What Is DFM?

Design for Manufacturing (DFM) means designing parts so they’re easy, fast, and cheap to make — without sacrificing function. The best time to reduce manufacturing cost is at the design stage, not after you’ve sent the drawing to the shop.

These 15 rules apply across CNC machining, sheet metal, welding, and finishing. Violate them and your quote goes up. Follow them and your parts arrive faster, cheaper, and with fewer problems.

Machining Rules

1. Avoid tight tolerances unless functionally necessary

Every tight tolerance adds cost. ±0.005″ is standard. ±0.001″ might double the price on that feature. Ask yourself: does this surface actually mate with something, or is it just a wall?

2. Use standard hole sizes

Drill bits come in standard fractional, letter, and number sizes. A 0.250″ hole is a stock drill. A 0.237″ hole requires a special tool or reaming operation.

3. Avoid deep pockets

Pocket depth > 4× the tool diameter requires long, thin tools that deflect and chatter. If you need a 0.250″ wide slot, keep it under 1″ deep. Deeper = special tooling = more money.

4. Add radii to internal corners

End mills are round. Internal corners can’t be perfectly sharp. Specify a radius ≥ the tool radius + 0.010″ for clearance. A 0.125″ radius is the sweet spot for most features.

5. Minimize setups

Every time the machinist has to flip or re-fixture the part, it adds time and introduces error. Design features accessible from as few directions as possible. A part machinable from two sides costs half as much as one requiring five setups.

Sheet Metal Rules

6. Bend radius ≥ material thickness

Bending tighter than material thickness risks cracking (especially aluminum). For steel, 1× thickness works. For aluminum 6061, use 1.5–2× minimum.

7. Maintain consistent bend direction

Every change in bend direction requires a new setup on the press brake. Parts with all bends in the same plane are cheapest.

8. Keep holes away from bends

Holes too close to a bend line distort during forming. Rule: minimum distance = 2× material thickness + bend radius.

9. Use standard gauges

Stock sheet comes in standard gauges (16 ga, 18 ga, 20 ga, etc.). Specifying 0.055″ thick sheet when 16 ga (0.060″) is standard means special order material.

Welding Rules

10. Design for access

The welder needs to reach the joint with a torch, see the weld pool, and maintain proper angle. Enclosed corners, deep recesses, and tight gaps are expensive to weld and hard to inspect.

11. Don’t over-specify weld size

A 1/4″ fillet weld on 1/8″ plate is overkill — you’re paying for 4× the weld metal needed. Weld size should match the thinner member. A 1/8″ fillet on 1/8″ plate is full-strength.

12. Use weldable materials

Not all materials weld well. 7075 aluminum cracks. Free-machining steels (1215, 12L14) are terrible to weld. 4140 needs preheat. If welding is part of the process, choose a weldable alloy from the start.

General Rules

13. Specify finish only where needed

Calling out Ra 32 μin on every surface adds cost. Only critical surfaces (sealing, bearing, mating) need finish callouts. Let the shop use their standard process elsewhere.

14. Design for standard materials

Starting from a standard bar, plate, or tube size means less waste and easier sourcing. A part designed from 2″ × 2″ bar stock is cheaper than one requiring 2.125″ × 1.875″ — which doesn’t exist as standard stock.

15. Consolidate parts where possible

Two parts welded together = material for two + cutting two + fixturing for welding + welding time + grinding. One part machined from solid = one setup, one material, done. Obviously this depends on geometry and quantity, but always ask: can this be one piece?

The One Rule That Matters Most

Talk to your shop before finalizing the design. A 5-minute conversation about manufacturing can save thousands in production. Every good shop will do a free DFM review if you ask.

Want a DFM review? Send us your drawings — we’ll tell you how to make it better and cheaper.

Need help applying DFM principles to your design? PartSnap is a licensed P.E. firm offering product development, prototyping, and manufacturing transition services in the Dallas / Fort Worth area.