Make A Worm Gear

Well I don't know what I'm doing, but there is a article in the 1948 popular science about making worm wheels in detail giving the formula for figuring the proper wheel sizes .:confused:
 
This has reminded me that I have a 5/8 10 LH tap that I made decades ago. If I could get a salvaged cross feed screw from somewhere I could make a worm gear set rather easily.
 
I had no idea when I started this thread just what I was getting into.

This is my setup.

GEDC2003.JPG
Without doing any research I just assumed that at 6 threads per inch and wanting 72 threads on the gear I would divide 72 by 6 to get the circumference of the gear. Multiply the circumference by 3.14 to get the diameter of the gear. This is what I got.

GEDC2012.JPG

It looked great with well formed teeth all the same size and evenly spaced. I was patting myself on the back when I decided I had better count the teeth just to make sure there were 72 of them. After numerous counts I found there were only 69 teeth.
What happened to the other three? Do I need to increase the diameter of the blank by three teeth and start over with that?
Finally I decided I should probably check out some of the videos and sites suggested earlier in the thread. Each place I went for information seemed to explain it differently or didn't give a clear cut answer. I came back here and did a search for worm gear on the forum. This thread from January of 2015 (http://www.hobby-machinist.com/threads/worm-gear-diameter.31182/ )was very enlightening and the formulas were mostly understandable. (No where have a found an answer to why you have to add 2 to the number of teeth)

I think Mark Stephen said it best: " I am beginning to suspect the need to sacrifice small furry animals while standing on my head reciting incantations to pull of this bit of black magic. :lmao: There seems to be very little hard math behind it. " I will gash the teeth first.

Inr 729 Richard offered in this same thread a calculator for free. Well let me tell you it is great and thank you Richard very much. The calculator covers many different subjects and I will recommend it to all.

Later
Ray

Well I haven't given up and will be trying again soon
 
I had no idea when I started this thread just what I was getting into.

This is my setup.

View attachment 106740
Without doing any research I just assumed that at 6 threads per inch and wanting 72 threads on the gear I would divide 72 by 6 to get the circumference of the gear. Multiply the circumference by 3.14 to get the diameter of the gear. This is what I got.

View attachment 106743

It looked great with well formed teeth all the same size and evenly spaced. I was patting myself on the back when I decided I had better count the teeth just to make sure there were 72 of them. After numerous counts I found there were only 69 teeth.
What happened to the other three? Do I need to increase the diameter of the blank by three teeth and start over with that?
Finally I decided I should probably check out some of the videos and sites suggested earlier in the thread. Each place I went for information seemed to explain it differently or didn't give a clear cut answer. I came back here and did a search for worm gear on the forum. This thread from January of 2015 (http://www.hobby-machinist.com/threads/worm-gear-diameter.31182/ )was very enlightening and the formulas were mostly understandable. (No where have a found an answer to why you have to add 2 to the number of teeth)

I think Mark Stephen said it best: " I am beginning to suspect the need to sacrifice small furry animals while standing on my head reciting incantations to pull of this bit of black magic. :lmao: There seems to be very little hard math behind it. " I will gash the teeth first.

Inr 729 Richard offered in this same thread a calculator for free. Well let me tell you it is great and thank you Richard very much. The calculator covers many different subjects and I will recommend it to all.

Later
Ray

Well I haven't given up and will be trying again soon
It goes back to the question that I asked in post #2
.......Also how did you compensate for the reduction in diameter as the "cutter" went deeper into the work?
As you cut deeper the pitch diameter decreases. The diametral pitch remains the same (it's determined by your cutter) so the number of teeth has to decrease. Now, your cutter is driving the gear so you are probably starting with 72 teeth. The mystery to me is how you morph from 72 to 69 teeth without having some malformed teeth.

The pitch diameter of a gear is not the same as the outside diameter. the worm engages the gear teeth approximately midway along the tooth and that is where the pitch diameter is measured. The two extra teeth are what is needed to correct for using the outside diameter. Note that correction this is for spur gears. Your worm gear cuts deeper than a tooth depth which is probably why you need three extra teeth instead of two.

Boston Gear has a rather thorough white paper on gear theory. http://www.bostongear.com/pdf/gear_theory.pdf
 
You guys are getting into my question in the beginning of the post. How to figure the ratio. I ain't no gear expert, in fact I have always avoided them. I am trying to find how to figure the diameter of the worm blank AND the diameter of the worm for a given ratio. My brain smokes and seizes up trying to understand worm gears
 
Figuring the ratio is fairly easy. For a single start worm, each turn of the worm advances one tooth on the gear. The number of teeth on the gear determines your ratio. With in reasonable limits, the diameter of the worm doesn't figure in. Its kind of like a helical rack and pinon except the rack is wrapped around a shaft. The pitch diameter of the mating gear is determined by the pitch and the number of teeth.
 
Figuring the ratio is fairly easy. For a single start worm, each turn of the worm advances one tooth on the gear. The number of teeth on the gear determines your ratio. With in reasonable limits, the diameter of the worm doesn't figure in. Its kind of like a helical rack and pinon except the rack is wrapped around a shaft. The pitch diameter of the mating gear is determined by the pitch and the number of teeth.

This is kind of like the chicken or egg theory, which came first. Ok...I can see the worm diameter makes no difference. So, do you make a worm gear with the number of teeth needed and then make a worm to match or just pick a TPI and make the gears to match it?
 
This is kind of like the chicken or egg theory, which came first. Ok...I can see the worm diameter makes no difference. So, do you make a worm gear with the number of teeth needed and then make a worm to match or just pick a TPI and make the gears to match it?
I would think you need to make the worm. It would be cut like a screw thread but with the proper profile.

Conceivably you could cut the gear to match but the equipment required would be quite complex. The gear is essentially a helical gear and and has to be rotated in synch with the cutter. The OP's method of cutting self -rotates the gear to maintain the synch as it is cut. Here is a You Tube video the does a great job of explaining the process. The hobbing process at the end is very similar to what the OP is doing. The gashing part is what is missing and why the 72 teeth morphed into 69 for the OP.
 
More experiments in process.
I have found three different formulas for computing the diameter of the worm wheel but most of the formulas end up very close to the same result and do add 2 teeth in the formula.
Richards calculator not only gives numbers for the wheel but also the screw if you want to make that. The calculator even gives figures for an envelope worm gear.

Mark I am going to keep going until I find the answer one way or the other.
I would like to find a process that is repeatable without gnashing the blank first.

Ray
 
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