How necessary is having modular / Diametric pitch capability on your lathe...

[RJSakowski]

Circular pitch, p, of a gear is the distance between teeth along the pitch diameter and is related to the diametral pitch by p = π/d.p. Circular pitch of a gear is the equivalent of lead or pitch in screw threads.

Diametral pitch is limited to whole numbers, and since the circular pitch, which is what would be of interest when cutting a worm gear, is π divided by a whole number, the change gear ratio must necessarily be an irrational number. Gears by their very nature must have rational numbers which means that to cut worm gears on lathe with a conventional lead screw, an approximation must be used for the gear train.

Looking at my chart of all possible threads that can be cut on a Grizzly G0602, it appears that there are a number of diametral pitch worm gears that could be cut to better than .1% accuracy using the stock change gears. If I add the 127 tooth transposing gear to the list, I would expect that a number of others would pop up.

 

[benmychree]

In my experience, all the worm gears that I have encountered have bee circular pitch, easy to gear up on most any lathe to cut the worm; the gear is quite another matter, and nearly always, the gear is the part that wears out, not the worm.

 

[ArmyDoc]

Looking at my chart of all possible threads that can be cut on a Grizzly G0602, it appears that there are a number of diametral pitch worm gears that could be cut to better than .1% accuracy using the stock change gears. If I add the 127 tooth transposing gear to the list, I would expect that a number of others would pop up.

Ok, so let me put it a different way. If I were to purchase a lathe like the PM 1340GT or 1440GT, which do not have a universal gear box, and I wanted to make something from scratch that was driven by a worm gear, could I do it? It sounds like I wouldn't be able to make all the possible worms and gears out there, as I would if I had a universal gear box, but that I would likely be able to make some, or even several.

In my experience, all the worm gears that I have encountered have bee circular pitch, easy to gear up on most any lathe to cut the worm; the gear is quite another matter, and nearly always, the gear is the part that wears out, not the worm.

As I understand it, the gear would usually be cut with a hob, not directly on the lathe. But the hob is made from the worm. So, if you could make the worm, you could make it into a hob and then use that to cut the gear?

 

[Lo-Fi]

If you're talking about worm wheel cutting, then yes, the tool is made the same way as the gear with some flutes and relief cut in.

Yes, you'll be able to to it. Like anything threaded, what pitches you can achieve depend on the change gears and/or gearbox.

 

[RJSakowski]

Ok, so let me put it a different way. If I were to purchase a lathe like the PM 1340GT or 1440GT, which do not have a universal gear box, and I wanted to make something from scratch that was driven by a worm gear, could I do it? It sounds like I wouldn't be able to make all the possible worms and gears out there, as I would if I had a universal gear box, but that I would likely be able to make some, or even several.



As I understand it, the gear would usually be cut with a hob, not directly on the lathe. But the hob is made from the worm. So, if you could make the worm, you could make it into a hob and then use that to cut the gear?

You should be able to cut any worm gear with a change gear lathe provided that you have the right combination of change gears. It will not be possible to find an exact combination as can be don for cutting metric threads on a lathe with an Imperial lathe. The inch is defined as exactly 25.4mm which can be achieved by the ratio of to whole numbers, typically 127/100 sd exact metric thread can be cut.

A ratio which contains pi as one of the multipliers can't ever be achieved by the ratio of two whole numbers. However by choosing the right combination of gears in a change gear train, you can come up with an approximation that is close enough for all practical purposes.

Different lathes use different strategies for achieving the change gear combinations to cut a particular thread. The change gear set is chosen by the manufacturer to cut all the common threads within a particular range. The change gear set for the G0602 provides 1280 possible combinations, not counting those if the 127 tooth transposing gear is used which would triple that number.

The classic approximation of 22/7 for pi gives a ratio that differs from the true value by .04%. This is easily attainable with common gear ratios, e.g., 44/14, 66/21, etc. A closer approximation would be 201/64 which is accurate to .03%. 201/64 = 3*67/2*32 or 67/32 * 45/30.