Welding

Realsoft Graphics Oy		Welding
	Chapter 4. NURBS Curves and Surfaces

The Welding tool connects two surface edges with a special purpose weld object. Basically, the weld object is a NURBS mesh with some additional intelligence so that it maintains a seamless joint between the specified edges.

Any two edges with different point counts, orders and other properties can be welded.

To connect two meshes, one polygonal and one smooth surface, by welding:

1. Create two NURBS meshes: a polygonal one and a cubic one.

2. Select both meshes. The control bar now shows you the weld tool.

Two NURBS meshes with a different order and density

Weld tool

Activate the tool. The tool automatically welds the two surfaces. You can change the weld by dragging a line over the mesh, near the edge you want to weld. The line you draw also defines the direction of the weld mesh.

	Note
	You have to drag over the meshes (inside the mesh boundaries), as shown in the example image

Mark two edges to be welded

If you accidentally marked a wrong edge, you can drag again to fix the error. There is no need to restart the tool.

3. When the desired edges are welded, click Accept to finish the tool. The two meshes are now connected.

You can modify the original meshes after welding. The shape of the welding surface is updated accordingly.

Tutorial project: 'tutorprojects\modeling\nurbs\weld\weld'

A polygonal and a cubic mesh welded

When welding closed surfaces (such as pipes, for example), there is an additional freedom to control the twist of the welding surfaces (the positions where the 'u=0' isoparametric curve meets the surfaces).

Let's connect two NURBS cylinders with each other to demonstrate this.

1. Use the NURBS/Cylinder tool to create two NURBS tubes (reset the Filled option).

Create two NURBS tubes

In order to see the twist effect, switch to the wire frame rendering mode.

2. Select both NURBS tubes. The control bar now automatically shows you the Weld tool. Activate the tool and drag along the edges to be welded, then Accept the tool.

Mark an edge from both cylinders. Drag points determine the start and end points for the first isoparametric curve of the welding mesh.

Welded tubes

Welding surfaces and trim curves

Welding tool also allows you to weld the edges of trimmed surfaces.

1. Create a NURBS surface.

2. Select the surface and select Unified/Create Curve from the Tools drop down menu. Set the Class option to Trim curve and enter a desired number of control points over the NURBS surface to trim it.

A NURBS surface

A trimmed surface

3. Use sweep or another appropriate tool to create a NURBS tube.

4. Select the trim curve and the tube and activate the Weld tool from the pull-down menu Tools/NURBS Construction/Weld Edges.

Select the trim curve and the tube mesh

	Note
	A mesh can contain any number of trim curves. You have to select the actual trim curve to which the weld mesh should be attached!

Mark an edge from both surfaces to weld them.

	Note
	Both edges are closed - by controlling the drag start points, you can control how twisted the weld surface will be.

Mark the edges to be welded

The trimmed surface edge welded with the tube

Welding two trimmed surfaces

Two trimmed meshes

The Weld tool can also weld two trimmed meshes with each other.

1. Create two trimmed NURBS surfaces somewhat parallel to each other.

2. Select both trim curves, click the Weld tool of the NURBS tool tab and mark edges to create a weld surface.

Two trim curves welded

The trimmed edges are now smoothly joined with each other. Let's play with this example a little bit more.

3. Select both trim curves and click the Invert tool in the control bar (it's located at the far right end).

This inverts the inside/outside property of the selected trim curves and they trim out the outer part of the surface.

Trim curves inverted

4. Select the weld surface and open the property window. Go to the Spec tab. Here you can see two Size fields, which define how sharp the weld surface is at its boundaries. The smaller the value, the sharper the joint.

The radius fields of the weld surface

Weld surface with negative size attributes

If you want completely sharp edges, set these values to zero. We just want to change the direction of the weld surface (the edges of the weld surface should point outwards rather than inside).

Change the sign of both the values (make them negative).

Tutorial project: 'tutorprojects\modeling\nurbs\weld\welded trims'

	Note
	You can create any number of trim curves to both surfaces and connect them with the weld tool. Also the weld surface itself can be trimmed and welded with another weld surface.

Two trimmed edges connected with the weld tool

Controlling the accuracy

When connecting the edges of the surfaces, the weld surface is capable of following the shape of both the edges exactly. However, when welding trim curves, it is mathematically impossible to create an exact weld surface for the trim curve.

If the joint is not accurate enough, increase the number of control points of the trim curves. The higher the point count of the trim curves, the more accurate the weld becomes.


Trimming NURBS Surfaces		Fillets