Last Updated: March 2026
Design for Manufacturability (DFM) Guide for CNC Machined Parts
Good DFM reduces your CNC machining cost, improves quality, and shortens lead time — before a single tool touches your material. The guidelines below cover wall thickness, internal corner radii, hole depth ratios, undercuts, thread specifications, and tolerance best practices. Apply them to your design before uploading to Rapid Manufacturing for a quote. Our team also performs a free DFM analysis with every submission and will flag anything missed.
Wall Thickness
Thin walls vibrate during machining (chatter), reducing surface quality and risking tool breakage. 1.5mm is a practical minimum for structural aluminium walls.
Steel is stiffer than aluminium but harder to machine. Thin steel walls deflect under cutting forces.
Plastics are flexible and prone to vibration. Thicker walls improve machining quality and part rigidity.
Internal Corner Radii
The tool radius must be smaller than the corner radius. Very small radii require tiny, slow tools. Increase radii to reduce cost.
R3mm allows a 6mm diameter end mill — a standard, economical tool. R6mm allows a 12mm tool — faster and cheaper.
The floor-to-wall corner can be radiused by a ball-nose tool without impact on the side-wall corner radius specification.
Holes and Drilling
Reliable and economical. Goes to 5D with extended drills at modest premium.
1.5× diameter gives reliable thread engagement. More is better for load-bearing threads.
Sub-1mm holes require micro-drilling — slower, expensive, and high breakage risk. Use ≥1.5mm where possible.
Drilling into an angled surface causes the drill to drift. If unavoidable, a flat spot must be milled first — specify this on your drawing.
Tolerances
Default for most dimensions. Achievable without special attention.
Achievable on most features across our supplier network without premium tooling.
Requires careful setup, slower feeds, temperature-controlled measurement. Increases cost — only specify where functionally required.
Standard metric thread class. Tighter classes (5H/5g) add cost for very little practical benefit in most applications.
Undercuts and Access
Undercuts require special tooling, extra setups, or 5-axis machining. Redesigning to remove them is almost always cheaper.
T-slot and dovetail cutters are standard but slower. Quote accordingly.
Deep, narrow cavities require long, thin tools that deflect and chatter. Keep cavities as shallow and wide as your design allows.
Threads
Never leave thread standard ambiguous. Specify size, pitch, class, and type.
Aluminium is weaker, so requires more thread engagement for equivalent strength.
Blind tapped holes have limited engagement depth and require careful depth control. Through holes simplify threading.
Pre-Quote DFM Checklist
- ☐All walls ≥1.5mm thick (aluminium) or ≥2.0mm (steel)
- ☐Internal pocket corner radii ≥R3.0mm where possible
- ☐Hole depths ≤4× diameter (standard drills); flag any deeper holes
- ☐Tight tolerances (≤±0.02mm) called out only on functionally critical dimensions
- ☐All threads specified with standard, pitch, class, and depth
- ☐No undercuts unless explicitly planned and priced for
- ☐Cavity depth:width ratio ≤4:1
- ☐Surfaces requiring different finish called out separately
- ☐Material specified to grade level (e.g., "6061-T6", not "aluminium")
- ☐2D drawing included with GD&T if applicable
Rapid Manufacturing's Free DFM Analysis
Every quote submission to Rapid Manufacturing includes a free DFM review by our engineering team. We check:
- →Wall thickness adequacy for the specified material
- →Internal corner radii and tool accessibility
- →Hole depth, threading specifications, and tap depths
- →Tolerance appropriateness — flag over-constrained dimensions
- →Undercuts and fixturing considerations
- →Surface finish specification completeness
- →Material suitability for the specified application
DFM recommendations are included in your quote at no charge. You're free to accept or decline any suggested modifications.
Get a Free DFM Review with Your Quote
Upload your STEP file to Rapid Manufacturing. Our engineering team reviews your design for manufacturability and returns pricing plus DFM feedback within 2 business days.
Submit for DFM ReviewFrequently Asked Questions
What is DFM (Design for Manufacturability) in CNC machining?
DFM is the practice of reviewing a design before manufacturing to identify features that increase cost, cycle time, or quality risk — and modifying them where possible. In CNC machining, this includes checking wall thickness, corner radii, hole depth ratios, tolerance callouts, undercuts, and fixturing considerations. A good DFM review reduces your quote cost and manufacturing risk before any metal is cut.
What is the minimum wall thickness for CNC machined aluminium?
For aluminium, the minimum recommended wall thickness is 0.8mm for very small parts, but 1.5mm or more is preferred for structural rigidity and to prevent vibration during machining (chatter). For steel and stainless, 1.0mm minimum but 2.0mm preferred. Walls thinner than 0.5mm in any metal are very difficult to machine without deflection or distortion.
What internal corner radius should I use for CNC machined pockets?
Internal corners in CNC machined pockets must have a radius because rotating cutting tools cannot produce sharp internal corners. The minimum radius is determined by the tool diameter needed to reach that corner. As a DFM guideline: use the largest radius your design allows. R3mm is a good default for most pockets. Larger radii (R6, R8) allow larger tools, which machine faster and cheaper. Very small radii (R0.5–R1.0) require tiny tools that machine slowly and break more often.
What is the maximum hole depth for CNC drilling?
Standard drill bits machine reliably to a depth of 3× the hole diameter (3D depth). Extended drills can reach 5–8D, and specialist deep hole drills can reach 20–30D, but cost and risk increase significantly beyond 5D. For blind holes, try to keep depth ≤4× diameter. For deep holes, consider using through-holes where geometry allows, or specify gun drilling for depths beyond 10D.
Should I apply tight tolerances to all dimensions on my drawing?
No. Applying tight tolerances (±0.01–0.02mm) to every dimension significantly increases cost without functional benefit. Use a general tolerance block (e.g., ±0.1mm unless otherwise specified) for non-critical dimensions, and only callout tight tolerances on functionally critical dimensions — bearing seats, thread engagement surfaces, sealing faces, press fit interfaces. Rapid Manufacturing's free DFM analysis will flag any tolerance callouts that appear excessively tight for the function.
Does Rapid Manufacturing provide free DFM analysis?
Yes. Every quote submission to Rapid Manufacturing includes a free DFM analysis. Our engineering team reviews your design for wall thickness, corner radii, hole depth ratios, undercuts, tolerance appropriateness, and surface finish specifications — and flags anything that may increase cost or manufacturing risk, with practical suggestions for improvement.
What are undercuts in CNC machining and how do I avoid them?
An undercut is a feature that a straight cutting tool cannot reach from above because part of the workpiece is in the way. Examples include T-slots, internal grooves, and recesses behind protruding features. Undercuts require special tooling (T-slot cutters, undercutting end mills), additional setups, or 5-axis machining — all of which add cost. The easiest way to avoid undercuts is to review your design from the machining direction and confirm every feature is accessible by a straight tool.