The CNC process
The CNC process was developed in the 1950’s and took a leap forward in the 1980’s with the addition of computerization. Unlike other production processes, CNC begins with a rendering by a computer, which creates a two or three dimensional representation of the part to be produced. CAD, computer aided design, is used to create the instructions that guide the CNC machine through the production process.
CAD software precisely defines the measurements and requirements for the part. Engineers and designers rely on it for the designing prototypes to be examined, enhanced, and approved. It provides a way to minutely examine their ideas without having the need to manufacture a prototype on a machine. CAD offers the benefits of safely working on a concept without a loss of time or labor. Many of the items on the market today were first created by 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 CNC milling process
The milling process begins by inserting the workpiece into the CNC machine’s work surface or holding device. This is followed by placement of the milling tools to the machine’s spindle. The directionality of the milling process can be either horizontal or vertical depending on the requirements programmed from CAD. Once the workpiece is secure, the programming downloaded, and the tool attached, the operator activates the program, and the milling process begins.
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 usually used as a finishing or secondary process for a workpiece that has already been machined where it is used to add special features like holes, slots, or threads. The general use of milling is the basic concept of the CNC process where material is gradually removed from the workpiece to create 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.
Types of milling operations
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 is the machining of a flat surface that is at right angles to the axis of the cutter. The cutters for this process have teeth on the periphery and tool face. Each set of teeth serves a specific function with the peripheral teeth doing the cutting while the face teeth do the finishing. Face milling produces higher quality finishes and can be used for 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 milling flat surfaces where the cutting tool’s axis is at an angle to the surface of the workpiece. A single angle milling cutter is used for angular surfaces, such as chamfers, serrations, and grooves. Dovetail milling is a typical example of angular milling where the angle of the cutter is 45°, 50°, 55°, or 60° based on the design. The tongue or groove is first roughed out using a side milling cutter, after which the angular sides and base are finished with an angle milling cutter.
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.
Specialized milling operations
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.
In straddle milling, two or more parallel vertical surfaces are machined at a single cut, which is accomplished by mounting two milling cutters on the same arbor, set apart at the exact width of the workpiece. The two sides of the workpiece are machined at the same time with the dimensions being exactly and precisely controlled. The workpiece is mounted on an indexing fixture or a vertical swivel vise.
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 is the machining of several surfaces of a workpiece at the same time by feeding it into multiple cutters that may be of the same or different diameters. The process can produce more intricate cuts for complex parts and shorten production time. If a workpiece requires several cuts such as a slot, flat, and angular groove, gang milling is the perfect choice.
These specialized milling functions are a small sampling of the total number of unique equipment available. As more complex designs and parts are developed, new types of milling methods will be invented. Though milling is an old process, it has become a basic part of modern product production.
The components of CNC milling equipment
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 where the milling tool is connected and held in place. It comes in different lengths and diameters, which is changed depending on the process and application to be performed. Arbors can be 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 milling machine 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.
Roughing End Mill Cutter
This type of cutter is used to remove large amounts of material to create a rough surface finish and has a wavy set of teeth. Referred to as a "rippa" cutter, it forms small chips resulting in a rough surface finish.
Slab Mill Cutter
Slab cutters are lightweight and remove material from slabs or large surfaces and can be used with horizontal or vertical milling machines. They are made from high speed steel and used in the gang milling processes.
End Mill Cutter
It has side teeth for drilling operations and is known as a bottom cutter able to cut in all directions with one or more flutes. There are two types – center and non-center. Center tools have the cutting edge on both sides while non-center has the cutting edge on one side.
Hollow Mill Cutter
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 are used to decrease stress. Known as ball end mills, they can cut three dimensional shapes.
Involute Gear Cutter
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.
Face Mill Cutter
The cutting teeth are on the side and circumference. They are made in several diameters and widths. The side teeth make unbalanced cuts. They are the earliest form of milling cutter.
Wood Ruff Cutter
Woodruff cutters are used to cut the keyway for a woodruff key. They are a hollow ground on the sides for relief. The teeth come in both straight and staggered varieties.
