This Article takes an In-depth look at the CNC Milling
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The CNC process originated in the 1950s and saw significant advancement in the 1980s with the integration of computer technology. Unlike traditional production methods, CNC starts with a digital rendering created by a computer, which produces a two or three-dimensional model of the intended part. CAD, or computer-aided design, is employed to generate the instructions that direct the CNC machine throughout the manufacturing process.
CAD software meticulously details the measurements and specifications needed for the part. Engineers and designers use it to design prototypes for review, refinement, and approval. This technology allows for detailed examination of concepts without the immediate need for physical prototypes, thus saving time and reducing labor costs. Many of the products available today were initially developed using CAD software.
Once a part has proven to be acceptable, it has to be converted to a programming language for the CNC machine. This is completed through the use of CAM, computer aided manufacturing, that changes the instructions for the CNC machine into either general or miscellaneous code, G-Code or M-Code.
The translated information is sent on to production for the setup of the CNC machine and downloading of the CAD programming. Set up of the equipment includes adding tools, inserting the raw material, and testing of the process. Once the CNC machine is programmed, the raw material or workpiece is inserted. The machine automatically performs the set of programmed instructions to create a finished part.
The CNC process is capable of reshaping a variety of materials such as metal, plastic, glass, or wood. It can be programmed to produce custom designed parts to exacting and accurate specifications in minutes. Modern manufacturing depends on CNC for the efficient and economical production of a varied number of products.
CNC milling is the most common form of CNC process and was the first type used with CNC machines in 1952. Milling is one of the oldest manufacturing processes that began in the early part of the 19th Century. It is a perfect partner for the CNC method since it is designed to remove material from a workpiece, which is at the heart of the CNC process.
The milling process starts with positioning the workpiece on the CNC machine’s work surface or holding device. Milling tools are then installed into the machine’s spindle. Depending on the programmed requirements from CAD, the milling can proceed in either a horizontal or vertical direction. After securing the workpiece, downloading the program, and attaching the tool, the operator initiates the program to commence the milling process.
Depending on the instructions from CAD, the workpiece is manipulated, shifted, rotated, and positioned for the work of the cutting tool. The cuts can come in three possible forms: a slow feed into a stationary tool, the tool moving across a stationary workpiece, or both methods combined. Most CNC machines use the climb milling operation where the workpiece and the cutter are moving in the same direction. The other method of milling, conventional, is when the cutter and workpiece are moving in opposite directions.
Milling is often employed as a finishing or secondary step for a workpiece that has undergone initial machining. It is used to add specific features such as holes, slots, or threads. The fundamental concept of milling within the CNC process involves the gradual removal of material from the workpiece to achieve the desired shape or form.
In the first phase of milling, the cutting tool makes small cuts to form an approximation of the shape to be produced. After the initial pass, the tool passes over the workpiece multiple times making accurate and precise cuts with each pass to achieve the exact features and specifications for the final part. Complex and intricate parts may require multiple machine setups to complete fabrication.
A milling cutter can move along multiple axes and make several shapes, slots, holes, and other impressions. The process is designed to move across a set of axes with X and Y representing the horizontal movement. The Z axis is vertical movement while the W axis is diagonal movement across a vertical plane. CNC milling machines have three to five axes and can produce parts that are impossible to create using any other method.
The accuracy of CNC milling makes it suitable for several industries in the production of a variety of items. Using a CAD program, CNC milling can create prototypes for examination, single parts, multiple parts, or complete small runs. Its versatility contributes to its popularity as a production process. CNC milling has four common milling processes.
Face milling involves machining a flat surface that is perpendicular to the cutter's axis. This process uses cutters with teeth located both on the periphery and the face of the tool. The peripheral teeth perform the cutting action, while the face teeth provide a finishing touch. Face milling is known for producing high-quality finishes and can be executed using either vertical or horizontal methods.
