The Bracket That Cost 3x to Machine
We designed a bracket for a sensor. The part had a deep pocket (50 mm deep, 10 mm wide) with a 0.5 mm internal fillet. On the drawing, it looked fine. The machine shop quoted 3x the expected price. The reason: the deep narrow pocket needed a long, thin end mill (which deflects and breaks). The 0.5 mm internal fillet needed a tiny ball end mill (slow, fragile). We redesigned: opened the pocket width, used a 3 mm internal fillet (standard end mill), and added a through-hole (avoided the deep pocket). The price dropped by 60%. The mistake was designing for the assembly, not for the machine shop.
Design for Manufacturing (DFM) designs parts that are easy and cheap to machine. This article covers the rules.
The DFM Mindset
DFM asks: “can the machine shop make this efficiently?” A part that looks good in CAD can be expensive to machine. Every feature (pocket, hole, fillet, wall) has a cost. Design for the process (milling, turning, bending), not just the function.
Step 1: Internal Fillets = End Mill Radius
An internal corner (a pocket’s inside corner) has a radius. The end mill that cuts it has a radius (its half-diameter). A 0.5 mm internal fillet needs a 1 mm diameter end mill (which is long and thin — it deflects and breaks). Use a standard end mill radius (R3 or R5, for a 6 mm or 10 mm end mill).
R_internal = end mill radius (half the tool diameter)
For a 6 mm end mill, R_internal = 3 mm. For a 10 mm end mill, R_internal = 5 mm. Don’t draw R0.5 unless you have to (and expect to pay for it).
The DFM fillet rule: Internal corners use a standard end mill radius (R3–R5). The bracket that cost 3x had R0.5 pockets (tiny end mills). Use R3 (standard 6 mm end mill). External corners can have any radius (the tool cuts them easily). Internal corners are the expensive ones.
Step 2: Wall Thickness and Depth
A thin wall (1 mm) in a deep pocket deflects (the cutting forces push it). It’s hard to hold tolerance. Use walls over 2 mm (for aluminum) or 3 mm (for steel). A deep pocket (depth > 3× the width) needs a long tool (deflects). Design the pocket shallow (or open the top).
| Feature | Expensive Design | Cheap Design |
|---|---|---|
| Internal fillet | R0.5 (tiny tool) | R3 (standard tool) |
| Deep narrow pocket | 50 mm deep, 10 mm wide | Shallow or open top |
| Thin wall | 1 mm wall | ≥2 mm (aluminum), ≥3 mm (steel) |
| Hole depth | 10× diameter deep | ≤3× diameter (standard drill) |
| Undercut | Internal groove | Avoid (needs special tool) |
Step 3: Hole Sizes (Standard Drills/Taps)
Use standard drill sizes and tap threads. A non-standard hole (e.g., 5.3 mm) needs an end mill (slow). Use a standard drill (5.0 mm, 5.5 mm, 6.0 mm). For threads, use standard M sizes (M3, M4, M5, M6). A custom thread (M4.5) needs a special tap (expensive).
For a blind hole (not through), drill deeper than the thread depth (the drill’s point adds extra depth). Design the hole deep enough.
Step 4: Tight Tolerances
Every tight tolerance costs (the shop must measure, use precise tools). A ±0.05 mm feature costs more than ±0.2 mm. Spec tight tolerances only where they matter (a locating pin, a sealing surface). Loosen the rest (±0.2–0.5 mm is fine for non-critical). Don’t put ±0.05 on every dimension (that’s how brackets get expensive).
Step 5: Ask the Shop
Before releasing the drawing, run it by the machine shop (or your supplier). They know what’s expensive. “Can you suggest a cheaper way to machine this pocket?” often saves 30–50%. The designer who never talks to the shop pays for it (in quote price).
A DFM Checklist
- Do internal corners use R3–R5? (Standard tool?)
- Are walls ≥2 mm (aluminum)?
- Are pockets shallow? (Depth < 3× width?)
- Are holes standard sizes? (Standard drill?)
- Are threads standard M sizes?
- Are tight tolerances only on critical features?
- Are undercuts avoided? (Or tool access?)
- Is the part fixturable? (Can it be held?)
- Did you ask the machine shop? (Quote feedback?)
- Is the part symmetric? (Reduces setups?)
The Bottom Line
Design for Manufacturing (DFM) designs parts the shop can make cheaply. The bracket that cost 3x had R0.5 pockets and thin walls. Use R3 internal fillets, standard holes and threads, walls over 2 mm, and tight tolerances only where needed. Ask the shop. The part that machined cheaply wasn’t the simplest one — it was designed for the process.
