CNC Part Design Checklist: 10 Rules for Lower Costs

1. Check Wall Thickness and Internal Cavities

Thin walls trap coolant and cause chip accumulation. They also flex during cutting, leading to poor surface finish and tool breakage. Keep uniform wall thickness wherever possible. If a thin section is required, aim for a thickness that the tool can support without deflection. For aluminum, thin walls under 1.5 millimeters often require slow feed rates and small tools. For steel, the minimum practical thickness increases significantly.

Internal cavities create air pockets that trap chips. They also hide defects from visual inspection. If a cavity is required, design it with a clear drain path or a breakaway tab. Avoid deep pockets with narrow entrances. A 10 millimeter deep cavity with a 5 millimeter opening is difficult to clear. A wider opening or a stepped entry allows standard tools to remove chips effectively.

Red flags to watch for:

• Walls thinner than 10 percent of part thickness
• Deep pockets with narrow entrances
• Internal voids with no access for chip removal
• Uniform wall thickness that varies across the part

2. Align Tolerances with Functional Requirements

Tighter tolerances cost more because they require slower feeds, better tools, and more inspection. If a feature does not need a tight tolerance, do not apply one. A flatness tolerance of plus or minus 0.1 millimeters on a non-critical surface adds time without adding value.

Use a tolerance table for each part. Group features by function. A mating surface might need a tight tolerance, while a decorative edge might not. If the drawing specifies a tolerance that is three times tighter than the function requires, ask why. Sometimes engineers copy a tolerance from a previous revision without checking the current need.

Red flags to watch for:

• Uniform tight tolerances across all surfaces
• Tolerances tighter than the inspection capability can verify
• No functional notes explaining why a tolerance is tight
• Tolerances that conflict with the material or process

3. Minimize Toolpath Complexity

Long toolpaths increase cycle time and tool wear. Complex internal geometries require multiple setups and different tools. A part with many small pockets and slots may take twice as long to machine as a simple block of the same material.

Look for features that can be combined or simplified. A series of small holes can sometimes be replaced with a single larger hole and a pin. A complex profile can be simplified if the function allows it. If a feature is purely decorative, consider removing it. If it is functional, consider whether a simpler shape serves the same purpose.

Red flags to watch for:

• Many small pockets or slots
• Complex internal profiles
• Features that require multiple setups
• Decorative elements that add no function

4. Use Standard Fastener Features

Non-standard fastener holes require special tooling and longer cycle times. Standard holes for M6, M8, M10, and common metric or imperial sizes are cheap to machine. They also make assembly faster and reduce the chance of damage during installation.

If a non-standard hole is required, justify it. Sometimes a unique size is needed for a specific load or interference fit. But most of the time, a standard hole with a proper fit class works. Use countersinks and counterbores in standard sizes. Avoid custom depths that require custom tools.

Red flags to watch for:

• Non-standard hole sizes
• Custom counterbore depths
• Holes too close to the edge for standard drilling
• No standard fastener pattern

5. Add Chamfers and Deburring Features

Chamfers protect edges from chipping during handling and assembly. They also reduce the cost of deburring. A sharp 90 degree edge is difficult to deburr because the tool cannot reach the corner. A 45 degree chamfer allows a standard tool to clean the edge efficiently.

Add chamfers to all external edges that will be handled. For internal edges, use a small radius or a chamfer that the tool can access. Avoid sharp internal corners. They are difficult to machine and tend to break tools. A small radius, even 0.5 millimeters, improves tool life and surface finish.

Red flags to watch for:

• Sharp 90 degree internal corners
• No chamfers on external edges
• Deburring notes that are vague or absent
• Edges that require manual deburring

6. Select the Right Material

Material choice affects cost, cycle time, and part performance. Aluminum is cheap and machines fast, but it does not hold tight tolerances well. Steel is stronger and holds tolerances, but it machines slower and costs more. Stainless steel is corrosion resistant but is the hardest to machine.

