Design for Manufacturing (DFM) is the practice of designing a part so it can be made well, reliably, and economically — without sacrificing what the part needs to do. The vast majority of a part's cost is locked in at the design stage, long before a chip is cut. A few geometry decisions can cut machining time in half or double it. This guide collects the DFM rules that matter most for CNC machined parts and shows how a DFM review pays for itself.
Why DFM matters
Machining cost is driven by time, and time is driven by geometry. Features that force long, slim tooling, tiny internal radii, multiple setups, or unnecessary tolerances all add cutting time and risk. The good news is that small design changes — often invisible to part function — can dramatically reduce cost. Catching them early is the entire point of design for manufacturing, and it is where a licensed engineering firm adds the most value; see engineering plus machining.
Internal radii & corners
A milling cutter is round, so it physically cannot cut a sharp internal corner — there is always a radius equal to the tool radius. Designing internal corners with a generous radius lets the shop use a larger, more rigid tool that cuts faster and leaves a better finish. Calling out tiny internal radii forces small tools, slow feeds, and more passes.
- Use the largest internal radius the design allows.
- Avoid sharp internal corners; specify a radius rather than a square corner.
- If a square corner is functionally required, consider a relief or dogbone.
Wall thickness & deep pockets
Thin walls flex under cutting forces, chatter, and are hard to hold to tolerance; very deep, narrow pockets require long tools that deflect. Keeping walls reasonably thick and limiting pocket depth relative to width keeps the part rigid and the tooling short.
- Avoid unnecessarily thin walls; they vibrate and distort.
- Limit pocket depth relative to width so tools stay short and rigid.
- Add fillets at pocket floors where function permits.
Holes & threads
Holes are cheapest when they use standard drill sizes and standard depths. Deep holes, flat-bottom holes, and threads taken to the very bottom of a blind hole all add cost and risk. Use standard thread sizes, and avoid specifying threads deeper than about 1.5 to 2 times the diameter — engagement beyond that adds little strength and a lot of cost.
- Prefer standard drill and tap sizes.
- Limit thread depth to roughly 1.5–2× the diameter.
- Avoid flat-bottom holes unless the function truly requires them.
- Allow a chamfer or relief at thread starts.
Tolerances & datums
Tolerance is the most commonly over-specified attribute on a drawing, and one of the most expensive. Default the drawing to a sensible general tolerance and tighten only the features that mate, locate, or seal. Use clear datums and GD&T so the shop knows exactly what matters. The tolerance guide covers this in depth.
Finishing & cosmetics
Decide early whether the part needs a surface treatment such as anodize, passivation, or plating, because it can affect dimensions and datum strategy. Specify cosmetic requirements only where they are visible or functional — a blanket high-finish callout on every face is needlessly expensive.
Get a DFM review
The single most cost-effective step you can take is a manufacturability review before you release the drawing. We will flag tolerances that are tighter than they need to be, features that fight the toolpath, and material choices worth a second look — feedback that routinely saves more than it costs. This is especially valuable when you are starting from a scan or sketch.
Send your model for a free design review, or request a quote and you can ask us to include manufacturability feedback with your pricing.