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road and workshop
Page 97
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)y Handyman
Elenchwise: lathe sense (30)
'HE screwcutting tool must be right for the )13—accurate in shape and with ample learances; shape is most important in that he thread must be at the precise angle of he nut thread, and a thread which has to be icked up and extended must have the same orm throughout. Thus the British Vhitworth V form which is at an angle of 5deg must be carefully copied on to the )ol and the best way to do this is to use the crewcutting tool template.
A single Whitworth gauge, or a template hat incorporates both the V form at 55deg nd the arrow head at the same angle, or a eneral gauge that covers Whitworth, BSF, wme and American screws can be ought—two popular makes are by Moore nd Wright, and Chestenman. These tools are ally essential to screwcutting operations. ot only for grinding the tool point orrectly, but also later to line the tool up xactly at right angles to the work. This is ecessary before starting up and is quite a imple operation, vis the screw gauge edges re truly parallel, thus if the edge is placed n the shaft to be threaded with the ppropriate V form towards the tool, it )1lows that the tool can be placed fully in le V of the gauge, and then tightened in the )01 post. This test can be applied at any me to ensure that the tool has not moved. While there is a great deal of quite !chnical formulae for grinding side learances, etc. particularly with tools for quare or acme forms, so that the tool skirt lay conform to a particular pitch. it is little se to the trainee or garage turner unless he as the special jigs, etc, that go with it.
Voricable threads He can, however, turn out quite sound nd workable threads by a very short period of trial and error, the main point being the correct V form to the laid-down angle, with perceptible side clearance but a definite 12deg backing-off below the tool point. This is probably more than is really needed and the turner can reduce this angle for deep threads, just as he may also slightly increase the side clearances in the same situation.
In practice, the curling swarf and his own ear for a clean cutting tool will tell him a great deal very quickly. The trainee has no need to bog himself down with masses of technicalities at the onset; the main thing is to have a go. but stick to the basic rules of thread form and ample clearances.
Whereas the tool bit itself can be in several forms, from a straight shank incorporating the formed tool point and gripped in the post. to the more sophisticated tool holder with a detachable bit of the required shape. there is, however, a particular type of tool holder much favoured by the experts. Because of its built-in characteristics, this tool holder is also useful to the trainee; this is the spring tool holder and it is an accepted aid to cutting a clean and smoothly finished thread.
In principle, it is a sturdy, square-sectioned bar, fitted with the tool bit clamp or pinch belt at the end: but about one inch inwards from the tool pinch belt or screw there is a saw cut or parting upwards for about half the bar thickness. In another version, the bar is slightly hump-backed near the tool clamp and reduced to near half-thickness in this area. 'so giving a small amount of spring against the downward thrust of the tool under load. This give or spring can be used to advantage in finishing a thread, just as I described much earlier when boring with a slightly springy tool, as the final run can be made with no extra feed other than that pressure or elasticity which is in the spring holder.
Shape of tool bit
A final word on the actual shaping of the tool bit. The top must be perfectly flat, otherwise if any form of rake is given this can effect the cutting form. Also when shaping the tool, check its form with the gauge against the light or a white card. The expert will give the extreme tip or point of the tool a light rub with an oil stone, in order ever-so-slightly to round the point because apart from the thread shape, it is not desirable to have anything in the nature of a cut or nick in the screw which could later promote a fracture.
Once having settled the business of shaping the tool to the required V form for a Whitworth thread, with the plain round bar which is to be screwed now mounted between centres, or from chuck to tail centre, and having lined up the tool by placing the gauge edge on to the round bar with the tool sent squarely into the V at exact work centre height, we are nearly ready to go. First, however, check that the correct gear wheels for the thread are mounted, or that the correct gearbox ratio has been selected. Secondly, engage the lowest back gear, and, thirdly, move the whole lathe carriage the full distance to make certain that there is unrestricted travel for the full length of the work to be cut.
Now engage the lead screw and ensure that the engaging lever goes fully home. If in doubt, move the head gear by hand as on occasions the split or clasp nut may hang up slightly, although most practised turners will hold the lever hard down for a second after starting in order to make sure. Start up for a trial cut at not more than .001in.; at the end of the run wind off the tool quickly and disengage the lead screw drive, then wind the carriage back for the next cut but this time a few "thous" deeper. Never overdo the depth of cut as this can tear and ruin the thread.
If the cross-slide is fitted with an index it is most valuable in that the cut can be set in "thous" and depth of thread established at any time. Where there is no index dial it is necessary to chalk mark the feed-screw handle to correspond with a similar thin mark on the cross-slide; by making these marks coincide the original setting can be duplicated. However, do note that it is necessary to re-mark the feed handle after each advance.