16 Types of Milling Operations: A Complete Guide for Engineers

In CNC machining, milling is one of the most common methods of material removal. However, what truly determines the machining quality of parts is not only machine tool precision or cutting tool brands but also the selection of appropriate milling operations. Before machining parts, you need to select the most suitable milling solution based on part structure, material properties, and precision requirements. This article will provide a detailed introduction to common types of milling operations to help you better understand and make choices.

What is Milling Operation?

Milling operations are specific processing methods used in milling. During the processing, the machine tool gradually removes material from the workpiece using a rotating multi-tooth cutter, ultimately machining it into parts of the required shape.

Three parameters:

• Spindle Speed: The speed at which the spindle drives the tool or workpiece to rotate.
• Feed Rate: The speed at which the cutting tool moves relative to the workpiece.
• Depth of Cut: The depth to which the cutting tool penetrates the workpiece material with each pass.

Common Types of CNC Milling Operations

So, what are the milling operations in milling machining? Next, I will explain them to you in detail one by one. Of course, I also included the corresponding milling process diagram.

Face Milling

One of the most common milling operations. Face milling is a milling operation used for machining flat surfaces or end faces. During machining, the tool axis is typically perpendicular to the workpiece surface. Cutting also primarily occurs at the end face and outer circumference of the tool. This process produces flat, smooth, and dimensionally consistent surfaces on the workpiece.

The main advantages of face milling are high machining efficiency and consistent surface quality, and it is well-suited for removing large amounts of material. Therefore, this process is also commonly used for machining large flat surfaces of parts, such as metal plate surfaces, housing reference surfaces, mold surfaces, etc. Moreover, in mass production, face milling can also improve the consistency of parts.

Additionally, in actual machining operations, face milling is divided into roughing and finishing. In the roughing stage, the tool needs to use a larger cutting depth to increase the material removal rate. In the finishing stage, you need to reduce the cutting depth and feed of the tool to control surface roughness and flatness.

Practical applications of face milling:

• Machining large flat surfaces
• Finishing rough-machined surfaces
• Creating part reference surfaces
• Machining mating surfaces and sealing surfaces
• Removing surface excess material from castings or forgings

Plain Milling / Slab Milling

Plain milling is a milling operation that uses the peripheral cutting edges of a milling cutter to machine flat surfaces. During machining, the tool axis is usually parallel to the surface of the workpiece. Material is also primarily removed by the cutting edges on the periphery of the tool. It is primarily used to machine long, wide flat surfaces.

Plain milling typically uses cylindrical milling cutters, face milling cutters or slab milling cutters. You need to select the appropriate tool width, number of teeth and materials based on workpiece dimensions, material hardness and machining allowance. For wider machining surfaces, you can choose a wider cylindrical milling cutter to improve machining efficiency. And for harder materials, you need to control the cutting depth and feed rate to reduce tool wear and vibration.

In addition, the advantage of plain milling lies in its stable cutting process, making it suitable for machining long, flat surfaces. This milling method can remove a larger amount of material in a single pass. For large parts or regular flat surfaces, it offers high machining efficiency and good surface flatness.

Practical applications of plain milling:

• Machining long, straight surfaces
• Machining flat areas on large workpieces
• Finishing the surfaces of metal sheets or blanks
• Machining guide surfaces, support surfaces, and mounting surfaces
• Removing large amounts of material on a horizontal milling machine

End Milling

End milling is a milling operation that uses end mills for machining. During machining, the tool can cut with its end face or its side cutting edge. Therefore, this process can machine flat surfaces as well as grooves, steps, contours, cavities, and complex edges.

In CNC milling, the end milling process uses end mills. Because this tool can move flexibly along the X, Y, and Z directions, the end milling process is also often used to machine parts with complex shapes.

The advantage of end milling lies in its high flexibility. For structural parts that cannot be machined using conventional face milling, end milling is usually the more suitable option.

Practical applications of end milling: 

• Machining the outer contours of parts
• Machining grooves, steps, and shoulders
• Machining cavities and pockets
• Trimming part edges
• Machining small flat surfaces
• Machining complex 2D or 3D contours
• Machining local features of molds, housings, brackets, and precision components

Side Milling

Side milling is a milling operation that uses the side cutting edges of a milling cutter to machine the sides of workpieces. In this process, material is removed using the outer circumference or the side edges of the tool. It is also commonly used for part features that require control of width, perpendicularity, and parallelism.

The advantage of side milling is that it can produce clean, perpendicular, and dimensionally stable side surfaces, such as the side walls of steps, the sides of grooves, shoulder surfaces, or workpiece edges. It is also ideal for controlling the width, edge profile, and side wall quality of parts.