Match the material to the function. If the part does not need corrosion resistance, do not use stainless steel. If the part does not need high strength, do not use a hard steel. If the part is lightweight and cosmetic, aluminum or a polymer may be better. If the part is a structural component, use a steel or aluminum alloy with the right yield strength.

Red flags to watch for:

• Material selected for cost without checking function
• Hard material used where a soft material works
• No material specification or standard reference
• Material that does not match the required strength or weight

7. Check Draft Angles and Undercuts

Undercuts are features that a straight tool cannot reach from the outside. They require special tools, multiple setups, or secondary operations. A draft angle of 1 to 3 degrees on a side wall allows a standard tool to remove material easily. A 90 degree vertical wall with an internal overhang is an undercut.

If an undercut is required, justify it. Sometimes a snap fit or a specific assembly sequence needs it. But most of the time, a draft angle or a different geometry solves the problem. For machined parts, avoid undercuts wherever possible. If they are needed, plan for multiple setups or a special tool.

Red flags to watch for:

• Internal overhangs
• Vertical walls with no draft
• Features that require a special tool
• No draft angle on external walls

8. Optimize Part Orientation

The orientation of the part on the vise or fixture affects cycle time and tool access. A part that sits flat on the table is easy to clamp. A part that must be hung or tilted is harder to fix and more likely to move during cutting.

Consider the natural shape of the part. If it has a flat base, use that. If it has a cylindrical profile, consider machining it in a chuck. If it has a complex profile, plan the orientation so that the most critical features are accessible. Avoid orientations that require flipping the part multiple times.

Red flags to watch for:

• No flat surface for clamping
• Orientation that hides critical features
• Multiple flips required for machining
• No fixture design notes

9. Review Inspection Requirements

Inspection adds time and cost. A part with a dozen critical dimensions that require CMM inspection takes longer than a part with two. If a feature is not critical, do not require inspection. If it is critical, specify the method.

Use a tolerance table that lists each feature, its tolerance, and its inspection method. For example, a bore diameter might be checked with a micrometer. A surface roughness might be checked with a profilometer. A flatness might be checked with a CMM. If the inspection method is not specified, the shop will choose the most expensive one.

Red flags to watch for:

• No inspection method specified
• Too many critical features
• Tolerances that are too tight for the inspection method
• No first article inspection notes

10. Run a DFAM Review Before Sending Files

A DFAM review is a final check before the files leave the design team. It is not a full DFM review. It is a focused check of the geometry against the machining process. Use the rules above as a guide. Check each rule against the part. If a rule is broken, ask why. If the answer is not functional, change the design.

A DFAM review is faster than a DFM review. It can be done in an hour. It catches the big errors that cost money. It does not replace a full DFM review, but it is a good first step. Use it for every new part. Use it for revisions. Use it when a part is sent to a new supplier.

Red flags to watch for:

• No DFAM review done before sending files
• No record of the review
• Design changes made after the review
• No feedback from the shop on the review

Quick Reference Table

Design Element	Rule	Red Flag
Wall Thickness	Keep uniform and above tool limit	Thin walls, no chip path
Tolerances	Match function, not habit	Uniform tight tolerances
Toolpath	Reduce complexity	Many small pockets
Fasteners	Use standard sizes	Non-standard holes
Chamfers	Add to all edges	Sharp 90 degree corners
Material	Match to function	Hard material for soft job
Draft	Use 1 to 3 degrees	Internal overhangs
Orientation	Use flat base	No clamping surface
Inspection	Specify method	No inspection notes
DFAM	Review before sending	No review record

Final Checklist

1. Check wall thickness and internal cavities for chip removal.
2. Align tolerances with functional requirements.
3. Minimize toolpath complexity by removing non-critical features.
4. Use standard fastener features and sizes.
5. Add chamfers and deburring features to all edges.
6. Select the right material for the function.
7. Check draft angles and avoid undercuts.
8. Optimize part orientation for clamping and access.
9. Review inspection requirements and specify methods.
10. Run a DFAM review before sending files to the shop.