CNC Machining

CNC 101: 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.

While each type of manufacturing process has its advantages and disadvantages, this article focuses on the CNC machining process, outlining the basics of the process, and the various components and tooling of the CNC machine. Additionally, this article explores various mechanical CNC machining operations and presents alternatives to the CNC machining process.

At a glance, this guide will cover:

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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 companyComputer-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.

Table 1: Linear Tolerances in CNC Machining

Dimension Range (mm) Fine (F)

+/-

Medium (M)

+/-

Coarse (C)

+/-

Very Coarse (V) +/-
.5-3 .05 .1 .2
3-6 .05 .1 .3 .5
6-30 .1 .2 .5 1.0
30-120 .15 .3 .8 1.5
120-400 .2 .5 1.2 2.5
400-1000 .3 .8 2.0 4.0
1000-2000 .5 1.2 3.0 6.0
2000-4000 2.0 4.0 8.0

 

Table 2: Angle Tolerances in CNC Machining

Dimension Range (mm) Fine (F)

+/-

Medium (M)

+/-

Coarse (C)

+/-

Very Coarse (V) +/-
0-10 1o 1o 1o 30’ 3o
10-50 0 o 30’ 0 o 30’ 1o 2o
50-120 0 o 20’ 0 o 20’ 0 o 30’ 1o
120-400 0 o 10’ 0 o 10’ 0 o 15’ 0 o 30’
400 0 o 5’ 0 o 5’ 0 o 10’ 0 o 20’

 

Table 3: Radius and Chamfer Tolerances in CNC Machining

Dimension Range (mm) Fine (F)

+/-

Medium (M)

+/-

Coarse (C)

+/-

Very Coarse (V) +/-
.5-3 .2 .2 .4 .4
3-6 .5 .5 1 1
6 1 1 2 2

 

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.

Machine Setup

Before the operator runs the CNC program, they must prepare the CNC machine for operation. These preparations include affixing the workpiece directly into the machine, onto machinery spindles, or into machine vises or similar workholding devices, and attaching the required tooling, such as drill bits and end mills, to the proper machine components.

Once the machine is fully set up, the operator can run the CNC program.

Machining Operation Execution

The CNC program acts as instructions for the CNC machine; it submits machine commands dictating the tooling’s actions and movements to the machine’s integrated computer, which operates and manipulates the machine tooling. Initiating the program prompts the CNC machine to begin the CNC machining process, and the program guides the machine throughout the process as it executes the necessary machine operations to produce a custom-designed part or product.

CNC machining processes can be performed in-house—if the company invests in obtaining and maintaining their own CNC equipment—or out-sourced to dedicated CNC machining service providers.

Types of CNC Machining Operations

CNC machining is a manufacturing process suitable for a wide variety of industries, including automotive, aerospace, construction, and agriculture, and able to produce a range of products, such as automobile frames, surgical equipment, airplane engines, gears, and hand and garden tools. The process encompasses several different computer-controlled machining operations—including mechanical, chemical, electrical, and thermal processes—which remove the necessary material from the workpiece to produce a custom-designed part or product. While chemical, electrical, and thermal machining processes are covered in a later section, this section explores some of the most common mechanical CNC machining operations including:

  • Drilling
  • Milling
  • Turning

CNC Drilling

Drilling is a machining process which employs multi-point drill bits to produce cylindrical holes in the workpiece. In CNC drilling, typically the CNC machine feeds the rotating drill bit perpendicularly to the plane of the workpiece’s surface, which produces vertically-aligned holes with diameters equal to the diameter of the drill bit employed for the drilling operation. However, angular drilling operations can also be performed through the use of specialized machine configurations and workholding devices. Operational capabilities of the drilling process include counterboring, countersinking, reaming, and tapping.

CNC Milling

Milling is a machining process which employs rotating multi-point cutting tools to remove material from the workpiece. In CNC milling, the CNC machine typically feeds the workpiece to the cutting tool in the same direction as the cutting tool’s rotation, whereas in manual milling the machine feeds the workpiece in the opposite direction to the cutting tool’s rotation. Operational capabilities of the milling process include face milling—cutting shallow, flat surfaces and flat-bottomed cavities into the workpiece—and peripheral milling—cutting deep cavities, such as slots and threads, into the workpiece.

