Face milling is often considered a routine operation in CNC machining. You square up the stock, clean up the surface, and go. Face milling in CNC machining is often the first operation that determines part flatness and reference alignment. But this one process sets the stage for all that follows. The flattening, parallelism, surface finish, and even the longevity of future tools depend on the quality of face milling.
In a manufacturing environment, minor decisions such as cutter type, insert shape, spindle load, or feed rate directly affect whether a job is completed successfully or becomes a never-ending rework cycle and a source of noise.
This guide discusses face milling in CNC machining, tools, and speeds and feeds. It also contains comparisons with end and peripheral milling. Let’s dive in!
What Is Face Milling in the CNC Milling Process?
Face milling is a CNC milling process that creates flat surfaces by using the front part of a spinning cutter. The cutter axis stands upright relative to the workpiece. In machining, workers use two types of machines for this task: a machining center or a milling machine.
The key difference between face milling and other milling methods is that the cutter axis is perpendicular to the workpiece surface. This means the top of the machine does the cutting on the workpiece, unlike in other milling methods, where the side does the cutting.
How It Works
- Tool Orientation
The face milling cutter spins about its axis perpendicular to the workpiece surface, allowing the cutter’s face to make contact with a large area of material.
- Advanced Engagement
Most of the material is removed by inserts located at the front of the cutter, while peripheral inserts help tidy up the edges and create a smooth surface.
- Machine Movement
The CNC machine moves the workpiece beneath the spinning cutter in a straight line, producing a flat surface with uniform material removal.
- Control of Depth and Width of Cut
The axial depth of cut determines the amount of material removed with each pass, while the cutter’s diameter establishes the effective cutting width and the degree of overlap.
- Surface Generation
The quality of the surface finish and its flatness depend on factors like insert shape, feed rate per tooth, how well the cutter runs true, and how sturdy the machine is.
Face Milling Compared to Other Milling Methods: End Milling and Peripheral Milling
| FEATURE | FACE MILLING | END MILLING | PERIPHERAL MILLING |
| Cutter axis orientation | At a right angle to the surface of the workpiece
| Straight (or a little tilted) to the surface | Even with the surface of the workpiece |
| Advanced engagement | Mainly, the inserts go on the cutter face, while the peripheral inserts help out. | Both the end teeth and the outer teeth are involved. | Primarily, the outer teeth operate along the cutter’s diameter.
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| Primary application | Making flat surfaces, squaring materials, and setting up reference planes. | Pockets, slots, grooves, and shapes with details. | Side walls, long flat areas, gears, or slim designs.
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| Material removal rate (MRR) | High: A wide cutter is able to strip away material very quickly.
| Moderate: This is based on the degree of overlap of a tool and the number of times the tool passes over the material.
| Moderate: This depends on how much the tool overlaps and how many times it passes over the material. |
| Surface finish / Flatness | Works well on large surfaces if the settings are adjusted properly.
| Does a decent job, but for smaller areas. It needs several passes to ensure flatness.
| Effective on narrow surfaces; less reliable over larger spaces. |
| Tool types / Variants | Indexable face mills, solid face mills, and shell mills are types of milling tools. | End mills come in different styles, like square end, ball nose, corner radius, and tapered. | There are also side and peripheral milling cutters.
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| Common depth of cut
| For each pass, the depth can be moderate to deep, depending on how sturdy the tool is. | For precise details, a shallow to moderate depth is best. | Usually, for shallow to moderate cuts, you will need to make several passes. |
| Feed strategy
| Control the feed for each tooth to ensure a smooth surface.
| The feed rate is based on the width of the slot, the size of the pocket, and the diameter of the cutter.
| Carefully managing step-over and feed is important to prevent scalloping. |
| Machine considerations
| You need a sturdy machine, enough spindle power, and stable workholding. | While it can work on various machines, high-speed milling can yield better results. | You should have some rigidity, as tool deflection may affect precision.
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| Strengths / Benefits
| Removes material quickly, achieves high flatness, and reduces the need for additional steps.
| Flexible and accurate, perfect for intricate shapes. | Works well for sidewalls, extended straight cuts, or slim details.
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| Limitations / Weaknesses
| Not great for small spaces or detailed designs.
| It takes longer to finish wide surfaces and doesn’t work well on large flat areas.
| It works less efficiently on large flat areas, and multiple passes may increase the time.
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Face milling is generally preferred for large flat surfaces, while end and peripheral milling are better suited for detailed features and side-wall machining.
Face Milling Cutters and Tooling Selection in CNC Machining
Types of Face Milling Cutters
- Solid Face Mills
○ Built in a one piece to ensure optimal rigidity and stability.
○ Applicable in roughing and in hard materials, where tool deflection has to be reduced to a minimum.
○ Powerful, tough, yet once worn, the cutter as a whole has to be replaced.
○ Has to have enough power on the spindle to make heavy cuts.
