# Make a 72 tooth worm gear without indexing?



## OrangeAlpine (Feb 11, 2013)

First of all, I'd like this thread to be a "how to", but I'm afraid the question mark in the title says it all.  At best we will learn how to make one with any indexing.  At worst, we will learn some things that will not work.  The current state of affairs requires that a person needing to make a 72 (or any other number) tooth gear has the ability to make 72 equal divisions.  That just does not sound right.

I have a thread on the Questions sub forum, but I was not asking many questions, so I'm starting this one.  

A quick preview of where I am:

This is my initial effort at making the hob.  7 tpi, fluted using a cutoff wheel in a Dremel tool.  The idea is for the hob to have continuous contact with the gear blank.







It worked fairly well.  The technique is to rock the hob back and forth while feeding the gear blank into the hob, then cutting the teeth full depth in one pass.  Here is the initial effort.  It is .100" hard aluminum.  While it made decent teeth, there were more than calculations predicted and I was less than happy with the tooth shape.  






So I decided to alter the hob.  Increased the number of flutes and lowered the tooth height, thinking that a cutter more resembling the Acme pattern would be better.  WRONG!  The second gear was made using .500" thick aluminum.  Things did not go well.  The cutter refused to really dig in and actually allowed the blank to quit turning at times.  Here is the grim result.  It looks doubly strange because I flipped it over and tried a second time.





So back to the beginning for try #2.  It will be 8 tpi in order to reduce the depth of cut.  Instead of reducing the depth of cut in order to alleviate cutting pressure, tooth width will be reduced.  The hob will have sections.  The first section will have 1/2 width teeth, with successive sections having increased tooth width.  Also need to come up with a more sophisticated 
flute system.

None of this has been made at this time, so any suggestions can be incorporated if they are made relatively soon.

Bill


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## OrangeAlpine (Feb 11, 2013)

Hate to respond to my own post, but I would like opinions about technique.  After making the initial cut (1/2 tooth width) would it be better to go immediately to feeding in a full width tooth section, hoping that would help to even out any tooth spacing irregularities.  I'm thinking that opening the teeth incrementally will simply preserve any spacing problems.

Bill


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## Herbiev (Feb 11, 2013)

Just wondering if you could use a 1" 8tpi tap as a hob.

[ame]http://youtu.be/ie7dNOLgno0[/ame]

Post Comparison
 	Just wondering if you could use a 1" 8tpi tap as a hob.
+	
[ame]http://youtu.be/ie7dNOLgno0[/ame]


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## f350ca (Feb 11, 2013)

I think for the hob to cut you need back clearance after the cutting edge. Your cuts with the dremel create a cutting face but the land behind it will ride up on the blank and stop the cut. similar to a drill bit with no clearance angle.


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## Swifty (Feb 11, 2013)

I have seen a worm made with a tap, I think it was about M8, worked really well. It was used on an astrological telescope. 
I have a feeling that there is not enough relief on your hob, causing it to rub too much. Bear in mind that most of the cutting is done with the lead in taper and first few threads, the rest of the threads are only used to keep the worm turning.

Paul.


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## OrangeAlpine (Feb 11, 2013)

Herbie, the 1" - 8 tap that I have has flutes 1/2" wide.  The key to this process is continuous cutter contact with the gear.  Maybe if I was making a 3/4" wide gear.  Perhaps if I made a 3/4" wide gear and faced it off to 1/2".  Whats happening in that video is exactly what I want to do.

Swifty and f350ca.  Yes, my "sharpening" left tons to be desired.  However, I'm thinking that if I mimic tap geometry it should work.  No relief,  same angles, narrow flutes for ample gear contact.

Bill


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## OrangeAlpine (Feb 11, 2013)

Herbie, you got me to thinking.  Checked into my tap collection.  I have some 3/4" - 10 taps.  The flute is about 3/8" wide and the 10 tpi should make a gear that is 2.29" dia, a workable size.  Gotta see what tomorrow brings.  

Also have a 7/8" - 9, but I would probably have to make the blank about 9/16" wide.  Doable.

