Understanding Stock Models in FreeCAD Path

In FreeCAD’s Path Workbench (also referred to as the CAM Workbench), the stock model represents the raw, unmachined block of material from which the final physical part is cut. This article explains the fundamental purpose of stock models, detailing how they define workpiece boundaries, optimize toolpath efficiency, enable accurate material removal simulations, and prevent dangerous machine collisions during CNC operations.

Defining Raw Material Dimensions

The primary purpose of a stock model is to define the physical volume and shape of the raw material before machining begins. FreeCAD allows users to define stock using simple bounding boxes, cylinders, extended offsets around the design model, or custom solid shapes imported from another body. Establishing accurate stock dimensions ensures that CAM operations reflect the real-world billet mounted to the CNC machine table.

Guiding Toolpath Strategies and Reducing Air Cutting

Many advanced CNC operations—such as adaptive clearing, facing, and 3D pocketing—rely on the stock model to determine where material exists. By knowing the exact stock boundary, FreeCAD calculates toolpaths that remove material only where necessary. This prevents "air cutting" (the tool moving through empty space unnecessarily) and ensures appropriate stepover and depth-of-cut parameters are maintained from the very first pass.

Work Coordinate System (WCS) Alignment

The stock model serves as the physical reference frame for locating the Work Coordinate System (WCS), typically output as G54, G55, etc. Machinists frequently probe the top surface or corner of the raw stock to set part zero rather than locating off a finished feature. FreeCAD uses the stock model's geometry to allow users to anchor the origin accurately at standard positions, such as the top-center or front-left-top corner of the raw blank.

Material Removal Simulation and Verification

Before sending G-code to a physical CNC machine, operators must verify tool motion digitally. FreeCAD Path uses the stock model as the canvas for its material removal simulation. As the virtual cutting tool follows the generated paths, the software visually subtracts volume from the stock. This provides a clear representation of the progressive machining process, allowing users to spot unmachined areas, incorrect depths, or unintended gouges into the finished geometry.

Collision Avoidance and Machine Safety

Safety is a critical reason for utilizing an accurate stock model. FreeCAD evaluates the stock model during rapid positioning movements (G0 moves). If rapid transit moves are planned below or through the unmachined stock volume, the software can flag potential crashes. Furthermore, defining realistic stock heights ensures the tool retracts to a safe clearance plane above the raw material rather than just above the finished part, preventing high-speed impacts with unmachined stock features.