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.
CNC Turning
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:
Types of Sheet Metal Fabrication Metals
Sheet Metal Fabrication Processes
Sheet Metal Fabrication Tools
Industries and Applications
Types of Sheet Metal Fabrication Metals
Sheet metal can come in a wide variety of types, and fabrication can adapt the

metal to whatever purposes you may need. Types of common metals used in sheet metal

fabrication include:
Steel
There are a multitude of types of steels for all sorts of purposes, such as

stainless steel, carbon steel, and galvanized steel, but the metal as a whole is

known for its durability and strength. Lower-carbon content steel may be found in

railings or fences, while medium carbon content steel is used for cars and

appliances. The highest carbon content sheet metal is frequently found in steel

wires. Stainless steel is used for cookware, medical instruments, and many other

products.
Aluminum
Aluminum is more lightweight while also sharing some of steel's strength.

It's good for lower temperatures, which partially accounts for its use in

aerospace and refrigeration. Aluminum sheet metal is also used for automotive parts,

electrical devices, and cooking vessels.
Magnesium
Magnesium is a structural metal with a very low density, excellent for when

stiffness is needed. It has been used as a structural metal in the transportation

industry since World War II and is used for automotive parts.
How common is mold in buildings?
Molds are very common in buildings and homes. Mold will grow in places with a lot of moisture, such as

around leaks in roofs, windows, or pipes, or where there has been flooding. Mold

grows well on paper products, cardboard, ceiling tiles, and wood products. Mold can

also grow in dust, paints, wallpaper, insulation, drywall, carpet, fabric, and

upholstery.
The most common indoor molds are Cladosporium, Penicillium, and Aspergillus. We

do not have precise information about how often different molds are found in

buildings and homes.
How do molds get in the indoor environment and how do they grow?
Mold is found both indoors and outdoors. Mold can enter your home through open

doorways, windows, vents, and heating and air conditioning systems. Mold in the air

outside can also attach itself to clothing, shoes, and pets can and be carried

indoors. When mold spores drop on places where there is excessive moisture, such as

where leakage may have occurred in roofs, pipes, walls, plant pots, or where there

has been flooding, they will grow. Many building materials provide suitable nutrients

that encourage mold to grow. Wet cellulose materials, including paper and paper

products, cardboard, ceiling tiles, wood, and wood products, are particularly

conducive for the growth of some molds. Other materials such as dust, paints,

wallpaper, insulation materials, drywall, carpet, fabric, and upholstery, commonly

support mold growth.
How do you know if you have a mold problem?
Large mold infestations can usually be seen or smelled.
How do molds affect people?
Exposure to damp and moldy environments may cause a variety of health effects, or

none at all. Some people are sensitive to molds. For these people, exposure to molds

can lead to symptoms such as stuffy nose, wheezing, and red or itchy eyes, or skin.

Some people, such as those with allergies to molds or with asthma, may have more

intense reactions. Severe reactions may occur among workers exposed to large amounts

of molds in occupational settings, such as farmers working around moldy hay. Severe

reactions may include fever and shortness of breath.