## Some Tutorials on Methodology (v0.17) - Master Sketch, Skeleton Geometry, Spreadsheets

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sjgallagher2
Posts: 7
Joined: Fri Mar 03, 2017 2:51 am

### Some Tutorials on Methodology (v0.17) - Master Sketch, Skeleton Geometry, Spreadsheets

I've been experimenting with methodologies for FreeCAD 0.17 lately, and while I'm taking notes, I realized the notes are similar to tutorials. So I thought I would share some thoughts on methodology in FreeCAD 0.17!

First, a brief description of methodologies I've come across:
There are three design methodologies commonly used with FreeCAD 0.17 (and they extend, as far as I can tell, to 0.18). The purpose of a design methodology is to put the design on solid footing in a way that allows maximum parameteric control. We don't want to have to redraw anything when we change important dimensions, such as spacing between parts. The design method must allow us to use a set of "master" dimensions, shared through all the parts, so that we can change dimensions shared between parts with ease.

The three approaches to this are:
1. Master sketch
2. Skeleton geometry
(4. Hybrid approach)

These all rely on using Expressions (directly or indirectly) to set important dimensions and constraints in the assembly.

In the master sketch method, we create one or two sketches that are only used to store the outline of the parts being created. We name constraints, and later we can reference them by either using the CarbonCopy (Copies geometry from another sketch) tool, when sketching, or using Expressions to reference the constraints for e.g. padding and offsetting sketches. All sketches are created mapped to the coordinate planes, never to a feature face, to avoid the topological naming problem.

In the skeleton geometry method, we use datum geometry (points, lines, planes) to control the dimensions of the parts. Sketches are created which reference the geometry, and sketches can be mapped to the datum planes. We can pad and pocket using the datum geometry by using Expressions. The datum geometry is called the "skeleton geometry."

For spreadsheet referencing, we use the Spreadsheet workbench, and create cells which are given aliases (by right-clicking, selecting Cell Properties, then going to Alias). These aliases can then be referenced in Expressions through the design. They can be used to control datum planes or master sketches for easier design.

The hybrid approach, of course, uses more than one of the above methods. We can combine things depending on what will be easiest for modeling. The decision of which method to use will become clearer as we get familiar with the limits and abilities of the various tools and methods FreeCAD has to offer.

FreeCAD 0.18 introduced variable tables, which provide a spreadsheet-like interface in a pop-up dialog sort of view. This provides a great boost in usability, as editing spreadsheets requires that we open the spreadsheet view, hiding the part design workspace.

Now, here are some examples which served as inspiration and reference:
For information about the topological naming problem

Some work from @ppemwm:
Basic:
With Assembly 4 (v0.18 or 0.19): https://forum.freecadweb.org/viewtopic. ... 40#p363215

From the Wiki (Skeleton Modeling):

Here is a previous tutorial I made to try to explain parametric modeling using the spreadsheet method: https://forum.freecadweb.org/viewtopic. ... 3&p=232625

Finally, here I have a PDF on Google Drive of another tutorial based on @ppemwm's experiment recreating the basic PartDesign Workbench tutorial using master sketches:

Zolko
Posts: 730
Joined: Mon Dec 17, 2018 10:02 am

### Re: Some Tutorials on Methodology (v0.17) - Master Sketch, Skeleton Geometry, Spreadsheets

sjgallagher2 wrote:
Mon Feb 17, 2020 3:05 pm
1. Master sketch
2. Skeleton geometry