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Benchwise: lathe sense (17)
HAVING reclaimed. the damaged taper and mated it successfully to a new companion flange with the aid of a test piece, it can be worth our while to take a look at the discarded worn flange and the possibilities of its recovery, should there be a supply problem. Even if these items can be obtained over the counter, there is no reason to throw the worn item away when it can be used for experimental purposes by the trainee.
Turning out a reclaimed female taper may not appear quite as straightforward as dealing with the male section, and whilst all the rules still apply as for turning a male taper, the job will be tackled somewhat differently. In the case of a companion flange, for instance, this can be' held and centred in either a threeor four-jaw chuck, although it is worth the extra time to use the four-jaw and true up by setting the dial gauge plunger against the unmarked and therefore unworn section of the oil seal seating, still found on most flanges. Normally on a used flange with the oil seal seating on the nose, the rub or score marks can clearly be seen but there is still an area left that is smooth and clear of marks to give a true reading. If by chance both the male and female tapers have been built up by welding, then it is wise to turn out the female taper first and check progress with the test taper which can be offered up without disturbing the work; thus if there should be a slight error of angle in the female, he necessary adjustment can easily be made when turning the reclaimed male taper, and final lapping can achieve the required fit.
Load the flange Regarding the actual turning operation, again this should be made in a series of fine cuts—and, again, test the finishing cut in advance as with the male taper. One other point to remember is that as the female taper is sent home on the shaft, special note should be taken of where the outer end face of the flange terminates in relation to the end face of the male taper, as it must be arranged that when the nut (and plain washer if fitted) is sent home, the load should be on the flange and not against the end face of the male taper. If by chance the two end faces of male and female are found to be vertically in line, no pressure can be exerted against the flange to secure it solidly on the taper. Therefore, the plain distance washer should have one side countersunk enough to ensure that it is at least bin. clear of butting against the shaft end face; if no washer is fitted and the threads go right up to the end of the shaft, kin. should be taken off the butt face or, if the nut is safely long enough, its approaching face should be countersunk by the same amount; but if this is going to cause the nut to move too far inwards from its split pin position, make up the countersunk distance washer instead.
This aspect of preloading the flange after reclaiming has been overlooked on occasions, with the result that whereas the nut is driven home tight, there is in fact no pressure on the flange, which is soon working free on the taper and relying entirely upon the key for drive, and you are back in square one.
I should again like to mention an old trick of the trade that is always of value when seeking to determine a hidden clearance without involved measurement or doubt, and this is a stick of child's plasticine, or soft lead wire, either of which materials can be placed in the clearance cavity, full load applied and then removed; the amount left can be measured either by micrometer or vernier, and any profile is also clearly indicated.
Mention should be made of the lapping of the two items, either to clear up any irregularities and achieve the required "nipping" fit, or to remove the final thou or so that one item is up or down on the other. For the purpose of taper lapping, valve grinding paste will do, although care is needed not to overdo it with the coarse paste unless there is a fair amount to clear up. A wash off in paraffin afterwards will make all safe, although if lapping is carried out with the work still "chucked" over the lathe bed, provision should be made to catch any droppings, either by covering the ways with poly sheet or paper, as one splash of lapping mixture can do a lot of damage to the lathe. Where some considerable lapping is required, and in particular with such items as gear shafts, it is wise to remove the work from the lathe altogether and put it in the vice between soft metal plates, then it is possible to bolt a pair of operating handles on the flange.
Remember also, that whereas grinding paste is applied to the face of one item only, the cutting action is affecting both, thus with tapers one is decreased and the other is enlarged in size. This must be borne in mind when within a thou or so of fit size with tapers, as an excess of grinding can quickly equal a cut in the lathe. The action of lapping one round item to another should be the same as with grinding in valves, the motion should be semi-rotary and the loose item should be frequently lifted clear, the paste levelled and then pushed firmly back on again for a further half dozen or so strokes in either direction under light load. Never apply heavy end load, and on no account attempt to rotate the item fully or drive it in the lathe, as continued full turns will pile the paste or move it into separate areas, with the result that continued full turns will ridge both newly turned items, ending in a condition of "misfit" that can never be truly secure under high torque.