Milling is a fundamental manufacturing process that employs a rotating multi-point cutting tool to remove material from a stationary or moving workpiece.
This subtractive method creates highly precise features, complex geometries, and custom parts from a solid block of raw stock.
The process relies on carefully controlled relative motion between the tool and the part to achieve the tight dimensional tolerances required for modern mechanical assemblies.
In this post, we’ll discuss the milling process and different types of milling operations using illustrations.
What is Milling?
Milling is a machining process that involves removing material from a workpiece using rotary cutters. It is widely used in manufacturing to shape, cut, and drill metal or other materials. The milling machine rotates the cutting tool while the workpiece is secured in place, allowing precise shaping of parts with various complex geometries.
Milling can be done in different directions and depths, producing flat surfaces, slots, gears, and more. It is essential in industries like automotive, aerospace, and construction, helping create components with tight tolerances and smooth finishes.
Milling Meaning:
Milling is a mechanical process where a rotating cutting tool removes material from a workpiece to shape or finish it. It is commonly used to create parts by cutting, drilling, or shaping metal or other materials.
This process allows precise and complex designs to be made, often used in manufacturing and engineering industries for producing components like gears, slots, and surfaces.
Milling Definition:
Milling is a machining process that uses a rotating cutting tool to remove material from a stationary workpiece. The tool moves along various axes to shape the workpiece into the desired form. This method allows for a wide range of operations, including cutting, drilling, facing, and contouring.
Milling machines can work on different materials such as metals, plastics, and wood, providing precise dimensions and finishes. It is a fundamental manufacturing technique used in industries like automotive, aerospace, and tool-making for producing complex parts with accuracy.
History of Milling
Traditionally, the crafting of complex shapes was done with manual hand filing. Hand filing created the requirement for a highly skilled laborer.
In the early 19th century, milling machines began to replace these processes. A milling machine eliminated the manual filing skill requirement. Instead, operators could use these machines with little expertise. Only short training on the machine was needed.
Initial milling machines found application in making rifle parts for the army. The milling machines were manually operated until the middle of the 20th century.
With the rise of computing technology, milling machines were integrated with CNC technology in the 1950s. This gave birth to the modern automated milling machines used industry-wide today.
Who invented the First Milling Machine?
Eli Whitney invented the first milling machine in 1818. The purpose of this milling machine was to manufacture rifles for the US government.
The basic design and features of this machine were so perfect that the same design was carried on for over 150 years. Incidentally, Eli Whitney is also credited with inventing the first cotton gin.
How Does Milling Work?
The main working part of a milling machine is the rotary cutting tool. This cutting tool is responsible for the material removal process. Milling machines can utilize both single-point and multi-point cutting tools.
The cutting tool in milling moves perpendicular to the rotational axis. For instance, if the cutting is rotating in the X-Y plane around the Z-axis, the movement of the cutter also occurs in the X-Y plane. The workpiece meets the cutter at the rotating tangent, resulting in the material removal process.
What are the Different Stages in the Milling Process?
Here is a step-by-step breakdown of the working process of milling machines:
- Workpiece Loading: The preliminary setup involves keeping the workpiece on the machine table feed and securing it. Wobbly fixtures will result in machining errors and poor precision.
- Tool Selection: Many different types of milling machine tools exist. Choose the right tool for the job, which depends on the workpiece materials and the required result.
- Machine Setup: Machine setup involves adjusting parameters like spindle speed, coolant flow, feed rate, cutting depth, etc.
- Milling Execution: The operator starts the actual milling operation once the setup is complete.
- Roughing: Roughing is the process of removing abundant material from the workpiece. This is done to get the workpiece into a vague resemblance of the required shape. This is done at a high cutting speed and feed rate.
- Semi-finishing: Once roughing completes, the speed of the milling machine is reduced. The workpiece is shaped identically to the final part.
- Finishing: Finishing occurs at a very slow feed rate and low depth of cut. The aim is to improve the dimensional accuracy of the part and make it as close as the machine possibly can.
