Make A Worm Gear

I would like to find a process that is repeatable without gnashing the blank first.
Let me make this simple... YOU WON'T!

The "correct" way to make a worm gear is to use a hobber that is made for it. If you don't have the special machinery, you can do what you have already done. However, it will NOT be repeatable. The way to have any chance to make it repeatable is to gash the teeth partially, to give the makeshift hob a guide to work with.

Otherwise it will be hit-or-miss.... (mostly miss - as you have discovered)
 
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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.
I think what we are disregarding is the contact area on the worm gear. What you have cut is much deeper than the depth of one thread on the "worm", and as such the actual pitch diameter of the worm gear is smaller than your calculated size. That is why the formula in the other thread added 2 teeth, to give you that good contact area. I think if you stopped at your original calculated depth, you would have 72 teeth, just not much contact area.
 
You could make a gashing tool by making a tool similar to the one you are using but with the correct TPI for the starting diameter. Start with it and then switch to a tool with the correct TPI for the finishing diameter. It might also help to cut a groove to match the minor diameter of the worm around the blank.
 
Idea:
1) Cut a groove in the blank the minor diameter of the worm and as deep as you want the minor diameter of the threads in the gear to be.

2) Make a tool just like the one you are using but two or three times the length. Then turn a taper on it so that the teeth go from full height somewhere in the middle to just a minor-diameter shaft toward the end.

3) Start with the blank running on the skinny end of the tool and gradually advance it into the tool until you are cutting full depth threads.
 
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

I think gashing the blank is an important step unless you are using a dividing head to make a gear.
 
If you look at the hob in the video at about 5:40 in, it is not that much different than your homemade hob The primary difference is that the teeth are staggered. When you use your hob, once every quarter turn, the hob is essentially disengaged from the gear. Since the gear is being driven by the hob, it can momentarily stop rotating, losing synch. This is most likely what causes the loss in teeth as the hob cuts deeper into the gear.

+1 John, I think you have hit the solution. It should solve the problem of the changing pitch diameter as the tool cuts deeper. The final remaining element will be to determine the diameter of the groove to make your tooth count correct. I expect that the spur gear formula Do = Dp +2/P = N/P + 2/P = (N+2)/P where Do is the diameter at the bottom of the groove, Dp is the pitch diameter, , P is the worm pitch, and N is the number of teeth will be very close.

The length of the taper will determine the chip load for each cutter tooth. If an Acme thread profile is assumed and 10 tpi, the tooth depth would be about .050" There are four cutter teeth per revolution and if the cutting depth was .0005", you would need 25 threads to cut from zero to full depth. Add a couple of full depth threads at the end and you would need 2.5 to 3" for the cutter.
 
I just watched the video. One day here, I am going to have to try this ( I am back in the darned hospital again right now),but it appears to me, you make the worm first. MAKE TWO! one preferably from say drill rod, the other from steel. The one made from drill rod gets grooves cut in it to make a hob like in the picture. Use it to cut the worm gear and you then have a " set" when you finished. Keep the hob you made , in case you ever want to make another. If you want , harden the hob you made and finish grind the grooves to make it better. Did I miss something?
 
I just watched the video. One day here, I am going to have to try this ( I am back in the darned hospital again right now),but it appears to me, you make the worm first. MAKE TWO! one preferably from say drill rod, the other from steel. The one made from drill rod gets grooves cut in it to make a hob like in the picture. Use it to cut the worm gear and you then have a " set" when you finished. Keep the hob you made , in case you ever want to make another. If you want , harden the hob you made and finish grind the grooves to make it better. Did I miss something?
Mark, you are darn close. What Aerohawk says is true if you use a conventional hob. You can't cut the whole tooth in one pass so there would be a change in the number of teeth as you cut deeper ending up with a messed up gear. This is because the hob is controlling the rotation. The gashing process in the video does a rough cut under controlled conditions (indexing fixture, depth of cut, angle of cut. This does two things. First, it removes most of the material so you can start close to the final diameter and second, it provides a pre-cut track to guide the hob as the hob rotates the gear.

However what John is proposing will work because you are starting at the finish diameter and cutting deeper and deeper teeth. Because the diameter isn't changing, the number of teeth doesn't change. So make a hob as John is proposing. As you state, make two worms ( or make one long one and cut in half). Taper the end of one cut the grooves to make the hob, and harden if you wish. The attached file is my conception of the hob. It is a 3D PDF so you should be able to rotate it and zoom in for close up detail.
 

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  • Worm Gear Hob.PDF
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I have success and failure.
Here are three worm rings that have 72 teeth each. The outside two are 0.125 deep and the center is 0.180 deep with a diameter of 3.929.

GEDC2033.JPG

I made three cuts to insure the process is repeatable and the deeper you go the tooth count stays the same but the teeth get narrower until they are cut out entirely.

The failure part is that "aerohawk" was right. I could not make a gear with a predefined number of teeth with out gnashing the blank first. One trail for 120 teeth came out 118, 122,and 119 all cut at the same outside diameter.
I used my rotary table to cut 72 slots about 0.025 deep to mark the teeth. I could not make the tooth count change even though I really drove the cutter into the blank easy or rough the tooth count stayed the same.
I think the most important part of making a worm wheel is cutting the tooth locations. The diameter of the blank can be a little small or a little large but you still get the same number of teeth.

Thanks for looking
Ray
 
I have success and failure.
Here are three worm rings that have 72 teeth each. The outside two are 0.125 deep and the center is 0.180 deep with a diameter of 3.929.

View attachment 106841

I made three cuts to insure the process is repeatable and the deeper you go the tooth count stays the same but the teeth get narrower until they are cut out entirely.

The failure part is that "aerohawk" was right. I could not make a gear with a predefined number of teeth with out gnashing the blank first. One trail for 120 teeth came out 118, 122,and 119 all cut at the same outside diameter.
I used my rotary table to cut 72 slots about 0.025 deep to mark the teeth. I could not make the tooth count change even though I really drove the cutter into the blank easy or rough the tooth count stayed the same.
I think the most important part of making a worm wheel is cutting the tooth locations. The diameter of the blank can be a little small or a little large but you still get the same number of teeth.

Thanks for looking
Ray

My thoughts are to make the cuts on the job at an angle and then the job will be engaged at all times. It would be similar to having the teeth staggered
 
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