Bill


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## Chriske (Feb 12, 2013)

Swifty said:


> I have seen a worm made with a tap, I think it was about M8, worked really well. It was used on an astrological telescope.
> I have a feeling that there is not enough relief on your hob, causing it to rub too much. Bear in mind that most of the cutting is done with the lead in taper and first few threads, the rest of the threads are only used to keep the worm turning.
> 
> Paul.



Paul,
You made an 'astronomical' mistake, telling you saw some astrological telescope.
To be clear astro*n*omy is a real and serious science. Astro*l*ogy is ab-so-lu-te-ly NOT...!

Chris


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## Swifty (Feb 12, 2013)

Chriske said:


> Paul,
> You made an 'astronomical' mistake, telling you saw some astrological telescope.
> To be clear astro*n*omy is a real and serious science. Astro*l*ogy is ab-so-lu-te-ly NOT...!
> 
> Chris



Oops, i stand corrected. Never into any of that star sign mumbo jumbo. Anyway it was a telescope for looking at the night sky.

Paul.


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## Chriske (Feb 12, 2013)

Swifty said:


> Oops, i stand corrected. Never into any of that star sign mumbo jumbo. Anyway it was a telescope for looking at the night sky.
> 
> Paul.




star sign mumbo jumbo....:rant:

...


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## dman (Feb 12, 2013)

I haven't attempted to hob on a lathe but it without the hob and part being driven by gears to stay synced there are some geometric problems that will make it difficult to. much like knerling there may be an ideal pressure/feed to prevent a double cut pattern. 

my guess is you need to find the pitch dia of the tap and the pitch dia of the gear that lines up to the tooth count then add the difference of the tap minor dia and pitch dia to the gear pitch dia to get the major dia as long as the circumference of the major dia doesnt increase enough for half an extra tooth to fit along it. it may be OK or even preferable to have an incomplete thread, infact the pitch diameter may also be the major dia and the sync problems may go away in that situation at the expense of only having half a thread formed. then find the ideal offsets for the pitch diameters to line up. it'll take some indicating but once you find the ideal position of the x from the spindle I'd lock it down and feed it in with the z. make plenty of blanks and experiment with aggressiveness of feed on the z. 

I know from knerling that you can get things to sync if the teeth land within the marks of the last pass but there seems to be an ideal pressure. but you will have to see if your major diameter causes the circumference to increase enough for more than half an extra tooth to be created. like I said you may need an incomplete tooth for this to work.


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## aarggh (Feb 12, 2013)

Herbiev said:


> Just wondering if you could use a 1" 8tpi tap as a hob.
> 
> http://youtu.be/ie7dNOLgno0


 
I'm a bit new to this, but that video is absolutely brilliant Herbie! You make it look so easy! Is the outside machined diamater of the gear simply multiples of the measurement from the tip of one thread to the next? Or another formula is used?

cheers, Ian


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## chipenter (Feb 12, 2013)

You haven't said how you are holding the blank , I recently cut a 72 tooth bronze worm using a 1\2" whitworth tap , but I used a slitting saw to index it to half depth first and fed the tap in until it looked even all round , you could just still see the slitting saw cuts on the outside .


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## Lakc (Feb 12, 2013)

http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml

http://www.astronomyasylum.com/gears.html

I knew I had seen this before 
If your making a hob to cut the worm, just cut the worm at the same time. You dont need to taper the front to look like a tap, but leave it square and gash it in a continuous depth.


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## OrangeAlpine (Feb 12, 2013)

Chriske said:


> Paul,
> You made an 'astronomical' mistake, telling you saw some astrological telescope.
> To be clear astro*n*omy is a real and serious science. Astro*l*ogy is ab-so-lu-te-ly NOT...!
> 
> Chris


Well, its pretty clear you've never talked to an astrologist.

By the by, I don't think that video counts.  All evidence says the wheel was generated using the gnash and form method and the video documents the final step.  Which is interesting.  They were lapping in the wheel.  As I understand lapping, the grit embeds in the softer material and polishes the harder material.  Looks like they mostly polished the worm.