CNC Turning

CNC Turning and Multi Spindle Machining
CNC Turning and Multi-Spindle Machining

Image Credit: Buell Automatics

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.

Table 1 – Characteristics of Common CNC Machining Operations

Note: Some CNC machining operation information provided courtesy of Metal Craft.

Machining Operation

Characteristics

Drilling

  • Employs rotating multi-point drill bits
  • Drill bit fed perpendicular or angularly to workpiece
  • Produces cylindrical holes in workpiece

Milling

  • Employs rotating multi-point cutting tools
  • Workpiece fed in same direction as cutting tool rotation
  • Removes material from workpiece
  • Produces broader range of shapes

Turning

  • Employs single-point cutting tools
  • Rotates workpiece
  • Cutting tool fed along the surface of the workpiece
  • Removes material from the workpiece
  • Produces round or cylindrical 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.

How Does a 5 Axis CNC Machine Work?

5 axis CNC machining describes a numerically-controlled computerized manufacturing system that adds to the traditional machine tool’s 3-axis linear motions (X, Y, Z) two rotational axes to provide the machine tool access to five out of six part sides in a single operation. By adding a tilting, rotating work holding fixture (or trunnion) to the work table, the mill becomes what is called a 3+2, or an indexed or positional, machine, enabling the milling cutter to approach five out of six sides of a prismatic workpiece at 90° without an operator having to reset the workpiece.

It is not quite a 5-axis mill, however, because the fourth and fifth axes do not move during machining operations. Adding servomotors to the additional axes, plus the computerized control for them – the CNC part –would make it one. Such a machine- which is capable of full simultaneous contouring- is sometimes called a “continuous” or “simultaneous” 5-axis CNC mill.  The two additional axes can also be incorporated at the machining head, or split – one axis on the table and one on the head.

CNC Lathe Operator Training

To handle a CNC lathe, a machinist should have completed a set amount of coursework and earned appropriate certification from an accredited industrial training organization. CNC turning machining training programs will usually involve multiple classes or sessions, offering a gradual instruction process broken up into several steps. The importance of adhering to safety protocols is reinforced throughout the training process.

Beginning CNC lathe classes might not include hands-on experience, but they may include familiarizing students with the command codes, translating CAD files, tool selection, cutting sequences, and other areas. A beginner CNC lathe course may include:

  • Lubrication and scheduling lathe maintenance
  • Translating instructions into a machine-readable format and loading them into the lathe
  • Establishing criteria for tool selection
  • Installing tools and parts for handling the material
  • Producing sample parts

Later CNC lathe training typically involves actual lathe operation, as well as machine adjustments, program editing, and the development of new command syntax. This type of lathe machine training can include courses on:

  • Figuring out where edits are needed from comparing sample parts to their specifications
  • CNC programming edits
  • Creating multiple cycles of test components to refine the results of edits
  • Regulating coolant flow, cleaning the lathe, and repair and replacement of tools

Other CNC Machining Operations

Other mechanical CNC machining operations include:

CNC Machining Equipment and Components

As indicated above, there is a wide range of machining operations available. Depending on the machining operation being performed, the CNC machining process employs a variety of software applications, machines, and machine tools to produce the desired shape or design.

Types of CNC Machining Support Software

The CNC machining process employs software applications to ensure the optimization, precision, and accuracy of the custom-designed part or product. Software applications used include:

  • CAD
  • CAM
  • CAE

CADComputer-aided design (CAD) software are programs used to draft and produce 2D vector or 3D solid part and surface renderings, as well as the necessary technical documentation and specifications associated with the part. The designs and models generated in a CAD program are typically used by a CAM program to create the necessary machine program to produce the part via a CNC machining method. CAD software can also be used to determine and define optimal part properties, evaluate and verify part designs, simulate products without a prototype, and provide design data to manufacturers and job shops.

CAMComputer-aided manufacturing (CAM) software are programs used extract the technical information from the CAD model and generate machine program necessary to run the CNC machine and manipulate the tooling to produce the custom-designed part. CAM software enables the CNC machine to run without operator assistance and can help automate finished product evaluation.

