What to Know When Preparing Your CAD Model for CNC Milling

Converting a CAD model into successfully machined parts requires careful consideration of design requirements, manufacturing capabilities, and optimizations to avoid toolpath issues. This guide covers key factors engineers should account for when setting up CAD models to ensure efficient, accurate CNC milling.

Designing Parts for Manufacturability

Designing parts with manufacturing in mind from the start avoids machinability issues:

Avoid Unnecessary Complexity

Simplify geometry and consolidate features where possible. Complex organic shapes usually offer no functional benefit but require slower machining.

Design for Minimum Operations

Reduce secondary operations like drilling and tapping by incorporating those features directly into the main milling operations if feasible.

Standardize Elements

Reuse common features, dimensions, and component sizes across families of parts to simplify fixturing and tooling requirements.

Utilize Standard Stock Sizes

Select raw material sizes that minimize required cuts. Consider the capabilities of the supplier’s available stock and in-house saw/shear capacity.

Account for Workholding and Access

Ensure adequate surface area for clamping and sufficient access for cutters to avoid collisions.

Facilitate Deburring/Finishing

Include chamfers, corner radii, and draft angles to ease secondary clean-up operations. Avoid deep pockets with no angles where tools can’t reach.

Minimize Thin Features

Thin walls and sections prone to chatter should be avoided or specified as non-critical areas.

By optimizing CAD models for manufacture, programming time and machining costs are reduced while maximizing quality.

Design Principles for Machined Parts

Additional guidelines to follow when designing parts for CNC milling:

• Specify generous internal radii at intersections to prevent stress concentrations.
• Call out all fillets, rounds, and chamfers to avoid sharp corners that require extra deburring work.
• Set wall thicknesses in increments suited to available cutter diameters.
• Where threads are needed, specify standard thread forms and sizes.
• Use shoulder bolts instead of dowel pins for more secure, adjustable locating.
• Avoid tiny drilled holes prone to tool breakage; specify minimum 12-15 hole diameters if possible.
• For holes, align axes to allow straight-through machining whenever feasible.
• Orient features consistently to minimize fixturing during machining.
• Design symmetrical patterns to maximize tool life and balance forces on the spindle.

Incorporating these best practices into CAD models will ease programming and machining.

Modeling Part Geometries Accurately

Creating CAD models precisely is critical for CNC machining:

• Model nominal part dimensions at exact values without tolerances. Tolerances should be specified only on drawings.
• Ensure models represent as-designed geometry without unintended gaps, surface defects, etc. that trip up CAM software.
• Account for the bend radius of sheet metal parts and model entsprechende stretched outer surfaces.
• Model threads, bosses, holes, etc. rather than simply placing cosmetic holes in models.
• Wireframe geometry should cleanly meet with zero gaps at intersections and vertices.
• Surfaces like cylinder sidewalls and cones should be geometrically precise, not faceted.
• Double-check models against 2D prints to verify all dimensions translated accurately from drawings.

Accurate solid models are essential for seamless CAM programming and to deliver correctly machined parts.

Organizing CAD Assemblies

Structure CAD assemblies logically to aid CAM programming and machining:

• Make each sub-component an individual part file referenced in the full assembly.
• Parts should be modeled in their final machined state, not as raw castings/drawings.
• Orient all parts consistently with respect to the main assembly coordinate system.
• Set part origins at the fixture datum to match the setup on the mill.
• Model parts in assembly context to detect any interferences or misalignments.
• Suppress irrelevant parts not being machined to avoid clutter and slow file performance.

Properly constructed CAD assemblies ensure efficient, error-free translation to finished workpieces.

Specifying Materials

Define model materials precisely to calculate tool speeds/feeds:

• Specify the exact material grade rather than the generic material type (e.g. Aluminum 6061 vs. just Aluminum).
• Include relevant material data like hardness, strength, and thermal properties.
• For plastics, provide melt/glass transition temperatures, chemical composition, and filler content.
• If using pre-hardened tool steel, indicate its hardness level in the model.
• For castings, denote grain structure – fine, medium, or coarse.

Accurate material information optimizes auto-generated speeds and feeds for tool life, power, and surface finish.

Modeling Cast or Forged Features

Account for requirements of casting and forging processes in models:

• Design generous draft angles and radii for casted parts to enable mold release.
• Allow extra stock around forged areas to be removed during CNC operations.
• Model porosities, parting lines, or chill effects typical in castings to avoid tool collisions.
• Set appropriate draft on forged bosses and holes for punch-out.
• Verify castability/formability of models with foundry/forge experts during design reviews.

This equips CAM programmers with complete information to machine semi-finished castings/forgings.

Detailing Critical Tolerances

Highlight critical dimensions and tolerances on models:

• Tolerance of all critical features on drawings and flag them in models using annotation.
• Tighter tolerances may require slower feeds and conservative depths of cut.
• Loosen non-critical tolerance bands for easier, faster machining.
• Avoid spec’ing unnecessarily tight tolerances like +/- 0.005 mm just because it sounds precise.
• Consider GD&T (Geometric Dimensioning and tolerancing) for complex tolerances.

This allows CAM programmers to select optimal tool paths and work-holding to achieve key tolerances.

Specifying Surface Finishes

Call out surface finish requirements on the CAD model:

• Provide surface finish value (e.g. 63 μin) or standard reference (e.g. #250 grit)
• Consider tool marks and lay patterns acceptable for non-critical cosmetic areas.
• Allow pebbling/orange peel finish on plastic parts if appearance is unimportant.
• Loosen surface finish standards where function allows to enable faster material removal rates.
• Tighter finishes may require reduced feeds, smaller stepovers, and spark-out cycles.

Accurate surface finish specs prevent machining issues like gouging while optimizing cycle time.

Modeling Fixturing and Workholding

Model actual fixtures and vises to be used along with parts:

• This allows checking for collisions between parts, work holders, and machine components.
• Fixtures should be located off machine origin and position parts for efficient tool access.
• Model clamps, vise jaws, straps, etc. to verify adequate holding strength and locate parts precisely.
• Add fixture components like riser blocks, parallels, angle plates, etc. that will support workpieces.

Simulating setups will prevent issues like insufficient clearance for tool changes or workpiece relationships.

CAD Model Review Checklist

Review models thoroughly before release to ensure they meet machinability needs:

• All part features based on engineering drawings are modeled accurately as solids.
• Standard hole sizes are specified where applicable.
• Minimum internal radii are defined per best practices.
• Draft angles meet casting process requirements if applicable.
• The datum scheme matches the planned milling fixture setup.
• Critical tolerances and surface finishes are defined.
• Parts/assemblies structured logically for CAM programming.
• Actual work-holding devices are modeled.

This comprehensive model review avoids delays, defects, and inefficiencies when milling parts.

Conclusion

Preparing CAD models requires considerable forethought to prevent issues during CNC programming and machining. Following manufacturing design principles, modeling parts precisely with proper GD&T, specifying machining requirements, and structuring data appropriately set up CNC programmers for efficient toolpath generation. Taking the time upfront to develop manufacturing-ready models streamlines the entire development process from CAD to finished milled parts. Leveraging your team’s collective expertise across design, machining, and quality helps build manufacturability into CAD models. This upfront DFM/DFA approach is well worth the effort to produce high-quality machined components faster and at a lower cost.