How Does CNC Machining Work?

CNC machining relies on computer programs to create the layout of the process in which the machine tool should function. Since users cannot directly communicate with the machine tools, Computer-Aided Design (CAD) software is used.

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CAD software creates the 2-dimensional and 3-dimensional models for the required CNC machined parts. With this design, the machine knows what the final part looks like.

The computerized controls do the calculation required for removing material, so the workpiece looks like the final part created in the CAD software.

Let us go through the breakdown of various processes that occur during CNC machining.

The Four Stages of CNC Machining

CNC machining occurs in four stages:

Stage 1: Creating the CAD Model

Before CNC machining begins, the 2D or 3D model of the final design is required. This model is created in CAD software. There are many CAD software programs available online, free and paid.

Creating CAD models is not difficult and can easily be learned. However, some complex parts might require more experience with CAD, for which expert designers can be hired.

Stage 2: Converting CAD Model to CNC File

No CNC machine understands CAD language directly. CNC machines only recognize movement based on coordinates. Therefore, the CAD model must be converted to a CNC understandable file called G code.

Many CAD software programs can write the output file directly in G code by using the particular setting before saving the file.

In other cases, converting the CAD design to G code will require dedicated software called Computer Aided Manufacturing (CAM). CAM software is a very functional tool when it comes to the automation of machine processes.

Besides using CAM software, many simple free tools can convert simple CAD designs to G code with the click of a button. However, they don't have the vast suite of features that CAM software offers.

Stage 3: Configuring the CNC Machine

Before starting manufacturing processes, the CNC machine must be set up the right way.

Think of this as configuring the printer before you print something. You need to feed the printer with pages and check specific settings. CNC machines operate similarly.

Before machining begins, there are many setup processes to complete. For instance, you must ensure the workpiece is properly positioned on the machine. The dies must also be set correctly, and other position settings.

Stage 4: machining operation execution

Once the configuration stage is complete, the machine operation can begin. For this, you can execute the program on the display panel of the CNC machine.

Depending on what you design, you might have to go through various program prompts to choose different types of settings and options.

Once the CNC program is executed, the machine keeps going till the end of the program. It only stops if switched off by the operator or in the case of an unexpected error or power disruption.

What are the Different Types of CNC Machining Processes?

A CNC machine is not one specific machine, as it is a group of different types of CNC machines working on various machining processes. Some of the most popular CNC machining operations include:

CNC Milling

CNC milling is one of the most popular types of CNC machining processes. In fact, many professional machine shops often use a CNC machining and CNC milling process interchangeably. Face milling and peripheral milling are two of the most frequently used CNC mill applications.

In a CNC milling machine, rotating cutting tools move relative to the workpiece to remove material.

The cutting tool (also called a milling tool) is fixed on a spindle that can rotate. The rotation and movement of the spindle give CNC milling machines the ability to perform three or more axes milling operations.

CNC Drilling

The CNC drilling process is a lot simpler than using milling tools or the turning process. In CNC drilling, the workpiece is held stationary while a drill bit moves over the workpiece and creates holes.

The purpose of drilling holes might be to add screw bolts, aesthetic requirements, or any other use.

CNC Grinding

CNC grinding machines use a rotating flat abrasive wheel for removing material from rough workpiece surfaces. This machine process is usually applied to create a smooth-finished part. The grinding wheel rotates at a very high speed.

CNC Routing

CNC routers are very similar to CNC milling machines. The main difference is that in a CNC router, the workpiece is always stationary, and the cutting tool moves in X, Y, and Z dimensions. CNC routers create faster cuts than milling machines without compromising accuracy and design complexity.

Other Types of CNC Machines and CNC Operations

Besides the various types mentioned above, there are other CNC processes too. Some independent fabrication machines are integrated with a CNC for automatic movement. Some of these additional CNC machines are:

Broaching

Broaching utilizes a toothed cutting head to create niche shapes on a workpiece. Broaching cuts are very consistent and highly accurate. These machines can be linear or rotary (with a rotating toothed cutting tool).

Sawing

Sawing utilizes a toothed blade for creating straight, linear cuts. The cuts are created by the removal of material due to friction with the saw blade. When operated with a CNC, this process is usually applied for the automated cutting of materials.

Honing

Honing is similar to grinding in that it is generally used for the secondary finishing of a material. In the honing process, an abrasive stone or wheel is used for controlled grinding of the workpiece, creating the desired shape, size, or finishing.