Practical applications of side milling:

• Machining the sides of parts
• Machining steps and shoulder features
• Trimming workpiece edges
• Machining the side walls of slots
• Machining two parallel surfaces
• Machining brackets, sliders, connectors, and mechanical guide surfaces
• Controlling part width and parallelism

Shoulder Milling

Shoulder milling is a milling operation used for machining steps, shoulders, and vertical sidewalls. This milling operation usually requires a specialized milling cutter capable of handling the intersection of the two surfaces without damaging the workpiece.

In CNC machining, shoulder milling is commonly used to create steps, edges, mounting surfaces, locating surfaces, and groove edges on parts. It has similarities with ordinary side milling, but shoulder milling places greater emphasis on the right-angle relationship between the bottom surface and the sidewall.

The advantage of shoulder milling is that it can efficiently machine stepped features and is suitable for a wide range of materials, from soft aluminum to hard steel. It is also well-suited for machining part features that require assembly fits.

Practical applications of shoulder milling:

• Machining part steps
• Machining 90° shoulder features
• Machining the bottom and side walls of grooves
• Machining mounting and locating surfaces
• Machining outer contour edges
• Machining molds, brackets, connecting plates, and machine housings
• Manufacturing right-angle features requiring precise assembly

Slot Milling

Slot milling is a milling operation used to machine slots, grooves, keyways, and narrow openings. During machining, the milling cutter cuts into the workpiece along a predetermined path, removing material through the bottom and side cutting edges of the tool to ultimately form slot structures with specified width, depth, and length. You can use this machining operation to create straight slots, keyways, guide slots, mounting slots, T-slots, or sealing slots on parts.

Compared to ordinary end milling, slot milling has a narrower cutting space. Since the tool may come into contact with the material on both sides, cutting resistance, chip evacuation, and tool rigidity become even more critical. It is worth noting that if chip evacuation is poor during machining, it can easily cause tool wear, surface scratching, or even tool breakage.

At the same time, the advantage of slot milling is that it can process groove structures with clear dimensions and regular shapes, and it has a wide range of applications. From simple keyways to complex mounting slots, you can choose different tools and toolpaths to meet your specific needs. In addition, its processing efficiency is also excellent.

Practical applications of slot milling:

• Machining straight slots and narrow slots
• Machining keyways
• Machining guide slots and slideways
• Machining sealing grooves
• Machining mounting slots and locating slots
• Machining preparatory structures for T-slots or dovetail slots
• Machining slot features on machine parts, fixtures, molds, and housings

Saw Milling

Saw milling is a milling operation that uses a circular saw blade for cutting. It uses a thin cutting edge to machine narrow slots, deep grooves or cut through material on the workpiece. You can use this machining method in applications that require narrow cuts, dividing workpieces, or machining thin grooves.

In addition, the cutting tools for saw milling are usually relatively thin. The thickness of the tool determines the kerf width, while the tool diameter affects the maximum cutting depth. Common tools used in saw milling include slitting saws, side milling cutters, and circular saw cutters. When selecting a tool, you need to consider factors such as the workpiece groove width, cutting depth, number of teeth, and tool rigidity.

In addition, the main advantage of saw milling is that it can cut very narrow grooves with minimal material waste. This machining method can produce straight and precise cuts, and it also offers good surface finish.

Practical applications of saw milling:

• Machining narrow slots and cutouts
• Cutting metal bars or sheet metal
• Machining retaining slots, spring slots, and clamping slots
• Separating multiple connected parts
• Machining openings in thin-walled parts
• Creating screwdriver slots, positioning slots, or release slots
• Machining narrow areas that are difficult to access with standard end mills

Pocket Milling

If you need to machine internal cavity areas of parts, then you can choose pocket milling.

Pocket milling is a milling operation used to machine enclosed or semi-enclosed cavities. During the process, the tool removes material layer by layer from the interior of the workpiece to form a cavity with a specified depth, shape, and dimensions. This milling operation allows you to machine internal spaces in housings, molds, brackets, mounting brackets, and lightweight parts.

The advantage of pocket milling lies in its ability to machine parts with complex internal geometries, and it is well-suited for highly automated CNC machining. With a reasonable CAM toolpath, this milling operation can stably remove a large amount of material while also maintaining good control over the quality of sidewalls, bottom surfaces, and corners.

