Helix and Sweep Problems

klaus_krämer

I have a helix on a cylindrical surface. Then there is an equilateral triangle attached to the helix. The triangle's lateral length is the same as the helix's helical pitch. That should be enough to cut a thread...

BUT:

Using the same lateral lenght for the triangle and at the same time for the helical pitch does not work always. Often and annoying enough Sweep won't work, when lateral length is the same size as helical pitch. Increasing helical pitch or decreasing lateral length of the triangle helps - but then the model isn't exact. In my oppinion there has to be a bug.

Even if the equilateral triangle's lateral length would be longer than the helix's helical pitch, there should be a visible result - even if it looks a bit weird...

mahir

@klaus_krämer
Like @konstantin_shiriazdanov said, you should have a two-part triangle. Even if the thread didn't have a sharp outer crest, it's generally a good idea to overbuild your sketches so that you're not adding or subtracting geometry exactly at the base geometry's borders/edges. In this case, treat your thread profile like an actual cutting tool. On a lathe the tool cutting the thread would be much larger than the thread itself. In the example below, the highlighted segments represent the effective profile, while the area to the right is there just to make sure your resulting geometry isn't line to line.

konstantin_shiriazdanov

If the triangle length is equal to helix pitch then you get the body of zero thickness, which should be subtracted from the cylinder. Zero thickness bodies are not supported, so try to devide the triangle into two parts - left and right and sweep them separately

ChuckKey

I have come across this too, when trying to model threads. A lot of thread forms have a profile that has a width equal to the helical pitch by definition because there are crest and root radii. IMHO, not being able to sweep these cases constitutes a (minor) bug and the work-around constitutes a work-around.

mahir

@klaus_krämer
Like @konstantin_shiriazdanov said, you should have a two-part triangle. Even if the thread didn't have a sharp outer crest, it's generally a good idea to overbuild your sketches so that you're not adding or subtracting geometry exactly at the base geometry's borders/edges. In this case, treat your thread profile like an actual cutting tool. On a lathe the tool cutting the thread would be much larger than the thread itself. In the example below, the highlighted segments represent the effective profile, while the area to the right is there just to make sure your resulting geometry isn't line to line.