Assuming you *are* going to grind in the lathe then there are certain precautions that should be taken to minimise the damage. Firstly, you want to cover as much of the machine as possible to prevent ingress of flying abrasive dust into slideways and bearings. Ideally, all that should be exposed are the workpiece and the grinding spindle. It's possible to get some way towards this using sheets of aluminium foil which is easily moulded to fit around awkward parts of the machine. It's even possible to wrap the chuck, poke a hole in the center for the workpiece, and this effectively prevents abrasive getting into the chuck's inner works. Similarly, the slides can be covered with overlapping sheets with holes cut for the grind spindle. Another alternative is to use plastic sheeting but this is less useful as it does not stay where it's put unlike the foil. You might ask how does a surface grinding machine survive then? Good question - and the answer is that special precautions are taken in their manufacture, with over-lapping slideways and protective felt strips, bearings which are properly sealed, and all manner of other devices to prevent grit reaching it's vital parts. Your lathe is not so protected, and entry of this grit will be fatal to bearings and slideways.
It cannot be argued that grinding is useful though. It not only allows hardened parts to be finished to size, but produces a finish which is superior to normal turning. The amount of metal that can be removed in a simple lathe setup is small, in the order of a few thousandths of an inch, but this is usually all that is required. The main considerations for successful work are:
The sort of rotational speed you should be looking for is at least 5000 rpm for a 4" wheel (probably about the maximum size for a light home machine), and proportionally faster for smaller wheels. The direction of rotation should throw the sparks and grit downwards - away from the operator. The lathe should rotate counter to the direction of the grindwheel, and at about 300rpm for a nominal 1" bar. The depth of cut needs be very small (substantially less than a thou), both for reasons of preventing the job springing, and also to preventing over-heating and produce a good finish. You will therefore appreciate the necessity for well-adjusted slides.
Internal grinding has it's own unique set of problems, not the least of which is the requirement to rotate a small stone at very high speeds. A 1/4" diameter stone (for example) needs a rotational speed of at least 20,000rpm to be effective. Flex in the system needs to be eliminated for the same reasons as stated above. Poor finish is almost certainly going to be due either to an eccentric running stone or loose bearings. I have had some success with a stone mounted in a high-speed motor clamped to the rear toolpost, though it must be said the finish is not perfect, probably because the motor bearings were never designed to work to the tollerances under the strain imposed.
The sort of jobs that can be successfully undertaken include re-grinding of hardened lathe centres, grinding of parallel shafts (gudgeon pins or even pistons), and internal grinding of short bores. I have succesfully re-ground the jaws of a 3-jaw lathe chuck using the internal grinding setup, and it must be said that there was probably no other simple method of doing this.
Purpose-built tooling combining motor and grinding head is available to carry out toolpost grinding, certainly Myford market a range of attachments for their machines, and third-party manufacturers regularly advertise their products in the Model Engineer magazine. This is clearly the way to go if you are serious about the job, but success can still be achieved using a separate grinding spindle (either purchased or home made) and a motor to power it. Sufficient speed can usually be attained through high gearing of the pulleys. I have recently installed an over-head drive system driven by a 1/4HP motor partly for the pupose of performing a limited amount of grinding. It's difficult to mount a sufficiently powerful mains motor on the cross-slide and a better alternative is to use one of the many powerful and compact 12/24V motors which usually have a higher speed rating and which doesn't therefore require high gearing.
(c) Chris Heapy 1996.
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