- Unloading: The operator removes the finished part from the milling machine.
- Inspection and Quality Control: The final part is inspected to ensure there are no flaws. In case of any defects or further machining requirements, the operator loads the part on the machine and goes through a further finishing pass. This stage repeats until the part meets the required standards.
- Post-processing: The part can undergo any secondary machining requirements after milling. Common post-processing steps are deburring, cleaning, grinding, surface treatment, etc.
Types of Milling Operations
The following are the different types of operations performed on milling machines:
- Plain Milling Operation
- Face Milling Operation
- Side Milling Operation
- Straddle Milling Operation
- Angular Milling Operation
- Gang Milling Operation
- Form Milling Operation
- Profile Milling Operation
- End Milling Operation
- Saw Milling Operation
- Milling Keyways, Grooves, and Slots
- Gear Milling
- Helical Milling
- Cam Milling
- Thread Milling
#1. Plain Milling.
This operation is specifically for machining flat surfaces and contours on the workpiece. Using cylindrical cutters with straight or helical teeth, it removes materials to create flat or stepped surfaces, slots, and recesses. The cutter’s axis aligns parallel to the workpiece surface.
Plain milling is noted for its consistent material removal capabilities among various machining operations. It is ideally suited for light machining or finishing touches. For instance, it first strips material from the outer layer of larger workpieces, setting the stage for further machining processes.
#2. Face Milling.
This operation flattens the workpiece surface. As part of vertical milling processes, the cutters used in this method have an axis of rotation perpendicular to the surface, aligning the tool’s face parallel to the workpiece. Consequently, the cutter’s sharp teeth remove material, while the end face smooths the surface.
One prime advantage of this method is its high material removal rate, attributed to its tool geometry. This allows for rapid part production with a smooth finish. Face machining is particularly effective for creating flat surfaces in applications such as automotive cylinder heads and heat sinks.
#3. Side Milling.
As the name says, it refers to machining a side of the workpiece using side milling cutters or end mills. The vertical and horizontal milling machines can perform this task.
During the operation, the lateral edges of the rotating cutter (side teeth) remove material to form flat vertical surfaces, edges, grooves, slots, complex contours, fins, and more.
The cutter features helical flutes specifically designed to target the sides of the material, in contrast to operations that focus on the top surface, such as plain machining.
This technique is particularly useful in a variety of applications, including creating automotive suspension mounts, machining channels or slots in aircraft components, producing molds for injection or casting, fabricating medical implants, and constructing heat sinks for electrical and electronic appliances.
#4. Straddle Milling.
This method stands out due to its capacity to simultaneously machine two parallel surfaces on a workpiece, maintaining uniform distances between them.
How does straddle milling manage to machine two faces in a single setup? It employs two or more side cutters mounted on a single arbor that remove chips from the material surface during the feed. As a result, this operation is ideal for creating parallel slots, grooves, and various profiles on a workpiece.
Application Examples: Jigs, fixtures, brackets, levers, gears, sprockets, transmission & axle housings for automotive, etc.
#5. Angular Milling.
Angular features in CNC-machined parts are often achieved through angle milling. In this operation, the cutter’s axis is angled relative to the machining surface, while the workpiece is held in the required orientation using an angle plate or fixtures.
The cutters precisely machine the workpiece at specific angles, which can be fixed, such as 45°, 60°, or 75°. This method is used for creating features like slight chamfers, bevels, T-slots, dovetail slides, and other intricate geometric profiles.
#6. Gang Milling.
Why is it called gang milling? It involves multiple cutters such as end mills, form cutters, and slab mills- mounted together on a single arbor to create complex features and intricate details.
This setup allows for various operations to be performed simultaneously on a workpiece, resulting in reduced machining time and increased efficiency.
Application examples;
- First, any milling application requires multiple features, such as slots, channels, grooves, and flat surfaces.