- Indexable Face Mills
○ Fitted with several removable inserts attached to the cutter face.
○ Individual inserts are replaceable and rotatable, which results in downtime and total tooling cost being lower.
○ Applicable in high volume production, whereby steady operation and finish of the surfaces are essential.
○ Enables insert geometry flexibility to suit other materials or cutting conditions.
- Shell Mills
○ Simply consists of a body and a removable face or an insert plate.
○ Can cut bigger diameters than solid face mills at less expense.
○ Medium rigidity is appropriate because they can be used in medium to high stock removal without extreme spindle load.
○ Mostly used in roughing and semi-finishing processes.
Tool Selection Considerations
- Diameter of Cutter and Number of Inserts
○ Greater diameters can serve more surface area in a single pass and save time in the number of passes.
○ The additional inserts raise the rate of material removal, but also require more power in the spindle.
- Add Geometry and Coatings
○ Positive rake inserts: The forces at the cutting edge are lowered, and the finish is enhanced.
○ Negative rake inserts: The cutting edge is more stable when cutting heavy.
○ Coatings such as TiN, TiAlN, or CVD enhance wear resistance and permit cutting at increased speed.
- Material and Stock Removal Rate
○ Aluminum, cast iron, and steel demand diverse insert grades and feeds together with speeds.
○ Feed per tooth and depth of cut should be maximized to achieve maximum surface finish as well as tool life.
- Machine Capability
○ With rigid machines, it is possible to use larger cutters and deeper cuts without chatter.
○ Spindle torque, stability, and workholding are important to attain flatness and uniform surface quality.
Face Milling Speeds and Feeds: Engineering Guidelines by Material and Tool Type
The following ranges represent typical starting values. Actual cutting parameters should be optimized based on tooling manufacturer recommendations, machine capability, and part geometry.
- Spindle Speed (RPM)
○ Definition: The revolutions per minute that the cutter makes.
○ Calculation: RPM = (Cutting Speed (m/min) × 1000) / (Diameter (mm) × π)
○ Note: 1000 converts meters to millimeters.
■ Aluminum: 600 – 1200 m/min
■ Mild Steel: 120 – 200 m/min
■ Stainless Steel: 80 – 150 m/min
- Feed per Tooth (fz)
○ Definition: The distance travelled by the piece of work per tooth revolution is defined.
○ Calculation: Table Feed (IPM/mm/min) = RPM × Number of Teeth × Feed per Tooth.
○ Example ranges:
■ Aluminum: 0.1 – 0.3 mm/tooth.
■ Steel: 0.05 – 0.15 mm/tooth.
■ Stainless Steel: 0.03 – 0.1 mm/tooth.
- Depth of Cut (ap)
○ Axial Depth: The depth that the cutter is making in the material each time.
○ Radial Width (ae): Width of contact of the cutter on the surface.
○ Guidelines:
■ Roughing: increased depth of cut, reduced feed per tooth.
■ Finishing passes: cut depth shall be low; an increase in the feed per cut will produce a smooth surface.
■ Always put into consideration cutter rigidity, machine power, and workholding stability.
Cutter and Insert Concerns
- Indexable face mills can work at higher radial engagement since they incorporate more than one insert, and this dispersion of forces evenly.
- Solid face mills are very powerful, allowing them to cut deeper without bending to a large extent.
- Positive rake inserts reduce the cutting forces, and negative rake inserts increase the stability in difficult materials.
- In production environments, engineers often adjust face milling parameters based on tooling wear, machine rigidity, and batch size to maintain stable machining performance.
Accuracy, Flatness, and Surface Finish Achievable with Face Milling
Normal Flatness and Tolerance Scale
- Flatness
○ This can be achieved based on the cutter diameter used, the type of insert, and the number of passes.
○ Depending on setup rigidity, cutter selection, and material, face milling can typically achieve flatness of approximately 0.01 – 0.05 mm over a 100 mm length.
- Dimensional Tolerances
○ For general-purpose parts: ±0.05 mm.
○ For components of high precision: ±0.01 mm.
- Factors Influencing
These include machine rigidity, cutter deflection, insert wear, spindle runout, and thermal expansion.
Expectations of Surface Finish (Ra)
- Roughing passes: Ra 3.2 – 6.3 μm, which is dependent on insert geometry and feed per tool.
- Finishing passes: Ra 0.8 to 1.6 μm can be completed with a small depth of cut and feed maximized.
- Influencing factors
○ Feed per tooth: excessive high increases scalloping.
○ Cutter geometry: chatter is reduced with positive rake inserts.
○ Machine vibration: flexible workholding reduces irregularities of surfaces.
Variables that affect Accuracy and Finish
- Selection of tools: Bigger diameter cutters decrease deflection, enhancing flatness.
- Inserts: The more the number of inserts, the more the forces of cutting are distributed evenly and the better the surface.
