In CNC machining, there are numerous machine configurations, conceivable designs, cutting speeds, dimensions, machinable materials. However, not all designs are compatible with all machining processes; likewise, not all features can be machined to whatever dimension, neither can all material types be machined at whatever speed. Combining incompatible designs and speeds or features and dimensions would most likely result in a safety hazard or one form of failure.
As a result of this, standards have been developed to serve as guidelines for machining processes. While some of these standards were derived from a combination of trial and error and experience, others were derived from elaborate experiments. Also, some standards are officially recognised by the International Standard Organisation (ISO) while others are unofficial but known standards that vary slightly from one CNC machining provider to the other.
Design standards are unofficial standards that guide you computer-aided design process when designing for CNC machining. This set of standards mostly has to do with the geometry of parts. It is important to know that these standards are only a recommendation and may vary by CNC machine capability and machining company.
During machining, the resulting vibrations may cause an insufficiently thick wall to break off or distort. This is especially true of low stiffness materials.. The standard minimum wall thickness for walls is 0.794 mm for metals and 1.5 mm for plastic.
Cavities that are too deep cannot be milled efficiently. Either deflection from excess overhang of the tool or tool deflection would occur. In some cases, the tool cannot reach the surface to be machined at all. Cavities should have a minimum depth of 4 x their width. This means that a 10 mm wide cavity should not be more than 40mm deep.
It is recommended that holes be designed according to the existing, standard drill bit sizes. For the depth of the hole, a standard 4 x of the nominal diameter is recommended. Although a maximum depth of 10 x the nominal diameter can be achieved.
For tall features such as walls, an important standard is the ratio of height to thickness (H:L). Features should have a maximum H:L ratio of 4. This means that a 15 mm wide feature must not be taller than 60mm. On the other hand, small features such as holes can be as small as 0.1mm. 2.5 however, is the recommended minimum standard.
The widely used average-sized CNC milling machines available can typically produce parts as large as 400 x 250 x 150 mm while their CNC turning counterparts can produce parts Φ 500 x 1000mm in size. For dimensions up to 2000 x 800 x 1000 mm, very large CNC machined should be used.
Tolerance is a very important aspect of any design. However, it should be limited to critical features, as over tolerancing leads to increased machining time and cost. The tolerances specified in a design should be dependent on the tolerancing capability of the CNC machine to be used. While ± 0.025 mm tolerance is achievable, 0.125 mm is the standard.
ISO standards are official standards recognised by the international standard organisation. There are hundreds of ISO standards that apply to CNC machining. The following are some of the most important of these standards. Others can be found on the official ISO website.
- ISO 230 – This is a 10-part series of standards which determine the geometric accuracy of machines operating under quasi-static or no-load conditions, the accuracy and repeatability of numerically controlled axes, the thermal effects on machine tools, noise emissions, and vibration levels during machining processes.
- ISO 229:1973 – This standard specifies the speeds and feeds of CNC machines.
- ISO 369:2009 – Certain symbols and depictions can be found on the body of CNC machines. These symbols and their corresponding meanings are specified in this standard.