Milling machines: An introduction
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Published: Tue, 25 Apr 2017
The ability of any nation to manufacture is essential but to deploy that ability in a certain nation is the trick as every nation defers from one another, so in order to enhance Arab nation ability to manufacture we will provide a better cheaper design for a CNC milling machine.
The Primary Objective of This Project is to present and develop a prototype for a substitution of the expensive hard to acquire machining facility for the Middle East industrial society; these machining facilities include the regular Turning, Drilling, Shaping, Milling, and Grinding Machines.
Our primary survey concluded that there always must be a turning machine beside a milling machine, but a milling machine (CNC milling machine) can include the rest of processes required within its operation to a good degree and the turning operation to an acceptable degree. So we chose the milling machine as our initial model to undergo our optimization plan. Therefore our Primary objective of choice is to Design a Basic Three Axis CNC milling machine. Also to manufacture a working Prototype for this machine fabricated all with nationally available components in order to minimize its cost but maintains an acceptable level of accuracy to suite a Computer Numerical Control Machine. Another Thing that we must point out is that in order to accomplish this project we need more than one trial to find the suitable combination of components and specs to pursue in order that this machine comes to light.
1.2 Proposed Approach
A new approach is needed that approach is based upon the 2010-2015 World Outlook for Computer Numerical Controls . The results of that survey were mainly based upon word wide research includes Middle East and North Africa. These results showed that any machine tool required for the Egyptian industrial society could be one of the following: Turning, Drilling, Shaping, Milling, and Grinding. So the first step was to pick up the operation over which this thesis will focus on. Milling can include in its operation: Drilling, Shaping, and Grinding. And might contain Turning under certain conditions. So milling was chosen. And this result might be verified by the fact that 90% of the working machine tools in the field of traditional CNC machines are either Milling or Turning Centers.
Now we will compare different configuration of milling machines
Case 1: A Universal Milling Machine.
Case 2: A regular CNC Milling Machine.
Case 3: Gantry style CNC Milling Machine.
Case 1: A Universal Milling Machine
From Cost point of view: Cost is high because of the usage of large material to build the bulky parts of the machine tool might be as high as a CNC Milling Machine
From Design point of view: Design level might be considered low compared with the design required in both the regular CNC machine and our machine.
From Accuracy Point of view: Accuracy is very low compared to the other two cases but could be difficulty approached with the use of very skilled labor and with high cost.
Case 2: A regular CNC machine
From Cost Point of view: Cost is much higher because of the usage of both large material amounts with high machining grade as will as the cost of the motion control equipments.
From Design Point of view: Design is better than the first case for every part is designed to enhance the entire machining operation.
From Accuracy Point of view: Attained accuracy is very high due to perfect guide ways assisted by ball screws along with feed back control.
Case 3: A gantry CNC milling machine
From Cost Point of view: Cost is mush less than the later case and might be less that the first case as the entire machine is only build from a metal sheet and some Bars.
From Design Point of view: Design is much sophisticated than the first and the second case as the design is optimized to hold the predetermined forces as a light weight structure.
From Accuracy Point of view: Accuracy is for sure less than the second case but higher than the first case as there is no feedback control technique employed.
Discussion of results:
For case 1: Universal Milling Machine
- Our results don’t imply that the use of that machine is not appropriate but we only mean that each application requires a certain machine and also each goal.
- The cost is very high in EGYPT as almost all of them is imported but not made in EGYPT but there maintenance and overhauling is available domestically.
For Case 2: three axis CNC milling machine Vertical Column type.
- The usage of this type of machines in any workshop requires a great deal of money so most of the manufacturers comprises between money and accuracy.
- The cost is definitely much higher as there is almost no one building or even providing maintenance for these machines in EGYPT.
For Case 3: Gantry style CNC milling machine.
- The usage of this machine is aimed to workshops who would like to get both cheap machining with good accuracy.
- The cost is very low as of the innovative design of putting the entire axis moving over each other rather than independent from each other as in the regular design of any CNC machine
2.1 Milling Machine
Milling is the process of machining flat, curved, or irregular surfaces by feeding the work piece against a rotating cutter containing a number of cutting edges. The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the work piece.
