What You Should Know About CNC Routers?
A CNC router is a CNC machine kit whose tool paths can be controlled via computer numerical control. It is a computer-controlled machine for cutting various hard materials, such as wood, composites, aluminium, steel, plastics, and foams. It is one of many kinds of tools that have CNC variants. A CNC router is very similar in concept to a CNC milling machine.
CNC routers come in many configurations, from small home-style "desktop" CNC routers to large "gantry" CNC routers used in boat-making facilities. Although there are many configurations, most CNC routers have a few specific parts: a dedicated CNC controller, one or more spindle motors, AC inverters, and a table.
CNC routers are generally available in 3-axis and 5-axis CNC formats.
The CNC router is run by a computer. Coordinates are uploaded into the machine controller from a separate program. CNC router owners often have two software applications—one program to make designs (CAD) and another to translate those designs into a program of instructions for the machine (CAM). As with CNC milling machines, CNC routers can be controlled directly by manual programming, but CAD/CAM opens up wider possibilities for contouring, speeding up the programming process and in some cases creating programs whose manual programming would be, if not truly impossible, certainly commercially impractical.
CNC routers can be very useful when carrying out identical, repetitive jobs. A CNC router typically produces consistent and high-quality work and improves factory productivity.
A CNC router can reduce waste, frequency of errors, and the time the finished product takes to get to market.
A CNC router gives more flexibility to the manufacturing process. It can be used in the production of many different items, such as door carvings, interior and exterior decorations, wood panels, sign boards, wooden frames, mouldings, musical instruments, furniture, and so on. In addition, the CNC router makes thermo-forming of plastics easier by automating the trimming process. CNC routers help ensure part repeatability and sufficient factory output.
Numerical control technology as it is known today emerged in the mid 20th century. It can be traced the year of 1952, the U.S Air Force, and the names of john parsons and the Massachusetts institute of technology in Cambridge, MA, USA. It was not applied in production manufacturing until the early 1960`s. the real boom came in the form on CNC, around the year of 1972, and decade later with the introduction of affordable micro computers. The history and development of this fascinating technology has been well documented in many publications.
In the manufacturing filed, and particularly in the area of metal working, Numerical Control technology has caused something of revolution. Even in the every days before computers became standard fixtures in every company and in many homes, the machine tools equipped with Numerical Control system found their special place in the machine shops. the recent evolution of micro electronics and the never ceasing computer development , including its impact on Numerical Control , has brought significant changes to the manufacturing sector in general and metalworking industry in particular.
DEFINITION OF NUMERICAL CONTROL
In various publication and articles, many descriptions have been used during the years, to define what Numerical Control is. Many of these definitions share the same idea, same basic concept, just use different wording.
The majority of all the known definitions can be summed up into relatively simple statement:
Numerical control can be defined as an operation of machine tools by the means of specifically coded instructions to the machine control system.
The instructions are combinations of the letters of alphabet, digits and selected symbols, for example, a decimal point, the percent sign or the parenthesis symbols. All instructions are written in a logical order and a predetermined form. The collection of all instructions necessary to machine a part is called an NC program, CNC program, or a part program. Such a program can be stored for a future use and used repeatedly to achieve identical machining results at any time.
NC and CNC Technology
In strict adherence to the terminology, there is a difference in the meaning of the abbreviations NC and CNC. The NC stands for the order and original Numerical Control technology, whereby the abbreviation CNC stands for the newer Computerized Numerical Control technology, a modern spin-off of its older relative. However, in practice, CNC is the preferred abbreviation. To clarify the proper usage of each term, look at the major differences between the NC and the CNC systems.
Bothe systems perform the same tasks, namely manipulation of data for the purpose of machining a part. In both cases, the internal design of the control system contains the logical instructions that process the data. At this point the similarity ends.
The NC system (as opposed to the CNC system) uses fixed logical functions, those that are builtin and permanently wired within the control unit. These functions can`t be changed by the programmer or the machine operator. because of the fixed writing of the control logic, the NC control system can interpret a part program, but it does not allows any changes must be made away from the control , typically in an office environment. Also, the NC system requires the compulsory use of punched tapes for input of the program information.
