CNC

Computer Numerical Control

        Computer numerical control (CNC) is the numerical control system in which a dedicated computer is built into the control to perform basic and advanced NC functions. CNC controls are also referred to as soft wired NC systems because most of their control functions are implemented by the control software programs. CNC is a computer assisted process to control general purpose machines from instructions generated by a processor and stored in a memory system. 

Elements of a CNC System:

       

                                                                                        A CNC system consist of the following 6 major elements:

Key Areas of Knowledge:            

               As with any subject, the more time you invest in learning about CNC and the related technologies, the more you will get from it. To achieve the best results, there are a few key areas which you should concentrate on:

          Computer skills One requirement common to all aspects of CNC work is how to use a computer to perform basic tasks. You will be working with computers and computer programs during almost all the steps of the process as you design your parts and need to understand basic operations such as starting and stopping programs, saving, copying and deleting files, finding files stored on your computer and installing programs and updates. Your CNC machine is also run by a computer, this may be a standalone PC or a dedicated Control Box. This guide will assume a basic knowledge of computers and the Windows operating system, if you don’t feel comfortable with your current computer skills or are new to running a PC then it would be well worth taking a basic course or buying a general guide to working with your PC. 

         

            Design & Tool path Software Before you can cut anything with a CNC, you need to first create the design layout that the machine is going to follow to cut the parts. The software you choose will play a significant role in successfully creating projects with your CNC. Simply put, the design software will allow you to transform “pencil and paper” ideas to a set of instructions used to run the machine. When done correctly, the end result will be a physical product you can touch and hold that has value and purpose and a great sense of achievement.

          Operating and Maintaining your CNC Machine If you currently own or use a CNC machine, you already know how important it is to keep it properly maintained and adjusted, to know and understand its limitations and how to set it up correctly to run a job. If you don’t own a machine yet, then it’s important to spend time thinking about what you want your machine to be able to produce, this can eliminate a lot of future frustration. Cost will always be an important factor, but realize that you need to balance that with capabilities, because nothing can be more expensive than a machine that cannot do what you need. For example, if you want to cut large sheet goods then a desktop model will probably not be your best choice. However, if you only have room for a small machine this may be your only option and you need to understand its limitations on how large a part it can cut. Only you can determine what this balance will be for your situation and budget.

            Some important considerations when researching the purchase of a machine or when looking at building one yourself include size, speed and accuracy and the technical support offered both before and after the purchase. As with software, the importance of a company’s reputation, support, and an active website and/or forum cannot be understated.

           Every CNC machine needs software to directly drive its movement; this is commonly referred to as the ‘Control Software’. Some common generic third-party packages that do this include “Mach3” and “WINCNC”. Many manufactures create and use their own proprietary systems specific to their own models and these may be installed on an external PC or be loaded onto a dedicated Control Box attached to the machine.

          Most control systems offer settings that can significantly improve the smoothness and accuracy of your machine when correctly set. While this goes beyond the scope of this guide, it is something worth investigating for your particular CNC. Remember, the best designed project will not cut well on an incorrectly “tuned” machine.

Knowledge of Materials and Tooling

            When it comes to obtaining the best possible results, you cannot forget the material you are working with or the tool you are using to cut it. The type of material will factor into every stage of the Project – from initial concept through final finishing. 

               The common materials people using CNC Routers work with include; wood, plastics, dense foam board and softer (non-ferrous) metals (brass, aluminum, etc.). If you are not already familiar with the type of material you want to use, there are many sources of information that can help you. 

             Typical questions you must answer for the type of material include proper tool (bit) selection, how fast you can move that tool through that material (Feed Rate and Plunge Rate), how much material you can remove at one time (Pass Depth and Cut Depth) and how fast the bit should be rotating (Spindle or Router speed). Typically suppliers of tooling offer information on the correct settings for the router bits they sell. 

Workflow Overview of a typical CNC project:

             When you step back and look at a complete project from start to finish, you can identify a series of major steps that will form the “Workflow” to complete it. Having a good understanding of this process will help you start to appreciate where the different software packages and setup procedures fit into the overall creation of parts with your CNC.

                 Concept This is the idea for what you are going to make. This may range from a specific customer requirement, something you have sketched on a napkin or a ready to go file that someone has already prepared. At this stage you need to try and think through the other processes in the job to help to get the best approach to achieving it. You should also assemble any reference material you will use to help design the part such as photos, data from the customer, design sketches etc. 

                   Design (CAD – Computer Aided Design) For the design you need create the computer data that will define either the 2D or 3D forms you want to cut on your CNC. This is done in what is typically called “CAD software” and you may also hear this type of software referred to as a drafting, drawing or design program. The finish point of the Design stage is to have prepared all the 2D data (Vectors) or 3D data (Components) you require to start calculating the specific movements the CNC machine will follow, these moves are typically referred to as the “Toolpaths”. Most of our customers use one of the Vectric products (VCarve Pro or Aspire) to do their design although there are many other design (CAD) programs available for either 2D drawing or 3D modeling and depending on the file format export options available, this data can be saved and imported into the Vectric programs for Too path creation. 

                    Toolpaths (CAM – Computer Aided Manufacturing) Once the design is complete, you will start to calculate the actual paths that will drive where the tool will move on the machine, as previously stated these are called “Toolpaths”. Creating your Toolpaths is the key stage in going from the virtual world of a computer design to the reality of the physical world. At this point you will start to take into account the shape and size of the tool, the type of movement you want the tool to make (the shape you want it to leave in the material) and appropriate settings for how fast the tool can be moved and how much material can be removed safely. Once the Toolpaths have been calculated the software will let you Preview how they will look in a virtual piece of material. This lets you check that they are doing what you expected. Once you are happy the Toolpaths are correct then they can be saved in a format that is appropriate for your particular CNC. 

                   Machining Once your tool paths have been saved then you transfer them over to the CNC. At this stage you need to set the CNC to match the job setup you specified in the Design/Machining software. This will involve setting up your material in the right orientation, and making sure it will be secure while you’re cutting it. Then you need to load the correct tool and tell the machine where the X, Y and Z reference position is for the tool tip (normally this is the zero position for each axis), again this will be to replicate how it was set in the software so all the positions and sizes you specified in the software will be replicated at the machine.

                         Finish and Assembly  are obviously going to vary dramatically depending on the type of job you are doing and the material you are cutting. We will not cover this in detail in this document but it is important to be aware of the finish you plan to use and where applicable use appropriate options in the software or on the machine to help minimize or aid with your finishing process. 

Applications of CNC machine: CNC machine I are widely used in the metal cutting industry and the best used to produce the following types of products:

• Parts with complicated contours.
• Parts requiring close tolerance and or good repeat-ability.
• Parts requiring expensive Jigs And Fixtures if produce on conventional machines.
• Parts that may have several engineering changes such as during the development stage of a prototype.
• In case where human error could be extremely costly.
• Parts that are needed in a hurry.
• A small batch lot or short production runs.