Computer Numerical Control Vertical Milling Machine Engineering Essay

Technology improvement has been rapidly sourced to introduce improvement in industrial and manufacturing sector. This improvement means a lot to those big companies where improvement such as faster producing rate and lower operating cost means profit and loss to the company.

Automations were being introduced to the world to help improve the efficiency and profit of new generation factories. With the help of new technology, operation cost of the industrial can be minimizing.

Automated machine are becoming a necessity in today world where the machines can operate themselves and produce product that are much accurate and faster compare to a manual or hand craft product.

Milling machine is a machining tool that mill or turn a solid block material into components that are then fix into a prototype or a product of machine. In the olden days, milling machine are operate manually where there are crank to determine the position of the milling bit. Technology improvement brings integration of computer into the milling machine where a computer is connected to motors and this controls the position of the mill bits. These integrations of computers bring a new definition of milling machine call Computer numerical control (CNC) milling machine.

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There are 2 basic type of milling machine, horizontal and vertical, which refers to the orientation of the main spindle. Unlike drill press which holds the workpiece stationary as the drill moves axially to penetrate the material, milling machines also move the workpiece radially against the rotating milling cutter which cuts on its sides as well as its tip. Workpiece and cutter movement are precisely controlled

Computer numerical control (CNC) milling machine is a common and much needed tool in industrial trade. These machines can operate themselves and produce a product repeatedly with almost perfect precision, compare to olden days where parts are mill manually with bigger range of error. This machine can reduce the operator cost and improve efficiency and also accuracy to product produce.

In today world once much expensive CNC milling are also becoming a much popular in hobbyist community. With technology improvement and wider knowledge of computing technology, CNC machine can be build with a much lower cost and by a wider range of people.

Objectives

The objective of this project is to build an affordable price table-top CNC vertical milling machine. And to demonstrate the use of Cartesian XY robot motion control concept with stepper motors, threaded rod and nut to perform the machining operation.

Scope of Work

3D CAD software modeling.

CNC movement and mechanism (kinematics calculation)

Fabrication of parts and assembly of the mechanical body.

Problem Statement

Cost of the CNC machine components and controllers are expensive. It has hence increase the cost to build a CNC machine.

Normally the CNC controller that is available comes in package including controller, servo amplifier and motor, etc, and replacement of the parts by third party products (easily obtained in the market) is not possible due to the differences in design and specification.

Besides that, the replacement of the mentioned parts me is very expensive and sometime it is difficult to source.

Propose Solutions

Arduino microcontroller will be used in this project which it is inexpensive, light and easy to use. Arduino is rich in function libraries to perform certain motion control however it would have slower speed in execution relative to industrial CNC controller.

The parts (electronic, electrical, and mechanical) that are to be used in this prototype are off the shelf and easily obtain in the market.

The designed parts will be much easier to be fabricated and replaced. Also, mechanical parts that are commonly used and sourced in the market will be used to reduce the maintenance cost and spare part inventories.

Open source code of the Arduino controller and common components or accessories of the machine will be the essential parts of this prototype.

Chapter 2

What Is CNC?

CNC is short form for Computer Numerical Control and CNC has been around since the early 1970’s. before this, it was called NC, for Numerical Control.

While people in most walks of life have never heard of this term, CNC is used in almost every form of manufacturing process in one way or another. If you ever join the manufacturing sector, it’s likely that you’ll be dealing with CNC on a regular basis.

Before CNC

A drill press can of course be used to machine holes. A person can place a drill in the drill chuck that is secured in the spindle of the drill press. They can then manually select the desired speed for rotation, and activate the spindle. Then they manually pull on the quill lever to drive the drill into the workpiece being machined.

There is a lot of manual intervention required to use a drill press to drill holes. A person is required to do something almost every step along the way. While this manual intervention may be acceptable for manufacturing companies if but a small number of holes or workpieces must be machined, as quantities grow, so does the likelihood for fatigue due to the tediousness of the operation. And do note that we’ve used one of the simplest machining operations (drilling) for our example. There are more complicated machining operations that would require a much higher skill level (and increase the potential for mistakes resulting in scrap workpieces) of the person running the conventional machine tool.

