Trying to hob a worm gear on a lathe

[dansawyer]

The project is to make a 90 tooth 10 tpi Acme aluminum gear. The tools are a South Bend 9a and a 1/2 inch Acme tap. The blank is a 2.9 inch diameter aluminum disc 1/4 inch thick. The first objective is to make a functional adapter to hold the blank. Here is how far I have gotten:

1. A 2 inch long T nut in the compound replacing the tool post.
2. A 2 inch long bolt with about about an inch of spacers to bring the gear blank up to the level of the tap.
3. The aluminum blank itself.

4.. A large washer to allow the bolt to be tightened while allowing the blank to rotate.
When tightened the assembly is solid and the blank can rotate. However it has too much up and down free play. It move about 1/8 of an inch down at the point of contact with the tap. This troughs the cutting position of the tap off.
Question: Does this method generally work or should the blank be held in some kind of a bearing jig? Is this an issue of needing to adjust the spacers to cut out free play or is the method too simple to work? The spacer is relatively thin, about 1 inch in diameter. Should I make a thicker spacer, say 2 1/2 inches in diameter to cut out the free play?

 

[markba633csi]

I think what I would do would be to add some brass washers on both sides of the blank to act as bearings. Fairly large in diameter, say 3/4". You may have to make them. Grease everything well, and fasten the whole assembly down with as little play as possible. You need to devise an adjustment mechanism that can remove the free play, maybe with a partially threaded shaft and locknut- you'd need to make this too.
The optimum of course would be to have a custom jig with ball bearings but I think the washer technique would work well enough.
Happy hobbing!
Mark
ps you may even consider a steel or brass sleeve in the blank to reduce wear also

 

[dansawyer]

Thank you. I thought about simple bearing thrust washers. I have seen them a couple of inches in diameter. Is there an issue with that concept?

 

[markba633csi]

Remember that the blank will wear not only on the sides but the center hole too. Especially the center hole. I think the sleeve I mentioned is going to be a must-have item, more important even than the side washers
Mark
ps you could instead make a pair of shoulder bushings like this out of steel or brass and use them to mount the blank:

 

[dansawyer]

The bolt only has threads on the bottom half. The top where the blank is is not threaded. I think a little grease there should protect it. I am working on making a better spacer assembly as well. I am thinking fender washers. I believe the current spacer is too narrow. It should be as close to the blank diameter as possible without interfering with the tap.

 

[dansawyer]

Below is an update:
On the second try the lower spacer was replaced with a section of 2 1/2 inch diameter PVC machined from an end piece that was mostly solid with a 1/2 inch center. This provided much more stability for the gear blank. The height was off by about 30 thou so the center of the gear was too low. This should be easy to correct.
The remaining problem is calculating the diameter of the gear blank. I am assuming from the article from the previous question the resulting effective diameter will be the inner diameter or d2. I am struggling with the question of does the total tooth count change ans the gear is hobbed? And if so how can this be accounted for?
The hobbing of the second attempt took a significant amount of material on the first pass. I am considering starting the hob near the entry end of the tap and moving the apron up the tap to increase depth. This is an alternative to starting the hob on the final portion of the tap and increasing the depth with the cross feed. Does anyone have advice on this? Thanks Dan

 

[rgray]

I am struggling with the question of does the total tooth count change ans the gear is hobbed? And if so how can this be accounted for?

I have never done a worm gear (just spur gears)
It interests me so I have followed along as others have done them. As I remember a big issue is that the blank needs gashed for the tooth count.
If not it will start with wrong number of teeth and make a mess of things or make a gear with the wrong tooth count.

 

[dansawyer]

This is a status update. The third try was a success, the outcome was a very usable B+ gear. The gear is 2.865 inches in diameter and is threaded from a 10 tpi acme tap.
First updates to the jig: The jig critical. It must be stable and accurate. Int must provide rigid support for the blank and a semi bearing surface for the blank to rotate. It will attach to either the compound feed or the cross feed. In this case it consists of a 3/8 double ended threaded rod.
The elevation of the gear blank is critical. If it is not aligned the gear teeth will be off center.
Finally measurement and technique. The tap was placed in the chuck and the end supported by a live center in the tail stock. The blank was turned to a calculated diameter. The calculated diameter is the desired number of teeth divided by pi divided by the tap tpi. The cut was initiated at the start end of the tap. The first pass cuts were shallow and used to ensure the cuts would align at the overlap between each pass.
As an additional note, the above is critical. On the second attempt I tried to improve the outcome by using the cross to feed the gear. The tap can actually act as a mill cutter. The tap can actually mill away material at a continuous rate. The result of this process is the gear teeth are too thin.
One final note, this process actually works quite well. It is sensitive to getting the correct measurements and to creating a proper jig.