Bill


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## OrangeAlpine (Feb 12, 2013)

What we learned from todays failure:

1. The flute being "just barely" narrower than the gear is not good enough.

2. At this scale, we need to keep the rear of the blank from lifting.  I had hoped not as the ball bearing thrust assemblies were not doing the job.  Maybe no bearing on the bottom, one on top is the combination.  Another reason for a smaller diameter gear.

3.  Using a 3/4 - 10 tap and cutting aluminum, pressures are high.  Will try 5/8" - 11.  Yeah, I know, they will be high also.  Flutes are narrower, so it is worth a try.

4.  Discovered that I have sprung the jaws on my 4 jaw chuck.

Actually, that is good news.  While I have no clue how I did that, it explains some funky things that were happening yesterday.  

Anybody know a good place to get jaws for Enco chucks?

Bill


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## OrangeAlpine (Feb 13, 2013)

chipenter, I'm holding the blank on a 9/16" bolt, allowing the blank to rotate.  My effort on the universal holding method.

Here is todays "gear", made using a 5/8" - 11 tap.





You can see things did not exactly come out even, and there are more teeth than calculated.  But I was able to generate a gear in thick aluminum using a hob.  I think the tooth count is wrong, in part, to the fact my calculations are based on the periphery of a circle.  However, the tap sees a a polygonal.  In this instance, a 102 sided polygonal.  As I am not smart enough to address this issue mathematically and I'm pretty sure there are other issues involved, I will proceed empirically.  Kentucky windage, if you will.

All this is leading me to try once again, using a home made hob.  It will be have 10 tpi and be two staged.  The first stage will have full depth, very wide teeth (narrow thread on the hob).  That should allow generating full depth teeth in one pass.  The second step will have normal profile thread.  The idea being to use the narrow thread profile to create a semi-finished blank which will be smoothed out with the second step.  Very much like the conventional method of making worm gears. 

Stay tuned.

Bill


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## chipenter (Feb 13, 2013)

72/11 = 6.5454R/Pie =2.291534 this is the pitch circle to get the overall size add 2 teeth 
74/11 = 6.7272/Pie = 2.141 inches diameter 
for 10 tpi 2.3551877 inches diameter 
mine was 12 tpi and it woked for me .


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## OrangeAlpine (Feb 13, 2013)

Thanks chipenter.  A very good place for me to start!

Here is today's activity.

Here I'm narrowing the tooth.  Am using a .060" necking tool, taking a plunge cut, a couple of thou per pass.






The thread cutting work all done.  The left hand portion will be used to form the teeth.






Cutting the flute.  The tool is a necking tool turned sideways and the saddle hand cranked the length of the hob.  Lots of cranking!  The bit is set so the top side is on center.  Good view of the altered "gnashing" thread form






The finished hob.  






Overall view of todays work.  Now to see how it performs on aluminum.





Bill


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## Herbiev (Feb 13, 2013)

Ian. Sorry for the late response. I was in hospital AGAIN. I have cut a few worm drives but forgot the formulas. I think I cheated and got it from Marv's site. 
Bill the 3/4 x 10 sounds the go. I usually use whatever tap is available and machine to size.


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## OrangeAlpine (Feb 13, 2013)

This is where is the gnashed teeth joined at one revolution.  Looks pretty good, huh?






And after forming with the full tooth portion of the hob.






I'm very happy with these results, but the tooth count is not as predicted.  The measured diameter is for 90 teeth, which gave an 88 tooth count for chipenter.  How about 93, Bunkie?  This is pretty consistent with my previous experience, which was two excess teeth.  I think I will try making the diameter for 70 teeth and see what happens.

Bill


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## OrangeAlpine (Feb 14, 2013)

Turned the blank to 2.228", which is the calculated diameter for a 70 tooth gear.  Here is the result:






The short gnash is the start of the 72 nd tooth.  Almost perfect.  The gnashes were made by cranking in .050" after making solid contact with the hob.  By solid, I mean unable to rotate the blank by hand. 

This blank was a test mule and has some quality issues.  A final gear will be made.  