The modern CNC system, but not the old NC system, uses an internal micro processor (I.e., a computer). This computer contains memory registers storing a variety of routines that are capable of manipulating logical functions. That means the part programmer or the machine operator can change the program of the control itself (at the machine), with instantaneous results. This flexibility is the greatest advantage of the CNC systems and probably the key element the contributed to such a wide use of the technology in modern manufacturing. The CNC programs and the logical functions are stored on special computer chips, as software instructions. Rather than used by the hardware connections, such as wires, that controls the logical functions. In contrast to the NC system, the CNC system is synonymous with the term `softwired`.
When describing a particular subject that relates to the numerical control technology, it is customary to use either the term NC or CNC. Keep in mind that NC can also mean CNC in everyday talk, but CNC can never refer to the order technology, described here under the abbreviation of NC. The letter `C` stands for computerized, and it is not applicable to the hardwired system. All control systems manufactured today are of the CNC design. Abbreviations such as C&C or C’n’C are not correct and reflect poorly on anybody that uses them.
This refers to the position of all the axes when they are located at the point where the sensors can physically detect them. an absolute zero position is normally arrived at after a home command is performed.
A fixed reference line about which an object translates or rotates.
A ball screw is a mechanical device for translating rotational motion to linear motion. it consists of a re-circulating ball bearing nut that races in a precision threaded screw.
Computer-aided design (CAD) is the use of a wide range of computerbased tools that assist engineers, architects and other design professionals in their design activities.
Computer-aided manufacturing (CAM) is the use of a wide range of computer-based software tools that assist engineers and CNC machinists in the manufacture or prototyping of product components.
The abbreviation CNC stands for computer numerical control, and refers specifically to a computer "controller" that reads g-code instructions and drives the machine tool.
A control system is a device or set of devices that manage, command, direct or regulate the behaviour of other devices or systems.
This is the distance between the lowest part of the tool and the machine table surface. Maximum daylight refers to the distance from the table to the highest point that a tool can reach.
Otherwise known as multi-drills, these are sets of drills usually spaced in 32 mm increments.
Or cutting speed is the speed difference between the cutting tool and the surface of the part it is operating on.
This is a value that represents the reference zero of a given fixture. it corresponds to the distance in all axes between the absolute zero and the fixture zero.
G-code is a common name for the programming language that controls NC and CNC machine tools.
This is the programmed reference point also known as 0,0,0 represented either as the absolute machine zero or a fixture offset zero.
Linear and circular interpolation is a method of constructing new data points from a discrete set of known data points. in other words, this is the way the program will calculate the cutting path of a full circle while knowing only the center point and the radius.
This is the default position of all the axes on the machine. When executing a homing command, all the drives move toward their default positions until they reach a switch or a sensor that tells them to stop.
It refers to the process of efficiently manufacturing parts from sheets. using complex algorithms, nesting software determines how to lay out the parts in such a way as to maximize the use of available stock.
It refers to the distance away from the centerline measurement that comes from the CAM software.
This is the term used to refer to air activated tools that are mounted beside the main spindle.
Software that provides some final processing to data, such as formatting it for display, printing or machining.
This is the reference point 0,0 specified in the program. in most cases it is different than the machine zero.
Rack and pinion
A rack and pinion is a pair of gears which convert rotational motion into linear motion.
A spindle is a high frequency motor fitted with a tool holding apparatus.
It is also known as the sacrificial board, it is the material used as a base for the material being cut. it can be made of many different materials, of which MDF and particleboard are most common.
This refers to the pressure exerted onto a tool while it is cutting through material.
It is also called the spindle speed, this is the rotational frequency of the spindle of the machine, measured in revolutions per minute (RPM).
Tooling, surprisingly enough, is often the least understood aspect of CNC equipment. given that it is the one element that will most affect the quality of cut and the cutting speed, operators should spend more time exploring this subject.
Cutting tools usually come in three different materials; high speed steel, carbide and diamond.
High speed steel (HSS)
HSS is the sharpest of the three materials and the least expensive, however, it wears the fastest and should only be used on non-abrasive materials. it requires frequent changes and sharpening and for that reason it is used mostly in cases where the operator will need to cut a custom profile in-house for a special job.
Carbide tools come in different forms: carbide tipped, carbide inserts and solid carbide tools. bear in mind that not all carbide is the same as the crystalline structure varies greatly between makers of these tools. as a result, these tools react differently to heat, vibration, impact and cut loads. generally, low cost generic carbide tools will wear and chip more rapidly than higher priced name brands.