In fact, the tool entry method for pocket milling is also important. Common toolpaths include:

• Ziga-zag pocketing: it is suitable for simple cavities
• Offset pocketing: machining layer by layer inward or outward along the contour
• Adaptive clearing: Suitable for efficient rough machining, able to maintain relativead
• Helical ramping: suitable for entering closed cavities
• Rest machining: used to clear corner material that cannot be reached by large tools

You can select the appropriate toolpath based on the specific cavity of the part.

practical applications of pocket milling:

• Machining closed cavities
• Machining the interior of housings
• Machining mold cavities
• Machining mounting recesses and locating cavities
• Machining lightweighting areas
• Machining electronic housings, aerospace parts, custom fixtures, and mechanical brackets
• Machining recessed structures with internal fillets and complex boundaries

Profile Milling

Profile milling is a milling operation that cuts along the contour path of a part. The tool moves along a path defined by CAD/CAM software to remove material from the edges or internal boundaries of the workpiece, ultimately forming a specified two-dimensional contour, three-dimensional surface, or complex shape.

The advantage of profile milling is that it can machine complex shapes and high-precision contours. Through CNC program control, the cutting tool can move along very complex paths, making this milling operation well-suited for manufacturing parts with complex shapes.

Practical applications of profile milling:

• Machining the outer contours of parts
• Machining the boundaries of internal contours
• Machining the edges of curves
• Machining complex 2D contours
• Machining 3D surfaces and mold surfaces
• Machining parts for the aerospace, automotive, medical, and mold industries
• Trimming the outer shapes of castings or forgings

Form Milling

Form milling is a milling operation that uses form cutters to machine special contours. The tool itself has a pre-designed shape. During cutting, the tool’s profile will be replicated onto the workpiece surface. Therefore, this milling operation can create arcs, grooves, convex surfaces, tooth profiles, semicircular slots, or other irregular shapes.

In CNC milling and conventional milling, form milling is commonly used to machine special shapes that are difficult to complete directly with ordinary flat-end mills or standard end mills. Its main advantages lie in excellent contour consistency and the ability to repeat the same shape.

Additionally, Form milling can also reduce the complexity of programming, as the shape is mainly determined by the tool rather than relying entirely on complex toolpaths.

Practical applications of form milling:

• Machining curved surfaces and semi-circular grooves
• Machining special grooves or raised contours
• Machining irregular curved surfaces
• Machining tooth profiles, splines, or special edges
• Machining camshafts, molds, guide rails and mechanical mating surfaces
• Batch machining of parts with repetitive features

Angular Milling

Angular milling is a milling operation used to machine inclined surfaces, angled grooves, chamfers or other non-90° surfaces. During the machining process, the cutting tool forms a specific angle with the workpiece. Alternatively, you can use an angled milling cutter to cut material, thereby creating geometric features with the specified angle.

In CNC machining, angular milling is commonly used to machine inclined surfaces, V-grooves, dovetail grooves, chamfered edges, and angular positioning surfaces on parts. Also, because angular milling can process angular structures that cannot be completed by ordinary face milling. Therefore, it is very important for parts with assembly, positioning, and guiding functions.

In addition, one point worth noting is that angular milling requires high angular precision. Tool angles, tool wear, clamping errors, and machine tool settings will all affect the final angle.

Practical applications of angular milling:

• Machining bevels
• Machining V-grooves
• Machining dovetail grooves
• Machining chamfers and bevels
• Machining angular locating surfaces
• Machining mold inclined surfaces and mechanical guide surfaces
• Machining parts structures that require specific assembly angles

Straddle Milling

Straddle milling is a milling operation that uses two side cutters to machine the surfaces on both sides of a workpiece simultaneously. The two cutters are mounted on the same tool holder and maintained at a fixed distance apart. As the workpiece passes between the cutters, material is removed from both sides simultaneously, creating two parallel surfaces.

Straddle milling has high machining efficiency and can well ensure the parallel relationship between two opposite surfaces. At the same time, this machining process reduces clamping errors and shortens machining time. Furthermore, through the straddle milling process, you can ensure the consistency of multiple parts.

Practical applications of straddle milling:

• Machining two parallel sides simultaneously
• Machining both sides of a groove
• Machining both sides of a boss or protrusion
• Machining opposing faces of brackets, sliders, and fixture blocks
• Controlling part width dimensions
• Batch machining parallel features of the same width

Gang Milling

If you need your machine tool to machine multiple part features simultaneously in a single cutting operation, gang milling is one of the milling operations you should consider.

