Double custom thread.

[bob.s]

Does a double thread, starting 180 degrees apart, make it easier for the user to mesh the threads and allow a greater travel distance with fewer revolutions?

[etfrench]

Perhaps and yes.

[chrisplyler]

A double-start thread gives the user more opportunity to start the thread, but the EASE of starting it depends more on the thread profile, size, fit, and the end-treatment of the hole and screw/bolt.

Of course the travel distance per revolution doubles relative to a single-start thread that otherwise has the same specifications.

[bob.s]

Hi there, thank you for your reply. I am working on a small project and just about ready to have my second prototype 3D printed, but still do not feel happy with the thread. I need it to fit as smoothly as possible as the container is holding hot liquids. Do you know of anyone who could change the thread to a custom thread designed for this purpose, if I sent them the files from Fusion 360. I have not acquired the skill yet to know how to complete the threads or know what would be the best type of thread to use. I would want the thread to travel 8.5 mm in 1.5 revolutions on a 90 mm diameter inside diameter lid, which would compress a 4 mm hollow paper o-ring by 1.5 mm to form a seal.

Thanks again for your reply,

Bob.

[etfrench]

Divide 8.5 by 1.5 to get the pitch for one revolution. 5.6666 is close to the standard M90x6 thread.  M90x6 thread will move the 8.5mm in 340 degrees.

[chrisplyler]

The ISO Metric thread M90x6 might suit your needs. It moves 6mm per revolution, so would do 9mm in 1.5 turns. Give a good chamfer to the top edge of the container and the bottom edge of the cap and it should be user friendly.

[bob.s]

Hi etfrench.

Thanks for your reply.

When using plastic, would I not be better off with a double thread to make it easier to get the threads to mesh, and if so, how would I do that? I am saying this because the container will hold hot liquid and therefor it is important to get the lid on as quickly as possible. I am not proficient in using Fusion 360, do you know of anyone who could do this for me if I sent them the Fusion Files? I hope to have this idea made by injection moulding if the prototype works properly. This is my third attempt with a 3D printed prototype.

Bob.

[bob.s]

Hi Chris, thanks again for your reply .

My last prototype only had a single thread and being so small it was difficult to mesh the threads quickly. I thought that at least with a double thread there was less chance of getting them on the wrong thread when there were two starting positions. Now when I look at plastic pill bottles ( I am 76 years old, so I have plenty to look at ) most have a single well spaced thread and the lid seems to seat itself easily and tightens in about one revolution or less over a similar distance of about 8 to 9 mm. I think they are custom designed for use with plastic and not so much for strength as with a nut and bolt.
Do you know anyone who could and would correct my files? I understand you are not able to do so as staff of Autodesk Fusion.

Bob.

[etfrench]

I'd start with a single metric trapezoidal thread, either 12 or 18 pitch:

Screencast will be displayed here after you click Post.

1e0a4f08-6980-49df-b7f1-627ea9e44e8f

Notes:

You'll need to add a chamfer along the top edge and also a relief on the bottom edge of the threads.

You can create custom threads by following the instructions here.  For a two start thread, use the TR90x12 or TR90x18 thread as the template and change the MajorDiameter, PitchDiameter, and MinorDiameter to match the TR90x6 threads.  The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing.

p.s. No Autodesk employees have posted to this thread.

[TheCADWhisperer]

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@etfrench wrote: 

I'd start with a single metric trapezoidal thread, either 12 or 18 pitch:

 The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing..

---

I would probably not use a trapezoid form.

There is no reason that a two start thread would need to have a smaller cross-section for this application.

[chrisplyler]

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@etfrench wrote:
The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing.

---

You could certainly choose to do things that way, but I think the typical implementation just increases the helical angle such that the the threads on the first helix is large enough to fit the threads of the second helix in between them.

[TheCADWhisperer]

I am looking at a 3-Pitch glass drink bottle (Snapple) at this very minute.

Not what I observe.

[etfrench]

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@TheCADWhisperer wrote:

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@etfrench wrote: 

I'd start with a single metric trapezoidal thread, either 12 or 18 pitch:

 The two start thread will give the teeth a smaller cross section, so that may not be the best for 3d printing..

---

I would probably not use a trapezoid form.

There is no reason that a two start thread would need to have a smaller cross-section for this application.

---

Actually, it does need a smaller cross section, otherwise there would be no room for the second start.  Is there a reason for not using the trapezoid form?

p.s. This is assuming the standard thread table (xml file) is used.

[TheCADWhisperer]

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@etfrench wrote:

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...  Is there a reason for not using the trapezoid form? 

p.s. This is assuming the standard thread table (xml file) is used.

---

I do not recall ever seeing a plastic bottle that used a trapezoid form.

P.S.  I would not use Fusion 360 for this helical form features - I would use something like Autodesk Inventor Professional.

[bob.s]

I don't understand what you mean, but thanks anyway.

[bob.s]

Thanks for your reply.

[bob.s]

Thanks, I will give it a try.

[bob.s]

Thanks for the info. I will try what you suggested.

[chrisplyler]

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@TheCADWhisperer wrote:

I am looking at a 3-Pitch glass drink bottle (Snapple) at this very minute.

Not what I observe.

---

Yeah, I'm pretty sure that the Snapple glass bottle (or similar) 3-start thread (the cap itself is not threaded, but makes use of three crimps to serve the same purpose) is not an ANSI , ISO or JIS (etc.) standard. Glass bottle manufacturers have their own standards. They make up whatever they want, and typically they want something that can be formed in the bottle making process, which precludes expensive glass machining processes that would be required to make profiles fitting some of the more common standards we are used to.

The typical thread specifications we are used to are built around the thread form itself, and if you want to turn any given size into a 2-start configuration, you will have to up the helix angle. Now of course your pitch will be doubled, and you might not want that, in which case you would have to select a smaller size in order to compensate. Is that what you mean?

[TheCADWhisperer]

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@chrisplyler wrote:

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...profiles fitting some of the more common standards we are used to. 

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My reading of the OP's problem statement leads me to believe that a standard screw thread for machined components is not the solution for their Design Intent.

Unless I miss-understand the Design Intent I would model a plastic bottle thread (in Autodesk Inventor Professional) rather than a 60° v-profile.

An automotive radiator cap or overflow tank cap or thermos cap also come to mind.