Wednesday, 17 October 2012

Cam Module - Work over the Weekend

Git Repo:

I have created a GIT Repo on github that the other guys are going to push back their changes to, so look here if you're interested in seeing our progress.

Weekend Work:

I spent the last weekend laying the ground work structure for the FreeCAD Cam Module - inadvertently helped by my student house mates being away, making it calm around here. Essentially the hardest or laborious task was creating the source structure. There are several classes which require both the declaration and definition just to work. This leads to a file tree in the module like so:


And there's more GUI stuff too!

This was created based on the design document we created over last week outlining a proposal for how all this stuff could fit together.



The Dependency Graph:

FreeCAD has a fairly intelligent system of managing updates through the document. Each document object can be referenced by a series of links which behave as dependency. When a Document Object changes or it's property. When this happens the property or Document Object is 'touched' and this indicates it should be recompute the hierarchy tree.



Any dependencies that reference this object will automatically be requested to recompute and update themselves. This is what drives parametric design but being transparent to the developer.

The first attempt at fitting these classes together wasn't entirely successful and in its process also made the discovery of some good fixes in the base FreeCAD application. The end result for the dependency chart (below) for the structure seemed logical but introduced several difficulties.


Logically the structure makes sense in terms of the object hierarchy. However, for the program it doesn't. When the StockGeometry, or CamPartList features update, the tool paths will not automatically update. It took me two hours to figure that one out and it didn't look pretty. Boo hoo.

I had two beers and a night off to think about this on Sunday. It became apparent that the CamFeature was simply a top level container that would provide a general interface for accessing and managing the other features - it didn't control the actual CAM process.

Instead I created an object hierarchy like so. Essentially GCodeFeature is the result from the combined results of the TPG. However we get the all the glorious updating mechanism with this.


The ToolPaths manages the Input and controls the process of updating the TPGFeatures eventually more intelligently.

The TPGFeatures are quite dumb. They behave like blackboxes and are given an input, do their thing and spit back a result. The nice thing is thing about this it will allow new TPG's to be more quickly developed since the developers don't have to set up the document feature, but only have to provide the input and in return can get the GCode back.