Best Practices in 3d Modeling - Jigs and Construction Geometry
Screenshot
	This workflow details methods for the use of construction geometry and "jigs" in 3d modeling as a way of ensuring consistently, accuracy, and agility in adapting to changing design parameters. The utility of construction geometry will be demonstrated through the case-study of modeling a 3d kayak based on 2d contour information.
Uses Tool(s)	Rhinoceros

This workflow will show how to model precising in Rhino using "jigs". First, you will need to set up construction lines in order to accurately line up the geometry. Next, you will create a "jig" which is linework based upon pure geometry (a circle) or traced linework (such as kayak curves which we will be exampling here). Then these curves, or jigs, should be moved to the correct location along the object using the construction lines. This will create a skeleton of your 3d model that you can use to loft your final surface. This workflow is closely related to an exercise on basic surface modeling

Steps

Setting up Construction Lines

The foundation of any 3d object begins with setting up proper construction lines. Construction lines help ensure that the linework you set-up for the surfaces is properly aligned. Without construction lines, lines may end up non-parallel to each other, OSnap may snap to the wrong point or view.
Construction lines should be dependent on the size and scale of your object. Sometimes it's helpful to draw a customized grid, other times it can be more helpful to draw a bounding box. This linework should be located within its own layer, titled "Construction Lines". This way, you can easily toggle on and off the construction lines by turning the layer on and off (or locking it so that you don't edit it accidently later on.) Start by drawing straight lines along the X and Y axis in the top view. Copy these lines upward in the Z direction by using the Right view. Additional construction lines can be created along the axis in which you will be copying the jig.

Set up Base Geometry

Using the construction lines previously drawn, set up the base geometry of the object. This geometry may be given to you, by tracing an image and scaled according to the construction lines, or started with simple pure geometry, such as circles or squares (if making a wine bottle for example). This base geometry is the first "jig" that we will be using to copy and edit throughout the skeleton of the model.
For the kayak example, we can trace the different curves drawn in the short section. To make the work easier, we can just trace one half of them because they are symmetrical (in the last step, we can mirror the surface to make it complete). Using the InterpCrv tool, we can easily trace the different curves. Keep these in place for the next step.

Copy Base Geometry Along Construction Lines

Using the geometry we just drew as our "jig", we can copy this linework along the construction lines to create skeleton of the 3d model. Be careful to copy the linework along the proper axis, by selecting the correct copy point. In the next step, we can go back to these lines and edit them as needed, the emphasis on this step is to create the correct spacing between each curve, or jig. The more jigs you draw, the more precise the 3d model will be.
This is where the construction lines prove helpful in the process. You may realize you need a center line throughout the middle of the object or division points along the construction lines to evenly space out the jigs.
For example, when creating the kayak, we can select the different curves we drew in the previous step and copy them into the correct location in the long section. Each curve is evenly spaces on the long axis of the kayak. When moving the curve, select on the corner box of the short section and move it to the reference point along the bounding box.

Edit Jigs According to Final 3d Model

Now that the jigs are in place along the model, you may need to edit them according to how the 3d model should look. For example, if creating a wine bottle, the circles you created along a vertical axis should now be scaled to create the skinnier neck of the bottle and the rounded bottom. To edit the curves individually, use the different viewports to ensure that you are selecting only the necessary curves that you would like to edit. When scaling or rotating, be sure that you can the necessary construction lines to help you scale / rotate from the proper point.
For the kayak example, all the curves we traced and moved were already scaled according to the different locations along the kayak so we do not need to make any further changes. However, you can always come back to this step if you would like to edit the appearance of the kayak.

Create the Surface by Lofting

Now that the skeleton of jigs is all edited, the final surface can be created. Select the jigs in the correct order that the surface should be created. (You can also select them quickly by holding down the mouse and moving over the geometry.) Use the Loft command to create a surface that attaches through all the selected geometry. The command options should pop up. Select the preview command to see the potential results.

Other Skeleton Models

The example used throughout this workflow, the kayak model, focused on creating a skeleton that works best for the Loft surface command. Depending on the different surface tools, different skeletons or jigs can assist in precise modeling. For example, the Revolve command can be specified further by revolving around a profile curve of your choice with Rail Revolve (rather than the default, a circle). The Extrude command can be specified further by using the command ExtrudeAlongCurve (rather than the default, extruding along the Z-axis).

Common Problems

• If the geometry that you lofted in Step 5 isn't looking correct, you can always go back to Step 4 and edit the model skeleton and then re-loft.

Resources

Curve Based Modeling

This is a great summary of the surface commands in Rhino from TU Delft.