Tool geometry

Most CNC machining cutting tools have a cylindrical shape with a flat or spherical end, restricting the part geometries that can be produced.

For instance, the internal vertical corners of a CNC part will always have a radius, no matter how small a cutting tool is used.

Tool access

Surfaces that cannot be reached by the cutting tool, cannot be CNC machined.

This prohibits, for example, the fabrication of parts with internal 'hidden' geometries, and puts a limit to the maximum depth of an undercut.

Workpiece stiffness

Due to the cutting forces and the temperatures developed during machining, it is possible for the workpiece to deform or vibrate.

This limits, for example, the minimum wall thickness that a CNC machined part can have and the maximum aspect ratio of tall features.

Tool stiffness

Like the workpiece, the cutting tool can also deflect or vibrate during machining. This results in looser tolerances and even tool breakage.

The effect becomes more prominent when the ratio of length-to-diameter of the cutting tool increases and is the reason why

deep cavities cannot be easily CNC machined.

Workholding

The geometry of a part determines the way it will be held on the CNC machine and the number of setups required. This has an impact on the cost, but also the accuracy of a part.

For example, manual repositioning introduces a small, but not negligible, positional error. This a key benefit of 5-axis versus 3-axis CNC machining.

Cavities & pockets

Recommended depth: 4 x cavity width

Feasible depth: 10 x tool diameter or 25 cm (10'')

Deeper cavities need to be machined with cutting tools with larger diameter affecting the fillets of the internal edges.

Internal edges Recommended: larger than ⅓ x cavity depth

For internal vertical edges, the larger the fillet the better.

Edges on the floor of a cavity should be either sharp or have a 0.1 mm or 1 mm radius.

Minimum wall thickness

Recommended: 0.8 mm (for metals)

Feasible: 0.5 mm

Recommended: 1.5 mm (for plastics)

Feasible: 1.0 mm

Decreasing the wall thickness reduces the stiffness of the workpiece, increasing vibrations and lowering the achievable tolerances.

Plastics are especially prone to warping and thermal softening, so a larger minimum wall thickness is necessary.

Holes Recommended diameter: standard drill bit sizes

Recommended depth: 4 x nominal diameter

Max. depth: 10 x nominal diameter

Holes with standard diameter are preferred, as they can be machined with a standard drill bit. Blind holes machined with a drill will have a conical floor.

Holes with non-standard diameter will be machined with an end mill tool and should be treated as cavities (see previous rule). Blind holes machined with an end mill tool will be flat

hreads Recommended length: 3 x nominal diameter

Recommended size: M6 or larger

Feasible size: M2

Choose the largest thread possible, as they are easier to machine. Threads longer than 3 times the nominal diameter are unnecessary.

Always design threads as cosmetic in your CAD package and include a technical drawing to your order.

Tall features

Recommended max. ratio: height / width < 4

Tall features are difficult to machine accurately, as they are prone to vibrations. Consider the overall geometry of the part: rotating the part by 90° degrees during machining changes the aspect ratio.

Small features Recommended: 2.5 mm (0.100'')

Feasible: 0.100 mm (.010'')

Cavities and holes down to 2.5 mm (0.1'') can be CNC machined with standard cutting tools. Anything below this limit is considered micro-machining and must be avoided unless necessary.