Plain Milling, Surface Milling, or Slab Milling is when the milling cutter axis is parallel to the surface being milled. It is done with the workpiece mounted parallel to the surface of the milling machine table with the cutter mounted on a standard milling machine arbor. The arbor is supported in a horizontal plane between the milling machine spindle and one or more arbor supports. The workpiece is clamped directly to the table. Plain milling tools have teeth on the periphery of the cutting tool. Either wide or narrow cutters can be used where narrow cutters make deep cuts, and wide cutters are used for large surfaces. If the workpiece needs the removal of large amounts of material, the operator uses a coarse-toothed cutter, slow cutting speeds, and fast feed rates to produce part‘s approximate geometry, which is followed by a finer toothed cutter, faster cutting speed, and slower feed rate for details.
Angular milling is used to machine flat surfaces where the axis of the cutting tool is angled relative to the surface of the workpiece. This technique employs a single-angle milling cutter to create angular surfaces such as chamfers, serrations, and grooves. Dovetail milling, for example, is a form of angular milling where the cutter's angle may be 45°, 50°, 55°, or 60°, depending on the design specifications. Typically, a side milling cutter is used to rough out the tongue or groove, followed by an angle milling cutter to refine the angular sides and base.
Form milling is a milling function for irregular surfaces such as a curved flat surface or all curves. It can complete the shaping in a single cut using a formed milling cutter or fly cutter shaped to the contour of the cut. Common form milling involves milling half-round recesses and beads or quarter-round radii on workpieces. The types of cutters for form milling are convex, concave, and corner rounding cutters that can grind to a required circular diameter. Form milling can create intricate patterns or mill several complex surfaces in a single cut. Hemispherical and semi-circular cavities, beads, and contours can also be shaped by this process.
A gear cutting machine is used to rough out and finish gear teeth, to fit gears, and to fine finish and round off the butt ends of gear teeth. They are used to make spur, helical, herringbone, and worm gears. Vertical models are made with either a movable table or stationary stand. Cutters come in different shapes and sizes depending on the number of teeth for the gear design.
Straddle milling involves machining two or more parallel vertical surfaces in a single pass. This is achieved by mounting two milling cutters on the same arbor, positioned at the precise width of the workpiece. Both sides of the workpiece are machined simultaneously, ensuring exact and precise dimensions. The workpiece is typically secured on an indexing fixture or a vertical swivel vise during this process.
Profile milling is used to rough machine or finish vertical or slanted surfaces with different milling cutters and can have multi-axes for 2D and 3D convex or concave shapes. The cutting tools can be parallel or perpendicular to the plane of the workpiece. Round inserts and radius concept cutters are normally used for roughing and semi-roughing while ball nose end mills are used for finishing and super-finishing.
Gang milling involves machining multiple surfaces of a workpiece simultaneously by feeding it into several cutters, which can be of varying diameters. This method is ideal for creating intricate cuts on complex parts and can significantly reduce production time. Gang milling is particularly useful for workpieces requiring multiple cuts, such as slots, flat surfaces, and angular grooves.
These specialized milling techniques represent just a fraction of the diverse range of milling equipment available. As designs and components become more complex, new milling methods will continue to be developed. Despite being an established technique, milling remains a fundamental aspect of modern manufacturing.
Regardless of the variety of milling machines, each of the various types have the same basic features – interface, column, knee, saddle, worktable, spindle, arbor, ram, and machine tool. The interface is the source of information for the machine and is where the data from CAD is downloaded to provide instructions for manufacturing the desired shape.
The various parts of the machine are its physical characteristics and structure. The column is the main support with a base and has the spindle attached to hold the arbor. The adjustable ram can slide back and forth increasing its movement along the X and Y axis. A secondary function of the column is the inclusion of additional components such as oil or a coolant reservoir. The supporting mechanism for the table and saddle is the knee. The saddle allows the worktable to move parallel to the spindle and be horizontally adjusted. The workpiece is placed on the worktable and is held in place by a form holding mechanism.