- Machine consistency: Vibration, or an ineffective spindle that is not rigid, may result in a rough surface and loss of flatness.
- Workpiece material: Softer materials (aluminum) can be deformed under the cutting forces. Harder materials (e.g., steel) may require more stable cutting conditions.
- Installation and fixturing: Chatter, springing, and moving of parts are undesirable and are eliminated by proper clamping.
Machine Tool Considerations for Face Milling in CNC Machining Centers
Spindle Power and Torque
- Significance: Sufficient spindle power will guarantee that the cutter will have consistent cutting velocity when under load.
- Effects: Weak spindles may result in the loss of feed rate, chatter, surface finish, and tool wear.
- Rule: Adjust the spindle horsepower and torque to the cutter diameter, cut depth, and material hardness. Larger face mills require higher spindle torque to continue cutting.
Machine Rigidity
- Structural rigidity: The machine bed, column, and spindle bearings should be able to resist deflection when cutting forces are acting.
- Vibration control: Low rigidity will result in chatter, topography variations, and deviations of tolerance.
- Workholding: Clamping plan and design of the fixtures are direct influencers of rigidity and, thus, flatness and accuracy.
CNC Set up: VMC vs HMC
- Vertical Machining Centers (VMC):
○ Ideal for face milling flat plates, general-purpose shop applications.
○ Very large cutters or heavy stocks cannot be cut off because of the rigidity of the spindle.
- Horizontal Machining Centers (HMC):
○ Better where high volume, multi-face machining, and large face milling is required.
○ Improved flow of the chips under the influence of gravity to enhance the tool life and surface finish.
Axis Configuration
- 3-Axis Machines:
○ Appropriate for simple geometries of standard face milling.
○ Limit: Cannot tilt the cutter on complicated surfaces or into narrow pockets.
- 5-Axis Machines:
○ Provides for cutting and workpiece rotation and tilting.
○ Allows sophisticated face milling strategies, less tool interference, and fine finishing of contour parts.
Cost, Lead Time, and Manufacturing Risk Implications
- Availability and Cost Structure of the Machine
- Face milling of large surfaces usually requires CNC machining centers with sufficient power, rigidity, and table area. Machining parts on standard vertical machining centers is generally cheaper than machining parts that require larger or more specialized machines, e.g., horizontal machining centers. Features that exceed the machine’s normal capacity should be specified, as this may significantly increase hourly rates.
- Setting Time and Efficiency in the Cycle
- The large surfaces are inherently best served with face milling, although machine rigidity and spindle capability would dictate the aggressiveness of the method. Higher-torque machines with stable fixturing require fewer passes and shorter cycle times. Limited machine capacity can necessitate reduced cuts, resulting in longer machining and lead times.
- Possibility of Distortion and Rework
- Face milling parts with thin areas or that are not well material distributed are more sensitive to the cutting forces. Machines that lack rigidity or have inadequate fixturing increase the risk of distortion, resulting in flatness deviation and rework. Scrap issues are mitigated by early correlation of machine capabilities with part geometry.
- Supplier Capacity and Scaled-up Capacity
- Not every supplier has the same variety of machining centers. Selecting face-milling strategies that match the most common types of CNC equipment enhances suppliers’ flexibility and scalability, should volume increase in the future or a second source be required.
- Influence on Predictability of Delivery
- This is achieved by selecting machines that fit well, reducing unforeseen process changes, tool wear, and missed inspections. The result is greater predictability in delivery schedules and a reduction in delays that machining refinements or corrective measures would have otherwise caused.
Conclusion
Face milling is a CNC machining process for producing both flat, precision surfaces and defining the data face in a wide range of materials, including aluminium, steel, stainless steel, and cast iron. The differences between face milling and end and peripheral milling allow the engineer to select the most effective approach for any feature, and prudent control of process parameters can achieve reasonable tolerances, flatness, and surface finish. Control of these factors enables high-quality, efficient, and accurate machining across a wide range of industrial applications.
Face milling plays a key role in precision CNC machining services where flatness, surface finish, and dimensional accuracy are critical. At FastPreci, the disciplined approach of engineering CNC machining is constructed upon. The company uses both modern CNC machining equipment and a deep understanding of the processes to create precise, repeatable, high-quality components for the most demanding applications. Contact us today to have your parts machined with efficiency, accuracy, and world-class quality.
FAQS
Can face milling be used for both roughing and finishing in CNC machining?
Yes. A heavier face mill pass is used to make the roughing pass, and then a light finishing pass with reconfigured feeds and inserts to enhance the quality of the surface.
In face milling, which is preferred between climb and conventional milling?
Climb milling is generally preferred on CNC machines due to better surface finish, lower tool wear, and improved dimensional control, assuming sufficient machine rigidity.
What is the performance of face milling with regard to insert geometry?
The rake angle, corner radius, and edge preparation are all inserted directly, affecting cutting forces, chip formation, and surface finish, particularly at increased feed rates.
