Modeling internal threads for a salt shaker cap

[Aracaifon]

So, my friend's wife has these custom collectible salt shakers that she holds very dear to her made of glass. the lid on one of them cracked and they asked me if I could design a replacement cap for it, to which I agreed. My issue is that the threading on the salt shaker isn't linear vertically, it narrows towards the top of the inside of the cap, and the threading isn't standard, so I need to design custom threads. I kind of screwed myself over, because I designed the cap first thinking I could use the thread tool on the inside of the cap. Then I realized I need custom threads, so I need to fit them into the already designed cap using a join operation. Fusion 360 doesn't let me inspect the distance between two peaks of the interior of the coil to know that the distance matches my real-world object. To make matters worse, the salt shaker is clear glass so it's not easy to take a picture and use as a reference photo like I've seen in some videos online, as the details are difficult to make out without up-close inspection. I am at a total loss, honestly. What do I measure on my real-world object that I can modify with the coil tool to ensure they will fit together? How do I make a coil diameter smaller towards the top than the bottom? 

[matthewZYM62]

It is unlikely these threads are completely custom. Does this help?

https://www.noupoi.net/bottle-cap-thread-calculator.html

[etfrench]

Create a sketch on an origin plane, then Project/Intersect the coil to the sketch.  Add driven dimensions to make it easier to modify the coil. 

One way to get an accurate dimensions of internal threads is to use clay or polymorph plastic to make a mold.

[HughesTooling]

See attached file for example custom thread. Edit the top 3 parameters to set the sizes and edit the third sketch in the timeline if you need a different profile. This method is more robust than using a coil. You might need to uncheck auto update, change all 3 parameters then apply all at once. 

[Aracaifon]

I think this does help but then how would I make the threads narrow towards the top and wider towards the bottom? 

[Aracaifon]

Thank you so much for this, I will give it a shot and see how it comes out

[HughesTooling · Accepted solution]

@Aracaifon I missed the bit where you said you needed a tapered thread. Attached is another file with a taper. Body1 is just used to create a tapered spiral and you can hide when not needed. Edit Sketch4 to match your profile and look in parameters to setup the thread pitch, diameter, etc.

[Aracaifon]

Quick question. I'm trying to adjust the parameters and I can't figure out how to actually turn the flat coil shape into threads that have shape. The threading on the interior of the cap has a triangular cross-section and I'm trying to add this in by filleting the coil shape, but it's not allowing me. What is the best way to attempt this?

EDIT: Nevermind, I see you added the thing about sketch 4 and I just forgot you said that. I finally got it to work thank you so much man!