Lapping

Lapping is also similar to grinding. But, lapping uses an abrasive paste, powder, or mixture instead of a grinding wheel to create a smooth surface finish. The abrasive mixture is inserted between two materials (one of which is the workpiece) and then rubbed against each other.

CNC lathes

CNC lathes are primary shaping tools used for machining metal or wood. In a lathe machine, the workpiece is rotated around a central axis, and the machining head moves linearly along the surface. CNC lathes can perform various functions, such as cutting, drilling, sanding, knurling, facing, and more. CNC lathes perform much better than manual lathes.

Plasma Cutters

Plasma cutters are an evolved form of cutting technology, using a high-temperature plasma jet to cut material. The plasma is created by an electrical arc, so this method applies to conductive materials only.

Laser Cutters

Laser cutters use a laser beam to cut through a material. Unlike plasma cutting, laser cutting is not limited to the cutting of electrically conductive materials. Laser beams can cut through anything by adjusting the laser parameters.

Flame Cutters

Flame cutting uses an Oxy-acetylene (also known as Oxy-fuel) gaseous mixture to cut through metals. When the Oxy-fuel stream is narrowed and ignited, it creates an ultra-high temperature flame that can easily cut through metal.

Press Brakes

The purpose of Press Brakes is to bend metal plates and sheets. The material is placed between a V-shape or a U-shape die. Then the die is pressed, resulting in the bend as required.

Electric Discharge Machines (EDM)

Electrical Discharge Machines (EDMs) are used for cutting conductive materials. In EDM, electrical pulses are emitted by a cutting head near the material, which creates an electrical arc. This arc melts and removes the material at the required position resulting in a cut.

Water Jet Cutters

Waterjet cutters utilize ultra high-pressure water for the cutting action. These cutters can cut through anything: metal, alloy, wood, stone, or glass. The water jet stream is controlled by CNC and moved according to the software.

Vertical CNC Machining

Horizontal CNC Machining

Precision Wire EDM

Selecting the right CNC machine for the job

When making CNC machined parts, it is important to figure out which type of machine is most suitable for the parts in question.

Every machine has its pros and cons, but it is difficult to find every kind of CNC machine in one place ' except maybe at a machine trade show, or in a CNC shop willing to invest lots of money.

If the most suitable machine is not available, you need to find a way to make the parts with the machines that are available. Here are some explanations of vertical vs horizontal milling and turning machines.

Figuring out the best way to load the workpieces

When you have chosen the CNC machine to make your parts, the next step ' before programing ' is to find the best way to load your workpieces in order to gain the best machined result. I believe this stage is more important than making the tool paths, but in my experience, most machinists have a difficult time with it, and it prevents them from moving forward smoothly.

As a prototype machining company, we try to hire people who have an open mind and are able to find solutions in this area, since we come across different CNC parts every day. (Of course, some people quit the job within two weeks!) Check out the different CNC workholding methods.

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Knowing what type of cutting tool to use

After choosing the machines and deciding on the best way to make the parts, selecting the right cutting tools will help to achieve a tighter tolerance and better surface finish. In short, suitable cutting tools result in better components.

Here's an example: milling ribs with draft might take hours using normal sphere cutting tools, but would require just a few minutes with a taper cutter.

So imagine how much time you could save when milling 10 pieces or more. Find out the differences between types of CNC cutting and machining tools.

What are the Advantages of CNC Machining?

CNC machining technologies have revolutionized the manufacturing industry by minimizing manual work and allowing for unparalleled levels of consistency and accuracy. It is often considered a modern boon due to its numerous advantages.

As a product designer, it is vitally important to know whether to stick with CNC machining or to design the parts for another manufacturing process.

Here is a brief overview of some of the key benefits:

Production Speed

Production speed is one of the primary reasons which led to the vast and rapid spread of CNC machining. With CNC machining, it is possible to speed up production exponentially since it removes the limitations of human labor.

Consistency

Computer Numerical Control machining ensures that all the parts created look and work the same. There is no possibility of human error. This leads to the fabrication of precision parts that serve their purpose as intended.

Reduction of Rejections

In conventional manufacturing processes using manual labor, there was a lot of human error, which resulted in rejections during quality control. This wasted a lot of time and resources. With CNC machining, the whole process is automated, which leads to fewer rejections.

Cost Saving

Labor costs include the salary paid to the labor, downtime during breaks, and the added benefits payments.

