Rotating centre.
I normally use a carbide tipped half-center in the tailstock, which I made myself by grinding away almost half the diameter of an ordinary tipped center. I don't know if this is a commercial item but the carbide tip offers lower friction and is much less prone to damage than ordinary tool steel. I have a rotating center for high-speed work but it should be recognised that rotating centers always have a certain amount of run-out, even if only a few 10ths. The run-out may or may not be important dependent upon the quality of work (you may be grinding to fine tollerance), and how much you paid for the rotating center...
Rotating centre.
It's wearisome pointing it out yet again, but, the lathe must be accurately aligned before you will be a ble to turn out (pun not intended) accurate work. In fact, turning between centers is the method used to assess whether the lathe is aligned in the first place (see initial lathe setup).
The sort of work done between centers is the machining of precision shafts such as model locomotive axles. Care must be taken that the dead centre is properly lubricated (high pressure molybdenum grease is useful here) and that if the work gets hot and expands, that the pressure so produced is released by slackening and re-tensioning the tailstock. If this is not done the headstock bearings will be subject to considerable strain and damage may result. In any case, if working to fine dimensional tollerances it's best to let the work cool and contract before any final sizing operation.
Carbide tipped half-centre.
I mentioned the half center earlier, this is probably of more use to the average worker than the normal full center. The half center provides clearance for the lathe tool so that machining can take place on narrow work right to the end of the bar. There are several other types of centre available but which you will likely never need, such as square centres, hollow centers and female centers (small ones are used a lot by clock makers though).
Centers are useful for aligning work too. Take the problem of setting up a casting for boring on the faceplate, the casting has two center punch marks either side marking the position of the intended bore. First job is to clamp lightly to an angle plate which itself is attached to the faceplate, and use a wobbler to set the front mark so that it aligns with the lathe center. Clamp the angle plate firmly to the faceplate. Next, slide the casting aside and place a center in the headstock socket and another in the tailstock socket - move the casting so it is caught between the two centers with the two center punch marks located by the points of the lathe centers. Clamp the casting firmly. Unscrew the faceplate so that the headstock center can be removed, then replace the faceplate. The casting is now setup for boring the hole truly.
Another example is quartering driving and coupled wheels for a model locomotive. I always use the method of mounting the axle and wheels between lathe centers and use 2 blocks of steel to set each pair of wheels with exactly the same lead. One crankpin rests on the top of a bar at center height ( bar height = center height less half the diameter of the crankpin), the other block is mounted on the cross-slide and moved forwards to a point where the axle center and crankpin center are vertically in line. This setup is left in position for the remaining pairs of wheels. In this case the lathe and centers act as a precision measuring device.
One final type is the pump center, also a very old design. This center consists of a hollow taper shank containing an accurately fitting cylindrical center-body with a 60 degree point. The center-body is free to slide in and out the taper shank under control of a strong spring. It has uses in faceplate work and also on the rotary table where castings with a hole or center punch mark can be located by the pump center's point, and as the casting is bolted down the center is pushed down into the taper shank. This leaves the work with the datum hole or punch mark aligned with the faceplate's or rotary table's own center. A pump center is quite a straightforward DIY project for a rainy day. In the example above where the casting is setup on the faceplate, a pump center in the headstock would allow an additonal axial movement for adjusting the overhang in front of the angle plate.
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Commercial taper mandrels are of course available, and this tool is again of antiquarian design. It's not worth buying one new (if you can find one) but if you happen to drop across one cheap it would be worth aquiring.
The next type to consider are expanding mandrels of various sorts. As considerable work goes into making these it is necessary to arrange for them to be re-usable in that they have to be mountable and dismountable without loss of concentricity. The only viable way of doing this is to provide them with a taper shank to fit the lathe spindle bore (2MT for example). This only adds to the work in making them of course. Several designs exist consisting of either a split body with a taper plug which when tightened expands the body by a small amount (like a collet in reverse), others use a sliding wedge arrangement with a slightly larger working range. As the commercial items all are precision made from tool steel, hardened and ground all over, they are expensive. Again, not really worthwhile buying new but look for used bargains. The type of work where such a tool is useful is where you have a precision bore (made first) and it's necessary to machine the outside of the workpiece concentric to it. Imagine a piece of steel 8" long and 1-1/2" O.D. with a 1" bore, it would be tough maintaining concentricities and true boring to such a depth normally. One method would be to use a between-centers boring bar first to give a true bore, then mount on an expanding mandrel to machine the outside.
Small special mandrels can be quickly made for machining washers , all it requires is a rod with a threaded hole through the centre and held in the chuck, and a threaded rod with a cone on the end which screws into it. The washer is trapped by the cone passed through it's bore and the outside can then be machined to size.
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Photos show (left) typical setup using a fixed steady to support a large diameter bar whilst it is machined, and a travelling steady (middle and right) to support a thin workpiece. Both steadies shown are standard Myford factory products which, whilst adequate for the job, lack some of the refinements of other designs.
