spline through points with end blended tangentially onto a circle

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spline through points with end blended tangentially onto a circle

bcs616
I want to represent a 2D section of a wing aerofoil shape.  The upper surface is defined as a set of data points, with a circular radius defined near the wing leading edge, and a flat surface for over the rear part of the wing.  I have used Spline through Points to produce a very credible looking smooth line through the data points.  By defining a complete circle at the leading edge, plus one additional point in the spline near the leading edge, I can set Snap to Entity and move the additional point onto the circle.  By moving the point with the mouse, the point slides around the circle, and I can set it by eye so that the spline looks to blend tangentially to the circle. Very pleasing. My question is please, can I get Librecad to determine the blend point automatically and so get the spline truly tangential to the circle?  Same question also for the rear part of the spline, where it is mean to blend with the falt surface, i.e. straight line.    
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Re: spline through points with end blended tangentially onto a circle

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

bcs616
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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