Machining center

By comparison, the CNC equivalent for a drill press can be programmed to perform this operation in a much more automatic fashion. Everything that the drill press operator was doing manually will now be done by the CNC machine, including: placing the drill in the spindle, activating the spindle, positioning the workpiece under the drill, machining the hole, and turning off the spindle.

How CNC works

As you might already have guessed, everything that an operator would be required to do with conventional machine tools is programmable with CNC machines. Once the machine is setup and running, a CNC machine is quite simple to keep running. In fact CNC operators tend to get quite bored during lengthy production runs because there is so little to do. With some CNC machines, even the workpiece loading process can be automated.

Motion control

All CNC machine types share this commonality: They all have two or more programmable directions of motion called axes. An axis of motion can be linear or rotary. One of the first specifications that implies a CNC machine’s complexity is how many axes it has. Generally speaking, the more axes, the more complex the machine.

The axes of any CNC machine are required for the purpose of causing the motions needed for the manufacturing process. In the drilling example, these (3) axis would position the tool over the hole to be machined (in two axes) and machine the hole (with the third axis). Axes are named with letters. Common linear axis names are X, Y, and Z. Common rotary axis names are A, B, and C.

Programmable accessories

A CNC machine wouldn’t be very helpful if all it could only move the workpiece in two or more axes. Almost all CNC machines are programmable in several other ways. The specific CNC machine type has a lot to do with its appropriate programmable accessories. Again, any required function will be programmable on full-blown CNC machine tools. Here are some examples for one machine type.

Machining centers

Automatic tool changer

Most machining centers can hold many tools in a tool magazine. When required, the required tool can be automatically placed in the spindle for machining.

Spindle speed and activation

The spindle speed (in revolutions per minute) can be easily specified and the spindle can be turned on in a forward or reverse direction. It can also, of course, be turned off.

Coolant

Many machining operations require coolant for lubrication and cooling purposes. Coolant can be turned on and off from within the machine cycle.

The CNC program

A CNC program is nothing more than another kind of instruction set. It’s written in sentence-like format and the control will execute it in sequential order, step by step.

A special series of CNC words are used to communicate what the machine is intended to do. CNC words begin with letter addresses (like F for feedrate, S for spindle speed, and X, Y & Z for axis motion). When placed together in a logical method, a group of CNC words make up a commandthat resemble a sentence.

For any given CNC machine type, there will only be about 40-50 words used on a regular basis. So if you compare learning to write CNC programs to learning a foreign language having only 50 words, it shouldn’t seem overly difficult to learn CNC programming.

The CNC control

The CNC control will interpret a CNC program and activate the series of commands in sequential order. As it reads the program, the CNC control will activate the appropriate machine functions, cause axis motion, and in general, follow the instructions given in the program.

Along with interpreting the CNC program, the CNC control has several other purposes. All current model CNC controls allow programs to be modified if mistakes are found. The CNC control allows special verification functions to confirm the correctness of the CNC program. The CNC control allows certain important operator inputs to be specified separate from the program, like tool length values. In general, the CNC control allows all functions of the machine to be manipulated.

Variant of Milling machine.

Bed mill This refers to any milling machine where the spindle is on a pendant that moves up and down to move the cutter into the work, while the table sits on a stout bed that rests on the floor. These are generally more rigid than a knee mill. Gantry mills can be included in this bed mill category.

Box mill or column mill Very basic hobbyist bench-mounted milling machines that feature a head riding up and down on a column or box way

C-Frame mill These are larger, industrial production mills. They feature a knee and fixed spindle head that is only mobile vertically. They are typically much more powerful than a turret mill, featuring a separate hydraulic motor for integral hydraulic power feeds in all directions, and a twenty to fifty horsepower motor. Backlash eliminators are almost always standard equipment. They use large NMTB 40 or 50 tooling. The tables on C-frame mills are usually 18″ by 68″ or larger, to allow multiple parts to be machined at the same time.