Silicon carbide crystals are embedded in a cobalt binder to form the tool. When the tool is heated, the cobalt binder loses its ability to hold on to the carbide crystals and it becomes dull. at the same time the hollow space left by the missing carbide fills up with contaminants from the material being cut, amplifying the dulling process.
This category of tooling has come down in price in the last couple of years. its remarkable abrasion resistance makes it ideal for cutting materials such as high pressure laminates or Mdf. some claim that it will outlast carbide by up to 100 times. diamond tipped tools are prone to chip or crack if they hit an embedded nail or a hard knot. some manufacturers use diamond tools for rough cutting abrasive materials and then switch to carbide or insert tooling for the finishing work.
The shank is the part of the tool that is held by the tool holder. it is the part of the tool that has no evidence of machining. the shank must be kept free of contamination, oxidation and scratching.
This is the diameter or the width of the cut that the tool will produce.
Length of cut
This is the effective cutting depth of the tool or how deep the tool can cut into the material.
This is the part of the tool that augers out the cut material. the number of flutes on a cutter is important in determining the chip load.
There are many profiles of tools in this category. the main ones to consider are upcut and downcut spirals, compression spirals,
rougher, finisher, low helix and straight cut tools. all of these come in a combination of one to four flutes.
The upcut spiral will cause the chips to fly upward out of the cut. this is good when doing a blind cut or when drilling straight down. this geometry of tool however promotes lifting and tends to tear out the top edge of the material being cut.
Downcut spiral tools will push the chips downward into the cut which tends to improve part holding but can cause clogging and overheating in certain situations. this tool will also tend to tear out the bottom edge of the material being cut.
Both the upcut and downcut spiral tools come with a roughing, chip breaker or a finishing edge.
Compression spirals are a combination of upcut and downcut flutes.
Compression tools push the chips away from the edges towards the center of the material and are used when cutting double sided laminates or when tear out of the edges is a problem.
Low helix or high helix spiral bits are used when cutting softer materials such as plastic and foam, when welding and chip evacuation are critical.
The most important factor for increasing tool life is to dissipate the heat that is absorbed by the tool. the fastest way to do this is by cutting more material rather than by going slower. Chips extract more heat away from the tool than dust does. as well, rubbing the tool against the material will cause friction which translates into heat.
Another factor to consider in the quest to increase tool life is to keep the tool, the collet and the tool holder clean, free of deposits or corrosion thus reducing vibrations caused by unbalanced tools.
The thickness of material being removed by each tooth of the tool is called the Chip Load.
The formula for calculating chip load is as follows:
Chip Load = Feed Rate / RPM / # Flutes
When the chip load is increased, tool life is increased, while decreasing the cycle time. furthermore, a broad range of chip loads will achieve a good edge finish. it is best to refer to the tool manufacturer’s chip load chart to find the best number to use. recommended chip loads usually range between 0.003" and 0.03" or 0.07 mm to 0.7 mm.
This is an option that is becoming more and more popular in the industry especially since CNC machines are becoming more integrated into the whole business formula. The controller can be connected to the sales or scheduling software and part labels are printed once the part is machined. Some vendors use labels to identify left over material for easy retrieval in the future.
Otherwise known as bar code wands, they can be integrated into the controller so that a program can be called by scanning a barcode on the work schedule. This option saves valuable time by automating the program loading process.
These measuring devices come in a variety of forms and perform many different functions. Some probes merely measure the surface height to ensure proper alignment in height sensitive applications. other probes can automatically scan the surface of a three-dimensional object for later reproduction.
Tool length sensor
A tool length sensor acts like a probe that measures the daylight or the distance between the end of the cutter and the surface of the workspace and enters this number in the control's tool parameters. This little addition will save the operator from the lengthy process required each time he changes a tool.
These devices were first seen in the furniture industry in CNC leather cutters. A laser projector mounted above the CNC work table projects an image of the part about to be cut. This greatly simplifies positioning the blank on the table to avoid defects and other issues.
A vinyl knife attachment is often seen in the sign industry. this is a cutter that can be attached to the main spindle or on the side with a free turning knife whose pressure can be adjusted by a knob. This attachment permits the user to turn his CNC router into a plotter to make vinyl masks for sandblasting or vinyl letters and logos for trucks and signs.