2.1.2 Milling Machine History
Unlike lathes, which have been known for thousands of years, milling machines are less than two hundred years old. Because they require much more power than hand-driven lathes, their introduction had to wait for the invention of industrial water and steam power. Also, all their mechanical components had to first be made available, such as accurately fitted slides, large castings to resist cutting forces, calibrated lead screws, and hardened steel cutting tools.
Eli Whitney is credited with inventing the first milling machine about 1818, but the knee-and-column support arrangement of the universal milling machine of Joseph A. Brown (later of Brown and Sharpe) dates from 1862 and marks an important step in the machine’s development. During the last half of the nineteenth Century, milling machines gradually replaced shapers and planers which have lathe-type, single-point tool bits that move over the work in a straight line and scrape off metal one stroke at a time. Milling machines, with their continuous cutting action, not only remove metal faster than shapers and planers, they perform additional operations like cutting helices for gears and twist drills. Today, milling machines greatly outnumber shaping and planning machines. Americans in New England and later the Midwest continuously added features leading to the modern milling machine.
Another important development came in the 1930s when Rudolph Bannow and Magnus Wahlstrom brought out the Bridgeport-style vertical milling machine. This design offers versatility and economy in place of the higher metal removal rates of traditional horizontal milling machines. Because of this versatility, there are more Bridgeport-style mills in existence today than any other milling machine design. Horizontal mills are now usually reserved for production applications where high metal removal rates on identical parts are needed, not prototyping and short runs. Bridgeport-style machines are also called knee-and-column machines and turret mills.
For production applications, there are large, expensive milling machines with three or more axes under computer control. Some machines perform all operations including automatic tool changing. However, today there is an intermediate step between a manual mill and a fully automated one. Adding a computer, digital readouts, and actuators to the X- and Y-axes of a Bridgeport-style mill does this. Not only can this enhanced machine tirelessly perform all its existing repetitive functions, it also has added new capabilities.
Now the mill can engrave (drive the tool to cut numbers and letters in various sizes and fonts), cut radii and angles without a rotary table, make islands, pockets, and cut ellipses, and frames. Entering the position, diameter and number of holes, automates cutting a bolt-hole pattern; the system does the math. The computer can automatically compensate for the reduced diameter of re sharp end milling cutters, saving time and money. The system can be manually programmed through its control panel, use stored programs, “learn” new tasks by memorizing a series of manual operations as the operator makes the first part, or accept files from CAD programs.
2.1.1 Type of MILLING MACHINES
Milling machine are classified on the base of the position of their spindle. The spindle operates in either a vertical or horizontal position or on base of construction. In a following section we represent the different type of milling machine.
Column Knee-type Milling Machine Fig 2 machines are characterized by a vertically adjustable worktable resting on a saddle which is supported by a knee. The knee is a massive casting that rides vertically on the milling machine column and can be clamped rigidly to have column in a position where the milling head and milling machine spindle are properly adjusted for operation. The plain vertical column knee type machines are characterized by a spindle located vertically, parallel to the column face, and mounted in a sliding head that can be fed up and down by hand or power. Modern vertical milling machines are designed so the entire head can also swivel to permit working on angular surfaces. The plain horizontal milling machine’s column contains the drive motor and gearing and a fixed position horizontal milling machine spindle. An adjustable overhead arm containing one or more arbor supports projects forward from the top of the column. The arm and arbor supports are used to stabilize long arbors. Supports can be moved along the overhead arm to support the arbor where support is desired depending on the position of the milling cutter or cutters.
UNIVERSAL MILLING MACHINE The basic difference between a universal milling machine and a plain column knee type milling machine is the addition of table swivel housing between the table and the saddle of the universal machine. This permits the table to swing up to 45° in either direction for angular and helical milling operations. The universal machine can be fitted with various attachments such as the indexing fixture, rotary table, slotting and rack cutting attachments, and various special fixtures.
RAM-TYPE MILLING MACHINEThe ram-type milling machine is characterized by a spindle mounted to a movable housing on the column to permit positioning the milling cutter forward or rearward in a horizontal plane. Two popular ram-type milling machines are the universal milling machine and the swivel cutter head ram-type milling machine.
