Notes on making a tailstock 3-jaw spindle

A quick perusal of the drawing might leave you wondering what possible use this attachment might be. It's main purpose is to mount a standard 3- or 4-jaw chuck in the tailstock, allowing it to spin freely within the built-in bearings. A thrust bearing is provided as the main loading is going to be axial. The free spinning chuck can now be used to hold the end of any size of tube within the capacity of the inside or outside jaws (inside jaws for larger diameter tubes), and in combination with the headstock chuck allows machining of long tubes without the use of over-size fixed steadies. Moreover, the tube need not be round - it could be hexagonal or square (using a tailstock mounted 4-jaw) - something a fixed steady can't help you with. Of course, solid barstock can be held too as well as tubes, and it's probably the only way of effectively supporting large section odd-shaped stock for machining where a center can't be used. So now I hope you appreciate it's value!

Like most of my attachments it's designed with simplicity in mind, a more sophisticated job could be done with ball or roller bearings, and provision made for taking up end-float. Well, that's up to you to modify my simple design. My version is easy to make though, and I believe it will do the job adequately - which is the main thing. I have specified plain oilite bearings as extended high speed use is not likely, and you should bear this in mind. You will also notice there is nothing preventing the spindle from moving forward in it's bearings, this is perceived not to be problem though as the tailstock is used to provide the axial loading to hold it in place. Larger diameter bearings would have been nice but I'm forced to work within the dimensions of the 2MT shank in the Super 7 tailstock.

Perhaps the best strategy for machining the taper shank would be to mount a length of 3/4" F/C mild steel in the 3-jaw and cut the taper first, using whatever method you find best for setting the top-slide (a gauge if you have one, or the DTI). Then drill right through 23/64" and finish bore to a couple of thou over 3/8" to give clearance to the spindle (say, 0.377"). Then bore the recess in the narrow end to 0.501" diameter for the oilite bush which is Loctited in place with #601 retaining compound. You might consider using an interference fit for the bush but if you do this you will likely find you will have to ream the hole again as it will close up slightly. Note that the end of the rear bush overhangs the end of the taper shaft a little, this offers some protection for the shank - an alternative would be to to make the bush to dead length and turn the end of the shank narrower by 10 thou or so (this prevents a knock on the end pushing up a burr which would otherwise interfere with the fit of the shank in it's socket). Now you can remove the shank from the 3-jaw and insert it in the headstock socket for boring the recess for the front bush (I hope you have a sensible lathe with similar tapers in headstock and tailstock - otherwise a re-think will be necessary!). This machining procedure will result in the maximum degree of concentricity achievable and should avoid excessive run-out.

The spindle itself should be turned between centers to obtain a similar high degree of concentricity, the 3/8" portion turned first and then turned around to machine the spindle nose thread and register. Aim for a good finish on the 1-1/8" x 12 TPI threaded portion - use a sharp tool and perhaps a mandrel handle for the finishing cuts. The 5/8" bore in the end is not critical and is there just to allow objects to be gripped in the chuck projecting as far as possible through the chuck body. If you don't fancy chewing out the entire thing in one piece from a length of 1-1/2" bar (and I didn't, being tight-fisted with my stock of metal)it's possible to build it up from a shorter length of 1-1/2" for the spindle nose, and a length of 3/8" silver steel for the spindle shaft. This method has something to offer in that the silver steel shaft would be automatically dead to size and probably offers better wear characteristics, but you have the extra machining operation of fitting the two parts together accurately (again, Loctite would be good enough). Yer pays yer money...

The spindle nose thread needs to be a good fit in the chuck (i.e., no slop), as does the 1.250" plain register just behind the thread. However, it is not necessary to provide a 'wringing' fit to get good accuracy. Either a couple of spanner flats or a hole for a tommy bar needs to be provided on the 1-1/2" diameter part to facillitate mounting and removing chucks.

Mention should perhaps be made regarding the method of making the thrust bearing. Face the end of a short length of 1-1/2" brass rod and drill the ring of holes (about 0.130" dia - to give clearance to the 1/8" balls) drilled using whatever form of dividing attachment you commonly use. Part off a 3/32" thick slice to form the disc. Use a sharpened hollow punch made from hardened silver steel (drill rod), with a bore a little larger than the 0.130" holes, to turn the edges of the hole inwards. This will prevent the ball bearings falling out through that side of the disc. Turn the disc over and insert the balls, then repeat the operation on the other side (some folk use a centre punch on this side to make 3 indents around the ball which serves the same purpose). Neither the spindle nor the hollow shank is specified as being hardened, and whilst this would be a definite advantage for the thrust bearing the difficulties of hardening these large parts (with the inherant risk of distortion) probably precludes this. I guess you could use hardened steel washers either side of the thrust bearing to act as seats but I doubt it's worth it unless you intend using the device a lot. If you cannot drill a neat ring of holes you could use a plain bronze or gunmetal washer to act as a thrust surface.

As a complete supprise to me (honest!) I just saw in the latest ME mag a short article mentioning that a similsr device to the one described above is available commercially from Mercer Skilled Crafts, W. Yorkshire (01274-8745596). Obviously there are s few differences but I want to know how come all my best ideas have been thought of before?

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