The arbor is the component where the milling tool is connected and held securely. It comes in various lengths and diameters, which can be adjusted based on the specific process and application. Types of arbors include standard, screw, slitting saw, end milling, and shell end.
The final part of a milling machine is the tool, which is held by the spindle and removes the material from the workpiece. There are several types of cutters that are chosen depending on the part to be fabricated. Milling tools come in a variety of sizes, shapes, and designs. They are classified by several categories that include number, arrangement and spacing of their teeth, material composition, length, diameter, and geometry. New uses for CNC milling has increased the kinds, styles, and variety of tools.
CNC machine tools are designed to produce specialized parts to exacting specifications and details. Though they may bear characteristics similar to handle tools, they are capable of performing complex intricate operations as a part of CNC programming.
This cutter is designed to remove significant amounts of material, resulting in a rough surface finish. Known as a "rippa" cutter, it features a wavy arrangement of teeth that produce small chips, which contributes to its ability to create a rough surface texture.
Slab cutters are designed to be lightweight and are used for removing material from slabs or large surfaces. They can be employed with both horizontal and vertical milling machines. Constructed from high-speed steel, these cutters are commonly utilized in gang milling operations.
This cutter features side teeth for drilling operations and is referred to as a bottom cutter, capable of cutting in all directions. It comes in two variations: center and non-center. Center tools have cutting edges on both sides, whereas non-center tools have a cutting edge on only one side.
Hollow milling cutters, or hollow mills, are "inside-out end mills" and look like a pipe with cutting edges on the inside. As turret lathes and screw machine devices, they are an alternative to turning with a box tool on a milling machine or a drill press.
Ball cutters, also known as ball end mills, are designed to reduce stress and can cut three-dimensional shapes. Their rounded ends allow for precise and versatile shaping.
These gear cutters mill at a pressure angle of 14.5o and have several teeth. There are eight types of these cutters that can cut gears of 12 teeth and more.
Side and circumference cutting teeth characterize these early milling cutters, which come in various diameters and widths. The side teeth produce uneven cuts. These cutters are among the earliest designs used in milling processes.
Woodruff cutters are specifically designed for cutting keyways for woodruff keys. They feature hollow ground sides for relief and are available with either straight or staggered teeth configurations.
The thread mill cutter functions similarly to an end mill cutter but with a helical movement to remove material from the workpiece. It is used for cutting both internal and external threads through a process known as "thread milling."
A Fly Cutter is a single point cutter used for fine surface finishes at a slower speed. The size of a fly cutter determines the quality of the finish with smaller ones producing a fine finish. In some ways, a fly cutter is similar to a face mill cutter.
Available in various sizes and shapes, these cutters feature teeth along the peripheral and side areas. Side and face cutters are particularly effective for cutting one side of a workpiece and are suitable for creating long, deep, open slots. Like other milling cutters, they can also be configured for gang cutting operations.
Hobbing is a machining process used to cut gears, sprockets, and splines. It involves a series of incremental cuts made by a hob, which progressively shapes the workpiece. This method is both cost-effective and precise, suitable for producing various components, especially spur and helical gears.
A dovetail cutter is used on wood and metal to create a strong joint. The cuts are either 45 degree or 60 degree. It is an end mill cutter that leaves a dovetail slot.
These are just a few examples of the CNC milling cutting tools currently available. As technology advances and new processes are developed, additional tools will likely emerge. The tools mentioned here represent some of the fundamental types commonly used with CNC milling machines.
In vertical milling, the spindle is oriented perpendicular to the workpiece, whereas in horizontal milling, the spindle is parallel to the workpiece. Both configurations are widely used in CNC milling, with the choice between them depending on the specific shape to be produced and the machine setup. Key factors to consider when deciding between vertical and horizontal milling include the type of milling application, the number of surfaces to be machined, and the design requirements of the part.
