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# continuity within a spline

Genius
To my knowledge, in creating a spline, NX would automatically add knots to divide the spline into multiple segments virtually and fulfill: points = degrees + segments

But I can't find some information about how NX controls the continuity at each knot within a spline.

Below are some of my rough ideas:

• A single-segment spline (B-curve) is best for achieving a high-quality surface
• At a knot of a multi-segment spline, the continuity is ≥G1.
• To build a high-quality surface, a multi-segment spline would be much better than a curve string (arcs/conics/splines...) connected with manually specified continuity.
• "Divide-By Bounding Objects" only cuts a spline for handling separately, without affecting the basic mathematics.

Are they correct? Hope to have your suggestions. Thank you!
5 REPLIES 5

# Re: continuity within a spline

Solution Partner Phenom

you can set for each point in the spline what the continuity is.

select the point of the spline point and go back to the spline dialog.

in the constraint tab you can select the point or is already selected by clicking in your design space.

the you can set by Continuity type the type that you want.

Ruud van den Brand
Pre-sales NX CAD
cards PLM Solutions

# Re: continuity within a spline

Genius

Hi @ruud_vandenbrand, I mean the contiunity at each knot (rectangle points) of a spline. Thanks!

# Re: continuity within a spline

Siemens Phenom

By default when creating a spline from scratch it will create a C2 knot. Creating a spline derived from other geometry it will depend on the quality and type of input geoemetry, for example blend to plane, arc to line cannot be better than C1.

And yes if you are looking for quality surfaces use splines and not strings of arcs.

Steve V

# Re: continuity within a spline

Siemens Phenom

• A single-segment spline (B-curve) is best for achieving a high-quality surface

That's what the folklore says, certainly.

• At a knot of a multi-segment spline, the continuity is ≥G1.

Correct, roughly. If you have a knot of multiplicity r on a spline of degree m, then the join will be C(m-r). So, for example, at a double knot (r=2) on a cubic spline (m=3), the join is C1.

• To build a high-quality surface, a multi-segment spline would be much better than a curve string (arcs/conics/splines...) connected with manually specified continuity.

Well, "quality" is hard to define. But if you use lines and arcs, there will be a lot of places where the curve string is not G2. If the jumps in curvature are large, they will be visible, which might be undesirable.

• "Divide-By Bounding Objects" only cuts a spline for handling separately, without affecting the basic mathematics.

Correct.

The basic equation is this:   number of knots = number of poles + degree + 1.

If you build a spline using "Studio Spline" then NX will use single knots. So, if you build a curve of degree 5 with 7 points, you will get 2 segments. The join will be C4.

Interactive NX hides the knots. But you can play with them using SNAP or NX/Open.

yamada

# Re: continuity within a spline

Genius

Hi @Yamada: Thank you for the detailed comments!

Hi @StevenVickers: Thank you for your continous help!