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:

• Input Device
• Machine Control Unit
• Machine Tool
• Driving System
• Feedback Devices
• Display Unit 

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.

Classification of CNC Machine:

Unconventional Machining Process

Introduction

Unconventional manufacturing processes is defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tools as it needs to be used for traditional manufacturing processes.

Extremely hard and brittle materials are difficult to machine by traditional machining processes such as turning, drilling, shaping and milling. Non-traditional machining processes, also called advanced manufacturing processes, are employed where traditional machining processes are not feasible, satisfactory or economical due to special reasons as outlined below.

• Very hard fragile materials difficult to clamp for traditional machining.
• When the work piece is too flexible or slender.
• When the shape of the part is too complex.

Several types of non-traditional machining processes have been developed to meet extra required machining conditions. When these processes are employed properly, they offer many advantages over non-traditional machining processes.

CM Process V/s UCM Process

Conventional Machining Processes mostly remove material in the form of chips by applying forces on the work material with a wedge shaped cutting tool that is harder than the work material under machining condition.

The major characteristics of conventional machining are: 

• Generally macroscopic chip formation by shear deformation.
• Material removal takes place due to application of cutting forces – energy domain can be classified as mechanical.
• Cutting tool is harder than work piece at room temperature as well as under machining conditions.

Non-conventional manufacturing processes is defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tools as it needs to be used for traditional manufacturing processes. 

NEED FOR UNCONVENTIONAL MACHINING PROCESSES

Ø  Extremely hard and brittle materials or Difficult to machine material are difficult to machine by traditional machining processes.

Ø  When the work piece is too flexible or slender to support the cutting or grinding forces when the shape of the part is too complex.

CLASSIFICATION OF UCM PROCESSES:-

Abrasive Jet Machining

In abrasive jet machining, a focused stream of abrasive particles, carried by high pressure air or gas is made to impinge on the work surface through a nozzle and the work material is made to impinge on the work surface through a nozzle and work material is removed by erosion by high velocity abrasive particles.

Abrasive water jet cutting systems (abrasive jet) use a combination of water and garnet to cut through materials considered "unmachineable" by conventional cutting methods. Using small amounts of water while eliminating the friction caused by tool-to-part contact, abrasive jet cutting avoids thermal damage or heat affected zones (HAZ) which can adversely affect metallurgic properties in materials being cut. The ability to pierce through material also eliminates the need and cost of drilling starter holes. Because abrasive jet cuts with a narrow kerf, parts can be tightly nested thus maximizing material usage.

Ultrasonic Machining

Ultrasonic machining (USM) is a mechanical material removal process used to erode holes and cavities in hard or brittle work pieces by using shaped tools, high frequency mechanical motion, and an abrasive slurry. . A relatively soft tool is shaped as desired and vibrated against the work piece while a mixture of fine abrasive and water flows between them. The friction of the abrasive particles gradually cuts the work piece.

It is also known as Ultrasonic impact grinding is an operation that involves a vibrating tool fluctuating the ultrasonic frequencies in order to remove the material from the work piece. The process involves an abrasive slurry that runs between the tool and the work piece. Due to this, the tool and the work piece never interact with each other. The process rarely exceeds two pounds. All the operations done with the ultrasonic machining method are cost effective and best in results. Ultrasonic machining is an abrasive process which can create any material into hard and brittle form with the help of its vibrating tool and the indirect passage of abrasive particles towards the work piece. It is a low material removal rate machining process.

Water Jet Machining

A water jet cutter is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure. It is often used during fabrication or manufacture of parts for machinery and other devices. It has found applications in a diverse number of industries from mining to aerospace where it is used for operations such as cutting, shaping, carving, and reaming.

The most important benefit of the water jet cutter is its ability to cut material without interfering with the materials inherent structure as there is no "heat affected zone" or HAZ. This allows metals to be cut without harming their intrinsic properties.

Abrasive Water Jet Machining

AWJM is a well-established non-traditional machining process. Abrasive water jet machining makes use of the principles of both abrasive jet machining and water jet machining. AWJM is a non conventional machining process where material is removed by impact erosion of high pressure high velocity of water and untrained high velocity of grit abrasives on a work piece.

This technology is most widely used compare to other non-conventional technology because of its distinct advantages. It is used for cutting a wide variety of materials ranging from soft to hard materials. This technique is especially suitable for very soft, brittle and fibrous materials. This technology is less sensitive to material properties as it does not cause chatter. This process is without much heat generation so machined surface is free from heat affected zone and residual stresses. AWJM has high machining versatility and high flexibility. The major drawback of this process is, it generate loud noise and a messy working environment.