The accuracy, speed, efficiency, and automation of CNC machining reduce manufacturing costs by minimizing production times and labor hours. These savings can be passed along to customers, creating a competitive advantage and providing an opportunity for business reinvestment.

However, other factors affect CNC machining costs, like quantity, material selection, and geometry.

Material Versatility

CNC machining can be performed on practically any material with sufficient hardness.

Manufacturing Data Tracking

A CNC machine feeds manufacturing data that allows manufacturers to track the entire process for every part. They can learn about the exact machines every part went through during manufacturing. In case of a fault, the exact cause can be tracked immediately.

Accuracy

With CNC machining, it is possible to achieve accuracy on a micro level. Manufacturers can even push that limit with appropriate tools. Such a level of accuracy is not possible with manual operations.

Read our detailed article covering the benefits of CNC machining for a deep dive into all its advantages.

A Brief Explanation of CNC Machines and How They Work

For more in-depth info than is covered here, look at the following:

The Basics of Computer Numerical Control
by Mike Lynch, CNC Concepts, Inc.

CNC basics

To better understand the problems involved to successfully use your Rhino data for a CNC-controlled machining or cutting type operation, you need to understand the CNC process and how it works. Hopefully, this little primer will help.

First, a couple of definitions

CNC ' Computer Numerical Control ' Taking digitized data, a computer and CAM program is used to control, automate, and monitor the movements of a machine. The machine can be a milling machine, lathe, router, welder, grinder, laser or waterjet cutter, sheet metal stamping machine, robot, or many other types of machines. For larger industrial machines, the computer is generally an on-board dedicated controller. But for more hobbyist types of machines, or with some retrofits, the computer can be an external PC. The CNC controller works together with a series of motors and drive components to move and control the machine axes, executing the programmed motions. On the industrial machines there is usually a sophisticated feedback system that constantly monitors and adjusts the cutter's speed and position.

Desktop CNC ' There are many smaller modelmaker-hobbyist style desktop CNC machines. In general these are lighter weight, less rigid, less precise, slower, and less expensive than their industrial counterparts, but can do well for machining objects out of softer materials such as plastics, foam, and wax. Some desktop machines may run a lot like a printer. Others have their own closed command system and perhaps even dedicated CAM software. A few will also accept standard G-code as input. Some industrial standard desktop machines do exist with dedicated controllers for doing precise small work.

CAM ' Computer Aided Machining or Manufacturing ' Refers to the use of various software packages to create toolpaths and NC code to run a CNC controlled machine, based on 3D computer model (CAD) data. When the two are used together, this is generally referred to as CAD/CAM.

Note: CAM does not actually run the CNC machine, but just creates code for it to follow. It is also not an automatic operation that imports your CAD model and spits out the correct NC code. CAM programming, like 3D modeling, requires knowledge and experience in running the program, developing machining strategies, and knowing what tools and operations to use in each situation to get the best results. While there are simple programs that for the inexperienced user to get started without too much difficulty, more sophisticated models will take an investment in time and money to become proficient.

NC code ' A special relatively simple computer language that a CNC machine can understand and execute. These languages were originally developed to program parts directly at the machine keyboard without the aid of a CAM program. They tell the machine what moves to execute, one by one, as well as controlling other machine functions such as spindle and feed speeds, coolant. The most common language is G-code or ISO code, a simple alphanumeric programming language developed for the earliest CNC machines in the 70s.

Postprocessor - While G-code is considered the standard, each manufacturer can modify certain parts such as auxiliary functions, creating a situation where G-code made for one machine may not work for another. There are also many machine manufacturers, such as Heidenhain or Mazak, that have developed their own programming languages. So, to translate the CAM software's internally calculated paths into specific NC code that the CNC machine can understand, there is a bridge software piece software called a postprocessor. The postprocessor, once configured correctly, outputs the appropriate code for the chosen machine, so that in theory at least, any CAM system can output code for any machine. Postprocessors may be free with the CAM system or added cost extras.

Here is a summary of the steps required to get a digital model to a CNC milling machine.

CNC controlled machines, general

CNC machines can have several axes of movement, and these movements can be either linear or rotary. Many machines have both types. Cutout machines like lasers or waterjets generally have just two linear axes, X and Y. Milling machines usually have at least three, X, Y, and Z, and can have more rotary axes. A five axis milling machine is one that has three linear axes and two rotary, allowing the cutter to operate in a full 180º hemisphere and sometimes more. Five axis lasers exist as well. A robot arm might have more than five axes.