Cool air guns or cutting fluid misters are used with a wood router to cut aluminium or other non-ferrous metals. These attachments blast a jet of cold air or a mist of cutting fluid near the cutting tool to ensure that it remains cool while working.
Engravers are mounted to the main spindle and consist of a floating head holding a small diameter engraving knife that turns between 20,000 and 40,000 RPM. The floating head ensures that the engraving depth will be constant even if the material thickness changes. This option if most often found in the sign making industry although trophy makers, luthiers and millwork shops use it for marquetry.
A rotating axis set along the x or the y axis can turn the router into a CNC lathe. Some of these rotating axes are simply a rotating spindle while others are indexable which means they can be used for carving intricate parts.
Floating cutter head
Floating cutter heads will keep the cutter at a specific height from the top surface of the material being cut. This is important when cutting features onto the top surface of a part that might not present an even surface. An example of this is cutting a v-groove on the top of a dining room table.
Plasma cutters are an add-on to some machines and allow the user to cut sheet metal parts of varying thicknesses.
Aggregate tools can be used for many operations that a straight cutter cannot perform.
CONVENTIONAL AND CNC MACHINING
What makes the CNC machining superior to the conventional methods? Is it superior at all? Where are the main benefits? If the CNC and the conventional machining processes are compared, a common general approach to machining a part will emerge:
1. Obtain and study the drawing
2. Select the most suitable machining method
3. Decide on the setup method (work holding)
4. Select the cutting tools
5. Establish speeds and feeds
6. Machine the part
The basic approach is the same for both types of machining. The major difference is in the way how various data are input. A feed rate of 10 inches per minute (10 in/min) is the same in manual
Or CNC applications, but the method of applying it is not. The same can be said about a coolant – it can be activated by turning a knob, pushing a switch or programming a special code. All these actions will result in a coolant rushing out of a nozzle. In both kinds of machining, a certain amount of knowledge on the part of the user is required. After all, metal working, particularly metal cutting is mainly a skill, but it is also, to a great degree, an art and a profession of large number of people. So is the application of Computerized Numerical Control. Like any skill or art or profession, mastering it to the last detail is necessary to be successful. It takes more than technical knowledge to be a CNC machinist or CNC programmer. Work experience, intuition and what is sometimes called a `gut-feel` is much needed supplement to any skill.
In conventional machining, the machine operator sets up the machine and moves each cutting tool, using one or both hands, to produce the required part. The design of a manual machine tool offers many features that help the process of machining a part-levers, handles, gears and dials, to name just a few. The same body motions are repeated by the operator for every part in the batch. However, the word `same` in this context really means `similar` rather than `identical`. Humans are not capable to repeat every process exactly the same at all times-that is the job of machines. People cannot work at the same performance level all the time, without a rest. All of us have some good and some bad moments. The results of these moments, when applied to machining a part, are difficult to predict. There will be some differences and inconsistencies within each batch of parts. The parts will not always be exactly the same. Maintaining dimensional tolerances and surface finish quality are the most typical problems in conventional machining. Individual machinists may have their fellow colleagues. Combination of these and other factors create a great amount of inconsistency.
The machining under numerical control does away with the majority of inconsistencies. It does not require the same physical involvement as machining. Numerically
Controlled machining does not need any levers or dials or handles, at least not in the same sense as conventional ma-chining does. Once the part program has been proven, it can be used any number of times over, always returning consistent results. That does not mean there are no limiting factors. The cutting tools do wear out, the material blank in one batch is not identical to the material blank in another batch, the setups may vary, etc. These factors be considered and compensated for, whenever necessary.
The emergence of the numerical control technology does not mean an instant, or even a long term, demise of all manual machines. There are times when a traditional machining method is preferable to a computerized method. For example, a simple one time job may be done more efficiently on a manual machine than a CNC machine. Certain types of machining jobs will benefit from manual or semiautomatic machining, rather than numerically controlled machining. The CNC machine tools are not meant to replace every manual machine, only to supplement them.
In many instances, the decision whether certain machining will be done on a CNC machine or not is based on the number of required parts and nothing else. Although the volume of parts machined as batch is always in important criteria, it should never be the only factor.
Consideration should also be given to the part complexity, its tolerances, the required quality of surface finish, etc. often, a single complex part will benefit from CNC machining, while fifty relatively simple parts will not.