Gantry Milling Machinefigure 5is simplest construction of milling machine is usually use in CNC machine for cutting wood plastic and light metal
Rotary-Table Milling MachinesThese are also called continuous milling machines, as the work picas are set up without stopping the operation. Rotary-table machines are highly productive; consequently, they are frequently used for both batch and mass production. The Work prices being machined are clamped in fixtures installed on the rotating table (2) figure 6. The machines may be equipped with one or two spindle heads (1).When several surfaces are to be machined; the Work pieces are indexed in the fixtures after each complete revolution of the table. The machining cycle provides as many table revolutions as the number of surfaces to be machined.
2.1.2 MILLING CUTTERS
The milling cutters are selected for each specified machining duty. The milling cutter may be provided with a hole to be mounted on the arbor of the horizontal milling machines, or provided with a straight or tapered shank for mounting on the vertical or horizontal milling machine. Figure 7 visualizes commonly used milling cutters during their operation. These include the following:
- Plain milling cutters are either straight or helical ones. Helical milling cutters are preferred for large cutting widths to provide smooth cutting and improved surface quality (Figure 7 a). Plain milling cutters are mainly used on horizontal milling machines.
- Face milling cutters are used for the production of horizontal (Figure 7 b), vertical (Figure 7 c), or inclined (Figure 7 d) at surfaces. They are used on vertical milling machines, planer type milling machines, and vertical milling machines with the spindle swiveled to the required angle.
- Side milling cutters are clamped on the arbor of the horizontal milling machine and are used for machining of the vertical surface of a shoulder (Figure 7 e) or cutting a keyway (Figure 7 f).
- Interlocking (staggered) side mills (Figure 7 g) mounted on the arbor of the horizontal milling machines are intended to cut wide keyways and cavities.
- Slitting saws (Figure 7 h) are used on horizontal milling machines.
- Angle milling cutters, used on horizontal milling machines, for the production of longitudinal grooves (Figure 7 I) or for edge chamfering.
- End mills are tools of a shank type, which can be mounted on vertical milling machines (or directly in the spindle nose of horizontal milling machines). End mills may be employed in machining keyways (Figure 7 j) or vertical surfaces (Figure 7 k).
- Key-cutters are also of the shank type that can be used on vertical milling machines. They may be used for single-pass milling or multi pass milling operations (Figures 7 l and 7 m).
- Form-milling cutters are mounted on horizontal milling machines. Form cutters may be either concave as shown in (Figure 7 n) or convex as in Figure (7 o).
- T-slot cutters are used for milling T-slots and are available in different sizes. The T-slot is machined on a vertical milling machine in two steps: 1-Slotting with end mill (Figure 7 j) 2-Cutting with T-slot cutter (Figure 7 p).
- Compound milling cutters are mainly used to produce compound surfaces. These cutters realize high productivity and accuracy (Figure 7 q).
- Inserted tool milling cutters have a main body that is fabricated from tough and less expensive steel. The teeth are made of alloy tool steel, HSS, carbides, ceramics, or cubic boron nitride (CBN) and mechanically attached to the body using set screws and in some cases are brazed. Cutters of this type are confined usually to large-diameter face milling cutters or horizontal milling cutters (Figure 7 q).
- Gear milling cutters are used for the production of spur and helical gears on vertical or horizontal milling machines (Figures 7 r and 7 s). Gear cutters are form-relieved cutters, which are used to mill contoured surfaces. They are sharpened at the tooth face. Hobbing machines and gear shapers are used to cut gears for mass production and high-accuracy demands.
2.1.3 Milling operation
Milling is the removal of metal by feeding the work past a rotating multi toothed cutter. In this operation the material removal rate (MRR) is enhanced as the cutter rotates at a high cutting speed. The surface quality is also improved due to the multi cutting edges of the milling cutter. The action of the milling cutter is totally different from that of a drill or a turning tool. In turning and drilling, the tools are kept continuously in contact with the material to be cut, whereas milling is an intermittent process, as each tooth produces a chip of variable thickness. Milling operations may be classified as peripheral (plain) milling or face (end) milling (Figure 8).
In peripheral milling, the cutting occurs by the teeth arranged on the periphery of the milling cutter, and the generated surface is a plane parallel to the cutter axis. Peripheral milling is usually performed on a horizontal milling machine. For this reason, it is sometimes called horizontal milling. The appearance of the surface and also the type of chip formation are affected by the direction of cutter rotation with respect to the movement of the WP. In this regard, two types of peripheral milling are differentiable, namely, up-milling and down-milling.
