LibreCAD - Re: spline through points with end blended tangentially onto a circle

Re: spline through points with end blended tangentially onto a circle

Posted by bcs616 on
URL: https://forum.librecad.org/spline-through-points-with-end-blended-tangentially-onto-a-circle-tp5720652p5720658.html

Hi LordOfBikes, thank you indeed for your reply, very informative. As you suggest, I will try using tangent or orthogonal construction lines to see if the blend point can be located more positively. Also, you raise the issue of the features the CNC can cope with. I need to get the CNC spec to find out. If splines are out, what options are there in Librecad please? Do I need to create an entity showing the outline of the part in any case for it to be acceptable for CNC?

The wing section is from an antique aircraft design, so I guessed that splines would be as close to the original construction, which I guess was templated by drawing with ship curves etc.

Thanks again for your reply.

Bernard

On Fri, May 14, 2021 at 10:40 AM LordOfBikes [via LibreCAD] <[hidden email]> wrote:

Welcome bcs616!

I understand your issue, but sadly this is not possible with LibreCAD.
It lacks some spline math for this.

What I can imagine is to use a circular arc approximation instead of splines.
But this is also not available in LibreCAD yet.
I once had this problem in another project with elliptical arcs, which would need similar math.
Many CNC's can't handle ellipses or splines and this is where approximation with circular arcs can help.
The problem is, that the radius in an ellipse or spline changes in every point.
The solution is to divide the original entity into several arcs with constant radii and G1 continuity (tangent).
This brings a little deviation, which could be reduced by more dividing, but the result is a smooth curve too.
The result could also be converted into a polyline which makes the curve a single entity.

It is possible to solve this graphically with some efforts too.
I once used tangent and orthogonal construction lines in the endpoints, some kind of bisecting and tangent circles for this.
Finally I wrote a piece of code for ellipses to do that, which I want to implement in LibreCAD one day.
But I can't say yet, when this will be realized.

Armin

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