with ultra-fine emery paper until it barely fit the reamed hole in the taper shank (oiled with a thin oil to stop it jamming in place). You should aim for a tight push fit without the slightest amount of play. The 5/16" diameter was turned to 0.3125" using a 3/32" parting tool and then a couple of 10ths polished off to match my 5/16" reamer which is older and cuts dead to size. To make a new plate backstop to fit the carrier I only need to use my reamer knowing it will be an exact fit. It should be clear that your lathe needs to able to cut parallel for this procedure to be successful, my S7 turns dead parallel as far as I can measure over a couple of inches, and only a bare few 10ths out over 6 inches. It took considerable time shimming the lathe feet and repeatedly measuring to achieve this (and also the lathe is new and well adjusted), but it's a worthwhile job to know you can turn a true cylinder.
The plate backstops themselves are easily turned up from various diameters of mild steel rod according to the sizes required. You might feel like doing a decent job and equiping yourself with a stepped range of sizes from about 1/8" to 2" or so, but I've only made 4 for my own use at the moment. The center of the working face is relieved by about 1/32" depth to give a ring contact, and this edge is then lightly skimmed whilst mounted on the carrier.
That about completes the tool suitable for the majority of uses to which it is likely to be put, GHT additionally describes an alternative carrier for holding tapered pins for mounting very small nuts in the chuck, but I can't see myself needing to do this so have not bothered to make it.
(c) Chris Heapy 1996.
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