10 DFM Tips for 5-Axis CNC Machining That Will Cut Your Cost by 30%

Written by the engineering team at Robocon CNC Pvt Ltd, Pune, India. We provide free DFM review on every quote.

5-axis CNC machining can produce extraordinarily complex parts — but design choices that look harmless in CAD can multiply your machining cost significantly. These are the ten most common issues we flag in DFM reviews, and how to fix them before you go to quote.

1. Minimum Wall Thickness

Thin walls vibrate during machining, causing chatter marks and tolerance failures. Minimum wall thickness depends on material and height:

2. Internal Corner Radii

A sharp internal corner (0 mm radius) is impossible to machine — cutting tools are round. Specifying an internal radius that is too small forces the use of tiny end mills, which are slow and break frequently.

Rule: Internal corner radius should be at least 1/3 of the pocket depth, with a minimum of 1 mm. For deep pockets, increase the radius proportionally. A 5 mm deep pocket should have at least 1.7 mm internal corner radius.

3. Pocket Depth-to-Width Ratio

Deep, narrow pockets require long, slender end mills — which chatter and break. The maximum practical depth-to-width ratio for a pocket is 4:1 (depth no more than 4 times the width).

Fix: If you need a deep narrow feature, consider adding draft angle to the walls, increasing width, or splitting the pocket across two machined faces.

4. Through-Holes vs. Blind Holes

Through-holes are faster and cheaper than blind holes. If a blind hole can be redesigned as a through-hole with no functional loss, do it. Blind threaded holes also require controlled thread depth — a through-tapped hole eliminates that concern.

5. Hole Depth-to-Diameter Ratio

Standard drill bits work reliably up to 3:1 depth-to-diameter ratio (3× the drill diameter in depth). Deeper holes require special tooling and slow pecking cycles.

Rule: Holes deeper than 5× diameter require a note on your drawing. Holes deeper than 10× diameter require gun-drilling — significant cost adder.

6. Undercuts

5-axis machining can access undercuts that 3-axis cannot, but only if the tool can physically reach. Check that your undercut has enough clearance for the tool body, not just the cutting edge. A 2 mm undercut with a 20 mm diameter tool shank won't machine without a special T-slot cutter.

7. Text and Logos

Engraved text is expensive. Embossed text (raised from the surface) is even more expensive — it requires machining away the surrounding material. For identification purposes, consider laser marking after machining as a much cheaper alternative.

If you must engrave: minimum character height 3 mm, minimum line width 0.5 mm, maximum depth 0.5 mm.

8. Number of Setups

5-axis machining shines when it can complete a part in a single setup. Each setup re-introduces fixture location error and adds labour cost. Design your part so that all critical features can be machined from one orientation if possible.

Ask your machinist: "How many setups does this part need?" If the answer is three or more, redesign to consolidate setups — the savings are substantial.

9. Surface Finish on Non-Critical Surfaces

Don't specify Ra 0.4 μm (fine finish) on a surface that only needs to look clean. Ra 1.6 μm or even Ra 3.2 μm as-machined is adequate for most non-sealing, non-bearing surfaces and costs significantly less.

Apply tight surface finish callouts only where the function requires it: sealing grooves, bearing seats, precision fits. Use a general surface finish note in the title block for everything else.

10. Standard vs. Non-Standard Thread Sizes

M3, M4, M5, M6, M8, M10, M12 and UNC/UNF standard sizes are stocked in every machine shop. Non-standard thread pitches (M5×0.5 instead of M5×0.8) require special taps or thread mills — add 15–25% to thread feature cost per hole.

Rule: Only specify non-standard thread pitch if your mating component genuinely requires it.