Make a 72 tooth worm gear without indexing?

[OrangeAlpine]

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

 

[OrangeAlpine]

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

 

[Herbiev]

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

 

[f350ca]

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.

 

[Swifty]

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.

 

[OrangeAlpine]

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

 

[OrangeAlpine]

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

 

[Chriske]

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 astronomy is a real and serious science. Astrology is ab-so-lu-te-ly NOT...!

Chris

 

[Swifty]

Paul,
You made an 'astronomical' mistake, telling you saw some astrological telescope.
To be clear astronomy is a real and serious science. Astrology 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.

 

[Chriske]

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:

...

 

[dman]

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.

 

[aarggh]

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

 

[chipenter]

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 .

 

[Lakc]

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.

 

[OrangeAlpine]

Paul,
You made an 'astronomical' mistake, telling you saw some astrological telescope.
To be clear astronomy is a real and serious science. Astrology 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

 

[OrangeAlpine]

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

 

[OrangeAlpine]

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

 

[chipenter]

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 .

 

[OrangeAlpine]

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

 

[Herbiev]

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.