Bill


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## Lockstocknbarrel (Feb 15, 2013)

Hi Bill 
You may find this interesting.

http://easyweb.easynet.co.uk/~chrish/cancelled account/worms.htm

Regards 
Beagles


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## Lakc (Feb 15, 2013)

Lockstocknbarrel said:


> Hi Bill
> You may find this interesting.
> 
> http://easyweb.easynet.co.uk/~chrish/cancelled account/worms.htm
> ...


I was looking for that site earlier, printed it out years ago but hard to link to a pile of paper in your office.


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## Lockstocknbarrel (Feb 16, 2013)

Hi Lakc,
Try attachment this it is condensed.
Just trying to help.
Chris Heapy is a true *Home Model Engineer.*

Kindest Regards
Beagles 

View attachment Worm Drive Machining Techniques.doc


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## OrangeAlpine (Feb 16, 2013)

That link is of no value to a person wanting to make a worm gear without indexing.  

Requires gnashing (what I'm avoiding)
No tooth calculation information (info I could have used)
Says to use bearings to support the gear blank (not needed)
Calls for hardening of hob. (not needed)
Contains improper hobbing instructions for hobbing a given number of divisions.

I say that in order to prevent confusion by anyone deciding to use my method and should not be construed as a critcism of anyone.  

Here is a summary of my technique:

Make sure the hob flute width is no more than 1/2 the length of the tooth to be produced on the gnashing pass.

Make gnashing pass as deep as possible.  I fed the blank into the hob the full depth (.050") while rocking the hob back and forth while located over a flute.   After depth of cut is established, lathe power can be used.

Make only one pass with the gnashing hob in order to leave material for the finish hob to smooth any irregularities.

Use several passes of the finishing hob to allow it to work out any imperfections.

A greased piece of steel nearly the diameter of the blank makes an adequate surface for the blank to rotate against. 

It was necessary to use an upper bearing surface in order to prevent gear "lifting".

I used WD 40 to lube the blank and wire brushed like crazy to control chip build up.

Use chipenter's instruction for calculating blank diameter,  only subtract 2 teeth, not add.  I think he got confused as he crossed "The Pond".  In my case: 70 divided by 10, divided by Pi was _perfect_. 

Use aluminum.  I have no idea of the forces that would have to be overcome while making the gear of brass.  It may work, it may not, most likely dependent upon your lathe's rigidity.  

Bill


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## OrangeAlpine (Feb 19, 2013)

I decided it would be best to have a concave shape to the gear teeth, in order to maximize tooth contact and wear.   So I made a fancy blank:







Problem:  The tooth count went to 78 and tooth contact with worm is not good due to the decreased tooth spacing. 




Tooth spacing is 11.4 tpi instead of 10.  I cannot make an 8.5 tpi hob, has to be either 8 or 9.  Any ideas how well a gear made with an 8 tpi hob (probably would give 9.5 tpi) would mesh with a 10 tpi worm?  

Any other suggestions, other than to forget the concave teeth?

Bill


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## OrangeAlpine (Feb 20, 2013)

Just thinking out loud.  While its true I cannot make an 8.5 tpi hob, I can make a 9.5 tpi worm, as well as 10, 11 and  12.  So I'm debating the wisdom of making an 8 tpi hob and selecting the best pitch worm to mate with it.  

Or should I just go with the flat face gear that I know I can make?  Anyone really interested?

Bill


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## OrangeAlpine (Feb 20, 2013)

As I fiddled with the finished gears, I realized the tooth form was really bad.  The hob had been made with a rather large flat bottom and small flats on top.  Looking at the gears, especially the concave ones, it became apparent a totally different tooth pattern was needed.  The space between the gears was lobbed out at the bottom and the teeth were short and sharp.  

The best I could do was to make the hob wtih a wider Vee at the bottom of the thread and sharp on top.  Here is a photo of the revised hob.  If you look on the left side of the hob you can get a pretty good feel for what I ended with.






This improved  gear tooth enough that the 10 tooth worm would engage without slop on the concave gear.  Previously a 13 tooth was required.  