Gang milling is a milling operation in which multiple milling cutters are mounted on the same tool holder, and multiple machining features are completed simultaneously in a single pass. Moreover, multiple tools can have different widths, diameters, or shapes. These tools can be used to simultaneously machine flat surfaces, grooves, steps, sides, or other composite structures.

Furthermore, gang milling is commonly used in mass production. When you need to machine multiple adjacent features on a single part at the same time, changing tools or cutting for each feature separately would undoubtedly increase machining time and positioning errors. However, through gang milling, multiple cutting operations can be completed in a single pass, thereby improving machining efficiency and dimensional consistency.

Practical applications of gang milling:

• Machine multiple flat or side surfaces in a single operation
• Machine multiple grooves or steps simultaneously
• Mass-produce repetitive parts
• Machine mechanically fixed components
• Reduce tool change and multiple pass times
• Improve the positional consistency among multiple features

Gear Milling

Gear milling is a milling operation used to machine gear tooth profiles. During the process, the cutting tool removes material according to the gear’s module, number of teeth, helix angle, and tooth profile requirements. After machining, tooth slots and tooth faces are formed around the outer circumference or inner bore of the workpiece.

In mechanical machining, gear milling is mainly used to manufacture transmission components. You can use this milling operation to manufacture spur gears, helical gears, sprockets, splines or other toothed structures.

In addition, the flexibility of gear milling is relatively high, making it suitable for machining non-standard gears, small-batch gears or special tooth profiles. However, it has high requirements for indexing accuracy. If the indexing error is too large, it will result in uneven pitch in the finished parts.

Practical applications of gear milling:

• Machining spur gears
• Machining helical gears
• Machining transmission gears
• Machining sprockets
• Machining spline shafts or spline grooves
• Machining non-standard tooth profiles
• Small-batch gear manufacturing
• Repair or custom transmission parts
• Machining mechanical parts, automotive parts, robot components, and industrial transmission parts

Thread Milling

Threads are a common feature in parts, and we often use the thread milling process to machine them. Thread milling is a milling operation that uses a thread milling cutter to create threads. During machining, the tool moves along a helical path, gradually cutting the thread profile inside a hole or on the outer surface of a cylinder.

Thread milling is a milling operation that uses thread milling cutters to machine threads. During machining, the tool moves along a helical path, gradually cutting out the thread shape inside holes or on the outer surface of cylinders. This milling operation supports machining various thread structures for you, including internal threads, external threads, blind hole threads, large diameter threads, and so on.

Additionally, thread milling has excellent safety and flexibility. As long as you ensure that the pitch matches the tool specifications, the same thread milling cutter can machine threads of different diameters within a certain range. Moreover, its cutting force is relatively small, and chip evacuation is easier to control. Therefore, thread milling is also very suitable for machining thin-walled parts, hard materials, and high-precision threads.

Practical applications of thread milling:

• Machining internal threads
• Machining external threads
• Machining blind hole threads
• Machining large-diameter threads
• Machining non-standard threads
• Machining precision threads on high-value parts
• Machining difficult-to-machine materials such as stainless steel and titanium alloys

Plunge Milling

Plunge milling is a vertical milling operation. During machining, the tool inserts into the workpiece vertically or nearly vertically along the Z-axis direction, removing material through continuous plunging.

The main advantage of plunge milling is that it can reduce radial cutting forces, thereby improving deep machining stability. This milling operation is also suitable for rough machining materials with large allowances, especially in large molds, aerospace structural parts, or deep cavity components.

Practical applications of thread milling:

• Deep cavity roughing
• Deep groove machining
• Long tool overhang machining
• Large mold roughing
• Material removal for difficult-to-machine materials
• Machining scenarios with insufficient rigidity of machine tool or workpiece

Quick Comparison Table of Milling Operations

To help you understand each milling operation more clearly and directly, I’ve also prepared this comparison chart of milling operations for you.

How to Choose the Right Milling Operation

This is probably a topic that many of you are interested in. Indeed, choosing the right milling process is not an easy task. During this period, you need to comprehensively consider the geometric shape, material, tolerances, surface roughness of the workpiece, the rigidity of the machine tool, and the production batch of the parts.

For this reason, I have summarized the following simple and easy-to-understand rules and tips for you. You’ll understand them at a glance.

1. Consider The Shape

Part features determine the basic process.

• For large flat surfaces, you should choose face milling.
• For long flat surfaces or horizontal machining scenarios, you can consider plain milling.
• For slots, steps, or localized contours, end milling is a more common choice.
• For enclosed cavities, we usually choose pocket milling.
• For outer contours or complex surfaces, profile milling is the more suitable choice.
• For threads or gear teeth, consider thread milling and gear milling, respectively.