Electrochemical Machining

Electrochemical machining (ECM) also uses electrical energy to remove material. An electrolytic cell is created in an electrolyte medium, with the tool as the cathode and the work piece as the anode. A high-amperage, low-voltage current is used to dissolve the metal and to remove it from the work piece, which must be electrically conductive. ECM is essentially a depleting process that utilizes the principles of electrolysis. The ECM tool is positioned very close to the work piece and a low voltage, high amperage DC current is passed between the two via an electrolyte. Material is removed from the work piece and the flowing electrolyte solution washes the ions away. These ions form metal hydroxides which are removed from the electrolyte solution by centrifugal separation. Both the electrolyte and the metal sludge are then recycled.

Electrochemical Grinding

Electro-chemical grinding (ECG) is a variant process of the basic ECM. It is a burr free and stress free material removal process, wherein material removal of the electrically conductive material takes place through mechanical (grinding) process and electro-chemical process. The abrasive laden grinding wheel is negatively charged and the work piece is positively charged. They are separated by an electrolyte fluid. The fine chips of the material that is removed from the work piece (debris) stays in the electrolyte fluid, which is further filtered out. Electrochemical grinding and electrochemical machining are similar processes with a difference that a wheel substitutes the tool used in ECM. The wheel shape is similar to the desired work shape.

The main feature of electrochemical grinding (ECG) process is the use of a metallic grinding wheel which is embedded with insulating abrasive particles such as diamond, set in the conducting material. Copper, brass, and nickel are the most commonly used materials while aluminium oxide is a typical abrasive used while grinding steels.

Electro Jet drilling               

Electro jet drilling (EJD) process is picking up noticeable quality in the machining of miniaturized scale and full-scale openings in hard to-machine materials utilized as a part of aviation, gadgets, and PCs, medicinal, and car ventures. As the pattern towards scaling down proceeds with, this procedure is increasing expanding significance as it has demonstrated its prevalence over other contemporary non-traditional miniaturized scale and large scale gap penetrating procedures. This paper exhibits a two-dimensional limited component demonstrate for the investigation of the EJD procedure utilizing quadrilateral (rectangular) components. The created display predicts the penetrating rate and spiral over cut. The test comes about show close concurrence with the mimicked comes about.

Electrical Discharge Machining

Electrical Discharge Machining (EDM), also known as spark erosion, employs electrical energy to remove metal from the work piece without touching it. A pulsating high- frequency electric current is applied between the tool point and the work piece, causing sparks to jump the gap and vaporize small areas of the work piece. Because no cutting forces are involved, light, delicate operations can be performed on thin work pieces. EDM can produce shapes unobtainable by any conventional machining process.

Laser Jet Machining

Laser-Jet machining (LJM) is accomplished by precisely manipulating a jet of coherent light to vaporize unwanted material. LJM is particularly suited to making accurately placed holes. It can be used to perform precision micromachining on all microelectronic substrates such as ceramic, silicon, diamond, and graphite. Examples of microelectronic micromachining include cutting, scribing & drilling all substrates, trimming any hybrid resistors, patterning displays of glass or plastic and trace cutting on semiconductor wafers and chips.

Electron-beam machining

In electron-beam machining (EBM), electrons are accelerated to a velocity nearly three-fourths that of light (~200,000 km/sec). The process is performed in a vacuum chamber to reduce the scattering of electrons by gas molecules in the atmosphere. The electron beam is aimed using magnets to deflect the stream of electrons and is focused using an electromagnetic lens. The stream of electrons is directed against a precisely limited area of the work piece; on impact, the kinetic energy of the electrons is converted into thermal energy that melts and vaporizes the material to be removed, forming holes or cuts.

Typical applications are annealing, welding, and metal removal. A hole in a sheet 1.25 mm thick up to 125 micro m diameter can be cut almost instantly with a taper of 2 to 4 degrees. EBM equipment is commonly used by the electronics industry to aid in the etching of circuits in microprocessors.

Chemical Milling

Chemical Milling aides in the manufacture of light gauge metal parts. The photo etching process (also called chemical etching and chemical milling) allows people to produce intricate metal components with close tolerances that are impossible to duplicate by other production methods. It is also known as chemical machining.

Chemical Milling is utilized in the manufacturing of encoders, masks, filters, lead frames, flat springs, strain gauges, laminations, chip carriers, step covers, fuel cell plates, heat sinks, shutter blades, electron grids, fluidic circuit plates, reticles, drive bands, haptics, and shims.

Photochemical Machining

Photochemical Machining (PCM) components are produced by the photo-etching technique using a wide array of metal and alloys. This technique avoids burrs, no mechanical stresses are built into the parts and the properties of the metal worked are not affected. Hardened and tempered metals are machined as easily as regular metals. The technique is ideal for machining thin metals and foils. Parts with very precise and intricate designs can be produced without difficulty. The photo chemical machining/milling processes can precisely etch lines and spaces on all types of metals (alloys: kovar, nickel, brass, beryllium, copper, stainless steel, aluminium, and others) with detailed accuracies. This is used for creating specialty flex circuits, plus in engineering of other rigid technologies. This results in a burr free part with very close tolerances.