Spline command does not provide desired result

[rharris]

I am trying to use the spline command in AutoCAD LT 2020 with end tangencies and the resultant spline is not what I expect.

Using an isosceles triangle as a basic starting shape, the resultant spline is not symmetrical. Sometimes, the curve goes well outside of the three control points. How do I get it to work like it does/did in previous versions?

In the attached pdf, I expect the shape on the right. However, the result is the shape on the left.

I have used several versions of AutoCAD LT over almost 20 years and I can not get this command to work like it has in the past.

What setting needs to be changed?

[steven-g]

It would help if you posted a dwg so we can see the spline, a picture doesn't tell us much. Does thie one work for you?

[rharris]

Your spline is what I expect it to look like. It is also what I get using the command the exact same way in older AutoCAD LT versions.

I have attached the dwg file. I hope you can see what I need to change to get what you did.

I generated the shape on the right using BLEND and tweaking the control points. If you look at it closely you'll see it is not exactly symmetrical.

[steven-g · Accepted solution]

You will have to forgive me I very rarely use splines, and I have to be honest I had never heard of 'Blend' before seeing your last answer (fascinating). I probably used a completely different workflow to the method you have been using, so I couldn't really help with what is causing your problem (maybe you could explain the steps) the way I did it was starting with the four lines as the basis, and then ran the spline command, picking point 1 as the start point then points 2 & 3 finishing the spline command with <enter>. Then highlight the spline and just hovering over the endpoints selecting "tangent direction" from the popup and from there picking point 4 & 5 respectively for the directions.

The description is difficult, so see if this screencast makes more sense

[steven-g]

Something is happening (actually it's not happening) with the screencast functionality, see if this link works.

[rharris]

Perfect! Thanks.

This is different from the previous version.

When you started the spline, did you notice in the command line there is an the option for "start tangency" after you selected your first point? This is the way I have used this command for years: Click the first point, type "T", select a point on the line for the direction of tangencey, click on point 2, then point 3, and then type "T" for "end tangency and pick a point on the line for the direction of tangency.

Interesting the options for start and end tangency are still in the command line when they don't work properly.

Thank you very much for showing me this!

[rharris]

Oh, and, I was not aware of the blend command until researching this spline problem. It is a nice little tool I'm sure I can use.

Thanks again.

[steven-g]

I can get the same results using that method, though it does depend on where you click for the tangent points. Picking the middle of that left-hand line for the first tangent direction works, but picking the end of that line creates some rather odd shapes.

[rharris]

I tried numerous line lengths in an attempt to achieve what you did so easily. I could not get a symmetrical shape. Some splines were close, but none were perfectly symmetrical.  And, as you discovered, some odd shapes are developed with some points.

Some CAD software allows the user to change the tangency vector properties. AutoCAD LT does not. Once the spline is drawn, the ends are either tangent or not.

When I use the start and end tangency options at the command line and develop the oddly shaped spline, I can easily fix it with your technique of hovering over the end points and specifying the tangent direction. Presto!