CAEComputer-aided engineering (CAE) software are programs used by engineers during the pre-processing, analysis, and post-processing phases of the development process. CAE software is used as assistive support tools in engineering analysis applications, such as design, simulation, planning, manufacturing, diagnosis, and repair, to help with evaluating and modifying product design. Types of CAE software available include finite element analysis (FEA), computational fluid dynamics (CFD), and multibody dynamics (MDB) software.

Some software applications have combined all of the aspects of CAD, CAM, and CAE software. This integrated program, typically referred to as CAD/CAM/CAE software, allows a single software program to manage the entire fabrication process from design to analysis to production.

What is a CNC Machine? Types of CNC Machines and Machine Tools

Depending on the machining operation being performed, the CNC machining process employs a variety of CNC machines and machine tools to produce the custom-designed part or product. While the equipment may vary in other ways from operation to operation and application to application, the integration of computer numerical control components and software (as outlined above) remains consistent across all CNC machining equipment and processes.

CNC Drilling Equipment

Drilling employs rotating drill bits to produce the cylindrical holes in the workpiece. The design of the drill bit allows for the waste metal—i.e., chips—to fall away from the workpiece. There are several types of drill bits, each of which is used for a specific application. Types of drill bits available include spotting drills (for producing shallow or pilot holes), peck drills (for reducing the amount of chips on the workpiece), screw machine drills (for producing holes without a pilot hole), and chucking reamers (for enlarging previously produced holes).

Typically the CNC drilling process also utilizes CNC-enabled drill presses, which are specifically designed to perform the drilling operation. However, the operation can also be performed by turningtapping, or milling machines.

CNC Milling Equipment

Milling employs rotating multi-point cutting tools to shape the workpiece. Milling tools are either horizontally or vertically oriented and include end millshelical mills, and chamfer mills.

The CNC milling process also utilizes CNC-enabled milling machinery, referred to as mill machines or mills, which can be horizontally or vertically oriented. Basic mills are capable of three-axis movements, with more advanced models accommodating additional axes. The types of mills available include hand milling, plain milling, universal milling, and omniversal milling machines.

CNC Turning Equipment

Turning employs single-point cutting tools to remove material from the rotating workpiece. The design of the turning tool varies based on the particular application, with tools available for roughing, finishing, facing, threading, forming, undercutting, parting, and grooving applications.

The CNC turning process also utilizes CNC-enabled lathes or turning machines. The types of lathes available include turret lathesengine lathes, and special-purpose lathes.

What is a Desktop CNC Machine?

Companies that specialize in manufacturing CNC machines often offer a desktop series of smaller, lightweight machines. Desktop CNC machines, although slower and less precise, handle soft materials well, such as plastic and foam. They’re also better for smaller parts and light to moderate production. Machines featured in a tabletop series resemble the larger industry standard, but their size and weight make them better suited to small applications. A desktop CNC lathe, for example, that features two axes and can handle parts up to six inches in diameter, would be useful for jewelry and mold-making. Other common desk CNC machines include plotter-sized laser cutters and milling machines.

With smaller lathes, it’s important to differentiate between a benchtop CNC lathe machine and a desktop lathe. Benchtop CNC lathes are generally more affordable, but also smaller and somewhat limited in the applications they can handle. A standard CNC benchtop lathe generally includes the motion controller, cables, and basic software. A standard CNC desktop lathe, with a similar basic package, costs slightly more.

CNC Machining Materials

The CNC machining process is suitable for a variety of engineering materials, including:

  • Metal (e.g., aluminum, brass, stainless steel, alloy steel, etc.)
  • Plastic (e.g., PEEK, PTFE, nylon, etc.)
  • Wood
  • Foam
  • Composites

The optimal material for selection to apply to a CNC manufacturing application is largely dependent on the particular manufacturing application and its specifications. Most materials can be machined provided that they can withstand the machining process—i.e., have sufficient hardness, tensile strength, shear strength, and chemical and temperature resistance.

