Draw a thread / spiral

[Elias_isaksen]

Hi, i'm trying to draw this thread / spiral in fusion but i can't seem to get the spacing and angle correct. Anyone got a good idea on how to do it? 

I've tried using both the Coil and Thread function. 

[MRWakefield]

That looks like a very badly worn worm so it might not be a trivial matter to replicate it. However, if you could provide your current model along with some dimensions we might be able to help. If you could provide the following:

• Major diameter
• Minor diameter
• Pitch
• If possible, the flank angle

[Elias_isaksen]

Major  68mm.

Minor 44mm

Pitch 15mm

Flank angle 60

I appreciate it!

[MRWakefield]

I'm unable to model it with those dimensions. I'm guessing it's the flank angle that's the problem as that's the hardest to measure. Here's an attempt:

As you can see: the minor diameter is 2mm oversize. I've added a small root radius (which does increase the minor dia. but is definitely present in the physical item). I think the angle needs to be less than 60° (that's if it is indeed just a simple angled face and not of involute or other generated form) to achieve the dimensions stated. Can you provide any more information about the item? What machine is it a component of? What does it mate with? The more information we have, the more likely it will be that we can help.

[Elias_isaksen]

It's for a gear box for a crane, it mates with a ring gear. 

I'm unsure on how to actually measure the flank angle, i took a guess that it was 60.

After measuring again i would say the major is actually 70mm. 

It doesn't have to perfect just pretty close to what it was before, the ring gear is also quite worn.

[MRWakefield]

Ok, I thought it might be something like that. Gearing is a very specialised field so as I mentioned previously: this might not be a trivial task. You'll find a really good document on gearing here which you might find useful. You'll find worm gearing in section 9 (starts on page T37). You'll need to know which of the 4 types of worm it is, the pressure angle, module/DP, and the distance between the worm and ring gear axes among other things.

I've created a thread file and attached it along with an edited version of your original model. This may well not be the correct form but it does satisfy the major, minor and pitch values of the original. It will need some tweaking where the ends of the thread run into the two bosses at each end but I'll leave that up to you 😉.

I hope you find this useful. Keep us updated with the project and if you need further help don't hesitate to shout.

EDIT: Sorry, I forgot to say that this is a 50° thread, which is equivalent to a 25° pressure angle. This is one of the common pressure angles used I believe.

[TheCADWhisperer]

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@Elias_isaksen wrote:

It's for a gear box for a crane; it mates with a ring gear. 

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@Elias_isaksen 

Do you have some specifications for the Ring Gear?

Number of teeth on the Ring?

Center to center distance of the centerlines of the shafts?

Approximate diameter of the Ring Gear?

[Elias_isaksen]

Thank you very much for all the help! I'll keep you updated! 

[Elias_isaksen]

I will see what information i can find and get back to you! 

[Elias_isaksen]

Hi again! 

Took a little break from this project, but now i'm trying to figure out how to do this in CAM.

I was planing on doing a threading cycle with a neutral holder and a VBMT insert. But fusion dosn't support it.

Any ideas? 

[MRWakefield]

Sorry, I can't help with the CAM side of things. When I was CNCing it was over 30 years ago and all our G-coding was done by hand! You could always insert a G76 (or whatever your threading cycle G-code is) manually but you'd need one hell of a big machine and tool to screwcut it! I'm no expert in gearcutting techniques but I would expect that this part would originally have been thread-milled, hobbed, ground or cut on a whirling machine.

[Elias_isaksen]

Alright thank you!