Side Milling: Types, Parameters, and Key Applications

What is Side Milling?

Let us begin with a technical answer to the question at hand. Side milling is essentially a machining process, milling in particular, where the side edges of a milling cutter remove material from a workpiece. This results in vertical or angular surfaces on the workpiece side.

This contrasts with face milling, where the face-cutting edges perform the cutting and side edges do not engage.

Side milling is widely used in CNC machining to produce 3D features like shoulders, slots, steps, pockets, and other complex geometries with high precision.

The mechanics and performance of side milling, as with any machining operation, are dictated by some machining parameters. Being a standard milling operation, the cutting parameters for side milling are quite straightforward.

1. Cutting Speed

Cutting speed (m/min or ft/min) relates to how fast the cutting edge moves across the workpiece surface. The choice of cutting speed depends on factors like workpiece material, cutter material, cutter geometry like helix angle, and quality requirements like throughput and surface finish.

2. Feed Speed

Feed speed (mm/rev) dictates how quickly the tool traverses across the workpiece. This is not to be confused with cutting speed, which is the local speed at the cutting edge-material interface, while feed is how the entire tool moves as a whole. Higher feeds relate to higher productivity, but low surface quality and thermal stability.

3. Axial Depth of Cut

The axial depth of cut (mm) is how deep the tool goes into the workpiece in its axial direction. Higher depth of cut increases material removal rate but also the cutting forces. Typically, the strength of the tool limits the axial depth of cut, since deeper cuts can cause tool deflections and even breakage.

4. Radial Depth of Cut

Radial depth of cut (mm) links to how much of the tool’s diameter penetrates the workpiece’s uncut surface. As with axial depth of cut, deeper radial depths of cut increase forces, temperature, and productivity. It can range from as low as 5-10% of cutter diameter in finishing side wall cuts to 100% in slot milling operations.

5. Stepover

The stepover is more of a toolpath planning parameter than a direct machining parameter. It is a measure of how much the tool moves into the workpiece after each cutting pass. Smaller stepovers are time-consuming but generate a smoother surface finish.

Main Side Milling Formulae

Side milling is a very systematic process with fundamental governing equations assisting engineers and technicians in decision-making on the shop floor.
Some of these side milling formulae are listed below.

Spindle Speed

It is important to determine the correct spindle RPM according to the cutting speed guidelines.

• n = Spindle speed (RPM)
• Vc = Cutting speed (m/min)
• D = Cutter diameter (mm)

Feed Rate

The feed rate is a necessary input to a CNC milling program. Thus, its calculation is according to the cutter geometry and the desired feed per tooth.

• fz = Feed per tooth (mm/tooth)
• n = Spindle speed (RPM)
• z = number of flutes in cutter

Material Removal Rate

Manufacturers are highly concerned about the material removal rate, which is a direct indication of their production line’s throughput. Computing the MRR and optimizing it for maximal results is a matter of profit and loss for manufacturing enterprises.

• MRR = Material removal rate (mm3/min)
• Ap = Axial depth of cut (mm)
• Ae = Radial depth of cut (mm)
• F = Feed rate (mm/min)

Side Milling vs. Other Milling Techniques

Side milling distinguishes itself from other CNC milling techniques in various ways. Let us make a short comparison of side milling with some common milling processes to highlight these differences.

 Cutter EngagementSurface FinishApplicationsSide MillingOnly the side edges of the cutter with partial radial engagement.Good finish (better with climb milling)Vertical walls, steps, and shouldersFace MillingOnly the bottom edges of the cutter.Excellent finish.Flat faces/planesSlottingBoth sides and the bottom edges of the cutter with full radial engagement.Requires optimization and careful planningFull-width slots, keyway slotsEnd MillingBoth sides and the bottom edges of the cutter with partial radial engagement.Variable, depending upon process plan and toolpathGeneral-purpose contours

Types of Side Milling Operations

Side milling is a versatile machining process. Machinists use it in several configurations for a variety of features. The following sections will discuss these types of side milling operations.

Plain Milling

Plain milling, also commonly called slab milling, is the most basic form of side milling. It uses a side milling cutter to machine a flat side surface. Generally, it utilizes a straight toolpath, and the output is a large, planar surface along the vertical direction or an angular face.

Angular Side Milling

Angular side milling produces a flat surface, but with the tool set at an angle other than 90 ° from the reference workpiece face. This is quite useful in producing angular features like chamfers, bevels, or angular guideways.

In many cases, it requires specialized angular cutters with the desired angle already built into the tool design.

Side-and-Face Milling

Side-and-face milling is a type of hybrid milling process where the cutter’s side and peripheral edges simultaneously cut the vertical and horizontal surfaces of the workpiece. It is quite efficient for cutting slots/grooves as they have both floor and wall features.

Contour Milling

Contour milling is similar to plain milling, but along complex, curved toolpaths. This side milling technique is useful in machining parts with irregular contours in their vertical faces, such as mold cavities.

Straddle Milling

Straddle side milling utilizes two cutters mounted on either side of the workpiece. This configuration machines two vertical faces at the same time, achieving good parallelism, symmetry, and saving time.

