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Last Update: June 16, 2002
This page contains some of the tools and modifications I've made to and for my Myford Super 7B lathe. It goes without saying that modifying an expensive piece of precision exquiment should only be done after carefull consideration has been given to the consequences if something goes wrong. Be warned: I am no kind of trained machinist. If you are brave enough to follow any suggestions you find here, you probably deserve everything I got!
The only other thing I can add to the article (and you really need to be familiar with the design drawings to appreciate this) is to make sure that the small plunger is a free fit in its hole and that the top of the brass plunger is domed as shown. Athough he does not mention this, these features make it possible to disassemble the mechanism without resorting to brutality. Study the drawings and you'll see how this could easily become a un-reversable assembly process if the little plunger cannot drop to be entirely within the leadscrew shaft. I was lucky and got away with it during a trial assembly, but not withough some judicous pounding with a soft mallet.
Also, since my handwheel is the new, unattractive, un-spoked design, I decided to drill the cross hole for the plunger directly opposite the keyway (GHT says drill it in line with a spoke). This hole needs to go well past the axial hole to allow the little plunger to fall for dis-assembly. My choice meant I actually broke through into the keyway, which is not bad, but could be better. Also, I could not grip the handwheel in the 4 jaw chuck as GHT recommended - the jaws are just not tall enough to get past the radius on the periphery. Instead, I clamped it to a faceplate and clocked the inside of the shaft bore. In retrospect, mounting it on a stub mandrel between centers would have been a better idea to assure concentricity of bore and dial bearing surface. I was lucky though, and have got away with it.
I'd also recommend that you turn the 20 degree taper on the tailstock thrust face first, then make the diameter of the dial to suit. I followed GHT's instructions and went the other way, so I was limited in the length I could taper and still mate with my (second) dial. Look at the first picture to see what I mean. I was also a bit surprised and disappointed to find that the hole the oil nipple screws into (visible in the heading photo) had not been de-burred. Rather, the burr was just mashed flat by the nipple. Shame on you, Mr Myford. Drilling the tailstock leadscrew is accomplished by pushing the tailstock up the bed - since the handwheel is obviously unavailable during this period. This required an unexpected amount of physical exertion and anxiety. So if you can do this operation in a friend's lathe, your blood pressure will thank you for it - especially when you get near the point where the axial hole is about to break into the cross hole and you're pushing on the tailstock casting with both hands, feet braced against the nearest vertical surface!
So, all in all, an well designed, highly practical and usefull modification to an already excellent piece of machinary.
This led to research on an appropriate design. There must have been a million designs for this item published over the years in the British Model Engineer magazine. I started with George Thomas' book which had a simple design that used a drilled and tapped Morse taper plug to adjust the position of a long rod. Neat, but as Myford collets use that Morse taper as their closure mechinism, not suitable for anything but chuck backstops. However, Goroge's article mentioned a design by a talented New Zealander named Radford. Looking up Radford's design, I found basically the same arrangement. Oh well... Next I checked out ME circa 1995 when I *thought* I remembered a different design. Sure enough, there it was, better, but still not really suitable.
The device I needed was not hard to visualize: it would clamp to the left side of the lathe spindle like the nifty manual rotation handle (there are a million published designs for that too, including Thomas and Radford!)--it was the backstop adjustment and locking that stopped me going ahead. I could visualize a threaded adjustment mechanism with a lock nut, but how to arrange the adjustable rod so one rod would suit collets, faceplates and chucks? That was the problem. I did not want to have to make a series of rods as I just knew that each rod would be exactly the wrong length for the next job! I was just about reconciled to making those 20 x 0.625" slugs the hard way and started simply browsing old Model Engineer issues as a way of putting off the task when an add for ME's companion magazine Model Engineers Workshop caught my eye. On the illustrated cover was notice of a "Chuck Backstop" device. The date was not visible, but presumably it would be close to the date on the ME in my hand.
