Jusheng

Let's say that a machine shop engineer is machining a batch of custom CNC milling parts using a face mill on a CNC machine—a 20mm-thick flange from 45-gauge steel. He can't decide between cutting one 10mm cut or two 5mm cuts. Cutting too deep results in tool chipping, and cutting too shallowly is too time-consuming.

This is something that the majority of CNC milling professionals encounter on a daily basis. This article, from JS's expertise, provides an exhaustive solution to the problem of "how deep can a face mill cut?" It warns the readers against the major cutting depth influencing factors, optimize cutting parameters, and select the appropriate milling process to improve machining quality and efficiency.

Core Answer Summary

Comparison Aspects	High-Feed Milling	Traditional Square-Shoulder Milling
Core Objectives	High Metal Removal Rate (MRR), high material removal rate.	Accurate sidewall and bottom surfaces to complete.
Depth of Cut (Ap)	Light (usually 0.5-2.0 mm)	Heavy (half the cutter diameter or more)
Feed Speed ​​(F)	Very High (feed per tooth fz 1.5-3.0 mm)	Medium (fz usually 0.1-0.5 mm)
Radial Width of Cut (Ae)	Large (usually 70-100% of diameter)	Small to Medium (smaller to complete)
Essential Machine Equipment	High rigidity, high torque.	High precision, high stability.
Ultimate Results	Champion of efficiency, allowance on surface.	Champion of precision, direct access to final dimensions.

Why Highlight JS's Experience With Face Milling Cutters In Hand?

With customers visiting JS to place orders for heavy-duty industrial CNC milling parts measuring 3 meters in diameter, or require swift online CNC milling services for aluminum alloy parts, JS Precision Manufacturing is ever ready to move with urgency.

We have over 10 years' custom CNC milling manufacturing experience, including face milling work on a wide range of materials from mild steel and stainless steel through to titanium and aluminium alloys. We have manufactured very precise components with flatness tolerances of 0.1mm and made repairs to massive 50-ton components.

Our engineers have extensive experience with behavior of different types of face mills and are able to precisely set cutting depth planning in accordance with machine capacity and workpiece material.

This book is a compilation of our innumerable shop hands-on CNC milling experiences, and each recommendation has been field-tested and validated in our shop, so you can rely on its professionalism and ease of use.

If you need CNC milling parts, JS's on-site custom CNC milling manufacturing capability offers innovative solutions and complete assistance. Using efficient face mills, we can help you prevent machining problems and minimize lead times.

Face Milling Cutter: More Than Steel, A Precise Technology-Material Match

In CNC milling, the selection of the face mill has a direct bearing on cutting depth and quality of machining. Every face mill is best suited for specific applications and is most suitable in various applications, as illustrated below:

High-Feed Milling Cutter (The Speed ​​Demon)

Intended for high velocities, its insert geometry accommodates high metal removal rates at small depths of cut (Ap = 0.5-2mm). Applicable for roughing and finishing on comparatively stiff machines, cutting forces and vibrations reduced.

45-Degree Face Milling Cutter (The All-Rounder)

This is a universal face milling cutter for CNC milling. With 45° entering angle, symmetrical cutting force, and ability to cut cast iron, steel, and aluminum alloys, its cutting depth can reach up to 15%-25% of the tool diameter (for example, 24-40mm with φ160mm tool), in addition to supporting the roughing, stock removing, and semi-finishing.

Square Shoulder Face Milling Cutter (The Precision Master)

Accurate to mill 90-degree stepped surfaces with zero flatness and zero perpendicularity errors. Its edge insert wiper enhances surface finish. It is suitable for finishing cuts, where maximum depth of cut is 10%-20% of tool diameter (e.g., 20-40mm for a 200mm diameter cutter). It is used extensively for high-accuracy CNC machining parts.

Niche Options: Circular Insert Milling Cutter and Wave-Edge Milling Cutter

Circular insert milling cutters possess lengthy cutting edges with an expansion of forces of cut, thus being well suited for hard materials like stainless steel. Their greatest cutting depth is 80% of the insert radius at best.

Wave-Edge mill cutters are designed exclusively for cutting aluminum alloys and have a broad chip pocket to prevent clogging and a maximum depth of 1-1.5 times the tool's diameter, making them an ideal mill cutting tool for CNC milling aluminum alloys.

In custom cnc milling manufacturing, selecting the proper face mill is key to success. JS can offer full services from selection to machining. Welcome to contact for more details on online CNC milling services.

The Mystery Of Depth: Four Principles Influencing Milling Depth (Ap)

Principle 1: Tool Diameter is the Foundation

The larger the diameter, the higher the cutting force can be accommodated, and the greater the maximum depth of cut (better rigidity of cutter body and uniform force distribution).

