worm gear diameter

mce5802

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Question about making worm gears

I just had a question while we're somewhat on the subject, and as I can't seem to figure out how to start a new thread, I thought I'd ask it here. I'm building an indexer, and I decided to make the worm and worm gear. I ordered a 1 1/8 6 acme tap to cut the gear with. I need to figure the exact diameter of the gear for 90 teeth. I've googled it, I got one formula but it's clear as mud. Now simple math tells me for 90 teeth at 6 tpi I'd need a circumference of 15, diameter of 4.774. But that's PITCH diameter, right? Not the od of the blank? So do I add 2(.3183) or how do I go about getting that dimension? Not trying to hijack a thread here, I apologize in advance for any inconvenience.:nervous:
 
Re: dividing plates

I just had a question while we're somewhat on the subject, and as I can't seem to figure out how to start a new thread, I thought I'd ask it here. I'm building an indexer, and I decided to make the worm and worm gear. I ordered a 1 1/8 6 acme tap to cut the gear with. I need to figure the exact diameter of the gear for 90 teeth. I've googled it, I got one formula but it's clear as mud. Now simple math tells me for 90 teeth at 6 tpi I'd need a circumference of 15, diameter of 4.774. But that's PITCH diameter, right? Not the od of the blank? So do I add 2(.3183) or how do I go about getting that dimension? Not trying to hijack a thread here, I apologize in advance for any inconvenience.:nervous:


No inconvenience, but we do need to get this split out to it's own thread which I have done for you. To start a new thread, go into the forum where you wish to post, then on the left center of the screen you will see a button marked "+ Post New Thread" Click on that button and type away!!

As for your question, it looks like it will get a bit involved, so we will address it shortly. I have to do a little digging on this one, but I think I have your answer in my head. Just want to make sure before I commit it to a post.
 
I'm working on building an indexer utilizing a worm and gear. I've decided to try and make my own worm and hob a worm gear. I ordered a 1 1/8-6 acme tap to cut the gear. The question I have is how to figure the precise diameter the gear blank. If I use a 90:1 reduction I'd need a circumference of 15", dia. of 4.774 to get those 90 teeth. But thinking about it I'm fairly certain this would be PITCH diameter and the diameter of the blank needs to be larger than this. I've googled it and got a formula but in this formula I need the distance center to center of the worm and wheel and I'm not sure how to get this number. Could use some clarification on the whole formula for that matter. Thanks in advance for any help.
 
Looks like we had two threads going for the same thing. I merged them into one, that is why it looks the way it does.
 
Re: Question about making worm gears

I need to figure the exact diameter of the gear for 90 teeth. I've googled it, I got one formula but it's clear as mud. Now simple math tells me for 90 teeth at 6 tpi I'd need a circumference of 15, diameter of 4.774. But that's PITCH diameter, right? Not the od of the blank? So do I add 2(.3183) or how do I go about getting that dimension?

Your calculation of 4.7747 is correct and it is the pitch diameter or PD.

We know that the number of teeth (N) is 90 and that the calculations will be in inches

To calculate the outside diameter or OD, we need to first calculate the diametral pitch or DP.

DP = Number of teeth / PD DP = 90/4.7747 = 18.8493

So we now have the DP of 18.8493

OD = (N+2)/DP OD = (90+2)/18.8493 = 4.8808

Look good??
 
Looks like we had two threads going for the same thing. I merged them into one, that is why it looks the way it does.

The original post was in another thread, and I posted that it should be in a separate thread, then went back and moved the original post to a new thread. The OP must have started a new thread while I was taking those steps. Thanks for merging them, Randy!
 
I just stumbled on something else, and it has to do with the way in which you plan to make your worm gear and worm wheel. If you plan for your worm gear to be at 90° to the worm wheel, you will need to cut the worm wheel in a helical fashion which changes EVERYTHING and the OD and PD will need to be a bit larger. I am just trying to find out how to calculate that part of it because I don't have it handy or know it off the top of my head. If you plan to set the worm gear at the helix angle so that the teeth on the worm wheel will be at 90° to the wheel, then your worm wheel could be done the same as a spur gear and the dimensions I gave would be correct.

I also found that the oddball DP of 18.8493 will be a bit problematic with the proper calculations for this.

So, how do you plan to do this??

Additionally, you might want to get a great book on making gears: Gears and Gear Cutting by Ivan Law. It includes everything you might want to know about making your own gears in a hobby machine shop. You can find it on Amazon and probably on eBay too. I highly recommend getting a copy of it!
 
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Thank you Terry, I am going to give this math a try this evening. I have had a real miss and miss with this, hobbing a worm wheel with a tap. Was shooting for 40 tooth and first try ended up with 44 teeth. Second try after trying some other math, 36 and maybe 1/2 teeth.:dunno:

What you have provided is completely different than anything else I have found on the subject. I'll now be shooting for a 36 tooth with a 1/2" - 13 worm.

If I did your math correct, I get a PD=0.8814 / DP=40.8441 / OD=0.9303.

Here's to hoping this time it works out.

One thing, how far will i want to advance the cutter (tap) into the work? Does 0.047 sound about right? (major diameter - minor diameter) / 2 or for a class 2A thread, (0.4985-0.4041)/2=0.0472

Again, thanks for your input on this.

Mark - (who is learning something new today)
 
The math is entirely different for worm gears than for spur gears because of the helix angle of the worm gear that is involved, and I haven't had the time to go through all of the information on worm gears and worm wheels, it is quite a bit of reading. I have not studied them in any detail before now because I have not had any projects involving worm gears, and don't envision any in the foreseeable future. That is why I suggested getting the book that I mentioned earlier, as I think you will find it very helpful. Your numbers do appear to be pretty close, however.
 
OK, got ya. I must of been typing when you made that post before mine. (happens to me all the time). Found a pdf of the book on line and am looking through it now. Thanks again.

Mark
 
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