Floor mill These have a row of rotary tables, and a horizontal pendant spindle mounted on a set of tracks that runs parallel to the table row. These mills have predominantly been converted to CNC, but some can still be found (if one can even find a used machine available) under manual control. The spindle carriage moves to each individual table, performs the machining operations, and moves to the next table while the previous table is being set up for the next operation. Unlike other mills, floor mills have movable floor units. A crane drops massive rotary tables, X-Y tables, etc., into position for machining, allowing large and complex custom milling operations.

Gantry mill The milling head rides over two rails (often steel tubes) which lie at each side of the work surface.

Horizontal boring mill Large, accurate bed horizontal mills that incorporate many features from various machine tools. They are predominantly used to create large manufacturing jigs, or to modify large, high precision parts. They have a spindle stroke of several (usually between four and six) feet, and many are equipped with a tailstock to perform very long boring operations without losing accuracy as the bore increases in depth. A typical bed has X and Y travel, and is between three and four feet square with a rotary table or a larger rectangle without a table. The pendant usually provides between four and eight feet of vertical movement. Some mills have a large (30″ or more) integral facing head. Right angle rotary tables and vertical milling attachments are available for further flexibility.

Jig borer Vertical mills that are built to bore holes, and very light slot or face milling. They are typically bed mills with a long spindle throw. The beds are more accurate, and the handwheels are graduated down to .0001″ for precise hole placement.

Knee mill or knee-and-column mill refers to any milling machine whose x-y table rides up and down the column on a vertically adjustable knee. This includes Bridgeports.

Planer-style mill Large mills built in the same configuration as planers except with a milling spindle instead of a planing head. This term is growing dated as planers themselves are largely a thing of the past.

Ram-type mill This can refer to any mill that has a cutting head mounted on a sliding ram. The spindle can be oriented either vertically or horizontally. In practice most mills with rams also involve swiveling ability, whether or not it is called “turret” mounting. The Bridgeport configuration can be classified as a vertical-head ram-type mill. Van Norman specialized in ram-type mills through most of the 20th century. Since the wide dissemination of CNC machines, ram-type mills are still made in the Bridgeport configuration (with either manual or CNC control), but the less common variations (such as were built by Van Norman, Index, and others) have died out, their work being done now by either Bridgeport-form mills or machining centers.

Turret mill More commonly referred to as Bridgeport-type milling machines. The spindle can be aligned in many different positions for a very versatile, if somewhat less rigid machine.

Usage of CNC machine

In the metal removal industry:

Machining processes that have traditionally been done on conventional machine tools are now possible with CNC.  This include all kinds of milling face milling, contour milling, slot milling, drilling, tapping, reaming, boring, and counter boring.

In similar fashion, all kinds of turning operations like facing, boring, turning, grooving, knurling, and threading are done on CNC lathe.

Grinding operations of all kinds like outside diameter (OD) grinding and internal diameter (ID) grinding are also being done on CNC grinders. CNC has even opened up a new technology when it comes to grinding. Contour grinding (grinding a contour in a similar fashion to turning), which was previously infeasible due to technology constraints is now possible with CNC grinders.

In the metal fabrication industry:

In manufacturing terms, fabrication commonly refers to operations that are performed on relatively thin plates. Think of a metal filing cabinet. All of the primary components are made of steel sheets. These sheets are sheared to size, holes are punched in appropriate places, and the sheets are bent (formed) to their final shapes. Again, operations commonly described as fabrication operations include shearing, flame or plasma cutting, punching, laser cutting, forming, and welding. Truly, CNC is heavily involved in almost every facet of fabrication.

