Engineering Support

Guidelines to Design Machining Parts

Good part design for CNC machining automatically results in cheaper, more accurate components. Considering internal radii, machining tolerances, and the capabilities of a milling machine and lathe at the design stage reduces cost, lead time, and the chance of non-conformances.

MACHINED FACES

For each face to be machined, the part must be removed from the machine, repositioned, and re-clamped. This directly increases set-up time and cost. Each new clamping also introduces the possibility of positional deviations between features machined on different surfaces.

Using 4-axis and 5-axis CNC milling machines reduces the number of repositionings needed, which is why 5-axis machining can be more economical for complex parts despite the higher hourly rate.

Design Advice
Reduce the number of machined faces wherever function allows. Consolidate features onto as few surfaces as possible. Where multiple faces must be machined, group features on opposite faces to keep setups to a minimum.

INTERNAL CORNER RADII

The internal corner radius dictates the maximum diameter of end mill that can be used. A sharp 90° corner forces a very small tool, meaning far more passes to clear the same pocket area, directly multiplying cycle time and cost.

Larger radii allow larger tools, fewer passes, and faster feed rates. This is one of the most impactful single design changes to reduce machining cost. See our CNC milling capabilities.

Using 4-axis and 5-axis CNC milling machines reduces the number of repositionings needed, which is why 5-axis machining can be more economical for complex parts despite the higher hourly rate.

Design Advice
Design internal corner radii to be at least ⅓ of the cavity depth. Even r = 2 mm is far better than r = 1. A radius of r = 8–12 mm allows large, efficient tooling on most pockets.

DEPTH OF THE CAVITY

CNC mills have a limited effective cutting depth. The longer a milling tool is relative to its diameter, the less rigid it becomes and the more prone it is to deflection and vibration under cutting loads. This results in a rough surface finish, dimensional inaccuracy, and risk of tool breakage.

Deep, narrow pockets force the use of special extended tooling with very slow feed rates and multiple roughing passes. They are one of the most common DFM issues found when reviewing parts for CNC machining.

Design Advice
Limit cavity depth to a maximum of
4× the tool diameter
If deeper pockets are unavoidable, stepped pockets or EDM machining may be better alternatives. Discuss this with your machining partner at the design stage.

ALIGNMENT OF MACHINED FEATURES

The positions of holes, pockets, and contours directly affect how long the CNC machine spends on rapid (non-cutting) moves between features. Features scattered at arbitrary positions force the machine to traverse long, unpredictable paths before each cut.

Features aligned on a regular grid allow the machine to sweep efficiently row by row. Symmetry also simplifies fixturing and improves dimensional consistency across the part.

Design Advice

Align holes and pockets on common X/Y grid lines wherever function allows. Introduce bilateral symmetry to simplify fixturing. Avoid rotated features unless the part geometry demands them.

DRAWINGS VS 3D FILES

A 3D model defines geometry, it does not define manufacturing intent. Critical information such as thread specifications, surface finish Ra values, general and specific tolerances, material grade, and post-process requirements is almost never embedded in a CAD file in a usable form.

A proper 2D manufacturing drawing alongside the 3D model saves time for estimators and CNC programmers, significantly reduces mistakes, and provides a legal reference document if parts are non-conforming.

Design Advice
Always supply a 2D manufacturing drawing with your CAD model. It must include: all critical dimensions with tolerances, a general tolerance class (e.g. ISO 2768-m), thread callouts (size, pitch, depth, fit class), Ra surface finish values on functional surfaces, material specification, and any post-process notes.

TOLERANCES

Machining cost rises sharply as tolerances tighten. Narrow tolerances require slower feed rates, dedicated fixtures, additional in-process inspection, and may need specialised grinding or lapping operations — all of which multiply cost and lead time.

Modern CNC machinery holds ±0.05–0.1 mm as standard. Most surfaces function perfectly well at this level. Read our full CNC machining tolerances guide for a detailed breakdown.

MATERIALS AND SURFACE TREATMENT

Machinability varies enormously between materials. Aluminium 6061 machines roughly 4–5× faster than stainless steel 316, and the difference in machining cost is equally dramatic. Choosing a material for machinability as well as mechanical performance can significantly reduce unit cost.

Surface treatments add process steps, external vendor lead time, and cost. If a part’s function does not require a coating or finish, specifying one unnecessarily wastes money. Our full materials guide for CNC machining covers the trade-offs between machinability, strength, and corrosion resistance.

Design Advice
Select materials for machinability as well as performance. Verify that your chosen material is compatible with any required surface treatment. If a part does not need a finish, don’t specify one. Working with an experienced manufacturer like Davantech during the design phase ensures the right material choice from the start.