CNC vertical machines have cylindrical cutters that are useful for plunge cuts and drilling considered to be ideal for die sinking. Tools can be manipulated up to five axes for the fabrication of shapes, slots, holes, and details for three-dimensional parts. There are multiple applications for vertical milling that include lathes, machining centers, and five axes to produce three dimensional designs.
Vertical milling is excellent for projects that are single sided such as large metal plates. The process of vertical milling is simpler and less complex than horizontal milling since the spindle doesn‘t have to move in two directions. Also, it is more economical and affordable than horizontal milling.
Vertical machines are the most common milling format because of their affordability of the parts they produce, which makes them a good investment. The low upfront cost makes the production of parts less expensive and more attractive. Due to the popularity of this format, there are several trained operators available.
Vertical milling machines offer the advantage of allowing operators to easily detect and correct errors during the milling process. Their simpler design makes programming more straightforward and user-friendly, which enhances efficiency by reducing setup and programming time. This allows for greater focus on meeting the specific requirements of the project and the precision of the part.
There are two basic vertical milling machines – turret and bed. With turret milling, the spindle moves perpendicular and parallel to the axis. The process is versatile since the position of the workpiece can easily be manipulated. The bed type of milling machine has the . The spindle is fixed to the vertical axis and can only move up and down. The bed method is used for large, heavy pieces.
The primary distinction between horizontal and vertical milling lies in the spindle orientation. In horizontal milling, the spindle is positioned horizontally, allowing the use of thicker and shorter cutting tools that can handle deeper and heavier cuts. This orientation facilitates precise cutting by enabling the workpiece to be angled differently. Additionally, the horizontal setup helps chips from the cutting process to fall away from the workpiece, resulting in a cleaner work surface.
Horizontal milling machines are designed for high-production jobs, with robust components built to withstand extended use. Their durable construction reduces machine vibration, contributing to a smoother milling process. The inclusion of a fourth axis in horizontal mills enables them to produce complex and detailed parts with greater efficiency.
By using cutting heads similar to small circular saws mounted on the horizontal arbor, a horizontal milling machine can perform multiple cuts with one pass when the arbor is stretched across the length of the bed. Some mills restrict the movement of the bed to the horizontal plane, while others have a rotating bed. Unlike vertical machines, horizontal machines can make cuts at several different angles.
Horizontal milling machines are ideal for complex projects that involve multiple cuts due to their versatility and capacity. They are particularly suited for handling heavy workpieces and materials of various sizes and weights. When a job demands cuts on multiple planes or on both sides of the workpiece, horizontal milling equipment can efficiently perform these operations without the need for repositioning or moving the material.
Both vertical and horizontal milling configurations offer distinct advantages. The key factors in choosing between them include the number of planes to be machined and the shape and size of the workpiece. Horizontal milling is generally preferred for heavier workpieces with multiple sides and complex cuts, while vertical milling is more suited for die sinking and tasks requiring precision on single surfaces. In some cases, either configuration can achieve comparable quality, depending on the specific requirements of the project.
CNC milling applies to vertical and horizontal milling since they conform to the demands of repetitive processes. The same cutting tools can be used for either process and are interchangeable. Also, heads can be modified and adjusted to fit the desired application, which can be a cost saving factor when cutters are infrequently used.
CNC milling machines come in several varieties according to the manufacturer and the type of operation required. They can be specially engineered to perform specific jobs according to the type of part to be produced. The reason milling machines are so popular is their ability to perform complicated operations accurately and precisely. Described below are typical milling machines.
The column knee milling machine is one of the most common types. In this machine, the table is mounted on the knee casting, which is then mounted on the vertical slide of the main column. The knee can be adjusted up and down to accommodate different heights based on the specifications of the workpiece.
The plain milling machine features a horizontal spindle, earning it the alternate name of horizontal spindle milling machine. The table can be fed in either a longitudinal or vertical position. It moves longitudinally when the table shifts at a right angle to the spindle.
