Machined parts

 Tolerances

Tolerances let the manufacturer know how closely a part’s dimensions must match those specified in the design or technical drawing. Tighter tolerances mean the dimensions must be almost exact, while looser tolerances indicate that speed and cost are more important than micrometer-level accuracy.

Most CNC machines can achieve fairly tight tolerances (± 0.025 mm is standard), but ultra-tight tolerances require more labour and are therefore more expensive.

Remember that not all part dimensions demand the tightest tolerances. If a part does not form part of a mechanical system, for example, a matter of micrometers may make little difference to the part’s function. Prototypes, too, can often be CNC machined with fairly loose tolerances, which can then be tightened up for the final part if necessary.

Cavities

Many CNC machined parts incorporate cavities for mechanical or fitting reasons, or to reduce mass. However, the dimensions of the machine’s cutting tool determine how deep these cavities can go.

In general, a machined cavity should be no deeper than four times the width of the cavity. This helps to avoid tool deflection and prevents chips getting stuck in the cavity and causing problems.

Wall thickness

Although CNC machines can make very fine cuts in the workpiece, machining very thin walls can result in a lack of stability. When machining plastic, very thin walls can easily deform, so it is often best to keep walls a reasonable thickness.

How thin these walls can go ultimately depends on the machine and material used. Metals are more resilient, and can — at a stretch — go as thin half a millimeter. Walls machined from plastics, on the other hand, should be around twice that thickness at a minimum.

Threads

CNC machined parts often have threads for screws or other fasteners, so that multiple parts may be attached together. Unlike 3D printed parts, for example, machined parts are highly amenable to threads, since there are cutting tools made specifically for them.

These thread mills can create a highly accurate thread in either plastic or metal, but are not the only way to create threads. Other tools for creating threads include cut taps and form taps.

As with holes and cavities, threads made using taps should only go so deep into the machined part.

Holes

Machined parts can incorporate round holes, which are created using a drill bit or end mill. There are limits to the depths of these holes: more than ten times the diameter becomes more difficult and may contribute to a higher cost.

In general, end mills create holes with a better surface finish and offer greater flexibility in terms of hole size. They are, however, limited in length.

Text

Adding text to a machined part is a surefire way of making it identifiable or brand-specific. It is also a surefire way of increasing cost, and is therefore considered something of a luxury for functional parts.

Recessed text is faster and cheaper to write than raised text, and each character should be at least half a millimeter apart to avoid deformation. Fonts should always be sans serif, without unnecessary embellishments. (Remember: text does not need to be added using machine tools; it can be painted or added to a sticker.)