Keep in mind that numerical control has never machined a single part by itself. Numerical control is only a process or a method that enables a machine tool to be used in a productive, accurate and consistent way.
NUMERICAL CONTROL ADVANTAGES
What are the main advantages of numerical control?
It is important to know which areas of machining will benefit from it and which are better done the conventional way. It is absurd to think that a two horse power CNC mill will win over jobs that are currently done on a twenty times more powerful manual mill. Equally unreasonable are expectations of great improvements to cutting speeds and feedrates over a conventional machine. If the machining and tooling conditions are the same, the cutting time will be very close in both cases.
Some of the major areas where the CNC user can and should expected improvement:
1. Setup time reduction
2. Lead time reduction
3. Accuracy and repeatability
4. Contouring of complex shapes
5. Simplified tooling and work holding
6. Consistent cutting time
7. General productivity increase
Each area offers only a potential improvement. Individual users will experience different levels of actual improvement, depending on the product manufactured on-site, the CNC machine used, the setup methods, complexity of fixturing, quality of cutting tools, management philosophy and engineering design, experience level of the workforce, individuals attitudes, etc.
Setup Time Reduction
In many cases, the setup time for a CNC machine can be reduced, sometimes quite dramatically. It is important to realize that setup is manual operation, greatly dependent on the performance of CNC operator, the type of fixturing and general practices of the machine shop. Setup time is unproductive, but necessary – it is a part of the overhead costs of doing business. To keep the setup time to a minimum should be one of the primary considerations of any machine shop supervisor, programmer and operator.
Because of the design of CNC machines, the setup time should not be major problem. Modular fixturing, standard tooling, fixed locators, automatic tool changing, pallets and other advanced features, make the setup time more efficient than comparable setup of a conventional machine. With a good knowledge of modern manufacturing, productivity can be increased significantly.
The number of parts machined under one setup is also important in order to assess the cost of setup time. If a great number of parts are machined in one setup, the setup cost per part can be very insignificant. A very similar reduction can be achieved be grouping several different operations into a single setup. Even if the setup time is longer, it may be justified when compared to the time required to setup several conventional machines.
Lead Time Reduction
Once a part program is written and proven, it is ready to be used again in the future, even at a short notice. Although the lead time for the first run is usually longer, it is virtually nil for any subsequent run. Even if an engineering change of the part design requires the program to be modified, it can be done usually quickly, reducing the lead time.
Long lead time, required to design and manufacture several special fixtures for conventional machines, can often be reduced by preparing a part program and the use of simplified fixturing.
Accuracy and Repeatability
The high degree of accuracy and repeatability of modern CNC machines has been the single major benefit to many users. Whether the part program is stored on a disk or in the computer memory, or even on a tape (the original method), it always remains the same. Any program can be changed at will, but once proven, no changes are usually required any more. A given program can be reused as many times as needed, without losing a single bit of data it contains. True, program has to follow for such changeable factors as tool wear and operating temperatures, it has to be stored safely, but generally very little interference from the CNC programmer or operator will be required, the high accuracy of CNC machines and their repeatability allows high quality parts to be produced consistently time after time.
Contouring of Complex Shapes
CNC lathes and machining centers are capable of contouring a variety of shapes. Many CNC users acquired their machines only to be able to handle complex parts. Good examples are CNC applications in the aircraft and automotive industries. The use of some form of computerized programming is virtually mandatory for any three dimensional tool path generation.
Complex shapes, such as molds, can be manufactured without the additional expense of making a model for tracing. Mirrored parts can be achieved literally at the switch of a button, templates, wooden models, and other pattern making tools.
Simplified Tooling and Work Holding
No standard and homemade tooling that clutters the benches and drawers around a conventional machine can be eliminated by using standard tooling, specially designed for numerical control applications. Multi-step tools such as pilot drills, step drills, combination tools, counter borers and others are replaced with several individual standard tools. These tools are often cheaper and easier to replace than special and nonstandard tools. Cost-cutting measures have forced many tool suppliers to keep a low or even a nonexistent. Standard, off-the shelf tooling can usually be obtained faster than nonstandard tooling.
Fixturing and work holding for CNC machines have only one major purpose – to hold the part rigidly and in the same position for all parts within a batch. Fixtures designed for CNC work do not normally require jigs, pilot holes and other hole locating aids.