Up-milling is accomplished by rotating the cutter against the direction of the feed of the WP (Figure 9 a). The tooth picks up from the material gradually; that is, the chip starts with no thickness and increases in size as the teeth progress through the cut. This means that the cycle of operation to remove the chip is first a sliding action at the beginning and then a crushing action takes place, which is followed by the actual cutting action. In some metals, up-milling leads to strain hardening of the machined surface, and also to chattering and excessive teeth blunting. Advantages of up-milling include the following:
- It does not require a backlash eliminator.
- It is safer in operation (the cutter does not climb on the work).
- Loads on teeth are acting gradually.
- Built-up edge (BUE) fragments are absent from the machined surface.
- The milling cutter is not affected by the sandy or scaly surfaces of the work.
Down-Milling is accomplished by rotating the cutter in the direction of the work feed, as shown in Figure 9b. In climb milling, as implied by the name, the milling cutter attempts to climb the WP. Chips are cut to maximum thickness at initial engagement of cutter teeth with the work, and decrease to zero at the end of its engagement.
The cutting forces in down milling are directed downward. Down-milling should not be attempted if machines do not have enough rigidity and are not provided with backlash eliminators. Under such circumstances, the cutter climbs up on the WP and the arbor and spindle may be damaged.
Advantages of down-milling include the following:
- Fixtures are simpler and less costly, as cutting forces are acting downward.
- Flat WPs or plates that cannot be held can be machined by down-milling.
- Cutter with higher rake angles can be used, which decreases the power requirements.
- Tool blunting is less likely.
- Down-milling is characterized by fewer tendencies of chattering and vibration, which leads to improved surface finish.
- up milling
- down milling
In face milling, the generated surface is at a right angle to the cutter axis. When using cutters of large diameters, it is a good practice to tilt the spindle head slightly at an angle of 13° to provide some clearance, which leads to an improved surface finish and eliminate tool blunting. Face milling is usually performed on vertical milling machines; for this reason, the process is called vertical milling, which is more productive than plain milling.
2.2 Computer Numerical Control
The abbreviation CNC stands for computer numerical control, and refers specifically to a computer “controller” that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via a software load program.
2.2.1 Historical overview
CNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.
Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and now is commonly tied directly into plant networks. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements.
The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action has been dramatically reduced.
With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required changing the machine to produce different components.
2.2.2 Production environment
A series of CNC machines may be combined into one station, commonly called a “cell”, to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator’s mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.
2.2.3 CNC Components and Control System
The two main components of a CNC machine are the machine tool and the machine control unit (MCU). The MCU is the computer that operates the machine tool. There are many manufacturers of these control units, so program codes may vary somewhat between machines.
Figure 10 shows three types of control systems for NC and CNC machines. The least expensive type is called a point-to-point (PTP) control system. A PTP control system can only move a tool in a straight line and is limited to hole operations (drilling, boring, reaming, etc.) and spot welding. The tool path between points is not controllable in a PTP system.
A straight-cut control system is capable of moving the cutting tools in a straight line between points. However, only one axis drive motor can move at one time, so arcs and angles are not possible.
The contouring control system is the most expensive and flexible of the three systems. With the recent advances in electronics, the price difference between these systems has narrowed so much that every CNC machine is now manufactured with a contouring control system. A contouring system is capable of doing what the other systems can do while also being able to cut arcs and angles.
2.2.3 Positioning Formats
A CNC program is written using the Cartesian coordinate system. The CNC tool can be moved inside the rectangular coordinates in either two or three dimensions. Lathes are programmed two-dimensionally using only the x- and y-axes, while milling machines are programmed three-dimensionally using the x-, y-, and z-axes.
The z-axis is always the axis that determines the depth of cut. The x-axis is parallel to the longest length of the machine table, and the y-axis is parallel to the shortest length of the machine table. CNC programs can be written in one quadrant or four quadrants.
