Problems with helical curves in SW 2003

[Spurs]

I was accustomed to modeling helical patterns (like a thread) in SW2000, but now that I have upgraded to SW2003 I have been having problems.

The problem is that I cannot get the root diameter of the thread to match perfectly to the root cylinder created by extruding a circle of the same radius. The mismatch shows us as very small steps and lines like they are mismated surfaces.

The procedure I use in SW2993 is as follow:

Draw circle at the thread pitch diameter on the front plane

Insert >Curve >Helix/Spiral & set the parameters

Insert >Reference Geometry >Plane - Select the Helix and the starting point of the helix to locate the plane - this should be the "normal" to the helix plane to my way of thinking.


Select the new plane -sketch on that plane the profile of the helical thread - to problerly align the thread along the axis, construct a line from the original origin to the new sketch origin. This is perpendicular to the thread axis. To simplify this example, instead of drawing a trapazoid thread, just draw a rectangular box with a known Major diameter.

Sweep the new sketch along the helical path

Now to check that the major diameter of the Swept Helical surface is in the correct position, select the original front plane and draw a circle for that same major diameter value, then extrude it. The Helical diameter wont match the extruded diameter?

Did you have the same problem? What am I doing wrong?

 

[Theophilus]

In your thread profile, are you adding a pierce relation to your helical path? If I make a trapezoidal thread profile, I'll do several things to get what I want.

1. Extrude the main shaft on which I want my threads.

2. Create the helical curve as you noted in your post.

3. Select the end point of the helix and then the helix to create the plane for my profile.

4. Sketch my trapezoidal profile. I don't sketch this in place. I sketch it in such a way that the geometry is stable if I drag it all around--angles and lines dimensioned. I also add construction lines to represent the point of contact with the walls of my shaft (I intersect the shaft walls somewhat with my profile to avoid the problems you mentioned) and with the helical guide curve (trim each construction line to one another to create this point).

5. Pierce my profile to my helical guide curve and my profile will snap to the profile where I want it.

6. Exit the sketch and create the sweep.

This should work fine.

Now, if you need an exact inner diameter of floating threads (no shaft, or like a spring) the same methods above work, but you don't have a shaft's walls to intersect. The ID of your coil should match what you specify with your profile sketch. For instance, if you sweep a circle by piercing it's centerpoint to your helix, remember you're reducing your coil ID by your profile radius times two (for thickness).

You should have exactly what you want.




Jeff Mowry
Industrial Designhaus, LLC

http://www.industrialdesignhaus.com

 

[Spurs]

Jeff

It hasnt really solved my problem. The result I get is identical to before.

I want to get the root diameter of the thread to be modelled accurately, and this is giving me errors. If for example, the root cylinder that I create is 3.5 mm in diameter, then when I create the trapazoid tooth with the root at the equivalent 3.5 mm size in the sketch, once the sketch is swept, it turns out being slightly larger in diameter than the original root cylinder. The swept profile is also concave shape relative to the wall of the cylinder - odd!

Anyone have any ideas?

 

[Bouke]

Is it only larger on your screen or also when you pick a dimension on it?

 

[Spurs]

Bouke

I can use the measure tool to measure the size of the step. It is there.

The step does not have a constant offset around the profile. It seems like there is a slight hyperbolic shape to the swept profile.

If you provide me with an email address, I can send a model to show what I mean.