road and workshop
Page 33
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y Handyman
9enchwise: lathe sense (28)
LTHOUGH the ability to turn a thread in e lathe—screwcutting—is part of the )prentice turner's training, it can remain a ystery to the part-time turner who has airily gained experience of straight turning his own trial and error methods without Therefore, even if his machine is suitably nipped, as I will later describe, he may a wish to venture into this class of work it may appear to be difficult to him. This itlook is not, however, justified, as ovided his lathe is rigged for the job, a an who has the skill to carry out accurate )rk from the chuck or between centres can ually well turn his attention to cutting a read with the same accuracy. I will outline approach he should take and if possible, nplify the technicalities.
First—the lathe itself: to enable a thread be cut in a plain round bar, the lathe tool ist be able to travel along the lathe, and der control in relation to the speed of lation; to achieve this the lathe is equipped th a long, threaded shaft, normally sitioned on the operator's side and ming parallel to the ways. This is known, the "lead" screw, and it is coupled into, d driven by, the lathe madrel, ie the adstock shaft, usually through either :achable gear-wheels or a complete irbox with speed change.
In certain of the lighter range of lathes this has been arranged through Vee belts and pulleys. However, the general practice is detachable wheels or speed change. The variable speed is utilized so that the lead screw can propel the tool at any required set speed in relation to the live mandrel, and as the tool starts its cut it will take that cut the full length of the work and maintain exact thread form throughout.
In addition the lead screw performs as a mechanical traverse, which is used extensively in general long turning; also it usually provides a better finish than by manual traverse and a fine feed can be available as required through the gearing. More expensive or sophisticated lathes have a separate drive shaft for the self-act traverse, quite independent of the lead screw. Obviously there will be a regular need to run the lead screw in a reverse direction and provision for this is either built into the gearbox, or an extra gear wheel can be meshed to reverse the screw—in the case of gearbox lathes this provides forward, reverse or neutral positions.
As an example of what happens when a round bar is mounted in a lathe with a pointed tool in the holder, and a gear ratio of 1 to I selected which means that lathe mandrel and lead screw will revolve at the same speed, the lathe tool will then begin to cut or trace a thread to the exact pitch of the lead screw. It can be understood that if the gear ratio between lathe mandrel and lead screw is varied in either direction, so will the tool trace a more coarse or fine thread accordingly.
Another requirement in connection with the lead screw is the need to disengage the drive as necessary, but to be able to pick up the start of the thread again for a second run. Here the common practice is to fit a lever-operated split nut to the saddle apron that will open and close on the lead screw as required. This is a positive method, but where the lead screw nut is a solid one, the drive is likely to be released or engaged by a pin or dog clutch, hut the important point is that engagement must only occur at one point in the rotation of the lead screw.
Gearing system
The gearing system can puzzle the novice until he understands the meaning and use of gear trains, as gear charts do not always convey much more than ratio data. First, think of the gear on the mandrel shaft as the driver, and the gear on the lead screw shaft as the driven wheel. If you mesh a 70-toothed wheel with a 35 toothed wheel and the 70-gear is the driver, the 35-gear will revolve twice for one turn of the 70-gear. If those two are fitted to mandrel and lead screw on their own, then you have geared up the lead screw to a 2 to 1 ratio. Reversing the position of the gears on the shafts will give a geared-down ratio of 2 to 1, i.e. lead screw now turning at half mandrel-speed.
It can now be seen how you can couple various size wheels to cover a whole range of lead screw speeds; all gears are at the same pitch and mesh correctly. You will, however, find that a third or intermediate gear is found in these gear trains to transmit drive from one wheel to the other; this gear will also reverse the rotation of the lead screw, but no matter what size this wheel has to be to mesh with the gears above and below, the ratio between driver and driven remains the same. Therefore in the normal range of work, you select the two main gears needed to provide a given lead screw speed, and insert the appropriate idler wheel to fill the gap and give drive.
Obviously, single wheels meshed directly cannot provide the full range of ratios called for, thus a compound gear train is required, otherwise wheel size would be out of all proportion and unmanageable. A compound gear train is best explained by visuli7ing a single gear on the driving shaft in mesh with an idler gear, but this idler gear has been made with two gear wheels joined side by side.
The driving wheel meshes with the largd gear of the idler pair, the smaller wheel of the idler meshing with the lead screw wheel—therefore a situation now exists where a 35-toothed driver is meshing with a 60-toothed idler. But as the second wheel on the idler has only 20 teeth, with this small wheel in mesh with the 70-toothed lead screw wheel you have multiplied the original 2 to 1 ratio by 3, i.e. it is now a 6 to 1 ratio.