Universal milling machine - This type of machine can be adapted to perform a variety of milling operations. The table can be turned to angles up to 45 degrees its normal position. Universal machines allow four different movements and can complete helical milling. The term universal refers to the different attachments that can be added, which include vertical, rotary, slutting, and dividing head attachments.
Fixed bed milling machines are typically very large and heavy. The table has limited movement and is fixed at a right angle to the spindle axis, unable to be adjusted for cross or vertical changes. These machines can be equipped with single, double, or triple spindles.
The planer or plano milling machine is designed for heavy-duty work and is usually quite large. It features adjustable spindle heads that can move in vertical or transverse directions. The machine includes a cross rail that can be raised or lowered to support the cutter. With the capability of having multiple cutting spindles, it allows for simultaneous processing of several surfaces, thereby reducing production time.
For optimal performance, CNC milling machines must be manufactured to high standards. Fortunately, several companies excel in producing top-quality CNC milling machines. Below, we highlight some of the leading manufacturers in this field.
The Haas Automation VF-4 is a flexible vertical machining center designed for diverse milling tasks. It offers a spacious work area, accommodating larger components or several smaller pieces simultaneously. The VF-4 is equipped with a robust spindle that delivers high torque and quick acceleration, ensuring effective cutting and enhanced productivity. It features Haas' user-friendly CNC control system and a high-capacity tool changer for streamlined and efficient performance.
The DMG MORI DMC 635 V is a precise and compact vertical machining center ideal for small to medium-sized components. It boasts high rigidity and excellent thermal stability, which ensures consistent accuracy and repeatability during machining. The machine is equipped with a high-speed spindle and quick tool change capabilities, enhancing production efficiency. It also includes DMG MORI's sophisticated control system, featuring intuitive programming and monitoring options.
The Mazak VCN-530C is a high-performance vertical machining center renowned for its sturdy build and versatile capabilities. It comes with a variety of spindle options to accommodate different materials and applications. Equipped with Mazak's SmoothG CNC control system, the VCN-530C offers advanced programming and real-time monitoring. It also features an automatic tool changer with a tool magazine and an optional coolant-through spindle for enhanced operational efficiency.
The Okuma MB-5000H is a horizontal machining center built for precision and durability. Its robust design and thermal stability ensure consistent accuracy throughout extended production cycles. The machine is equipped with a high-speed spindle and a large-capacity tool magazine, which supports continuous and efficient machining. Additionally, it incorporates Okuma's advanced technologies, including collision prevention and tool breakage monitoring, to improve operational reliability.
The FANUC ROBODRILL α-D21MiB is a high-speed, compact CNC milling machine ideal for environments with limited space or high-density production needs. It boasts a small footprint while delivering powerful spindle performance and rapid tool changes for efficient machining. This model features FANUC's advanced CNC control system, which includes AI-driven functions to enhance process optimization and productivity.
Keep in mind that the specifications and models mentioned are representative examples. Manufacturers typically offer a range of models with different features and capabilities. For the most precise and current details about specific machines, it is best to consult with the manufacturers or their authorized distributors.
CNC milling is an efficient manufacturing process that can be used to finish a product or create a new one. It has limitless applications. When making the decision to mill a material, there are considerations that need to be taken to ensure the material is appropriate and fits the process. Generally, various metals, plastics, elastomers, ceramics, various composites, and glass can be shaped and formed using milling.
When deciding whether milling is the appropriate production method, it's crucial to evaluate the material's properties, such as hardness, tensile and shear strength, as well as its resistance to chemicals and temperature. The key consideration is whether milling offers a cost-effective solution. Milling may not be suitable for all applications due to its limitations and constraints, which can include the material composition and the form of the final product.
The production method can easily be determined during the design phase of any project since computer simulations make it possible to develop prototypes and test the necessary components of an item. This advancement is the one element that has made the difference in modern production providing critical information and data advancing milling from the 19th Century into the 21st.
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