CNC back gages are commonly used with shearing machines to control the length of the plate being sheared. CNC lasers and CNC plasma cutters are also used to bring plates to their final shapes. CNC turret punch presses can hold a variety of punch-and-die combinations and punch holes in all shapes and sizes through plates. CNC press brakes are used to bend the plates into their final shapes.

In the electrical discharge machining industry:

Electrical discharge machining (EDM) is the process of removing metal through the use of electrical sparks which burn away the metal. CNC EDM comes in two forms, vertical EDM and Wire EDM. Vertical EDM requires the use of an electrode (commonly machined on a CNC machining center) that is of the shape of the cavity to be machined into the workpiece. Picture the shape of a plastic bottle that must be machined into a mold. Wire EDM is commonly used to make punch and die combinations for dies sets used in the fabrication industry. EDM is one of the lesser known CNC operations because it is so closely related to making tooling used with other manufacturing processes.

In the woodworking industry

As in the metal removal industry, CNC machines are heavily used in woodworking shops. Operations include routing (similar to milling) and drilling. Many woodworking machining centers are available that can hold several tools and perform several operations on the workpiece being machined.

CNC milling using industrial mills

Computer Numerical Control (CNC) Milling is the most common form of CNC. CNC mills can perform the functions of drilling and often turning. CNC Mills are classified according to the number of axes that they possess. Axes are labeled as x and y for horizontal movement, and z for vertical movement, as shown in this view of a manual mill table. A standard manual light-duty mill (such as a Bridgeportâ„¢) is typically assumed to have four axes:

Table x.

Table y.

Table z.

Milling Head z.

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Components of CNC machine

Linear movement

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Ball Bearing Slides

Also called “ball slides”, ball bearing slides are the most common type of linear slide. Ball bearing slides offer smooth precision motion along a single-axis linear design, aided by ball bearings housed in the linear base, with self-lubrication properties that increase reliability. Ball bearing slide applications include delicate instrumentation, robotic assembly, cabinetry, high-end appliances and clean room environments, which primarily serve the manufacturing industry but also the furniture, electronics and construction industries. For example, a widely used ball bearing slide in the furniture industry is a ball bearing drawer slide.

Commonly constructed from materials such as aluminum, hardened cold rolled steel and galvanized steel, ball bearing slides consist of two linear rows of ball bearings contained by four rods and located on differing sides of the base, which support the carriage for smooth linear movement along the ball bearings. This low-friction linear movement can be powered by either a drive mechanism, inertia or by hand. Ball bearing slides tend to have a lower load capacity for their size compared to other linear slides because the balls are less resistant to wear and abrasions. In addition, ball bearing slides are limited by the need to fit into housing or drive systems.

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Roller Slides

Also known as crossed roller slides, roller slides are non-motorized linear slides that provide low-friction linear movement for equipment powered by inertia or by hand. Roller slides are based on linear roller bearings, which are frequently criss-crossed to provide heavier load capabilities and better movement control. Serving industries such as manufacturing, photonics, medical and telecommunications, roller slides are versatile and can be adjusted to meet numerous applications which typically include clean rooms, vacuum environments, material handling and automation machinery.

Consisting of a stationary linear base and a moving carriage, roller slides work similarly to ball bearing slides, except that the bearings housed within the carriage are cylinder-shaped instead of ball shaped. The rollers crisscross each other at a 90° angle and move between the four semi-flat and parallel rods that surround the rollers. The rollers are between “V” grooved bearing races, one being on the top carriage and the other on the base. The travel of the carriage ends when it meets the end cap, a limiting component. Typically, carriages are constructed from aluminum and the rods and rollers are constructed from steel, while the end caps are constructed from stainless steel.

Although roller slides are not self-cleaning, they are suitable for environments with low levels of airborne contaminants such as dirt and dust. As one of the more expensive types of linear slides, roller slides are capable of providing linear motion on more than one axis through stackable slides and double carriages. Roller slides offers line contact versus point contact as with ball bearings, creating a broader contact surface due to the consistency of contact between the carriage and the base and resulting in less erosion.

