Composite Curves - Threads

[Cockroach]

I am drawing a 4 1/2 IF Pin and would like to show the vanishing threads. To do this, I would like to joint two helix, the first is simply the tapered path (taper=2in/ft) up to the cylinder diameter (14 thds at 0.25 in pitch). The second helix is simply a spiral which is also tapered, but at a greater rate to simulate tool pull out on a metal lathe. This is 3 threads at 15 deg taper. My problem is getting the two helix into a composite curve.

I set up a pierce plane at the end of the first helix, 14 thds X 0.25 = 3.500 inch offset. The first helix has a pitch diameter of 4.3992 inches so that the run-out after 14 thds X 0.25 in pitch would give me 4.97854 inch. That would be the start of the vanishing threads in my offset plane, the pitch diameter for the second helix. My problem is that I can't join the two helix into a composite curve. I've confirmed the start and end points are inline with eachother and the directions of either helix are identical.

The method works beautiful for a straight thread, I thought the taper thread would be simply a significant digit issue at the mating joint. But I can't get it even though measurement gives me the correct XYZ coordinates for the finish of helix 1 and start of helix 2.

There is got to be a better method. Any suggestions?

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[rollupswx]

Can you attach the file here?

 

[CorBlimeyLimey]

Can you not do that in one helix using the Variable option?

Have you tried converting the helices into a 3D sketch before creating the Composite Curve?

 

[Cockroach]

I've seen the variable option in the helix command, wanted more control rather than tabularize the thread height.

I got it. The pitch diameter for the second helix was out by 0.000002 inch, it should be 2.978538 inches rather than 2.978540 inches. Probably a trucation/rounding process in the background, I thought five decimal significance was good enough.

But the methodology is correct, you can splice two helix together provided the endpoints are sufficiently close to one another. You'd think I could pick one helix and then the other and have a sub routine that would make it happen, i.e. end of one = start of two for these helix.

Maybe a program suggestion.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[CorBlimeyLimey]

Take a look at the attached Variable Pitch example.