CNC Milling

A. What is a CNC Router/Mill?

A CNC mill uses rotating cutting tools to precisely machine materials

CNC Router

A CNC router uses a high-speed rotating cutting tool to cut and mill sheet materials, similar to a large automated engraving machine. It's typically used to machine softer materials like wood, plastic, and aluminum.

CNC Mill

A CNC mill is more precise and rigid than a router, capable of machining harder metals (such as steel and titanium). Mills can perform more complex 3D machining operations.

Why Do Robotics Teams Use CNC?

• Precision metal plates — Machining accuracy down to 0.05mm
• Custom mounting plates — Precise hole positions and contours
• Gearbox housings — Structural parts requiring precise fits

Common Machining Operations

• Aluminum plate cutting — Cutting aluminum sheets along a contour
• Hole drilling — Precise hole placement
• Pocket milling — Milling recesses and pockets into sheet material (to reduce weight)

B. CNC Design Tips

CNC design must account for tool radius, workholding, and machine work envelope

Workholding

Parts must be securely clamped before machining. Design considerations include:

• Leave clearance for clamps — the area occupied by fixtures cannot be machined
• Consider using bolt holes for fixturing (drill fixture holes first, then machine the contour)
• Avoid designing shapes that cannot be clamped

Machine Work Envelope

Every CNC machine has a work envelope — the maximum part size it can handle. Before designing, find out the work envelope of the CNC machine available to you. A typical small CNC router has a work envelope of about 300mm x 200mm.

Sharp Corners vs. Fillets

A round cutting tool cannot produce sharp internal corners — design internal corner radii larger than the tool radius

Available Tools

Different types and sizes of endmills — know what tools you have available and design your parts accordingly

Different tool diameters determine the minimum feature size you can machine:

• Large tools (6mm+) — Faster machining, but larger minimum feature size
• Small tools (1-3mm) — Can machine fine features, but slower and more prone to breaking
• Design with features that can be completed by larger tools first; only use small tools when necessary

C. Exporting from Onshape for CNC

Choose DXF or STEP export format based on machining requirements

Face DXF

Suitable for 2D profile machining (cutting aluminum plates, etc.):

1. Right-click the machining face of the part
2. Select Export as DXF
3. The DXF file contains all contours and hole positions for that face

Sketch DXF

If you need to export specific toolpaths (rather than the part face contour), you can export a sketch directly as DXF:

1. Right-click the sketch tab
2. Select Export as DXF

STEP Files

Suitable for CAM software that needs a 3D model (for pocket milling and other 3D operations):

1. Right-click the part
2. Select Export
3. Choose STEP as the format

Common CAM Software

CAM (Computer-Aided Manufacturing) software generates the toolpaths for CNC machining:

• Fusion 360 CAM — Full-featured, free for students
• Mastercam — Industry-grade CAM software
• Onshape Built-in CAM — Some CAM features are available directly within Onshape