It's not that straightforward setting up a fixed steady, care needs be taken otherwise it's possible to mistakenly set the workpiece so that it is not in line with the lathe axis, yet a DTI is of little use here as it will not show any run-out. If the work is off-axis then the only result can be the cutting of a taper. My own method of setting up a long-ish bar for machining with the support of a fixed steady is as follows:
The previous paragraph may give you a clue to another use for the fixed steady - that of ensuring drilled centers are truely concentric to the outer surface of a workpiece. Imagine for a moment that you have a length of bar held in the chuck, centered one end for support by the tailstock. The part is finish-turned and then parted off, but you really want to put a center in the other end too. One way might be to reverse the piece in a 4-jaw independent chuck and use a DTI to set it running true. An easier and quicker way is to use the procedure just described, with the workpiece (which needs to be about 3" or longer) just held in the tip of the chuck jaws and the fixed steady supporting the far end. The steady will ensure the center is drilled concentric to the outer diameter. Note in this example that the workpiece may NOT be co-incident with the center of the lathe spindle, the steady may in fact force the workpiece away from parallel to it, but this does not matter for the purpose of producing the center. With that drilled, the piece can now be supported with a tailstock center and further machined as required. Purists may argue that the cone angle (should the workpiece not be parallel) will not be 60 degrees, but I would say the difference would be negligible under anything other than extreme circumstances.
The steady jaws are usually made of bronze, or sometimes brass, and will not harm steel work provided lubrication is used in the form of machine oil or molybdenum grease. Brass or light alloy work on the other hand can be easily marked by the steady jaws and some added protection is needed. Simplest way is to wrap a piece of oiled card around the work and trap the free ends in the join where the two halves of the steady body clamp together. This will offer sufficient protection to even soft aluminium for quite prolonged machining operations. Use a hard thin card rather than a porous card.
Travelling steady jaws can be set much the same way, by pressing them against the work as close to the chuck jaws as possible. However, if the work is being turned down then the final size has to be reached in one cut or the steady jaws will need to be re-set to the new, smaller diameter, after a roughing cut.
Another major application for steadies is the cutting of accurate threads. If you have already read the thread cutting notes in - 12.2 V Threads then you will appreciate the value of the set-over threading tool for cutting cleanly. The straight-in feed of a normal tool throws up burrs onto the crests of the threads and this plays havoc with the bronze steady jaws (applies particularly for cutting threads in steel and other tough materials). Brass jaws in the same circumstances will gain their own thread before you're finished!
Some of the refinements referred to earlier include the ability to adjust the jaws with a screw thread. This is useful provided that there is sufficient 'feel' so that the action of tightening the thread does not push the work out of line. You need to be able to register quite a light contact, and a fairly coarse thread is probably best. Another refinement is to use roller bearings instead of plain bronze bearings at the points of the steady jaws, but bear in mind that such bearings are always hardened and as such will mark the work to a greater or lesser degree. It's often necessary in model locomotive boiler making to trim the ends of quite large section copper tube. This is way beyond the capacity of normal fixed steadies, but castings are available to make your own large-capacity steady (about 5" if I recall) from Model Engineering Services.
I once saw in a very old Model Engineer Magazine (circa 1932) a photograph of a beautifully made self-centering fixed steady. It worked along similar lines to a camera lens iris, with 3 jaws connected by linkages, and a lever to close all three jaws at once. It seemed like a good idea yet I've never seen a commercial equivalent.
(c) Chris Heapy 1996/99.
4.7 Mandrels
Mandrels in their most basic form are chunks of turned metal held in the chuck onto which the workpiece is jammed for machining. There are many refinements to this basic idea. Stub mandrels, as the name suggests, are short turnings whose outer diameter is usually a close fit in a bore of the workpiece. The workpiece may either be held by friction alone or a threaded hole or stud may be machined on the end for a retaining bolt or nut. In the former case, it is usual to make the mandrel very slightly tapered (a 'tapered mandrel' by definition) and I made one of these to machine the cylinder cover bolting faces on the first set of cylinders I ever machined. By 'very slightly' tapered I mean that the O.D. is machined so that the bore will barely fit onto the mandrel, the end third is then polished down until the workpiece will twist on part way and is held firmly. The second option is easier to make as a normal precision fit is used, the workpiece being secured by the bolt (or nut) - note that in this case a shoulder can be machined so that the workpiece can be removed and replaced with considerable accuracy. An example of this type is the fitting used to locate locomotive wheels for final profiling. Both types are usually machined in place whilst held in the chuck, in this way good concentricity is maintained, but once removed from the chuck it's pretty much useless. Make sure you are very clear about the machining steps involved in completing the job before you start, you don't want to have to remove the mandrel half way through.
Special expanding mandrel to hold a changewheel on the end of the lathe spindle.
4.8 Steadies
Two types of steady are commonly associated with lathe work, these are the fixed and travelling steadies.
Assuming that one end of the bar will be held in the 3-jaw chuck, I would first mark out and, as accurately as possible, mark the center of one end of the bar with a punch. The ultimate accuracy of the setup will not be dependent upon the positional accuracy of the punch mark but care should be taken to work to a reasonably close tolerance. The un-marked end would be placed in the chuck jaws, and the tailstock centre brought up to support the other end (the center engaging the punch mark). The fixed steady would be put in place as near to the chuck jaws as possible (you may have to put it in place before mounting the work with some designs), and the steady jaws brought into contact with the work and locked in place. I would then slide the steady down the bed to the far end (with the tailstock center retracted) to the position required. Done this way, the work will be centered and in line with the lathe axis to the same degree of accuracy as that exhibited by the 3-jaw chuck. A more accurate job could be done using the 4-jaw and a DTI to set running true close to the chuck jaws.
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