But I think I'm going to abandon the idea of concave gear.  Don't think I will be able to wear out the flat gear, especially with the improved tooth shape.  In addition, the concave gear is proving very difficult.  

Time to move on.

Bill


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## OrangeAlpine (Feb 21, 2013)

My "keeper" gear.  Note the depth of tooth formation.  





I had been afraid that if I went to deep, the change in pitch diameter would cause the hob to walk along the periphery, creating a new tooth count.  Does not happen.  The hob did create multiply patterns the first few times around the blank, but once it settled down, I never destroyed the pattern.  The hob was fed into the blank about 40 thou farther than the previously posted completed gear.  The two gears are the same width.

Bill


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## cwebs (Mar 6, 2013)

For what it's worth, I made some worm gears with a 1" 8 tap where the gear material was thinner then the flute  by sandwiching the blank between two peaces of plastic to keep the cut constant. I also did one using wood on each side of the blank. Worked well for me. Carl


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## robcas631 (Mar 7, 2013)

Do you have a picture?


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## Chriske (Mar 7, 2017)

It's an old thread I know, but I thought to show how I made a wormwheel to be used in a startracker I designed.
The assembly runs *very* slow, the wormwheel itself runs at  a whoping1 rpday. So I choose SS for the worm. A SS bolt(M6) that has been cleaned up a bit serves as worm.
To make the wormwheel is basically nothing more than a M6 tap pushing against a brass's disk perimeter. I made(3D printed) two brackets that will hold two ball bearings(8x22x7). The wormwheel has the same size hole as the bearings. Once the assembly is fixed on the milling  machine, the wormwheel can rotate freely around the 8 mm axis.
Then the rotating tap is pushed very gently against the perimeter of the wormwheel. If you go for it, again go slow..!
As you see there's no indexing involved, yet it turned out to be a perfect wormwheel, very sharp and clean teeth.

EDIT : forgot to mention diameter of the wormwheel : 50mm (2")  Ratio : 1:156


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## Nick Hulme (Mar 7, 2017)

The OP seemed to be asking how to generate a wheel with a chosen tooth count without any indexing. 
He should have asked how to generate a wheel without understanding - without understanding that gashing (not gnashing  ) using some form of indexing is the way to set tooth spacing and thus set tooth count ;-)


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## Chriske (Mar 7, 2017)

Hi Nick,
Just wanted to demonstrate a quick and easy way to make a rather good quality wormwheel.
Maybe I should have started a new thread for this one...


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## velocette (Mar 7, 2017)

Hi 
One method that has not come up is to use a "Spiral Flute Tap" to cut the worm wheel.
This will give much better cutting action and a lot less load.
Search "Model Engineers Workshop" There is an article by Neil Wyatt on making precision  worm gears.
http://www.model-engineer.co.uk/forums/latest_posts.asp

Eric


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## Wizard69 (Mar 7, 2017)

Having followed amateur telescope making since i was a kid, this is just another example of the get her done attitude in this group.   Ive seen some amazing work from these dedicated hand builders of telescope hardware.  Often this work is done in the most meager of shops.  

Thanks for the post.


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## kiwi2 (Mar 8, 2017)

Hi Chriske,
                  Did you start with a plain disc with a flat edge or did you start with a groove with a diameter equal to the tapping drill size for an m6 thread?
Regards,
Alan C.


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## Chriske (Mar 8, 2017)

Hi Alan,

I started with a flat perimeter. It probably would work faster if I cut a groove first. 
I'll need an extra set of these wormwheels for another project very soon, Maybe I'll try it.
On the other hand it's maybe not worth the effort because the teeth are not very deep for a M6 tap. It only takes me literal a few minutes to cut one wormwheel. I can imagine if I would cut a wormwheel with a very large tap, say M24(about 1"), it'll work way faster to have that groove first.

