Internal 13/16" 20 TPI not available in Thread tool?!?

[mirageman01]

I am trying to model a 13/16 x 20 TPI internal thread on a .762" bore ID and it automatically changes it to .750"! I have modeled 13/16" 20 TPI external threads on another design. So what's the issue here?

[MRWakefield]

Are you sure you're picking a 13/16 x 20 and not 13/16 x 16?

If possible can you attach your Fusion file for us to review?

[mirageman01]

There is no option for anything other than 3/4” thread pitches. That’s the problem. 

[mirageman01]

• Here is the file. The Capture is the surface I'm trying to create the thread on. I've also included the specs for a 13/16" tpi internal thread in a capture. I'm also trying to create another 16th inch size thread on the other end of the part and the same thing is happening. It won't allow a 15/16" 28TPI.

[MRWakefield]

That's odd!

[mirageman01]

Those are the specs for external threads. Is fusion somehow trying to make an external thread on an internal diameter???

[HughesTooling]

Fusion expects you to make the hole the major diameter not the minor. Because you've used the minor Fusion is using the nearest size, 3/4. You can just change the diameter on the thread dialog, but a lot easier to just make the part with the hole the diameter of the thread.

[HughesTooling]

@mirageman01 Please don't attach pictures, just paste them into the message.

[HughesTooling]

@mirageman01  Just a bit of general advice. For this part Revolve would be a lot less work and a lot less features.

Just a single sketch, a revlove then add the threads. Note the use of a centreline in the sketch so you can dimension diameters. See attached file, please check the sizes.

[MRWakefield]

Sorry, it was getting very late here when I posted my last reply and had to retire for the night. We also must have posted at the same time! Anyway, @HughesTooling has given the same advice that I would have so I'll just ditto what he said.

[mirageman01]

I understand what you’re saying but why in the world would they do that? In the real

world you bore the hole the minor and then cut the threads to the major. If you measure an internal thread you measure the minor because how in the world could you measure the major? You cut threads you don’t add material. So it’s like telling fusion to add material back for the threads by having to specify the major. It doesn’t do this on an external thread model. Ok rant mode off. lol

[HughesTooling]

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@mirageman01 wrote:

I understand what you’re saying but why in the world would they do that? In the real

world you bore the hole the minor and then cut the threads to the major. If you measure an internal thread you measure the minor because how in the world could you measure the major? You cut threads you don’t add material. So it’s like telling fusion to add material back for the threads by having to specify the major. It doesn’t do this on an external thread model. Ok rant mode off. lol

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Here's an obvious reason, do you know all the minor diameter for all threads without looking them up? Everyone knows the OD (well perhaps not for pipe threads) so it's far easier to make the hole the size of the OD of the thread you want then let the thread tool do all the work for you.

[mirageman01]

I am a total noob to Fusion and just getting started learning it from scratch. That said I'm all for simplifying the process. Most of the parts I want to model an relatively simple compared to what Fusion can do. So thanks for the help and suggestions. Now if I can just figure out how you did what you did using the sketch and revolve tool. LOL

And just to clarify when modeling an internal thread I have to make the hole the largest major diameter of the external thread that mates with it???? I'm assuming that if you want a specific minor to override Fusion you have to use the create custom thread thing?

I think I should mention that I am not machining anything on CNC equipment. I do all my machining on a manual lathe so what might make sense for someone that does use CNC doesn't translate totally to me. I can't just tell the machine to make a 13/16 20 TPI thread. 

Another thing that came up while looking at the file you made... Whenever I machine a thread I create a lead in chamfer. I've gone in and edited the project you posted by adding chamfers at the start of the threads. Is it better to add the chamfer to the shoulder of where the thread will start before modeling the thread or after? Maybe you draw the chamfers when making the 2D sketch before revolving??? Here's my issue if the program expects the bore dimension to be that of the major of the mating external thread how does one specify where the lead in chamfer is? IOW on that specific part, on the 13/16 20 thread I want the lead in chamfer, which also is the ID for an O-ring, to stop .085" from the edge of the part.

I'm sure all this is very basic stuff to those that are familiar with the program so I apologize for all the questions. I guess I think in terms of how I'm going to actually machine the part. For instance to machine the 13/16 thread end of the part I would drill to rough diameter, cut lead in chamfer and O-ring ID with a 45 degree internal chamfer tool, bore ID to minor thread diameter, then machine the threads.