- Automotive engine blocks and transmission housing.
- Fames, brackets, gears, sprockets, and housings for numerous types of machinery and tools.
- Die prototyping and manufacturing.
#7. Form Milling.
Do you know how CNC machines create irregular contours? This is achieved through form milling, which involves using cutters shaped as the negative of the desired contour. For instance, to machine a turbine blade, a cutter is first made with a geometry that mirrors the blade’s shape. This method allows for precise and custom shaping of complex designs.
When the cutter rotates, it traverses across the workpiece, or the workpiece moves under the cutter. Then, the cutter’s edges cut away the material to create the desired geometry.
Application Examples: Dome-topped pistons, orthopedic implants, turbine blades, die-making, guitar bodies, custom prototypes, etc.
#8. Profile Milling.
Profile milling is the operation of reproducing an outline of a template or complex shape of a master dies on a workpiece. Different cutters are used for profile milling. An end mill is one of the widely used milling cutters in profile milling work.
#9. End Milling.
It is one of the most popular ones among different types of milling operations. The end mills execute this operation by feeding the work into it in a perpendicular or angled direction.
Meanwhile, the multiple cutting edges on the faces and periphery of the end mill remove the material while feeding the workpiece.
The end milling is extremely favorable in the machining conditions where you need: An intricate profile, precise edges, slots, and grooves with varying depths, and a smooth finish on the workpiece surface.
#10. Saw Milling.
Saw milling is executed by a large cutter with teeth around its circumference. This method is particularly useful for creating narrow slots and dividing the workpiece into two parts. The cutter moves downward, slicing through the material with continuous feeding.
However, Saw milling operations are generally slower than other methods due to the large size of the cutter, which can generate significant heat during rapid material removal, potentially causing thermal damage to both the work material and the cutter.
Moreover, CNC saw operations extend beyond just slots and parting-off tasks typical of traditional machines. They are also capable of producing other geometries, such as slight curves or profiles.
#11. Milling Keyways, Grooves, and Slots.
The operation of producing keyways, grooves, and slots of varying shapes and sizes can be performed in a milling machine.
It is done by using a plain milling cutter, a metal slitting saw, an end mill, or a side milling cutter. The open slots can be cut by a plain milling cutter, a metal slitting saw, or a side milling cutter. The closed slots are produced by using endmills.
#12. T-slot Milling.
A dovetail slot or T-slot is manufactured by using special types of cutters designed to give the required shape to the workpiece. The second slot is cut at right angles to the first slot by feeding the work past the cutter.
A woodruff key is designed by using a woodruff key slot cutter. Standard keyways are cut on the shaft by using side milling cutters or end mills. The cutter is set exactly at the center line of the workpiece, and then the cut is taken.
#13. Gear milling.
Gear milling is a specialized operation used to refine manufactured gears to precise dimensions and surface roughness, or to create detailed tooth profiles. For instance, it can refine an extruded bevel gear to achieve a surface roughness of Ra 1.2 µm. In some cases, it can also manufacture a complete gear from scratch.
The tools used in gear milling include gear cutters, gear hobbing machines, and form mill cutters. These tools enable highly accurate shaping of gear teeth, surpassing the precision offered by other gear generation techniques.
Moreover, this operation can handle almost every type of gear, regardless of shape and complexity, such as spur gears, bevel gears, helical gears, and rack and pinion setups.
#14. Helical Milling.
Helical milling is the operation of producing helical flutes or grooves around the periphery of a cylindrical or conical workpiece. The operation is performed by rotating the table to the required helix angle.
And then by rotating and feeding the workpiece against the rotary cutting edges of a milling cutter. Production of the helical milling cutter, helical gears, cutting helical grooves or flutes on a drill blank or a reamer.
#15. Cam Milling.
The CAMS are essential components in different mechanical systems and machinery for converting linear motion into rotational or vice versa. Meanwhile, the CAM milling operation produces these components with a diving head tool.
