How do I model 3D components for rotary projects, and how do I handle wrapping distortion?

Overview

Rotary projects work by designing on a flat canvas that the software wraps around a cylinder for machining. 3D modeling in a rotary job follows most of the same rules as flat modeling, with extra considerations for distortion, the modeling plane and toolpath generation.

Setting Up a Rotary Job

When you create a new job, choose the Rotary job type. Key settings are the diameter of the cylindrical blank, its length along the rotation axis, and which axis is the rotation axis (X or Y).

For modeling a shape's body or cross-section, the modeling plane must sit at the center of the cylinder. In the Material Setup form, move the modeling plane slider until Gap Inside Model reads 0. If it does not, the model has an incorrect diameter and the cross-section becomes rounded.

The software automatically adds a Zero Plane component when a rotary job is created.

Three Approaches to 3D Rotary Modeling

Approach 1: Clipart on the Cylinder Surface

The simplest method. Drag clipart from the Clipart tab into the rotary job. When the 3D model is empty, the software asks whether you want to use the component to decorate a surface. Answer Yes to confirm it should sit on the surface rather than define the body of the column. The message is not shown again in that project.

Choose settings for the 3D Roughing and 3D Finishing toolpaths with the rotating axis in mind. This can matter if the rotary axis is slower than the linear axes.

Clipart sits proud of the surface, so the flat areas around it end up recessed after 3D machining. Existing 2D toolpaths should be recalculated with Project toolpath onto 3D model ticked.

Distortion warning: clipart placed on a tapered part of a column is distorted because the circumference changes. To compensate, stretch the clipart component in the wrapped dimension. As a starting point, a section narrower than the wide end needs the clipart stretched by roughly the inverse of the diameter ratio; check it in the 3D view.

Approach 2: Two Rail Sweep for Turned and Cross-Section Shapes

Turned shape (chess piece, baluster, table leg): you only need the side profile and Two Rail Sweep.

1. Draw or import the side profile (for example a chess pawn).
2. Open Two Rail Sweep. When a rotary job is created, the software adds a layer called 2Rail Sweep Rails containing two blue lines on the sides of the job, perpendicular to the rotation axis.
3. Select both rails and click Use Selection.
4. Select the profile vector and click Apply, then check the 3D view.

Cross-section shape (hexagon, star): use the Vector Unwrapper.

1. Draw the cross-section (for example with Draw Polygon). It can be anywhere in the 2D view. Its radius cannot exceed the radius of the material block.
2. Select it and open Vector Unwrapper. A crosshair shows where the rotation axis crosses the profile and a circle shows the diameter of the blank. For a shape whose true center is not the center of its bounding box (a star), drag the rotation center to the true center.
3. Click Apply to create the unwrapped vector. It has legs on each end, which are needed for the correct height in the next step. You can tick Simplify unwrapped vectors to fit Bezier curves instead of many short line segments.
4. The tool creates a layer called Unwrapped Vectors Drive Rails with two blue lines parallel to the rotation axis. Open Two Rail Sweep, select the top and then the bottom rail (left and right if Y is the rotation axis), and click Use Selection.
5. Click the unwrapped vector and Apply. The modeling plane must be at the cylinder center (Gap Inside Model = 0) or the cross-section comes out wrong.

Approach 3: Single-Sided Modeling, Then Import

For complex shapes such as detailed furniture legs, model half the shape with normal single-sided techniques (Two Rail Sweep with several cross-sections), then:

1. Make sure no other components are visible, including the automatically added zero plane.
2. Hold Shift while opening Export Model. This opens the export tool in single-sided mode instead of rotary mode.
3. For a half model, use the Close with inverted front option.
4. Import the STL back into the rotary project as a Full 3D model.

The imported model has a seam where the two halves joined. Smooth it with the smoothing in the Sculpting tools.

Wrapping Distortion: The Critical Issue

Wrapped toolpaths match flat toolpaths only at the surface of the blank. The closer to the rotation axis (the deeper) the toolpath is, the more it is compressed, because the circumference at a smaller radius is shorter. On a uniform cylinder this is acceptable. On a tapered piece it is a real problem. If there is a substantial difference in diameter between parts of the model, one 3D toolpath for the whole model is overly compressed.

Fix for tapered work: draw boundary vectors around regions with significantly different diameters and calculate a separate 3D toolpath for each region, choosing the stepover for that region. There is no diameter box on the 3D toolpath forms.

Tapered column body: double-click the zero plane component to open Component Properties, enter a Base Height (the example uses 0.8), tick Tilt, click Set, switch to the 2D view and click at the middle left and then the middle right, and set the angle (the example uses 3 degrees). Then readjust the modeling plane in Material Setup until Gap Inside Model is 0 again, so the body is not inflated. Clipart in the narrower parts will then need stretching in the wrapped dimension.

Spiral Wrapping (Level Wrapping)

To wrap a design spirally around a column:

1. Draw lines at an angle to the rotation axis that extend past the 2D job boundary. The number of wraps is set by the length of the vertical leg. In the example, a line made with a 90 degree segment of length y * 4 wraps four times.
2. Build the strip design in the flat view as a component and position it between the lines. The manual example uses Create Texture Area, Set Size and Copy Along Vectors to do this.
3. Create a new Level and move the component onto it.
4. Right-click the Level and choose Wrapping. The design wraps around the blank, and wrapping is available for rotary jobs only.

Wrapping can be enabled on any Level and combined with Mirror Mode. If a Level wraps onto itself, the intersecting areas are merged regardless of the Level's combine mode. For a woven pattern, put left-hand and right-hand spirals on separate Levels, both with wrapping on. A third Level set to Merge can hide the spiral at the column ends.

Twisted Shapes

Twisted shapes use Level Wrapping together with the Vector Unwrapper.

1. Draw the cross-section (for example a 5-armed star) and unwrap it with the Vector Unwrapper, placing the rotation center at the true center of the shape.
2. Draw angled rail lines the same way as for a spiral (a length of y * 2 gives two revolutions).
3. The rails extend past the job boundary, so a sweep made in the rotary job is cropped. To get around this, make a bounding box around the rails, note its size, save the project, and create a new single-sided project slightly larger than the box. Import the rails as vectors, center them (F9), and run Two Rail Sweep there.
4. Save that file, reopen the rotary project, and use Import Component to bring the sweep in.
5. Move the component to a new Level and enable Wrapping on it.

Toolpath Tips for Rotary 3D

• Use the 3D Roughing and 3D Finishing toolpaths, remembering which axis rotates.
• Wrapped toolpaths match flat toolpaths only at the surface, so trust your preview but test the cut on scrap.