Design for Machining: 9 Rules That Save You Weeks in the Shop

Why DFM Matters Before You Hit the Shop

Design for Manufacturing (DFM) isn't just about making parts cheaper—it's about making them manufacturable at all. A beautiful CAD model can be impossible to machine, or require custom tooling that adds weeks and thousands of dollars to your prototype.

These nine rules represent the most common DFM issues we see in prototype CAD files. Follow them, and you'll avoid costly redesigns and iteration delays.

Rule 1: Respect Tool Access and Clearance

CNC machines cut material using rotating tools. If the tool can't reach a feature, it can't be machined. Common access problems:

• Deep pockets with narrow openings: End mills need clearance. A 0.25" diameter tool can't cut a 1" deep pocket with 0.25" walls.
• Undercuts and overhangs: Standard 3-axis CNC can't cut features that require tool approach from below.
• Internal corners: All internal corners will have a radius equal to the tool diameter (typically 0.015"-0.125").

Fix: Add draft angles, widen pockets, or split parts into assemblies when tool access is limited.

Rule 2: Use Standard Hole Sizes

Custom hole diameters require custom drill bits, which add cost and lead time. Standard fractional, number, and letter drill sizes are readily available and cheap.

• Bad: 0.328" diameter hole (requires custom reaming or interpolation)
• Good: 0.332" (#O drill) or 0.3125" (5/16") hole

Fix: Use standard drill sizes for all holes, or design features that can be interpolated with standard end mills.

Rule 3: Design for Minimum Wall Thickness

Thin walls are fragile during machining and prone to vibration, deflection, and breakage. Minimum recommended wall thickness:

• Aluminum: 1.5mm (0.060") for cosmetic parts, 2.5mm (0.100") for structural parts
• Steel/Stainless: 2.0mm (0.080") minimum
• Plastics (ABS, Delrin): 1.5mm (0.060") minimum

Fix: Add ribs, gussets, or thicken walls to improve rigidity and machinability.

Rule 4: Specify Realistic Tolerances

Tight tolerances dramatically increase cost and lead time. Most features don't need precision tolerancing. Use these guidelines:

• Standard CNC tolerance: ±0.005" (±0.13mm) for general features
• Tight tolerance: ±0.002" (±0.05mm) for critical fits and interfaces
• Precision tolerance: ±0.001" (±0.025mm) for high-precision assemblies (requires grinding or secondary operations)

Fix: Only specify tight tolerances on critical dimensions (mating surfaces, bearing bores, etc.). Use ±0.005" for everything else.

Rule 5: Avoid Thin Floors in Deep Pockets

Deep pockets with thin bottom walls are prone to breakthrough, vibration, and poor surface finish. The issue: tool pressure can punch through thin material.

• Minimum floor thickness: 1.5mm (0.060") for aluminum, 2.0mm (0.080") for plastics
• Deep pockets: Floor thickness should be at least 1/3 of pocket depth

Fix: Increase floor thickness, reduce pocket depth, or split into an assembly.

Rule 6: Add Draft Angles to Vertical Walls

Even in CNC machining, draft angles improve tool life, surface finish, and ease of machining. Recommended draft:

• External walls: 1-2° draft improves finish and reduces tool wear
• Internal pockets: 2-3° draft reduces corner stress and improves chip evacuation

Fix: Add 1-3° draft to all vertical surfaces where possible. It won't affect function but will improve machinability.

Rule 7: Limit Thread Depth

Threaded holes deeper than 1.5x the thread diameter are difficult to tap and prone to breakage. Standard thread depth:

• Maximum recommended depth: 1.5x thread diameter (e.g., M6 threads = 9mm max depth)
• Deep threads: Use helicoil inserts for threads deeper than 2x diameter

Fix: Reduce thread depth, or design for threaded inserts (helicoils, PEM nuts).

Rule 8: Design Multi-Axis Features Intentionally

Features requiring 4-axis or 5-axis machining add significant cost and lead time. Use multi-axis operations only when necessary:

• 3-axis features: All surfaces accessible from top, bottom, or four sides
• 4/5-axis features: Angled holes, compound curves, undercuts

Fix: Split parts into assemblies to avoid multi-axis operations, or accept the cost and lead time increase.

Rule 9: Avoid Sharp External Corners

Sharp external corners are stress concentrators and difficult to achieve in CNC machining. Round external corners with a radius:

• Recommended radius: 0.5mm-1.0mm for most applications
• Sharp corners: Only achievable with secondary operations (grinding, wire EDM)

Fix: Add 0.5-1mm radii to all external corners for better strength and machinability.

Checklist: DFM Review Before Submitting CAD

• All features accessible with standard tool diameters?
• Hole sizes match standard drill sizes or can be interpolated?
• Wall thickness meets minimum requirements?
• Tolerances specified only where critical?
• Floor thickness adequate for pocket depths?
• Draft angles added to vertical walls?
• Thread depths within recommended limits?
• Multi-axis features justified and intentional?
• External corners rounded with appropriate radii?

Get a Free DFM Review

Not sure if your design follows these rules? We provide complimentary DFM feedback with every prototype quote. Upload your CAD files, and our engineers will identify potential issues and suggest optimizations to reduce cost and lead time.