Thread: Is this multi start thread possible ?

I need to make a worm gear for a film projector If it's at all possible.
Buying the part is out of the question because it's way too expensive for me.
It's nylon or similar and appears to be prob an 8 start thread but I won't know that til I pull the covers off the projector tomorrow and have a look.
At this stage I'm guessing it will be around 30-35mm diameter and I saw a photo where the pitch was measured at 2.5mm. So if I've got that correct, means the pitch is actually 20mm for 8 start ?
Here's photos from my lathe plate and the new part.
Is there some trick to making it on my lathe ?
Gear.jpgP1030768.jpg


PS Just found the other photos.
gear 2.jpg

Its a bit early for me to use my clapped out brain, i am thinking 8÷2.5=3.2.
Then if you could mount it on a shaft between centres that has a way to index it 8 times to cut each thread.
Does your lathe have module thread cutting ?

Originally Posted by shedhappens

Its a bit early for me to use my clapped out brain, i am thinking 8÷2.5=3.2.
Then if you could mount it on a shaft between centres that has a way to index it 8 times to cut each thread.
Does your lathe have module thread cutting ?

That's a really great question.
What's module thread cutting ?

Not gears, but threads articles and video on multi-start can be searched online:
"cutting multi start threads"

I can't see the pictures you have posted but if you are cutting a worm/ worm drive set, then pi (3.1415...) intrudes and module gears have that incorporated. You can do it with change gears though but it might be unusual ratios that you have not got.
Can you estimate the angle of the tooth? It may actually be a helical gear, and easier to produce using that process, although I would hope for more than 8 starts as the tooth profile would start getting a bit weird.

Michael

Can everyone else see my photos ?

Originally Posted by Michael G

I can't see the pictures you have posted but if you are cutting a worm/ worm drive set, then pi (3.1415...) intrudes and module gears have that incorporated. You can do it with change gears though but it might be unusual ratios that you have not got.
Can you estimate the angle of the tooth? It may actually be a helical gear, and easier to produce using that process, although I would hope for more than 8 starts as the tooth profile would start getting a bit weird.

Michael

https://ian-partridge.com/bh_worm.html

I can see your photos.

Given that it's nylon, I'd suggest this might be a good 3D printing project.
The original appears to have a brass hub, is the black part also metal?
You could model the worm part it in Fusion 360 or something similar, get it printed and use the original hub.
Cheers,
Greg.

Originally Posted by kwijibo99

Given that it's nylon, I'd suggest this might be a good 3D printing project.
The original appears to have a brass hub, is the black part also metal?
You could model the worm part it in Fusion 360 or something similar, get it printed and use the original hub.
Cheers,
Greg.

From videos I've seen I think it's an aluminium hub but it doesn't need a hub at all. It can be one piece.
I'd have no trouble designing it with autocad and printing it myself but there may be a problem.
Firstly I'm not too sure if 3Dprinted would be strong enough. The plastic is certainly strong enough but it's possible to get splitting layers.
Another thing is that the part is so deep in the mechanism of the machine that it's a major task to disassemble then reassemble the job just to test it.
Then there's the surface finish of the printed part. It will not be without little bumps along the thread.
I will definitely consider 3D printing though if there's no other way around.
If I knew someone with an SLA printer might be a different story.

I can see them now.
That part would best be done on a lathe but if you are measuring a 2.5mm pitch then you don't have to worry about pi. Your biggest problem will be getting the feed on the lathe right as the tool needs to travel 8*2.5mm per rev, so 20mm. That aspect is probably more suited to helical milling.

Two ways to index the starts - one is to cut a thread and then rotate around 1/8 of a turn. The other is to plunge cut the thread (as it is plastic there should not be much of an issue doing that) and for each start, move the compound along 2.5mm

I agree with your concerns over 3D printing although hand finishing with some wet&dry would probably deal with the surface finish issue.

Michael

Looks like the coarsest metric pitch you can do out of the box that fits into 20mm is 4mm. You are going to need to cobble together an additional 5:1 increase in feed speed. On a little lathe that probably isn't going to be fun.

Sounds like there's a fair bit of rooting around...you sure $80 to just buy one isn't a better deal?

Originally Posted by elanjacobs

Sounds like there's a fair bit of rooting around...you sure $80 to just buy one isn't a better deal?

Frankly, at this stage I don't know.

Out of curiosity what does this part do in the projector?