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Plain bearings

Plain bearings are very similar in design to rolling-element bearings, except they slide without the use of ball bearings.

Plain bearings can run on hardened steel or stainless steel shafting (raceways), or can be run on hard-anodized aluminum or soft steel or aluminum. The specific type of polymer/fluoro-polymer will determine what hardness is allowed.

Plain bearings are less rigid than rolling-element bearings.

Plain bearings handle contamination well and often do not need seals/scrapers.

Plain bearings generally handle a wider temperature range than rolling-element bearings

Plain bearings (plastic versions) do not require oil or lubrication (often it can be used to increase performance characteristics)

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Dovetail slides

Dovetail slides, or dovetail way slides are typically constructed from cast iron, but can also be constructed from hard-coat aluminum, acetal or stainless steel. Like any bearing, a dovetail slide is composed of a stationary linear base and a moving carriage. a Dovetail carriage has a v-shaped, or dovetail-shaped protruding channel which locks into the linear base’s correspondingly shaped groove. Once the dovetail carriage is fitted into its base’s channel, the carriage is locked into the channel’s linear axis and allows free linear movement. When a platform is attached to the carriage of a dovetail slide, a dovetail table is created, offering extended load carrying capabilities.

Since dovetail slides have such a large surface contact area, a greater force is required to move the saddle than other linear slides, which results in slower acceleration rates. Additionally, dovetail slides have difficulties with high-friction but are advantageous when it comes to load capacity, affordability and durability. Capable of long travel, dovetail slides are more resistant to shock than other bearings, and they are mostly immune to chemical, dust and dirt contamination. Dovetail slides can be motorized, mechanical or electromechanical. Electric dovetail slides are driven by a number of different devices, such as ball screws, belts and cables, which are powered by functional motors such as stepper motors, linear motors and handwheels. Dovetail slides are direct contact systems, making them fitting for heavy load applications including CNC machines, shuttle devices, special machines and work holding devices. Mainly used in the manufacturing and laboratory science industries, dovetail slides are not ideal for high-precision applications.

Homemade linear slide

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Assembled sliding element Sliding element – bearings and Teflon

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Stepper motors:

http://amci.com/tutorials/images/stepper-vs-servo.jpg

A stepper motor’s shaft has permanet magnets attached to it. Around the body of the motor is a series of coils that create a magnetic field that interacts with the permanet magnets. When these coils are turned on and off the magnetic field causes the rotor to move. As the coils are turned on and off in sequence the motor will rotate forward or reverse. This sequence is called the phase pattern and there are several types of patterns that will cause the motor to turn. Common types are full-double phase, full-single phase, and half step.

To make a stepper motor rotate, you must constantly turn on and off the coils. If you simply energize one coil the motor will just jump to that position and stay there resisting change. This energized coil pulls full current even though the motor is not turning. The stepper motor will generate a lot of heat at standstill. The ability to stay put at one position rigidly is often an advantage of stepper motors. The torque at standstill is called the holding torque.

Because steppers can be controlled by turning coils on and off, they are easy to control using digital circuitry and microcontroller chips. The controller simply energizes the coils in a certain pattern and the motor will move accordingly. At any given time the computer will know the position of the motor since the number of steps given can be tracked. This is true only if some outside force of greater strength than the motor has not interfered with the motion.

An optical encoder could be attached to the motor to verify its position but steppers are usually used open-loop (without feedback). Most stepper motor control systems will have a home switch associated with each motor that will allow the software to determine the starting or reference “home” position.

Servo motors:

There are several types of servo motors but I’ll just deal with a simple DC type here. If you take a normal DC motor that can be bought at Radio Shack it has one coil (2 wires). If you attach a battery to those wires the motor will spin. See, very different from a stepper already!. Reversing the polarity will reverse the direction. Attach that motor to the wheel of a robot and watch the robot move noting the speed. Now add a heavier payload to the robot, what happens? The robot will slow down due to the increased load. The computer inside of the robot would not know this happened unless there was an encoder on the motor keeping track of its position.