For example, a 200mm diameter face mill cutting 45-grade steel has a maximum depth of cut of 30-50mm, a 100mm diameter face mill a maximum depth of cut of 15-25mm. Overloading needs to be considered based on capacity of machine tool.

Principle 2: Machine Tool Power and Rigidity are the Ceilings

Power is directly accountable for the ability to drive deep cuts. There is little margin for lack of power before resulting in overload, stalling, or vibration.

For example, a 10kW machine with 160mm diameter face mill cutting 45-grade steel can manage up to a maximum depth of cut of 5mm or less, a 20kW machine can manage up to a maximum depth of cut of 8-10mm.

Poor rigidity of machine tools can exacerbate vibrations at large cutting depths,impairing quality of machining.

Principle 3: Material hardness of the workpiece is a limitation

The harder the material, the greater the cutting resistance and less the maximum depth of cut.

For example, while milling HRC20-30 mild steel, it is possible to mill 20mm with a square shoulder face milling cutter. While milling HRC40-50 alloy steel, with the same tool, the maximum depth of cut should be reduced to 5-8mm in order not to chipp the insert.

Principle 4: Insert geometry and grade are crucial

The wide groove blade has a large chip space and is suitable for deep cutting. Narrow groove type is suitable for shallow cutting, deep cutting, and high feed.

The maximum cutting depth of hard alloy blades is 30% -50% greater than that of high-speed steel.When processing high hardness materials with ceramic blades, the cutting depth ability is increased by about 20% compared to hard alloys.

The table below shows how the four basic rules affect milling depth and how to correct them:

Principle	Influencing Factor	Method
Tool Diameter	The smaller the diameter, the smaller the depth of cut.	Select a face mill of proper diameter based on your machining requirement.
Machine Tool Power and Rigidity	Smaller power and reduced rigidity result in lower depth of cut.	If power is insufficient, reduce the depth of cut or change machines to one with increased rigidity.
Hardness of Workpiece Material	The more critical the hardness, the smaller the depth of cut.	In case of hard workpieces, use layered milling in an attempt to reduce the depth of cut per layer.
Insert Geometry and Material	Thin geometry, high-speed steel, small depth of cut.	For large depths of cut, choose wide-geometry carbide or ceramic inserts.

JS's online CNC milling service will first calculate your machine power, workpiece material, and other aspects to design a reasonable face mill depth of cut plan for you, with a stable and efficient machining process and helping you successfully produce CNC milling parts.

How To Safely Achieve Maximum Depth Of Cut In Heavy Cutting?

Deep cutting with high depths of cut has significant hazards, and three points have to be considered with topmost priority:

Stability First

Stability is a prerequisite for heavy cutting:

• To ensure that the workpiece is securely clamped.Use special fixtures or pressure plates to impart clamping pressure and also safeguard against workpiece movement during machining.
• To check the clearance between the machine tool guide rails. Remove any excessive gaps in order to prevent vibration during cutting.
• Keep the tool extension as short as possible. A longer extension reduces tool rigidity and increases the risk of vibration.

For example, when machining 50mm thick steel, keep the face mill extension within 1.5 times the tool diameter for optimal stability.

Tool Selection

For sever cutting, use an impact-resistant and stiff face mill. Recommend the cutter body to be solid carbide, greater than 50% stiffer than welded cutters. For inserts, employ ultra-fine-grain carbide with good toughness in order to withstand high cutting forces.

Meanwhile, the diameter of the cutting tool should match the size of the workpiece. For example, to machine a workpiece whose diameter is 1 meter, a face mill of diameter 250-315mm should be selected to enable a deep cut and minimize the number of tool passes.

Path Strategy (Using Plunge Milling or Trochoidal Milling)

Conventional reciprocating milling involves focusing cutting forces at deep depths of cut, which causes vibration. Plunge milling and trochoidal milling successfully counter this problem.

Plunge milling involves using the cutting force downwards along the Z-axis, aligning the machine tool spindle. The machine tool has the strongest rigidity and is particularly suited to deep cavity or step surface deep cutting processing,with a maximum depth of cut of up to twice the tool diameter.

Trochoidal milling uses a helical cutting motion, decreasing the cutting contact area and distributing cutting forces, minimizing tool wear and efficient for deep depths of cut in hard materials.

The Heart Of Cutting Parameters: Finding The Ideal Ratio Of Depth Of Cut, Feed, And Speed

Depth of cut (Ap), feed (F), and speed (S) in face milling are interdependent and require an optimum ratio. This is specifically defined as below:

Depth of Cut (Ap) and Radial Width of Cut (Ae)

Metal Removal Rate (MRR) has a direct effect on efficiency of machining, and Ap and Ae are significant components of MRR. Larger Ap leads to increased MRR, but so do cutting forces proportionally. Ae represents width of radial contact between tool and workpiece, wider Ae, more cutting per revolution.