Chris


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## Chriske (Mar 8, 2017)

What puzzle me is the fact that using this technique I get a perfect wormwheel every time again, no matter what diameter the wormwheel is. I've tried this with smaller diameters to and every time I get a perfect result. Even with very small diameters. I even cut a wormwheel on a 8mm(about 1/3") smooth rod, to be used as a hobbed bolt in a 3D-printer. Perfect result, each time.
My brain says : the very first point were you start cutting on that perimeter is not necessarily going to meet exactly with the last cut on that perimeter after exactly 1 rotation. So I decided to challenge my brain and made a smaller wormwheel so I made a 49mm and a 48 mm and also a 45mm diameter wormwheel with that same M6 tap. Each time again I got a perfect result. And I can't still figure out why.

This is a 'wormwheel' I cut on a 7mm diameter smooth brass rod. Way to small for a wormwheel of course but a perfect hobbed bolt for a 3D-printer.


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## tornitore45 (Mar 8, 2017)

What Chriske said makes perfect sense.  Because: For a given pitch P there is an arithmetic series of diameters accommodating an integer number of teeth.
The diameters corresponding to 1 tooth increment differ P/pie.
Since the 1/2 depth of the tooth (from pitch diameter to top or bottom)
is  P x Sqrt(3)/4 < 0.5 x P/pie  there is no way that the process can be "confused" and start off with the "wrong" number of teeth.

Starting with a pitch diameter  Dp = N x P/pie 
and adding the tooth depth H = 0.5 x P x  Sqrt(3) will give the blank diameter Db

As the cutting starts on the Db it is bound to generate N + a fraction of tooth but less than N+1 teeth.  At the end the hobbing will  try to overrun the previously cut teeth but that is OK because on the Db the teeth have plenty of space (as long as we are not trying to cut too many teeth above N.

As the hob penetrates the blank it will reach a point where the hob pitch radius touches the gear pitch radius, that radius that allow exactly N teeth.

If you stop here you got perfect teeth and a concave groove on top of the worm gear.

Now the question is:   But how do we manage to have the same N at the bottom of the groove, at the pitch diameter and at the Blank diameter?

Visualizing the engagement between the Hob and the worm gear as a snug fit at the pitch-to pitch diameters, the gash at the bottom of the groove has zero width and is spaced exactly 360*/N, the spaces at the blank diameter can not be = P but must be larger.  When we were just starting out the Hob was overrunning the teeth at the Db cutting a bit larger space but never enough to fit one more tooth, ad the process continues the hob will continue to widen the opening at the Db above and beyond P but by then the drive contact is nearing the pitch to pitch point where the drive set exactly N tooth.

The secret is to start with a blank that is not oversized to the point of starting N+1 teeth.   If the number of teeth is not important but you only want full shape nice teeth then any old blank will do if you stop at the right spot before starting to chew up the N? teeth in an effort to place N?-1 teeth on a smaller diameter.  This will not work very well because however many N? teeth are there are not going to go away and allow a new set of N?-1 teeth to synchronize on the next allowable diameter.


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## Chriske (Mar 9, 2017)

Mauro,

I think my brain will be pleased to hear someone cracked it, I'll pass on the message.
Thanks...

Chris


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## Chriske (Mar 9, 2017)

On the other hand...
Suppose I make a wormwheel like I did for my project and end up with a 1:156 ratio.
Suppose The wormwheel is just under 6 mm thick, say 5.5mm.
Suppose I stay on it and keep pushing the tap further and further in the wormwheel.
Suppose I keep pushing the tap until the wormwheel has become nothing more than a rod with a few teeth(So it has become a hobbed bolt for my 3D-printer) Meaning the tap has 'consumed' most of the wormwheel, and I'm left with a 'wormwheel' diameter 7 mm and ratio 1:18
My brain is puzzled again...! Sorry Mauro...

My question : what will happen with the number of teeth while decreasing the diameter of the wormwheel as described here. How would it look like *while* milling...??

Chris


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## tornitore45 (Mar 9, 2017)

I am responding with a guess because I never tried and an analysis is really difficult.
Since this is basically a geometry problem, pictures are essential.
I have placed my thoughts into the attached spreadsheet.

In my previous post 


> Since the 1/2 depth of the tooth (from pitch diameter to top or bottom)
> is  P x Sqrt(3)/4 < 0.5 x P/pie  there is no way that the process can  be "confused" and start off with the "wrong" number of teeth.


