Polyline Bulge - why?

[Phoenix]

Need some help or suggestions to a long standing problem I have please.

I use Mechanical Desktop 6 (ancient I know!) to design bottles, jars, containers, etc.

For a variety of reasons my favoured method of construction has always been to loft surfaces over a series of cross sections to form half a model, which I then mirror to get the whole surfaced model.

 

My cross sections are created as polylines, and here's the big question......

When I mirror the surfaced half model I get a non-tangential 'ridge' all down the split line.

This non-tangential vertical ridge is extremely small, but nevertheless causes problems.

 

Now, I've searched various forums, 'Googled' and looked in the AutoCad manual and it appears that for some strange reason when, for example, an elliptical polyline is split in half it creates a 'polyline bulge' at the ends of the arcs.

 

So, why does AutoCAD create this bulge and how can I prevent it from happening every time I split a polyline shape?

 

Very grateful for any suggestions or pointers to this issue please!

 

Many thanks,

 

Phoenix.

[JD Mather]

... series of cross sections to form half a model, which I then mirror to get the whole surfaced model.

... mirror ... I get a non-tangential ..

Phoenix.

 

Do not do half and then mirror - for the very reason you are seeing.

 

For a more complete description of why not, search Google for Ed Eaton Curvy Stuff DiMonte Group tutorials.

 

The CAD program he uses is not MDT but the principles are the same.

[Phoenix]

OK. But my question still remains. Why are polyline bulges created. For what purpose exactly?

 

Thanks.

 

Phoenix.