Fortunately, this was an old issue that I had in my library and the simple design by Mr XXXX looked close to what I needed. It still used a Morse tapered plug to support the backstop rod in the middle of the bore at the chuck end of the headstock spindle, but that could be changed to a paralled sided support for use with the Morse taper collets. The rest of the device is similar to the Myford manual headstock handle that I had been thinking about. The aspect of the design that hooked me was the simple solution to the "one rod suits all" problem: grip the rod in a collet like device. Not as rigid as other designs, but more than good enough for a backstop! All the parts appear in this picture:
These photos show the stop being inserted into the spindle and fully inserted and locked. The gear cover has been left open merely to make the photograph clearer--in normal operation it would be closed. For repitition work, the first piece is set up in a collet (for example) and the unlocked back-stop inserted in the spindle. the rod is broght up be bear against the part and locked with the knurled nut, holding the right hand knurled part, if neccessary. If the saddle and top slide are now locked, parts finished on one end, but rought cut on the other may now be be quickly machined to the same length as fast as you can pop them in and out. The backstop rod is long enough to almost protrude from the jaws on the 3 jaw chuck when it is screwed onto the spindle. When fitted with a pad skimmed to be normal to the axis, very short, thin parts which have one side faced can be inserted into the chuck jaws and pressed against the stop to set them up to run true in the chuck. If you've ever tried to set-up pieces like this, you'll sure appreciate this feature.
A Tailstock Micrometer Collar
The main use for my lathe is making minature internal combustion engines. Naturally, this involves lots of drilling. Frequently these are blind holes to very precise depths - for instance drilling a spray bar to seat the needle valve. When doing this, I sometimes clamp a DTI (or digital caliper) to rest against the tailstock mounted drill chuck, but that's such a pain to setup. George Thomas (writing in Model Engineer) developed a neat little mod to the tailstock that mounts a micrometer collar, with appropriate friction device, to the tailstock hand-wheel as seen on my lathe in the accompanying photograph. For those who've not seen the article in question, the little knurled screw visible in the middle of the hand-wheel actuates a brass plunger that locks the micrometer dial. It is a very positive arrangement, much better than the friction devices supplied on the slide dials of the standard Myford product. Most of GHT's lathe modifications are collected in a book titled The Model Engineers Workshop Manual, from Tee Publishing, ISBN 1-85761-000-8. This includes a similar friction lock on the cross slide micrometer and a totally revised top slide which I'm now seriously considering.
The work is straightforward and GHT, as usual, provides a fine step by step description. Unfortunately, an error has either crept into the text, or perhaps it's been there all along. This picture shows my "first attempt" at the micrometer collar. GHT's text says it is to be calibrated with graduations that are "..50 in number.." - so that's exactly what I did (p 161). Well, better make that 150. The text goes on to note that one turn of the handwheel advances the tailstock barrel by 0.300in, hence each division represents 0.002in. I must have read that several times, but never stopped to think that 0.3 divided by 50 is not 0.002. Anyone want to buy a collar with 0.006in divison marks? Oh well, at least I hadn't stamped it before I found the mistake and the knurling on the second one is better too.
A Backstop for Chucks and Collets
This one does not require any actual white-knuckle modification of the lathe itself; it is simply to make certain repititous operations and some set-ups easier. The impetus for this project was the prospect of having to face back 20 tappets for a pair of Morton M5 engines. These are just little pieces of 5/32" drill rod, spherical on one end, flat on the other with a 3/32" spherical cavity for the valve push rod. The precision required is their length of 0.625" and the depth of the push rod socket. I'd planned to hold them in a 5/32" Myford collet and thought how nice it would be if there was a stop in the collet so the saddle could be locked and I could just pop them in one at a time, winding the facing tool in and out like a simple precision, mindless automiton.
Other designs (like the Radford/Thomas) use a screwed rod allowing precise adjustment and absolutely preventing axial loads from upsetting the back-stop position. While this stop is susceptible to end loads, that's not its purpose, so I'm not concerned. The flange on the end (glued on, not turned from solid--too much swarf involved) allows you to squeese the loosened nut and body together between thumbs and forefingers to break the taper loose without resorting to the normal violence associated with locked tapers.
