road and workshop
Page 41
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by Handyman
Benchwise: lathe sense (2)
A GREAT DEAL of vehicle-repair lathe work is done between centres as I mentioned earlier in this series, but there are points to learn and appreciate if a long-length job is to be turned up with accuracy and a good finish, as the work can be below standard if certain facts are not kept in mind regarding lathe centres and their adjustment.
During the process of a long-length turning job between centres adjustment may be needed several times, particularly in the early stages where a reclaimed shaft or spindle is having the rough taken off when there can be some vibration. When centring a job the work should spin freely by hand without any trace of shake, as within a short while of the cut starting the stationary centre will have bedded in to the work piece and it will need adjustment and lubrication.
As the turning proceeds the workpiece will begin to warm up and will expand endways—this will close the running clearance and can cause it to tighten on the centre, so heating up the cent-re and removing the lubrication. Special note should be taken of drops of oil put on the fixed centre, as any spluttering will indicate heat in excess of normal and the tail-stock wheel should be eased, the lathe stopped and the work again checked for free running.
Careful checks
Once the job is under way maximum expansion will be reached and no further centre adjustment should be needed, but as the final cut is reached, the centre should again be checked carefully and lubricated.
There are several opinions regarding lubrication for the dead centre, as it is known, and quite a number of the old hands still prefer a mixture of white lead and engine oil, as this is known to stay on the job under quite an amount of heat. Where I have had a large job to do between centres, an example being a long rear-suspension trunnion being fully reclaimed or made up from new material, I usually slip a few strong rubber bands over the centre first and then wind about three turns of oil-soaked felt around the centre and slip three or four bands over it. I find that this keeps oil on the job throughout quite a long operation, should I forget the oil can. A centre should never be neglected until it warms up and begins to squeak, as this means that the hardened point can be burnt out and useless until reground and hardened.
There is, of course, a more sophisticated answer to this problem where a large amount of heavy centre turning is carried out. I refer to the running centre, a device rather like a drill chuck on a tapered shank, with the centre running on ball bearings fitted inside its body. The device will run true to very fine limits indeed, but care is still needed in setting up. Do not overload via the tailstock wheel; use two fingers lightly, at the same time check that the work is perfectly free with no feeling of drag that would indicate preload.
One danger which must not be overlooked with running centres is that in the course of a long job, the work itself may heat up from the cutting area outwards, and this can cause the work to bend or bow. To offset this, ample coolant should be used at the cutting face.
Moving to the headstock end of the lathe, there are many vehicle-part reclaiming jobs that will be handled on the face plate, examples being, brake drums, bearing housings, fly-wheel facings, belt drive pulleys, etc, and quite accurate setting up is required.
Whereas I will deal with chuck and face plate work later on, I can cover one problem now, i.e. suitable face plate packings, of which one never seems to have enough, or of the right kind. With packings it is essential that they are uniform in size and thickness to a thou or so. Most ttirners have their own private stock of packings; these are usually steel cuttings, bar, strip or block in form, but It's not always easy to keep them for long clear of scars or damage, and the full-time turner is always on the lookout for new packings.
One item that fulfills this requirement is found in plenty in any garage or vehicle workshop—this is the old ball or roller race, inner or outer track. It is too hard to scar or bruise, it remains accurate and can be found in all the range of sizes that could be called for, from the small dynamo race to the large 4in. hub or diff bearing.
Separate track It is handier to aim for the separate track rather than use the built-up race, as the greater hole dimension is an advantage. These can be arranged in sets of four of each size, and there is no limit to their application on the face plate; one task, however, is to part the inner and outer tracks where the bearing is complete. Some bearings can be pressed apart, but with others it is necessary to sacrifice one track by cracking it. One tip to remember when breaking the outer track of a ball race by a hammer blow is to wrap the bearing in two or three layers of sacking or cloth, otherwise, if the bearing is given an uncovered cracking blow on the anvil, the wielder of the hammer and those nearby will be liable to be hurt by fast-moving shrapnel-like fragments.
One old turner in a large garage always had a watchful eye open for whenever a Gardner LW engine was being finally scrapped after, no doubt, several long lives; he would seek out the long cylinder block to main bearing bolts, and would confiscate the thick distance washers, which had a concave top face with a smaller convex floating washer in the top. These two pieces made first-class self-centring packings when a bolt was drawn through them on the face plate. Again he would lay claim to the tough and well-made clamps used on the sameengine, if I remember rightly, to secure the tappet blocks; these could also go straight to work on many face plate tasks.