2. Consider The Material

The material determines the cutting risk.

• Aluminum alloy has good machinability, so this material can be machined using a variety of milling operations.
• Stainless steel, titanium alloy, and high hardness steel are more prone to generating cutting heat, tool wear, and vibration.
• For difficult-to-machine materials, more attention should be paid to tool rigidity, cooling, chip removal, and cutting force control.

3. Consider Required Precision and Surface Finish

If you need to quickly remove material, you can choose pocket milling, plunge milling or roughing end milling.

If you need to control the final dimensions and surface quality of the part, you should schedule a finishing pass, such as face milling, contour finishing, or side wall finishing.

Note: Do not use the same toolpath to pursue both high efficiency and high surface quality. In actual production, rough machining is responsible for material removal, while finishing is responsible for controlling the dimensions and surface of the parts.

4. Consider Machine Tool and Clamping Rigidity

Machine rigidity, tool overhang, and workpiece clamping methods all influence your choice of milling operations.

If the workpiece is thin, the tool is long, or the clamping is unstable, excessive lateral cutting forces should be avoided.

When machining deep slots, deep pockets, and thin-walled parts, it is safer to use layered cutting, small radial depth of cut, or plunge milling.

5. Consider Volume and Cost

Small batches or complex parts are more suitable for flexible end milling, profile milling and thread milling.

For high-volume production, you can choose these milling operations, such as gang milling, straddle milling, or dedicated form milling. These milling operations can improve part consistency and production efficiency.

If part designs change frequently, custom form tools may not be cost-effective.

Conclusion

Milling operations are not simply classifications of machining processes. Each milling operation can produce parts with corresponding features. Choosing the right milling operation directly impacts the quality of the part.

Of course, if you still don’t know which milling operation is suitable for your part solution, you can also contact us. Our DSR company is a machining manufacturer with 14 years of experience, capable of providing professional CNC milling services. We can develop a customized manufacturing plan based on your drawings to ensure you receive high-quality CNC-machined parts.

FAQ

Q1: What is the process for vertical milling? 

A: The basic process for vertical milling operations is as follows:

1. You need to choose the appropriate milling cutter, fixture, and cutting parameters according to the drawings, then securely clamp the workpiece onto the workbench and align the reference surface. 

2. Next, you need to install the cutting tools and check the tool runout, spindle orientation, and coolant status. At the same time, you also need to perform tool setting to determine the workpiece coordinate zero point.

3. Once the spindle is started, the milling cutter rotates vertically to cut the material, gradually performing various machining operations.

4. During machining, you should also pay attention to controlling the cutting depth, feed rate, and coolant flow of the cutting tools.

5. After machining is complete, you also need to measure the dimensions and surface quality of the finished parts.

Q2: What is CAM milling?

 A: CAM milling refers to the use of computer-aided manufacturing (CAM) software to automatically plan the milling process based on a part’s 3D model or 2D drawing. This process includes selecting cutting tools, setting cutting parameters, generating toolpaths, simulating the machining process, and outputting CNC programs. The material is then removed by a CNC milling machine or machining center according to the program.

Q3: What is a cylindrical milling cutter?

 A: Cylindrical milling cutter is a milling tool with a cylindrical shape and cutting edges primarily distributed on the outer cylindrical surface. It is primarily used for machining flat surfaces, stepped surfaces, or wide surfaces.

Q4: What is angle milling?

A: A very practical machining process. This milling operation uses special shaped cutters such as single angle or double angle to precisely cut geometric features with specific angles, such as chamfers, V-shaped grooves, dovetail grooves, or tooth shapes, on the workpiece.

Q5: What are the disadvantages of thread milling?

A: 1. Equipment and tooling costs are relatively high.

2. Programming is relatively complex.

3. Machining efficiency is typically lower than that of tapping.

Q6: What types of milling cutters are there?

A: End mills, face mills, ball-nose end mills, angle mills, keyway cutters, T-slot cutters, form cutters, gear cutters, thread cutters and undercutting end mill.

Q7: What are common milling mistakes?

A: 1. The workpiece is not clamped securely,

2. Inappropriate tool selection

3. Spindle speed, feed rate, and cutting depth settings are unreasonable

4. Incorrect tool setting and coordinate setting

5. Failure to properly arrange the machining sequence

6. Improper cooling, lubrication and chip removal handling

Q8: Is drilling a milling operation?

A: No. Drilling is generally not considered a milling operation, but rather an independent machining method. The CNC drilling process mainly uses drill bits to create circular holes in workpieces.