Side Milling Requirements & Setup

A typical side milling operation, while basic in nature, requires a certain machine tool setup for proper execution. Machinists generally use checklist points like below before CNC milling a part.

• Machine Type: CNC machines come in various types and sizes. For instance, vertical milling centers usually offer higher maneuverability but are not as rigid as horizontal machining centers. Hard-to-cut materials require high spindle speeds and power, which is not available in every machine tool center.
• Tooling: The cutting tool and toolholder setup are critical in precision side milling. The cutting tool material (HSS, carbide), geometry (length, diameter, helix angle, number of flutes), and mounting configuration (tool overhang, shrink fit, or collet) are the main factors contributing to tool-tip rigidity and cutting ability.
• Fixturing: Workholding is important to rigidly hold the part. This may require special fixturing and controlling the part misalignments before machining. Poorly mounted parts are susceptible to unwanted deflections and vibrations.

Advantages and Limitations of CNC Side Milling

CNC side milling is a universal machining operation. However, it is not always the first choice for generating vertical prismatic geometries. It has its pros and cons.
Understanding these is necessary to weigh their benefits against alternative machining operations.

Advantages 

CNC side milling is preferable due to the following reasons:

• High precision: It is a straightforward operation with minimal external disturbances. It is ideal for accurately machining simple geometric features like slots, steps, and shoulders.
• Process versatility: Side milling is quite universal. It applies to heavy-duty parts with vertical walls/slots and small parts with delicate thin-wall features.
• Material flexibility: With advancing side milling cutter technology, side milling can handle hard-to-cut materials like Titanium and Nickel alloys at aggressive speeds and depths.
• Good surface quality: Optimal cutting parameters, climb milling, and fine CNC-level control allow highly precise, fine wall finishes for precision applications.

Limitations

The following factors limit the performance of CNC side milling:

• Tool deflection: Deeper cuts increase the cutting forces on the tool, which, owing to its slender geometry, is susceptible to bending during cutting. This deteriorates dimensional and form accuracy as well as tool life.
• Chatter vibrations: It is well-known that dynamic flexibilities cause unwanted vibrations and chatter during machining. These vibrations are more likely to occur in side milling operations due to high tool engagement and depth of cut.
• Complex toolpaths: Side milling toolpaths are often more time-consuming and difficult to plan and execute. CAM software takes longer to process changing features along a part’s depth, which is often more variational than its face features.

Key Considerations in Side Milling

The quality of a CNC side milling operation depends on several factors. Engineers and technicians need to look at their process plan from multiple angles and ensure there are no quality-deteriorating quirks.

1. Parameter Selection

Cutting parameter selection is the first and foremost factor when it comes to CNC machining processes. Choosing the optimal combination of cutting speed, feed, and depth of cut is necessary for part quality, MRR, and tool life management.

In production environments, it is common to use sophisticated theories, software, and technical experience to make these decisions.

2. Material Selection

Machinists have a range of side milling cutter materials and coatings to choose from. Generally, high-speed steel (HSS) is suitable for easy-to-cut materials, while carbide is preferable for hard-to-cut materials like steels and titanium alloys.

3. Cutter Selection

A side milling cutter has numerous specifications. Apart from the common ones like length, diameter, and flutes, the cutting-edge geometry matters a lot. This includes properties like the helix angle, rake angle, flank edge, chip breakers, and edge radius.

These specifications impact the cutting mechanics at the cutter-workpiece interface, determining the magnitude and direction of cutting forces, chips, and heat evacuation, and spindle power requirements.

4. Coolant/Lubrication Strategy

Coolants/lubricants improve the surface properties of the cutter and workpiece, decreasing friction and heat generation. They also cool down the cutting interface and carry away chips.

5. Toolpath Strategy

The toolpath strategy defines how the tool moves across the workpiece throughout the operation. CAM programmers control this with techniques like climb/conventional milling, multiple passes, toolpath techniques (helical, zigzag, trochoidal, etc.), and cutting parameters.

Best Practices for Side Milling

Experts share the following tips for optimal side milling performance:

• Climb milling: Climb milling achieves better surface finish and reduces tool wear in comparison to conventional milling.
• Minimize tool overhang: Smaller tool overhang keeps the tool assembly rigid, helping avoid deflection and vibration issues.
• Tool geometry: Choosing the correct cutter diameter, number of flutes, and helix angle is essential in maximizing productivity, surface quality, and tool life.
• Smooth entry/exit paths: CAM experts recommend using gradual exit/entry for smoother cuts and longer tool life.
• Use a light finishing pass: It may be tempting to remove the material quickly, but a light and slow finishing pass is the key to nailing the desired quality requirements, especially in applications like thin-wall machining.

With years of expertise in CNC milling services, WayKen offers advanced side milling solutions for creating precise slots, steps, shoulders, and complex contours. Our 3-axis and 5-axis capabilities, combined with optimized tooling strategies and professional fixturing, ensure high accuracy, smooth finishes, and efficiency across metal and plastic parts. From rapid prototypes to low-volume production, we deliver cost-effective and reliable results that meet the strict engineering and manufacturing requirements.

Side milling is a fundamental machining operation with several types, parameters, and considerations. Its operations have a wide range of industrial applications in areas like automotive, defense, aviation, and fabrication.