Cutting Time and Productivity Increase
The cutting time on the CNC machine is commonly known as the cycle timeand is always consistent. Unlike a conventional machining, where the operators skill, experience and personal fatigue are subject to changes, the CNC machining is under the control of a computer. The small amount of manual work is restricted to the setup and loading and unloading the part. For large batch runs, the high cost of the unproductive time is spread among many parts, making it less significant. The main benefit of a consistent cutting time is for repetitive jobs, where the production scheduling and work allocation to individual machine tools can be done very accurately.
The main reason companies often purchase CNC machines is strictly economic – it is a serious investment. Also, having a competitive edge is always on the mind of every plant manager. The numerical control technology offers excellent means to achieve a significant improvement in the manufacturing productivity and increasing the overall quality of the manufactured parts. Like any means, it has to be used wisely and knowledgeably. When more and more companies use the CNC technology, just having a CNC machine does not offer the extra edge anymore. The companies that get forward are those who know to use the technology efficiently and practice it to be competitive in the global economy.
To reach the goal of major increase in productivity, it is essential that users understand the fundamental principles on which CNC technology is based. These principles take many forms, for example, understanding the electronic circuitry, complex ladders diagrams, computer logic, metrology, machine design, machine principles and practices and many others. Each one has to be studied and mastered by the person in charge. In this handbook, the emphasis is on the topics that relate directly to the CNC programming and understanding the most common CNC machine tools, the machining centers and the lathes (sometimes also called the turning centers). The part quality consideration should be very important to every programmer and machine tool operator and this goal is also reflected in the handbook approach as well as in numerous examples.
TYPES OF CNC MACHINE TOOLS
Different kinds of CNC machines cover an extremely large variety. Their numbers are rapidly increasing, as the technology development advances. It is impossible to identify all the applications; they would make a long list. Here is a brief list of some of the groups CNC machines can be part of:
1. Mills and machining centres
2. Lathes and turning centres
3. Drilling machines
4. Boring mills and profilers
5. EDM machines
6. Punch presses and shears
7. Flame cutting machines
9. Water jet and laser profilers
10. Cylindrical grinders
11. Welding machines
12. Benders, winding and spinning machines, etc.
CNC machining centres and lathes dominate the number of installations in industry. These two groups share the market just about equally. Some industries may give a higher need for one group of machines, depending on their needs. One must remember that there are many different kinds of lathes and equally many different kinds of ma-chining centres. However, the programming process for a vertical machine is similar to the one for a horizontal ma-chine or a simple CNC mill. Even between different ma-chine groups, there is a great amount of general applications and the programming process is generally the same For example, a contour milled with an end mill has a lot in common with a contour cut with a wire.
Mills and Machining Centres
Standard number of axes on a milling machine is three-the X, Y and Z axes. The part set on a milling system is al-cutting tool rotates, it can move up and down (or in and out), but it does not physically follow the tool path.
CNC mills sometimes called CNC milling machines are usually small, simple machines, without a tool changer or other automatic features. Their power rating is often quite low. In industry, they are used tool room work, maintenance purposes, or small part production. They are usually designed for contouring, unlike CNC drills.
CNC machining centres are for more popular and efficient that drills and mills, mainly for their flexibility. The main benefit user gets out of a CNC machining centre is the ability to group
several diverse operations into a single setup. For example, drilling, boring, counter boring, tapping, spot facing and contour milling can be incorporated into a single CNC program. In addition, the flexibility is enhanced by automatic tool changing using pallets to minimize idle time, indexing to a different side of the part, using a rotary movement of additional axes, and a number of other features, CNC machining centres can be equipped with special software that controls the speeds and feeds, the life of the cutting tool, automatic in-process gauging and offset adjustment and other production enhancing and time saving devices.
There are two basic designs of a typical CNC machining centre. There are the vertical and the horizontal machining centres. The major difference between the two types is the nature of work that can be done on them efficiently. For a vertical CNC machining centre, the most suitable type of work are flat parts, either mounted to the fixture on the table, or help in a vise or a chuck. The work that requires machining on two or more faces in a single setup is more desirable to be done on a CNC horizontal machining centre. A good example is pump housing and other cubic-like shapes. Some multi-face machining of small parts can also be done on a CNC vertical machining center equipped with a rotary table.
The programming process is the same for both designs, but an additional axis (usually a B axis) is added to the horizontal design. This axis is either a simple positioning axis (indexing axis) for the table, or a fully rotary axis for simultaneous contouring.