So, in a DC motor, the speed and current draw is a affected by the load. For applications that the exact position of the motor must be known, a feedback device like an encoder MUST be used (not optional like a stepper).

The control circuitry to perform good servoing of a DC motor is MUCH more complex than the circuitry that controls a stepper motor.

Comparison between stepper motor and servo motor

Characteristics

Servo Motor (DC Brushed

Stepper (Hybrid)

Cost

The cost for a servo motor and servo motor system is higher than that of a stepper motor system with equal power rating.

This feature would have to go to stepper motors. Steppers are generally cheaper than servo motors that have the same power rating.

Versatility

Servo motors are very versatile in their use for automation and CNC applications.

Stepper motors are also very versatile in their use for automation and CNC applications. Because of their simplicity stepper motors may be found on anything from printers to clocks.

Reliability

it depends on the environment and how well the motor is protected.

The stepper takes this category only because it does not require an encoder which may fail.

Frame Sizes

Servo motors are availible in a wide variety of frame sizes, from small to large motors capable of running huge machines. Many of the motors come in NEMA standard sized.

Stepper motors do not have as many size selections as servo motors in the large sizes. However stepper motors may still be found in a variety of NEMA frame sizes.

Setup Complexity

Servo motors require tuning of the (PID) closed loop variable circuit to obtain correct motor function.

Stepper motors are almost plug-and-play. They require only the motor wires to be wired to the stepper motor driver.

Motor Life

The brushes on servo motors must be replaced every 2000 hours of operation. Also encoders may need replacing.

The bearing on stepper motors are the only wearing parts. That gives stepper motors a slight edge on life.

Low Speed High Torque

Servo motors will do fine with low speed applications given low friction and the correct gear ratio

Stepper motors provide most torque at low speed (RPM).

High speed High Torque

Servo motors maintain their rated torque to about 90% of their no load RPM.

Stepper motors lose up to 80% of their maximum torque at 90% of their maximum RPM.

Repeatability

Servo motors can have very good repeatability if setup correctly. The encoder quality can also play into repeatability.

Because of the way stepper motors are constructed and operate they have very good repeatability with little or no tuning required.

Overload Safety

Servo motors may malfunction if overloaded mechanically.

Stepper motors are unlikely to be damages by mechanical overload.

Power to Weight/Size ratio

Servo motors have an excellent power to weight ratio given their efficiency.

Stepper motors are less efficient than servo motors which usually means a smaller power to weight/size ratio.

Efficiency

Servo motors are very efficient. Yielding 80-90% efficiency given light loads.

Stepper motors consume a lot of power given their output, much of which is converted to heat. Stepper motors are usually about 70% efficient but this has some to do with the stepper driver.

Flexibility in motor resolution

Since the encoder on a servo motor determines the motor resolution servos have a wide range of resolutions available.

Stepper motors usually have 1.8 or 0.9 degree resolution. However thanks to micro-stepping steppers can obtain higher resolutions. This is up to the driver and not the motor.

Torque to Inertia Ratio

Servo motors are very capable of accelerating loads.

Stepper motors are also capable of accelerating loads but not as well as servo motors. Stepper motors may stall and skip steps if the motor is not powerful enough.

Least Heat production

Since the current draw of a servo motor is proportional to the load applied, heat production is very low.

Stepper motors draw excess current regardless of load. The excess power is dissipated as heat.

Reserve Power and Torque

A servo motor can supply about 200% of the continuous power for short periods.

Stepper motors do not have reserve power. However stepper motors can brake very well.

Noise

Servo motors produce very little noise.

Stepper motors produce a slight hum due to the control process. However a high quality driver will decrease the noise level.

Resonance and Vibration

Servo motors do not vibrate or have resonance issues.

Stepper motors vibrate slightly and have some resonance issues because of how the stepper motor operates.

Availability

Servo motors are not as readily available to the masses as are stepper m