worm gear question

[Mr Fixit]

Hi Group,

I have an old 1920's Westinghouse desk fan that has a worm gear for the oscillation of the fan. When the Previous owner had it they lost the worm part of the setup so it won't oscillate. I have checked the spur gear and it matches a 5/16" x 18 tpi bolt perfectly except the OD needs to be a smaller OD to fit the fan.

So the question is, can I single point the 5/16" tpi on a smaller OD stock and get the same tpi as the bolt that matched the spur gear. I realize the tops of the threads will need to be cut down to fully engage into the spur gear, this will not be used daily or under a heavy load so I'm hoping this will be good enough to get it back to original operation.

TX
Mr fixit for the family
Chris

[J Tiers]

The ones I have do not have a regular thread type form, they are somewhere closer to acme or square. As to whether it can be made to work with your fan, I do not know.

[Mr Fixit]

I might have miss written the question. What I want to know is can a 5/16" thread be cut on a smaller OD piece of stock and still keep the same TPI of 5/16" x 18 when I single point on the lathe.

TX
Mr fixit for the family
Chris

[plunger]

I would think if you can make it fit in the fan and you have sufficient clearance then it should work. ..

[J Tiers]

I might have miss written the question. What I want to know is can a 5/16" thread be cut on a smaller OD piece of stock and still keep the same TPI of 5/16" x 18 when I single point on the lathe.

TX
Mr fixit for the family
Chris

Sure, if I understand the question. As the diameter gets smaller, the same tpi will be a coarser-appearing thread, but it can be done. There is no magic about 5/16" and 18. You can cut a 1/4-18 thread, it will be a little coarser than the standard 1/4-20.

Obviously you cannot cut a 5/16-18 thread on a 1/4" rod, because it won't be 5/16". But you could cut the 1/4" rod with a 5/16-18 thread in the sense of it being 18 tpi.

You can even cut the thread as if the rod were the full 5/16", and you will get an incomplete form of thread that is 18 tpi. How incompletely formed depends on how much smaller the rod is.

[Mr Fixit]

HI J tiers,

Thanks for the responses. I will have to measure the actual diameter of the rod needed and see what I do get. Sounds like for want I need to do I might get something that will work. I'll go to the shop in the morning and see what I can make.

TX
Mr fixit for the family
Chris

[elf]

It doesn't matter what the diameter of the stock is. If your gear ratios are set to 18 tpi, you will get 18 tpi on any size stock.

[Paul Alciatore]

Short answer is yes. You can single point a 18 TPI thread on any diameter as long as it is large enough to leave a minor diameter after making the thread.

BUT, if you start with the correct OD for your worm then cut the thread and then cut the peaks of the thread down, your final thread will be of a smaller OD.

What you need to do is start with the correct OD and then just cut a thread to a partial depth, preserving the correct OD. No further trimming after making the thread.

Hi Group,

I have an old 1920's Westinghouse desk fan that has a worm gear for the oscillation of the fan. When the Previous owner had it they lost the worm part of the setup so it won't oscillate. I have checked the spur gear and it matches a 5/16" x 18 tpi bolt perfectly except the OD needs to be a smaller OD to fit the fan.

So the question is, can I single point the 5/16" tpi on a smaller OD stock and get the same tpi as the bolt that matched the spur gear. I realize the tops of the threads will need to be cut down to fully engage into the spur gear, this will not be used daily or under a heavy load so I'm hoping this will be good enough to get it back to original operation.

TX
Mr fixit for the family
Chris

[old mart]

The helix angle gets steeper as the diameter is reduced for the same thread pitch. That is a vital factor with worm gears.

[larry_g]

I would suggest that if you are not sure if things are perfect that you make a worm out of a softer material like Delrin so it may have a chance at working without messing up the gear.

Look at the search below and see if it is of any help,
https://www.google.com/search?q=worm...hrome&ie=UTF-8

lg