This tool facilitates the rotation of the workpiece to position it so the tool can remove the surface materials according to the designed CAM profile.
#16. Thread Milling.
The thread milling machine operations are used to produce threads by using single or multiple thread milling cutters. Thread milling operation is performed in special thread milling machines to produce accurate threads in small or large quantities.
The operation requires three driving motions in the machine. One for the cutter, one for the work, and the third for the longitudinal movement of the cutter.
When the operation is performed by a single-thread milling cutter, the cutter head is swiveled to the exact helix angle of the thread.
The cutter is rotated on the spindle, and the workpiece is revolved slowly about its axis. The thread is completed in one cut by setting the cutter to the full depth of the thread and then feeding it along the entire length of the workpiece.
When the thread is cut by multiple thread milling cutters, the cutter axis and the work spindle are set parallel to each other after adjusting the depth of cut equal to the full depth of the thread.
The thread is completed by simply feeding the revolving cutter longitudinally through a distance equal to the pitch length of the thread while the work is rotated through one complete revolution.
How to Choose the Right Type of Milling Operation?
Selecting the appropriate milling operation is crucial for optimizing performance, achieving precision, and managing costs. Each type of milling operation offers distinct benefits and is suitable for different applications, depending on a variety of factors.
Let’s get into the main tips on how to choose the right type of milling operation.
Material Characteristics
The type of material you are working with significantly influences the choice of milling operation. Hard materials like stainless steel may require specific types of milling operations, such as carbide milling, to effectively handle the material’s hardness.
Examples:
- Aluminum often requires sharp, polished cutting tools to prevent material from sticking to the tool.
- Hardened steels are best machined with high-speed steel or carbide cutters.
Desired Surface Finish
The surface finish required for the final product can dictate the milling operation selected. Operations like surface milling or slab milling can provide finer finishes. Aesthetic components might need a smooth finish achievable through high-speed milling.
Complexity of Part Geometry
The complexity of the part’s design, including the number of axes required for milling cutters to produce the shape, will affect the operation type. Complex aerospace components might require 5-axis CNC milling.
Tolerance Requirements
Precision is paramount in many industries, and certain milling operations are better suited for tight tolerances. Precision engineering components like engine parts require high-precision milling operations.
Parameters and Settings
The specific parameters and settings of a milling operation, such as speed and feed rate, are tailored based on the equipment and desired outcome. Higher feed rates are suitable for rough milling to remove material quickly.
Production Volume
The volume of parts needed can influence whether a more automated operation like CNC milling is more cost-effective than manual milling. Large production runs are more efficiently handled by CNC machines due to their automation and consistency.
Consider Cutter Choice
The type of cutter used is influenced by the material, the complexity of the cut, and the type of milling machine available. For example, end mills are used for profile milling, side milling, and face milling.
Cost Considerations
Budget constraints can affect the choice of milling operation, with some being more cost-intensive due to equipment or labor costs. Conventional milling is often less expensive than CNC milling but might not meet all precision or complexity requirements.
Available Machine Tools and Technology
The availability of advanced machinery can open up possibilities for more complex and precise milling operations. Access to multi-axis CNC machines allows for advanced drilling and 3D shaping.
Tool Availability and Compatibility
Ensure that the tools required for the milling operation are readily available and compatible with the milling machines. Some specialized operations might require custom or hard-to-find tools.
Operator Skill Level
The skill level of the operator can significantly influence the effectiveness of the milling operation. Complex CNC operations require highly skilled operators to program and monitor.
Safety Requirements
Safety is crucial, especially in operations involving high-speed machines or hard materials. Operations that generate substantial heat or debris might require enhanced safety protocols.
Conclusion
Milling operations are essential in shaping metals, and can be utilized even in woodworking for cabinetry and construction.
Each operation offers a unique capability, and to get your exact desired results, you need to figure out the best type of milling operation.
Figure it out using our guide, and pick the right operating machine for your project.