This handbook concentrates on the CNC vertical machining centres applications, with a special section dealing with the horizontal setup and machining. The programming methods are also applicable to the small CNC mills or drilling and/or tapping machines, but the programmer has to conceder their restrictions.
Lathes and Turning Centres
A CNC lathe is usually a machine tool with two axes, the vertical X axis and the horizontal Z axis. The main future of the lathe that distinguishes it from a mill is that the part is rotating about the machine center line. In addition, the cutting tool is normally stationary, mounted in a sliding turret. The cutting tool follows the contour of the programmed tool path. For the CNC lathe with a milling attachment, so called live tooling, the milling tool has its own motor and rotates while the spindle is stationary.
The modern lathe design can be horizontal or vertical. Horizontal type is far more common than the vertical type, but both designs exist for either group. For example, a typical CNC lathe of the horizontal group can be designed with a flat bed or a slant bed, as a bar type, chucker type or universal type. Added to these combinations or many accessories that make a CNC lathe is an extremely flexible machine tool. Typically, accessories such as a tailstock, steady rests or followup rests, part catchers, pullout-fingers and even a third axis milling attachment are popular components of the CNC lathe. A CNC lathe can be very versatile so versatile in fact, that it is often called a CNC turning centre. All text and program examples in this handbook use the more traditional term CNC lathe, yet still recognizing all its modern functions.
PERSONNEL FOR CNC
Computers and machine tools have no intelligence. They cannot think, they cannot evaluate a station in a rational way. Only people with certain skills and knowledge can do that. In the field of numerical control, the skills are usually in the hands of two key peopleone doing the programming, the other doing the machining. Their respective numbers and duties typically depend on the company preference, its size, as well as the product manufactured there. However, each position is a quite distinct, although many companies combine the two functions into a one, often called a CNC programmer/operator.
The CNC programmer is usually the person who has the most responsible in the CNC machine shop. This person is often responsible for the success of numerical control technology in the plant. Equally this person is held responsible for problems related to the CNC operations.
Although duties may vary, the programmer is also responsible for a variety of tasks relating to the effective usage of the CNC machines. In fact, this person is often accountable for the production and quality of all CNC operations.
Many CNC programmers are experienced machinists, who have had a practical, hands-on experience as machine tool operations they know how to read technical drawings and they can comprehend the engineering intent behind the design. This practical experience is the foundation for the ability to ‘machine’ a part in an office environment. A good CNC programmer must be able to visualize all the tool motions and recognize all restricting factories that may be involved. The programmer must be able to collect, analyze process and logically integrate all the collected data into a signal, cohesive program. In simple terms, the CNC programmer must be able to decide upon the best manufacturing methodology in all respects.
In addition to the machining skills, the CNC programmer has to have an understanding of mathematical principles, mainly application of equations, solutions of arcs and angles. Equally important is the knowledge of trigonometry. Even with computerized programming, the knowledge of manual programming methods is absolutely essential to the through understanding of the computer output and the control of this output.
The last important quality of a truly professional CNC programmer is his or her ability to listen to the other people – the engineers, the CNC operators, the managers. Good listing skills are the first prerequisites to become flexible. A good CNC programmer must be flexible in order to offer high programming quality.
CNC Machine Operator
The CNC machine tool operator is a complementary position to the CNC programmer. The programmer and the operator may exist in a single person, as is the case in many small shops. Although the majority of duties performed by conventional machine operator has been transferred to the CNC program, the CNC operator has many unique responsibilities. In typical cases, the operator is responsible for the tool and machine setup, for the changing of the parts, often even for some in-process inspection. Many companies expect quality control at the machine – and the operator of any machine tool, manual or computerized, is also responsible for the quality of the work done on that machine. One of the very important responsibilities of the CNC machine operator is to report findings about each program to the programmer. Even with the best knowledge, skills, attitudes and intentions, the "final" program can always be improved. The CNC operator being the one, who is the closest to the actual machining, knows precisely what extent such improvements can be.
Justifying the Cost of CNC
The cost of a CNC machine might make most manufacturers nervous but the benefits of owning a CNC router will most likely justify the cost in very little time.
The first cost to take into consideration is the machine cost. Some vendors offer bundled deals that include installation, software training and shipping charges. But in most cases, everything is sold separately to allow for customization of the CNC router.
