Overview of CNC Machining Process
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.
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 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.
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.
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.
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, like die-casting 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
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: Computer-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.
CAM: Computer-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, such as stamping parts, custom plastic parts, etc. CAM software enables the CNC machine to run without operator assistance and can help automate finished product evaluation.
CAE: Computer-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 custom CNC precision machining parts
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 turning, tapping, 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 mills, helical 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 lathes, engine 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.
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