Revolve command acts differently between Splines and Polylines

[omikron24]

Hello, I stumbled across a weird behavior of Revolve command that is unexpected and pretty annoying.  File is attached.

If I create a simple closed object with polylines - a 5 by 5 units  square for example, then I can simply revolve it around it's own edge to create a cylinder of 10 by 5 units.

If I create it with splines then I can't revolve it around it's edge anymore. Revolve just cancels as soon as I select desired axis. However revolve is still possible if I select axis a little bit further from the edge.

This creates 5 problems even with simplest parts:

1. The cylinder (or any object created by revolving) - has a hole in the middle.

2. The cylinder (or any object created by revolving) - is bigger since this hole has diameter. If my initial "spline square" had 5 by 5 unit size and my revolving axis was 1 unit apart from the edge, then my cylinder will be 12 by 5 units  instead of desired 10 by 5 units.

3. If consider this prior to designing a part hole stills needs to be patched afterwards which impacts workflow.

4. If part precision is desired and it contains curves - they become deformed and precision is lost.

5. Patching hole creates flat surface which is visible and needs additional finishing after part was produced.

I know that it is possible to convert splines to polylines but it introduces own problems especially if precision is required.

1. For example even if I use 0 precision I get a sh**load of mesh lines after revolving a large part.

2. Even if I try to ignore "mesh overload" - my hardware can't and it has a very significant impact on performance.

Does anybody know why REVOLVE acts like that and how to change this behavior if possible?

[chriscowgill7373]

In my opinion, splines are just plain bad and shouldnt be used for anything ever.  They are only good for a picture. If you need precision, you need to use a polyline.  It has to do with how the spline is handled by the software, the control points and such.    I dont know that I can provide a very good explanation of the difference.

Here is what some say on the subject:

I like this explaination:

https://www.landfx.com/kb/autocad-fxcad/features/item/4058-splines.html

You can search on Google for the differences between the two.

[neaton · Accepted solution]

If you must use a spline, create a REGION from it and revolve the region.

Nancy

[omikron24]

YOU!

I like you! 😃

[SEANT61]

Prior to AutoCAD 2007, the spline toolbox was very weak.  Now the workflow just need's additional training.

Some general statements could be:  If you get by with a polyline, use a polyline.  If you need splines, use splines.

A Spline is the only way to draw a true Parabola (Vertical Curve), where AutoCAD can acquire an accurate Perpendicular and/or Tangent Osnap.  Behind the scenes, AutoCAD uses NURBS/Spline mechanics for much of their standard AutoCAD entities (Helix, Lofts, Network, Blend, Patch, etc.).

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@chriscowgill7373 wrote:

. . . .

I like this explaination:

https://www.landfx.com/kb/autocad-fxcad/features/item/4058-splines.html . . . .

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That link is suspect.  I'm surprised the author didn't claim his car started getting worse gas mileage since his first introduction to splines, and their use in a drawing is the reason his unwed daughter got pregnant.