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Composite Curves - Threads

Composite Curves - Threads

Composite Curves - Threads

(OP)
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

RE: Composite Curves - Threads

Can you attach the file here?

RE: Composite Curves - Threads

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?

RE: Composite Curves - Threads

(OP)
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

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