What is CNC Machining?
The term CNC stands for 'computer numerical control', and the CNC machining definition is that it is a subtractive manufacturing process that typically employs computerized controls and machine tools to remove layers of material from a stock piece—known as the blank or workpiece—and produces a custom-designed part. This process is suitable for a wide range of materials, including metals, plastics, wood, glass, foam, and composites, and finds application in a variety of industries, such as large CNC machining, machining of parts and prototypes for telecommunications, and CNC machining aerospace parts, which require tighter tolerances than other industries. Note there is a difference between the CNC machining definition and the CNC machine definition- one is a process and the other is a machine. A CNC machine (sometimes incorrectly referred to as a C and C machine) is a programmable machine that is capable of autonomously performing the operations of CNC machining.
Subtractive manufacturing processes, such as CNC machining, are often presented in contrast to additive manufacturing processes, such as 3D printing, or formative manufacturing processes, such as liquid injection molding. While subtractive processes remove layers of material from the workpiece to produce custom shapes and designs, additive processes assemble layers of material to produce the desired form and formative processes deform and displace stock material into the desired shape. The automated nature of CNC machining enables the production of high precision and high accuracy, simple parts and cost-effectiveness when fulfilling one-off and medium-volume production runs. However, while CNC machining demonstrates certain advantages over other manufacturing processes, the degree of complexity and intricacy attainable for part design and the cost-effectiveness of producing complex parts is limited.
Overview of CNC Machining Process
Evolving from the numerical control (NC) machining process which utilized punched tape cards, CNC machining is 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 CNC machine 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. If, for example, a part absolutely cannot be larger than the measurement, it might have a specified tolerance of +0.0/-0.5 to show it can be slightly smaller, but no larger in that area.
Turning is a machining process which employs single-point cutting tools to remove material from the rotating workpiece. In CNC turning, the machine—typically a CNC lathe machine—feeds the cutting tool in a linear motion along the surface of the rotating workpiece, removing material around the circumference until the desired diameter is achieved, to produce cylindrical parts with external and internal features, such as slots, tapers, and threads. Operational capabilities of the turning process include boring, facing, grooving, and thread cutting. When it comes down to a CNC mill vs. lathe, milling, with its rotating cutting tools, works better for more complex parts. However, lathes, with rotating workpieces and stationary cutting tools, work best for faster, more accurate creation of round parts.
CNC Metal Spinning
Close cousins to lathes, CNC spinning lathe machines involve a lathe set with a blank (a metal sheet or tube) that rotates at high speeds while a metal spinning roller shapes the workpiece into a desired shape. As a “cold” process, CNC metal spinning forms pre-formed metal—the friction of the spinning lathe contacting the roller creates the force necessary to shape the part.
How Does a Swiss Machine Work?
Swiss machining, also known as swiss screw machining, uses a specialized type of lathe that allows the workpiece to move back and forth as well as rotate, to enable closer tolerances and better stability while cutting. Workpieces are cut right next to the bushing holding them instead of farther away. This allows for less stress on the part being made. Swiss machining is best for small parts in large quantities, like watch screws, as well as for applications with critical straightness or concentricity tolerances. You can find out more about this topic in our guide on how swiss screw machines work.
What is Sheet Metal Fabrication?
Sheet metal fabrication is an umbrella term for a group of processes that use sheet metal to build machines and structures. It can be a complex process involving many different professionals. Businesses that specialize in sheet metal fabrication are commonly referred to as fabrication shops, or fab shops for short.
Metal has been hand-hammered into sheets and used for architectural purposes since ancient times. Water-powered rolling mills replaced hammering in the late 17th century. This method of flattening metal used large rotating iron cylinders to press metal pieces into sheets. The metals used were lead, copper, iron, zinc, and later steel. Tin was frequently used to coat iron and steel sheets to prevent rusting. This tin-coated sheet metal was called "tinplate."
Sheet metal became popular in the United States in the 1870s, used for shingle roofing, stamped ornamental ceilings, and exterior fa?ades. With further advances in sheet metal production in the 1890s, the promise of being cheap, durable, easy to install, and fireproof gave the middle-class a significant appetite for sheet metal products.
Although we've already covered types of metal fabrication on a broader level, this article will focus on the materials, processes, and tools related to sheet metal fabrication.
Here's what this sheet metal guide will cover: