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genchwise: lathe sense (1 1 )
Page 49
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N VEHICLE or trailer repair, the lathe can lay a big part in making up new items in tee or brass, and a great deal of time and ioney can be saved if raw material and the ecessary lathe skill is available. With trailers a mind, trunnion shafts, shackle pins, torsion iar pins, etc, can be made up ready for use, nd a good turner can run off several pins rom one length of steel bar in one complete operation, needing only hardening attention oefore ready for use. However, a little more are and skill is called for when a 3in. diaaeter trunnion is to be turned up and parted off-from its parent material.
By parting off, we mean the task of cutting ight through the workpiece with the lathe ool on completion of a particular job. How.ver, there are certain tricks of the trade, ind also pitfalls that deserve a mention for he benefit of tile trainee turner who is cutting leep into a large diameter, or parting off a wavy, finished workpiece. The most obvious eason for a parting tool to seize up or break )ff when deep in a cut, is, of course, a lack
• Parting tools must be set correctly at exactly 90deg to the work. Parting off should always be done as close to the chuck as possible and in any case never between centres.
• The plan and end-elevations of the parting tool show how the blade must be ground to ensure that the tool does not bind in the cut and snapoff.
• When parting a job of large diameter make two or three cuts side by side to provide adequate relief for the tool to work in.
of side clearance. Despite the fact that the parting tool has been sharpened correctly, with back clearance arranged from the cutting edge, by which I mean that the cutting edge is wider than the rear of the blade, binding on the tool sides has occurred; so it is clear that the tool is not being offered squareon to the work but is going in on a slant. Therefore, one side is bound to bind as the tool goes deeper, and can and will jam up solid, often spoiling the whole job.
Another cause of jam-up is wear and slackness existing in the cross-slide. As the tool is going deeper into the work, this backlash is taken up in one direction by the feed pressure to a point where the tool will be pulled into the work and this can lock the tool and work solid. Normally, careful adjustment of the thrust will remove this back-lash or free play, but if the lathe is old and the back-lash is in the feed screw and nut, the only answer is to replace them both if you are to achieve accurate results.
Slack head-stock bearings are another cause of trouble when cutting deep, as the workpiece can be lifted up and pulled on to the tool—again this is a matter of careful bearing adjustment which will be found very worthwhile. If you have a deep cut to make in parting off a large diameter job and the tool is correctly ground and shaped with back clearance, the procedure is as follows: engage back gear and send the tool into the work with a steady, even pressure, and most important, avoid any slackening off; a slow speed and a coarse cut is necessary with ample lubrication to the base of the cut. Remember, of course, that the parting-off cut must be made as close as possible to the chuck jaws and never attempted between centres.
Where a really deep cut of three or mom inches is required, this should not be attempted in one operation, but in stages that amount to side-by-side cuts to give ample side clearance and also permit the swarf or chips to escape—also the wider gap is an aid to lubrication. The best method is to take the first cut in about one-fourth of the required depth then withdraw, and take a second cut alongside the first, making a wide slot. The tool is then sent in again to take a further cut down the original slot and this leaves a safe working clearance and will guarantee a trouble-free operation.
Roughing tool There can be a problem if the workpiece is cast iron and has a hard outer skin, as if this is not first removed the parting tool can be ruined or broken. Therefore it pays to deal with this area by first using a roughing tool to get through this hard skin, or scale as the Whereas the speed required is the same for steel or cast iron, dealing with brass is another matter, and techniques are somewhat different. First, a much higher speed is called for to turn brass, and the parting tool must be ground up without any top rake or lip, secondly, when brass is cut at speed, the swarf or debris is thrown off in the form of fine chips and is travelling at quite high speed, so much so that it can penetrate the skin on face or hands and be a real source of trouble, also a danger to the eyes.
Therefore, whenever you deal with brass turning, it pays to wear glasses, or set up a glass or plastics screen to travel on the tool post; also keep ungloved hands out of the debris stream, as the particles are almost in the form of tiny barbs and tend to work themselves deeper into the flesh. This sounds a bit dreadful, but the main risk is to the eyes, and your first brass job will teach you to take the other necessary precautions. Remember with brass, it is high speed, slow feed, fine cut—the same goes for copper. But above all use a really sharp lathe tool.