Some limitations of CNC controlled machines

Depending on their age and sophistication, CNC machines can be limited to the capabilities of their control and drive systems. Most CNC controllers only understand straight line movements and circular arcs. In many machines, the arcs are restricted to the principal XYZ planes as well. Rotary axis movements can be considered like linear movements, just degrees instead of distance. To create arc movements or linear movements that are at an angle to the principal axes, two or more axes must interpolate (move precisely in a synchronized manner) together. Linear and rotary axes can also interpolate simultaneously. In the case of five axis machines, all five must be perfectly synchronized ' no easy task.

The speed at which the machine controller can receive and process the incoming data, transmit commands to the drive system, and monitor the machine's speed and position is critical. Older and less expensive machines are obviously less capable in this, much in the same way that an older computer will work less well and more slowly (if at all) on demanding tasks than a newer one.

Interpret your 3D and spline data first

A typical problem is how to set up your files and do your CAM programming so that the machine executing your parts will work smoothly and efficiently with the data. Since most CNC controls only understand arcs and lines, any form that is not describable with these entities needs to be converted into something usable. Typical things that need converting are splines, i.e. general NURBS curves that are not arcs or lines, and 3D surfaces. Some desktop machine systems are not able to understand circular arcs either, so everything must be converted into polylines.

Splines can be broken up into a series of line segments, a series of tangent arcs, or a combination of both. You can imagine the first option as a series of chords on your spline, touching the spline on each end and having a certain deviation in the middle. Another way is to convert your spline into a polyline. The fewer segments you use, the coarser the approximation will be, and the more faceted the result. Going finer increases the smoothness of the approximation, but also dramatically increases the number of segments. You can imagine that a series of arcs might be able to approximate your spline within tolerance with fewer, longer pieces. This is the main reason for preferring arc conversion over simple polyline conversion, especially if you are working with older machines. With newer ones, there is less of a problem.

Imagine surfaces as the same kind of spline approximation, just multiplied many times in the across direction with a space between (usually called the stepover). In general, surfaces are done using all line segments, but there are situations where arcs or a combination of lines and arcs can also be used.

The size and number of segments are determined by the accuracy required and the method chosen, and will directly influence the execution. Too many short segments will choke some older machines, and too few will make a faceted part. The CAM system is usually where this approximation is done. With a skilled operator who knows what the user needs and the machine can handle, it is usually no problem. But some CAM systems may not handle splines or certain types of surfaces, so you might need to convert the entities in the CAD software first (Rhino) before going into CAM. The translation process from CAD to CAM (via a neutral format such as IGES, DXF, etc.) may also occasionally cause problems, depending on the quality of the import/export functions of the programs.

Common conventions used in describing CNC procedures

Your project can be:

2 Axis if all the cutting takes place in the same plane. In this case, the cutter does not have any capability of movement in the Z (vertical) plane. In general the X and Y axes can interpolate together simultaneously to create angled lines and circular arcs.

2.5 Axis if all the cutting takes place entirely in planes parallel to the principal plane but not necessarily at the same height or depth. In this case, the cutter can move in the Z (vertical) plane to change levels, but not simultaneously with the X,Y movements. An exception might be that the cutter can interpolate helically, that is, do a circle in X,Y while moving simultaneously in Z to form a helix (for example in thread milling).

A subset of the above is that the machine can interpolate any 2 axes together simultaneously, but not 3. This does make a limited number of 3D objects possible, by cutting in the XZ or YZ planes, for example, but is much more limited than full 3 axis interpolation.

3 Axis if your cutting requires simultaneous controlled movement of the X,Y,Z axes, which most free-form surfaces require.

4 axis if it includes the above plus 1 rotary axis movement. There are two possibilities: 4 axis simultaneous interpolation (also known as true 4th axis). Or just 4th axis positioning, where the 4th axis can reposition the part between 3 axis operations, but does not actually move during the machining.

5 axis if it includes the above plus 2 rotary axis movements. Besides true 5 axis machining (5 axes moving simultaneously while machining), you also often have 3 plus 2 or 3 axis machining + 2 separate axes positioning only, as well as in rarer cases 4 plus 1 or continuous 4 axis machining + a single 5th axis positioning only. Complicated, isn't it'

'MSH 28.10.07

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