DFM Guidelines for CNC Machining
Design for Manufacturability rules we use daily to reduce cycle time, avoid tooling issues, and cut costs without affecting function.
I've seen thousands of designs come through our shop. The ones that machine well have one thing in common: the designer understood what a cutting tool can and can't do. Here's what I wish every engineer knew before sending us a STEP file.
Design Parameter Reference Table
| Parameter | Good Design | Problematic | Impact | Recommendation |
|---|---|---|---|---|
| Wall Thickness | > 1mm (metal), > 1.5mm (plastic) | 0.3-0.8mm walls | Vibration, deflection, poor finish | Increase wall thickness or add ribs |
| Internal Corner Radius | > 3mm (16mm end mill) | Sharp internal corners | Requires EDM or smaller tool + slow feed | Design radius = cutter dia / 2 + 0.5mm |
| Hole Depth | < 3× diameter (standard drill) | > 10× diameter | Peck drilling, chip evacuation issues | Use gun drilling or step-drill |
| Pocket Depth | < 4× tool diameter | > 6× tool diameter | Tool deflection, poor surface, long cycle | Design shallow pockets or split into features |
| Thread Depth | 1.5-2× diameter | > 3× diameter | Tap breakage, slow threading | Use thread inserts for deep threads |
| Undercuts | None or minimal | Complex undercuts | Special tooling, 5-axis, or EDM | Design as separate part or use standard tool |
| Tolerance Density | 20% features toleranced | 80%+ features toleranced | Unnecessary inspection cost | Only tolerance critical features |
See: Tolerances Guide · Surface Finishes · GD&T Guide
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Submit Your Design →Geometry rules that cut cost without touching function
Avoid internal corners tighter than the cutter radius — a 90° internal corner forces a small cutter, slow feeds and long cycle time. A 1 mm radius instead of a sharp corner lets us use a larger tool at three times the feed rate. Same strength, same function, a quarter of the machine time.
Undercuts and deep pockets are the other cost driver. Every feature a standard tool cannot reach means special tooling or EDM. If the pocket is 5× the tool diameter deep, we are burning time and risking tool deflection. Design for standard tool lengths and the price drops immediately.
Tolerance and thread rules from the shop floor
Put tolerances where they matter and ISO 2768-m everywhere else. We see drawings with ±0.01 mm on a cosmetic face and ±0.1 mm on the mating surface that actually carries the load — reversed priorities that cost money for no function. Every tight tolerance is machine time and CMM time; make the drawing honest about what matters.
Threads: specify standard threads and standard depths. A blind M3 thread three diameters deep is a broken tap waiting to happen — in stainless it is a recurring nightmare. And if the part is plated or anodized, thread size must account for coating build-up, or the gauge pin will not go.