Low-end machines cost from $2,000 to $10,000. they are usually bolt-it yourself kits made of bent sheet metal and use stepper motors. They come with a training video and an instruction manual. These machines are meant for do-it-yourself use, for the signage industry and other very light duty operations. they will usually come with an adapter for a conventional plunge router. accessories such as a spindle and vacuum work holding are options. These machines can be very successfully integrated into a high production environment as a dedicated process or as part of a manufacturing cell. for instance, one of these CNC’s can be programmed to drill hardware holes on drawer fronts before assembly.
Mid-range CNC machines will cost between $10,000 and $100,000. these machines are built of heavier gauge steel or aluminium. They might use stepper motors and sometimes servos; and use rack and pinion drives or belt drives. they will have a separate controller and offer a good range of options such as automatic tool changers and vacuum plenum tables. these machines are meant for heavier duty use in the signage industry and for light panel processing applications.
These are a good option for start-ups with limited resources or manpower. They can perform most operations needed in cabinet making although not with the same degree of sophistication or with the same efficiency.
High-end routers cost upward of $100,000. This includes a whole range of machines with 3 to 5 axes suited for a broad range of applications. these machines will be built out of heavy gauge welded steel and come fully loaded with automatic tool changer, vacuum table and other accessories depending on the application. these machines are usually installed by the manufacturer and training is often included.
Transporting a CNC router carries a considerable cost. With routers weighing anywhere from a few hundred pounds to several tons, freight costs can range from $200 to $5,000 or more, depending on location. remember that unless the machine was built nearby, the hidden cost of moving it from europe or asia to the dealer’s showroom is likely included. additional costs may also be incurred just to get the machine inside once it is delivered as it is always a good idea to use professional riggers to deal with this kind of operation.
Installation and training
CNC vendors typically charge from $300 to $1,000 per day for installation costs. It can take anywhere from a half day to a full week to install and test the router. This cost could be included in the price of buying the machine. some vendors will provide free training on how to use the hardware and software, usually on-site, while others will charge $300 to $1,000 per day for this service.
SAFETY RELATED TO CNC WORK
One the wall of many companies is a safety poster with a simple, yet powerful message:
The first rule of safety is to follow all safety rules.
The heading of this section does not indicate whether the safety is oriented at the programming or the machining level. The season is that the safety is totally independent. It stands on its own and it governs behaviour of everybody in a machine shop and outside of it. At first sight, it may appear that safety is something related to the machining and the machine operation, perhaps to the setup as well. That is definitely true but hardly presents a complete picture.
Safety is the most important element in programming, setup, machining, tooling, fixturing, inspection, chipping, and-you-name it operation within a typical machine shop daily work. Safety can never be overemphasized. Companies talk about safety, conduct safety meeting, display posters, make speeches, call experts. This mass of information and instructions is presented to all of us for some very good reasons. Quite a few are passed on past tragic occurrences – many laws, rules and regulations have been written as a result of inquests and inquire into serious accidence.
At first sight, it may seem that in CNC work, the safety is a secondary issue. There is a lot of automation; a part program that runs over and over again, tooling that has been used in the past, a simple setup, etc. All this can lead to complacency and false assumption that safety is taken care of. This is a view that can have serious consequences.
Safety is a large subject but a few points that relate to the CNC work are important. Every machinist should know the hazards of mechanical and electrical devices. The first step towards a safe work place is with a clean work area, where no chips, oil spills and other debris are allowed to accumulate on the floor. Taking care of personal safety is equally important. Loose clothing, jewellery, ties, scarves, unprotected long hair, improper use of gloves and similar infraction, is dangerous in machining environment. Protection of eyes, ears, hands and feet is strongly recommended.
While a machine is operating, protective devices should be in place and no moving parts should be exposed. Special care should be taken around rotating spindles and automatic tool changers. Other devices that could pose a hazard are pallet changers, chip conveyors, high voltage areas, hoists, etc. disconnecting any interlocks or other safety features is dangers – and also illegal, without appropriate skills and authorization.
In programming, observation of safety rules is also important. A tool motion can be programmed in many ways. Speeds and feeds have to be realistic, not just mathematically "correct". Depth of cut, width of cut, the tool characteristics, all have a profound effect on overall safety.
All these ideas are just a very short summery and a reminder that safety should always be taken seriously.