# Best method to smoothly close this opening?

I’m creating a cylindrical post that tapers to a flat plate. I need to blend these surfaces together to create a smooth transition from a rounded post to a tighter filleted edge.

2019.10.15_roofsupportpost.3dm (187.7 KB)

Hello - make an arc on the lower gap - Start-End-Point on arc. `DupEdge` the upper gap (it will be an arc)
`Rebuild` both curves to degree 5/6pts - this holds an arc shape very nicely for 90 degrees, which is close to what you have at the top.
`Sweep2` and ask for tangency at the rails.

2019.10.15_roofsupportpost_PG.3dm (184.6 KB)

-Pascal

Thanks! That worked well. Home come you have to rebuild those arc curves? Why cant you just use them as your Sweep2 profiles in the first place?

Hello - as is, the arcs are 3 point curves in this case - you’l need at least four to hold tangency on each edge. 90 degree arcs rebuild very accurately to degree 5/6 points, so you get enough points to hold tangency and still make the ends of the sweep meet up pretty well with the arcs on the existing fillets.

-Pascal

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By default, Arcs are degree=2 curves with only 3 control points. The middle pont has a special weight value to make the curve circular.
Rebuilding the arc changes the degree, adds the additional control points, and provides the flexibility needed to smoothly transition between the start and ending shapes.

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Okay, that makes sense. When you say degree5/6 points…are you saying to rebuild the curve to degree 5 with 6 control points?

Yes, both of them, so they match. Rebuild if the curve structures do not match - the arcs do, sort of - degree 2 three points, here, but that is not enough points and as John points out they are rational curves (with different weights), and Sweep2 will not let you ask for tangency with rationals.

-Pascal

How come you go to degree 5 instead of 3 or 4? Is it an easy explanation?

Sorry for all the questions.

Hello - I like a single span (degree + 1 points) where possible - that is the simplest structure. A 90degree arc rebuilds very cleanly to 5/6 - the arc and rebuilt arc are very very close for 5/6 and not so much for a 3/4.

If you boost the point count to six for degree 3 you can of course get closer - but not as close, and it’s just not as simple a curve - internal knots - and, the curvature graph, though fine, is still not as good as the 5/6…

-Pascal

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