    Wrong!  the sign should be > 
Still the conclusion is not invalidated since the start involves a very small depth of cut.  The problem arises when N teeth are set and we force the hobbing beyond expecting to find a new happy spot at N-1 teeth.  Once N teeth are established there is no way to transition smoothly to N-1, the N-1 "potential" locations have already been compromised. 

View attachment Hobbing.xlsx


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## TSutrina (Mar 10, 2017)

I have been looking at hobbing gears since I do not have a rotary or index table.  I first calculated if I could cut a hob on my Atlas 10" lathe.  Atlas cast pot metal gears have a 16/in pitch diameter, module 0.0625 in, and (rack) 5.093/in circular pitch and pressure angle 14.5 deg the same as an ACME thread.  I adding to the tool steel lathe tool rounds of 0.25 of the module to the 2 module high rack thus 0.156 in tool.  The hob would be created as others have shown and the gears on the lathe set as close as possible to the rack, 5.0 TPI.  To create a spur rack the axis of gear blank to the hob axis is 168.975 deg pitch diameter (center of tooth) is 0.327 ", rood diameter is 0.171 ", and OD is 0.483.  Obvious deflection and potential breakage of this hob would be a problem.

To make a Hob an odd pitch thread speed in needed which is not with possible with standard lathe threads. Thus a means of making an odd pitch is needed. The Reference below is such an approach. We know that if you gash a gear blank a hob will automatic follow it gashes.  Thus that approach below create in effect a helix gash by adding material. This material can be totally machined away or removed once a cut gash of the forming tool tooth shape is made.  Spur Gear add 'V' face on side point PD so that the hob will create gashes properly spaced.  May need to taper front of Hob to PD of hob at the lead angle.
Ref:http://www.opensourcemachinetools.org/archive-manuals/lindsay_thread_follower.pdf
I chose the hob diameter at 1.65".  Thus pitch diameter (-1.25 Module) is 1.4937" and the root diameter is 1.337".  The hob circular pitch is PI x Pitch Dia / sqroot( (PI x PD / Circ Pitch) squared - 1) thus 0.197 in .  Inverse is the Hob TPI of the attached follower spiral thus 15.986/in.  A little trig the lead angle is 0.764 deg which is the angle that the hob or blank need to be placed at to cut a spur gear.  The Diametral pitch is the hob TPI  and the module is the inverse thus 0.0626".


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## JCSteam (May 8, 2017)

Chriske said:


> It's an old thread I know, but I thought to show how I made a wormwheel to be used in a startracker I designed.
> The assembly runs *very* slow, the wormwheel itself runs at  a whoping1 rpday. So I choose SS for the worm. A SS bolt(M6) that has been cleaned up a bit serves as worm.
> To make the wormwheel is basically nothing more than a M6 tap pushing against a brass's disk perimeter. I made(3D printed) two brackets that will hold two ball bearings(8x22x7). The wormwheel has the same size hole as the bearings. Once the assembly is fixed on the milling  machine, the wormwheel can rotate freely around the 8 mm axis.
> Then the rotating tap is pushed very gently against the perimeter of the wormwheel. If you go for it, again go slow..!
> ...




A LOT of what is discussed in this thread is well over the top of my head, however....
Seeing this has made me have an idea, which may work, I want to make a worm and pinion for a small traction engines steering, the engine will be around 3/4" scale, would making a small diameter wheel, and a brass bolt work using the method shown above? Essentially using something like a M4 tap on the wheel and a M4 bolt shortened and fastened to a steel rod? 

Simple answers for a simple mind please


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## Chriske (May 9, 2017)

First of all I would consider using a thicker tap, say M6 like I did. M4 is to thin imo, it cannot handle that kind of strain. Keep in mind to use brass for the wormwheel. There's no way you can make a steel wormwheel using the technique mentioned above. Maybe if you can find a way to support the tap at the very top, I suppose it can be done using steel.
Feeding the tap into the wormwheel, go slow, very slow..!


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