Modeling API tapered thread for injection mold

[tracy]

I have several parts that I need to model with actual (non-cosmetic) API round thread form tapered thread.

I'm using helical sweep, and the thread profile I'm using is similar to the profile of a tool to cut the threads on a lathe. That is, it incorporates a peak-to-peak profile.

The problem I'm running into is that while the helical sweep works, it leaves a ledge of a bit over a thousandth on the peaks of the threads, as seen here:
View attachment 6541

View attachment 6542

I've tried to figure out what I can do to mitigate this by trying out a completely sharp thread, but no dice.

I've tried aligning the thread form to the taper, and also to the axis of the part, but neither one seems to be much different as far as the final thread quality is concerned. I think one or the other of those gave me issues with a failed feature, but I can't remember which one.

I can't attach a file, since the forum limits me to 97.7 KB for a zip file, and the minimal example for this is 278.7 KB, zipped with max compression (it's due to the helical sweep).

I'd be willing to e-mail (or PM, if that's possible) the file to anyone who might be able to shed some light on how to fix this. I've been working on it off and on for a few weeks, and it's getting down to crunch time.

This part is destined, in the long run, for injection molding, but in the near term, it will be a lathe part.

Thanks!

Tracy

 

[dr_gallup]

Ive modeled screw threads using helical sweeps without any problems. I would have to see what you are doing to understand what is causing the step.

You can suppress all the features in your part before saving. This will make a very small file that you can upload without problem.

 

[zpaolo]

I've modeled threads and knurls in many part with no issues, for aesthetic reasons, so I agree with dr_gallup, you should upload the part so we can play with that. I usually use helical sweep or VSS with formulas to control the pitch which is probably more flexible.

Paolo

 

[tracy]

Sorry I didn't reply sooner. I've been busy with other work.

Thanks for the info! I have suppressed the helical sweep, so here's the part.

I hope you all can help me.

 

[silverado8405]

Quick glance at the file, it appears that the end points of the revolving sketch aren't in the same plane, hence why you are seeing the undercut on the tail side of the cut. It almost looks like one end is curved up a little more than the other. I tried playing with it by drawing a new sketch without the tails of the curve coming back around I got the undercut to go away. But it wasn't remotely close to the profile you are looking for.

 

[dr_gallup]

Quick glance at the file, it appears that the end points of the revolving sketch aren't in the same plane, hence why you are seeing the undercut on the tail side of the cut. It almost looks like one end is curved up a little more than the other. I tried playing with it by drawing a new sketch without the tails of the curve coming back around I got the undercut to go away. But it wasn't remotely close to the profile you are looking for.

The step is definitely between successive revolutions of the helical sweep. That is a majorly complicated section, I have no idea what all you have going on there but it violates the first rule of Pro/E which is keep sections simple. I usually do not put the radii in the sweep section, just the truncated pyramid of a nominal screw thread. Then if you really need the rounds they can be added later.

 

[tracy]

The step is definitely between successive revolutions of the helical sweep. That is a majorly complicated section, I have no idea what all you have going on there but it violates the first rule of Pro/E which is keep sections simple. I usually do not put the radii in the sweep section, just the truncated pyramid of a nominal screw thread. Then if you really need the rounds they can be added later.

Good point about simple sections. I will try that.

 

[tracy]

That worked. Part of what had me stumped was a misunderstanding of the shape of the thread form tool. It is stepped, so that the peak of the leading thread, as it's formed, is higher than the peak of the second thread as it's formed, so that it's a progressive process. Putting that difference in peak heights into the sweep section fixed the biggest problem, and then I put the rounds in after the basic shape.

Thanks!