Thread Mill Cutter
The thread mill cutter operates the same as an end mill cutter. It removes material from the workpiece by moving helically and is identified by its helical motion. They cut internal and external threads using a process called "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.
Slide and Face Cutter
They come in various sizes and shapes with cutting teeth in the peripheral area and sides. Side and face cutters are efficient at cutting one side and can handle long, deep, open slots. As with the other mill cutters, they can be configured for a gang cutting application.
Hobbing cutting is a process for cutting gears, sprockets, and splines. The teeth progressively cut into the workpiece with a series of cuts from the hob. This is an inexpensive process that is accurate and can be used for a variety of parts. It is mainly used for 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 a few of the CNC milling cutting tools available today. As new parts and processes are developed, another assortment of tools will be invented. The ones listed here are some of the basic types common for most CNC milling machines.
Vertical and horizontal milling
In the vertical milling process, the spindle is vertical to the workpiece while in the horizontal process the spindle is horizontal to the workpiece. Both configurations are commonly used in the CNC milling process. Which one is used depends on the type of shape to be produced and the configuration of the machine. When choosing between the two styles, things to consider are type of milling application, number of surfaces, and the design of the part to be fabricated.
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.
The design of vertical milling gives operators the ability to spot errors and make corrections during the milling process. Since vertical machines are less complex the programming of them is more user friendly, which increases efficiency since less time is wasted in set up and programming. Greater concern can be devoted to meeting the specifications for the project and 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 significant difference between horizontal and vertical is the position of the spindle. Horizontal milling features a variety of cutting tools that are thicker and shorter, which are able to perform heavier and deeper cuts. The workpiece can be positioned at different angles for precise cutting. The horizontal orientation allows the chips from a cut to fall off of the workpiece creating a cleaner work surface.
Horizontal milling is designed to produce more products for a production job since the machine parts are built to endure long runs and are longer lasting. Their sturdy construction means less machine vibration during operation leading to a smooth milling process. Since they have a fourth axis, horizontal machines are capable of efficiently producing complex and intricate parts.
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.
Milling projects that are complex and require multiple cuts are normally completed on a horizontal machine because of its flexibility. This is especially true for jobs that include heavy workpieces. Horizontal milling can handle materials of any size and weight. When a job requires cuts on multiple planes with cuts on both sides of the workpiece, horizontal milling equipment can perform the shaping without having to be repositioned or moved.
Making the choice between vertical and horizontal milling
There are advantages to both configurations. The major factors in the decision process is the number of planes to be worked and the shape and size of the piece. Heavier workpieces with multiple sides and cuts are better produced using horizontal milling while die sinking is more compatible with vertical milling. There are projects that can use either configuration and produce the comparable quality.
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.
The various kinds of milling machines
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.
Column knee milling machine - Of the different types of milling machines, the column knee kind is the most common. The table is mounted on the knee casting, which is mounted on the vertical slide of the main column. The knee on the column can be adjusted up and down to work at different heights depending on the specifications of the workpiece.
Plain milling machine - The plain type has a horizontal spindle, which gives a plain machine the alternate name of horizontal spindle milling machine. The table can be fed in a longitudinal or vertical position. It is longitudinal when the table moves 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 - Most fixed bed machines are very large and heavy. The table has very limited movement and is at a right angle to the spindle axis unable to be adjusted for cross or vertical changes. Fixed bed machines can have single, double, or triple spindles.
Planer or plano milling machine- The typical planer milling machine is very large and designed for heavy duty work. It has adjustable spindle heads that can move in vertical or transverse directions The machine has a cross rail that can be raised or lowered, which carries the cutter. Since it can have multiple cutting spindles, a number of surfaces can be worked simultaneously saving production time.
Materials shaped by CNC milling
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.
A main factor, when making the choice to use milling as a production method, is the careful consideration of the type of material. This is in regard to its hardness, tensile and shearing strength, and chemical and temperature resistance. The basic determination is if milling is the most cost effective method. Milling has a variety of constraints and limitations that make it unacceptable for certain types of production, which can include the composition and 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.