CNC machines can be positioned in two ways with respect to the Cartesian coordinate system. They can be positioned in an absolute format or an incremental format. In the absolute format, all machine locations are taken from starting point (0, 0), which is referred to as the home position. In incremental format there are no assigned or home coordinates. The cutter tool bit moves in spaces (increments) relative to the coordinates previously listed.
The absolute format is typically used for the main body of a CNC program. This allows the cutting tool to be given a home position to start and end its job. The incremental format is used in the middle of a program when a repetitious cycle (commonly referred to as a canned cycle) is needed. A canned cycle is a cutting pattern that can be programmed to repeat as many times as desired. It can reduce the size of a program tremendously, depending on the number of machining operations involved. The size of a program is reduced by a canned cycle because a programmer does not have to give the fixed coordinates of each move of the cutting tool. A programmer must only give the distance from each previous move of the cutter for one cycle. After that, a programmer merely has to list the number of times the cycle must be repeated. A programmer can also call up the canned cycle and use it again to do a similar machining operation in another part of the work piece.
2.2.4 CNC Programming
Because of the added feature of a computer’s memory, CNC programming is a lot easier and simpler than conventional NC programming. Most CNC machines can be programmed using a standardized list of codes that were developed by the Electronics Industries Association (EIA), as shown in Table 1. These codes include preparatory functions or G codes. Some of these functions include cutting an arc in a clockwise direction (G02 circular interpolation) and telling the machine that the dimensions will be given in millimeters (G71-metric programming).
2.2.5 Task Flow in CNC
The task flow that is needed for producing apart using a CNC machine can be summarizedas Fig.1.2.The tasks can be classified as the following three types:
- Offline tasks: CAD, CAPP, CAM.
- Online tasks: CNC machining, monitoring and On Machine Measurement.
- Post-line tasks: Computer Aided Inspection (CAI), post-operation.
Offline tasks are the tasks that are needed to generate apart program for controlling a CNC machine. In the offline stage, after the shape of apart has been decided, a geometry model of this part is created by 2D or 3D CAD. In general, CAD means Computer Aided Design
After finishing geometric modeling, Computer Aided Process Planning, CAPP, is carried out where necessary information for machining is generated. In this stage, the Selection of machine tools, tools, jig and fixture, decisions about cutting conditions, Scheduling and machining sequences are created. Because process planning is very Complicated and CAPP is immature with respect to technology, process planning generally depends on the know how of a process planner.
CAM (Computer Aided Manufacturing) is executed in the final stage for generating apart program. In this stage, tool paths are generated based on geometry information from CAD and machining information from CAPP. During tool path generation, interferences between tool and work piece, minimization of machining time and tool change, and machine performance are considered .In particular; CAM is an essential tool to generate 2.5D or 3D tool paths for machine tools with more than three axes.
Online tasks are those that are needed to machine parts using CNC machines. A part program, being the machine understandable instructions, can be generated in the above mentioned offline stage and part programs for a simple part can be directly edited in CNC by the user. In this stage, the CNC system reads and interprets part programs from memory and controls the movement of axes. The CNC system generates instructions for position and velocity control based on the part program and servo motors are controlled based on the instructions generated. As the rotation of a servo motor is transformed into linear movement via ball screw mechanisms, the work piece or tool is moved and finally, the part is machined by these movements.
In the online stage, the status of the machine and machining process may be monitored during machining. Actually, tool breakage detection, compensation of thermal deformation, adaptive control, and compensation of tool defection based on monitoring of cutting force, heat, and electric current are applied during machining. On Machine Measurement is also used to calculate machining error by inspecting the finished part on the machine, returning machining errors to controller to carryout compensation.
The post-line task is to carryout CAI (Computer Aided Inspection), inspecting the finished part. In this stage, inspection using a CMM (Coordinate Measurement Machine) is used to make a comparison between the result and the geometry model in order to perform compensation. The compensation is executed by modifying tool Compensation or by doing post operations such as re machining and grinding. Reverse engineering, meaning that the shape of the part is measured and a geometric model based on the measured data is generated, is included in this stage. As mentioned above, through three stages, it is possible for machine tools not only to satisfy high accuracy and productivity but also to machine parts with complex shape as well as simple shapes. Because CNC machines can machine a variety of parts by changing the part program and repetitively machine the same part shape by storing part programs, CNC machines can be used for general purposes.
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