The workpiece material and its physical properties are used to determine the optimal cutting speed, cutting feed rate, and depth of cut. Measured in surface feet per minute, the cutting speed refers to how fast the machine tool cuts into or removes material from the workpiece. The feed rate—measured in inches per minute—is a measure of how fast the workpiece is fed towards the machine tool, and the cut depth is how deep the cutting tool cuts into the workpiece. Typically, the workpiece will first undergo an initial phase in which it is roughly machined to the approximate, custom-designed shape and dimensions, and then undertake a finishing phase in which it experiences slower feed rates and shallower cut depths to achieve its more precise and accurate specifications.

CNC Size Considerations

The wide range of capabilities and operations offered by the CNC machining process help it to find application in a variety of industries, including automotive, aerospace, construction, and agriculture, and enable it to produce a range of products, such as hydraulic components, screws, and shafts. Despite the versatility and customizability of the process, the manufacturing of some parts—e.g., large or heavy components—present greater challenges than others. Table 1, below, outlines some of the challenges of machining large parts and heavy components.

Table 2 – Challenges of Machining by Part Size

Note: Some of the large part machining and heavy component machining challenges are provided by Technox Machine & Manufacturing Inc.

Part Size

Challenges of Machining

Large Part

  • Requires specialized equipment for positioning and processing
  • Requires operator training for specialized equipment
  • More complex machine set-up
  • May be too large for work area
  • Amplification of factors affecting accuracy
  • Larger amounts of heat generated during the process
  • Greater chance of stress-related distortion

Heavy Component

  • Requires specialized tooling and equipment for handling and processing
  • Requires operator training for specialized equipment
  • May be too heavy for work area
  • Greater amount of stress on the equipment

Alternatives to Using a CNC Machine

Although CNC machining demonstrates advantages over other manufacturing processes, it may not be appropriate for every manufacturing application, and other processes may prove more suitable and cost-effective. While this article focuses on the mechanical CNC machining processes which employ machine tools to produce the custom-designed part or product, CNC controls can be integrated into a variety of machines. Other mechanical CNC machining processes include ultrasonic machining, waterjet cutting, and abrasive jet machining.

Besides mechanical processes, chemical, electrochemical, and thermal machining processes are also available. Chemical machining processes include chemical milling, blanking, and engraving; electrochemical machining processes include electrochemical deburring and grinding; and thermal machining processes include electron beam machining, laser cuttingplasma arc cutting, and electrical discharge machining (EDM).

History of CNC Machining (Video)

Summary

Outlined above are the basics of the CNC machining process, various CNC machining operations and their required equipment, and some of the considerations that may be taken into account by manufacturers and machine shops when deciding whether CNC machining is the most optimal solution for their particular manufacturing application.

To find more information on domestic commercial and industrial suppliers of custom manufacturing services and equipment, visit the Thomas Supplier Discovery Platform, where you will find information on over 500,000 commercial and industrial suppliers.

Sources

  1. Non Traditional Machining and Thermal Cutting Processes
  2. CNC Plastic Machining
  3. Industries Making Use of High-Tolerance CNC Machining
  4. CNC Turning and Milling Services
  5. Swiss Screw Turning vs. Traditional CNC Machining
  6. 7 Reasons Why CNC Machining Wins Over Conventional Machining
  7. As the World (CNC) Turns – The Evolution of CNC Turning
  8. Types and Advantages of CNC Machining
  9. CNC Prototyping, The Way It Is Today
  10. Benefits of Contracting CNC Machining
  11. All About CNC Machining
  12. Aluminum Machined Components
  13. Firearms Parts Component Machining
  14. Tips for a Great Metal Spinning Design
  15. The Importance of Driveline Parts and Components for Off-Road Vehicles
  16. What is Swiss CNC Machining?
  17. What You Need to Know About Swiss Turning
  18. Aircraft & Aerospace at Ardel Engineering
  19. Communications at Ardel Engineering
  20. CNC Turning and Milling Services at Helander
  21. CNC Plastic Machining
  22. Milling Machines Vs. Lathe Machines
  23. CNC Swiss Machining
  24. What is CNC Machining?
  25. Guide to CNC Machining Tolerances
  26. How to Specify General Tolerances in Machined Parts

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