Evolving from the numerical control (NC) machining process which utilized punched tape cards, CNC machiningis a manufacturing process which utilizes computerized controls to operate and manipulate machine and cutting tools to shape stock material—e.g., metal, plastic, wood, foam, composite, etc.—into custom parts and designs. While the CNC machining process offers various capabilities and operations, the fundamental principles of the process remain largely the same throughout all of them. The basic CNC machining process includes the following stages:
Designing the CAD model
Converting the CAD file to a CNC program
Preparing the CNC machine
Executing the machining operation
CAD Model Design
The CNC machining process begins with the creation of a 2D vector or 3D solid part CAD design either in-house or by a CAD/CAM design service company. Computer-aided design (CAD) software allows designers and manufacturers to produce a model or rendering of their parts and products along with the necessary technical specifications, such as dimensions and geometries, for producing the part or product.
Designs for CNC machined parts are restricted by the capabilities (or inabilities) of the CNC machine and tooling. For example, most custom CNC machine parts tooling is cylindrical therefore the part geometries possible via the CNC machining process are limited as the tooling creates curved corner sections. Additionally, the properties of the material being machined, tooling design, and workholding capabilities of the machine further restrict the design possibilities, such as the minimum part thicknesses, maximum part size, and inclusion and complexity of internal cavities and features.
Once the CAD design is completed, the designer exports it to a CNC-compatible file format, such as STEP or IGES.
CNC Machining Tolerances Tables
When specifying parts to a machine shop, it's important to include any necessary tolerances. Though CNC machines are very accurate, they still leave some slight variation between duplicates of the same part, generally around + or - .005 in (.127 mm), which is roughly twice the width of a human hair. To save on costs, buyers should only specify tolerances in areas of the part that will need to be especially accurate because they will come into contact with other parts. While there are standard tolerances for different levels of machining (as shown in the tables below), not all tolerances are equal.
CAD File Conversion
The formatted CAD design file runs through a program, typically computer-aided manufacturing (CAM) software, to extract the part geometry and generates the digital programming code which will control the CNC machine and manipulate the tooling to produce the custom-designed part.
CNC machines used several programming languages, including G-code and M-code. The most well-known of the CNC programming languages, general or geometric code, referred to as G-code, controls when, where, and how the machine tools move—e.g., when to turn on or off, how fast to travel to a particular location, what paths to take, etc.—across the workpiece. Miscellaneous function code, referred to as M-code, controls the auxiliary functions of the machine, such as automating the removal and replacement of the machine cover at the start and end of production, respectively.
Once the CNC program is generated, the operator loads it to the CNC machine.
