Designing for CNC Machining: A Practical DFM Checklist for Mechanical Designers

Every drawing tells a story about cost, and it is usually written in the corner radii and the wall thicknesses. DFM, design for manufacturability, is not about dumbing down your design. It is about knowing which choices make machining hard, and which ones make it predictable. The machinist is not going to tell you your baby is ugly; the quotation will.

The Rules That Matter Most

Seven choices on a drawing decide most of the machining cost:

• Internal corner radii. A standard end mill is round, so an internal corner can only be as tight as the tool radius. Sharp internal corners mean small tools, slow passes, or EDM. A radius of 0.5–1 mm is cheap; a sharp corner is not.
• Wall thickness. Thin walls deflect and vibrate. The practical floor depends on material and feature size, but a wall that rings like a bell will not hold tolerance.
• Deep pockets. Depth-to-width ratio decides whether a standard tool reaches the bottom. Very deep pockets need long tools, which vibrate, or special processes.
• Hole depth. Standard drills have reach limits. Deep holes need pecking, coolant and patience; very deep holes are a process of their own.
• Threads. Specify thread depth in blind holes with the tap drill in mind. A thread that barely reaches the bottom of a hole is a broken tap waiting to happen.
• Tolerances. Apply tight tolerances only where function needs them. A ±0.01 mm callout on a non-critical cosmetic edge adds inspection cost and scrap risk for nothing.
• Datums. Give the part a datum scheme that makes sense to machine and measure. A part without clear datums gets measured differently by everyone who touches it.

What Ignoring These Costs

Small tools cut slowly and break often; that is machining time and tooling cost. Tight tolerances everywhere mean more inspection and more scrap. Thin walls mean distortion, rework and parts that fail at assembly, not at the machine. None of these problems show up in the CAD model, and all of them show up in the price.

What a Good Drawing Looks Like

From a machinist’s point of view, a good drawing package is:

• A complete 3D model plus a 2D drawing with tolerances; the 3D carries the geometry, the 2D carries the intent.
• Material grade specified, not “aluminum” but 6061-T6 or 7075-T651.
• Finish specified only where it matters: appearance, corrosion, wear, sealing.
• Critical features identified, with GD&T on the ones that must relate to each other.
• A revision number, so “the new version” is not a conversation.

How KGL Reviews Drawings

Drawing evaluation is the second step of the KGL workflow, before any quotation. The engineering team analyzes the structure, checks machining possibility and tolerance matching, and flags risks such as deformation, clamping trouble and out-of-tolerance dimensions. The suggestions come back as a DFM review, and the changes are made in CAD, where they are free.

Conclusion

DFM is a conversation, not a punishment. Send the drawing early, get the feedback, iterate once, and the part gets cheaper, faster and more reliable. The cheapest change you will ever make is the one made before the first chip is cut.

FAQ

Q: What is the minimum wall thickness for CNC machining?

A: There is no single number; it depends on material, wall height and feature size. Aluminum can hold thinner walls than steel or titanium, but a very thin wall will vibrate and distort no matter the material. A practical discussion with the machinist beats a rule of thumb.

Q: What internal corner radius should I specify?

A: Match the radius to the tool you expect to be used. Radii of 0.5 to 1 mm are typical for machined internal corners. Smaller radii force small tools and slow machining; larger radii are nearly free.

Q: Should I send the supplier a 3D model or a 2D drawing?

A: Both. The 3D model carries the geometry and the 2D drawing carries the tolerances, finishes and notes. A package that has both, with a revision number, is the one that gets quoted accurately.