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Belt set ups for cutting right hand and left hand screw threads.
These pulleys are for a 48 Thread/Inch pitch. The belts and pulleys are best changed by removing the 1/4"-20TPI hex head bolt holding the short arm of the pulley carrier frame to the 1/2" square cross member. The pulley cluster of N2 and N3 and both belts can thus be completely removed. Note that when the pulley frame is bolted back together in the RH thread cutting configuration, the belts can be included in any order since they can be interleaved after the pulley carrier frame is reassembled. No specific order has been specified for which belt runs between the N1,N2 set or the N3,N4 set of pulleys since it should be easy for the user to do this. In many cases either pair can be used on either set of pulleys but not in all cases. The best placement of the belts is that which makes the pulley frame closest to horizontal. For laft hand threading the longest single sided belt(50 tooth) is used in combination with the 60 tooth double sided belt. Note that the double sided belt can be removed completely from the pulley set up or installed in it without disassembling the pulley carrier frame.
The right hand setups is pretty straight forward but the left hand setups are more of a challenge since there are several things to make work together. For this the double sided belt should wrap around the N2 pulley as far as possible and the tensioner pulley should be as close to the N2 pulley as possible without actually tightly "clamping" the belt between them. The distance between the N2 and tensioner pulleys should be close enough, however, that the belt cannot slide between them. If the distance between the N2 and tensioner pulleys is too great, it may be possible to cause the belt to rise up on the cogs of the N2 pulley and lose registration with it under very heavy tension. The tension does not get this great in screw cutting but one can prevent the possibility of any loss in registration by having the pulleys as close as described.
Note also that the end of the nylon guide peg is lined up in the same plane as the flange of the tensioner pulley. This keeps the belt from drifting off the edge of the tensioner pulley and jamming between this pulley and the flange of the N2 pulley when the spindle is turning in the normal forward direction. In reverse rotation of the pindle the flange on the N2 pulley keeps the belt in position. The belts always drift to the left until a flange or the guide peg stops them. This is due to the slight angles on the shafts caused by belt tension flexing the pulley carrier frame.
The formula for calculating the threads per inch; TPI(cut), of the cut thread is:
TPI(cut) = TPI(leadscrew)* N2*N4/(N1*N3)
Since the pitch of the leadscrew is; TPI(leadscrew)=20
and the number of teeth on the spindle pulley is; N1=14
Then the above formula becomes;
TPI(cut) = 20 * N2*N4/(14*N3)
This formula should give the same numbers as the Table for threading setups gives.
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* * * SOME FINE POINTS * * *
1. It may be noticed that tightening the pulley carrier frame to the lathe headstock seems to increase the tension in the top belt(between pulleys N1 and N2) beyond that which was applied by hand prior to tightening. This depends upon how one grips the carrier frame while tightening it but is caused by the fact that the carrier frame tends to be rotated away from the spindle as well as being pulled up against the headstock when the drawbolt is tightened. Sometimes this added tension seems too great and a readjustment can be made by setting the tension on the loose side before tightening. In this one aspect setting up the left hand threading pulleys may be easier than setting up the right hand case since the upper belt, which is the double sided belt now, can be loosened by simply loosening and retightening the "left hand adaptor" arm.
2. There are some fine points about timing belt operation once you get beyond the basic "gear" ratio calculation caused by the different pulley tooth counts. First, it has been learned that the double sided neoprene belt is nylon coated on only the inside surface (as recieved). It can of course be turned inside out and it is hard to tell which is which. Observation and consideration of the details of the geometry between belt and pulley indicate that friction between the none toothed parts; i.e., the flat belt elements, of these surfaces results in a competition between the belt wanting to run as an ordinary smooth flat belt as opposed to running in the perfectly indexed and registered operation we would like. If the friction in the smooth belt mode is high enough, this can cause the belt to want to advance ahead or behind the position where it lays on the pulley and where the belt/pulley teeth match. When this happens the teeth on the belt tend to climb up out of the teeth on the pulley.
Lowering or eliminating the friction of the flat belt operating mode is the solution to this problem and is undoubtedly the reason that a number of changes have recently been made in the tooth shape design of timing belts and pulleys. These belts are not available in the configurations needed for the threader but the standard trapezoidal tooth shape is very satisfactory. The single sided polyurethane belts in the kit have a modified shape which helps to reduce this problem over the neoprene types. The uncoated side of the double sided neoprene timing belt can cause some problem and it is recommended that this belt be sprayed with a silicone type spray before and periodically during use. This spray can be bought at hardware and auto stores and does not have to be applied so much that the belts and pulleys feel wet. Since the application does not lead to any reasonable amount of messyness in the pulley setups, it is recommended that the mating parts of the whole pulley frame assembly be sprayed. This will also undoubtedly help the operation of the other belt and the nylon-graphite bearings in the setup. As pointed out in other places - these belts and pulleys can be washed in soap and water to keep them looking like new. The effects of these silicone sprays are very durable and may not be completely removed by ordinary washing but since the sprays are so nonintrusive, it is recommended to keep the belts and pulleys "conditioned" by a spray occasionally.
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