When processing steel parts using a 200mm face mill with Ap=10mm, Ae=100mm, and F=1000mm/min, MRR is 1,000,000 mm³/min. If Ap is raised to 20mm, MRR doubles, which guarantees sufficient machine power.

Feed per Tooth (fz)

Feed per tooth refers to the quantity of distance cut per blade revolution, directly influencing cutting force and surface finish.

• A high fz (0.2-0.5mm/tooth) generates high cutting force and is best suited for roughing (e.g., a high-feed milling cutter with an fz of 0.5mm/tooth for milling steel).
• A low fz (0.1-0.2mm/tooth) generates excellent surface finish and is best suited for finishing (e.g., a square shoulder face milling cutter with an fz of 0.15mm/tooth for finishing).

Cutting Speed ​​(Vc)

Cutting speed is the linear rate of motion of the tool's cutting edge and is closely related with tool life. Higher Vc leads to excessive wear, and lower Vc results in inefficient working. For aluminum alloys, Vc = 300-500m/min, for 45-grade steel, Vc = 100-200m/min, and for stainless steel, Vc = 80-120m/min.

The Golden Rule: "High feed with shallow depth of cut, high depth of cut with medium-low feed."

This rule is based on hundreds of CNC milling operations.

• High feed with shallow depth of cut,for example, Ap = 1-2mm and fz = 0.4-0.6mm/tooth, can reduce cutting forces while improving efficiency and may be used on machine tools of normal rigidity.
• High depth of cut with medium-low feed, for example, Ap = 15-20mm and fz = 0.15-0.25mm/tooth, can avoid vibration caused by overlarge cutting forces and is suitable for high rigidity machine tools.

The following table provides a reference for feed and speed at different depths of cut (for machining 45# steel using a φ160mm carbide face mill):

Deepth of Cut (Ap)	Feed per Tooth (fz)	Cutting Speed ​​(Vc)	Spindle Speed ​​(S)
1-2mm (Shallow Depth of Cut)	0.4-0.6mm/Tooth	180-200m/min	358-398r/min
8-10mm (Medium Depth of Cut)	0.25-0.35mm/Tooth	150-180m/min	298-358r/min
15-20mm (High Depth of Cut)	0.15-0.25mm/Tooth	100-150m/min	199-298r/min

JS's online CNC milling services will customize the depth of cut, feed, and speed ratio to your machining application, following the golden rule, and provide transparent CNC milling prices. Quoting and ordering is straightforward. Just provide us with the part drawings and we will quickly develop a plan.

Face Milling Is Not a Panacea: When Do You Have An Alternative Milling Process To Utilize?

Face milling is more appropriate for machining flat surfaces. Other operations are required for deep cavities and complex contours.

The table below gives a comparison of face milling and other milling processes:

Milling Process	Applications	Maximum Depth of Cut	Advantages	Disadvantages
Face Milling	Machining of flat and stepped surfaces.	Typically ≤ 50% of the tool diameter.	High efficiency, good surface finish.	Cannot machine deep cavities or curved surfaces.
Pocketing	Machining of internal grooves and cavities.	Up to 3 times the diameter of the cutting tool	Can machine deep cavities and flexible layering.	Less efficient than face milling.
Contouring	Milling of complicated contours and curved surfaces.	Deformability to the contour.	High accuracy, best suited for complicated shapes.	Complicated programming, lower efficiency.
Plunge Milling	Rough milling of cavities and deep steps.	Down to 3 times the tool diameter.	Low cutting force, minimum vibration hazard.	Poor surface quality, requiring finish machining afterwards.

JS offers not just face milling but also several other operations such as pocket milling, contour milling, and plunge milling. We can select the optimal process based on the structural characteristics of your CNC milling parts and create a custom cnc milling manufacturing solution to enable quicker and more precise machining of parts.

Case Study: The Rebirth Of a Giant Turbine Bottom Ring – A Race Against Time In Face Milling

Customer's Dilemma

One of the hydropower power plants replaced an 8-meter-diameter, 50-ton cast steel bottom ring (ZG270-500). The flatness required was ≤0.2mm and the turnover time was 72 hours.

The customer initially employed a large square shoulder milling cutter (Ap = 8mm, fz = 0.3mm/tooth, Vc = 120m/min). The massive cutting forces (15kN) caused workpiece displacement and tool cutting edge chipping. The machining took 96 hours, well beyond schedule, with tens of thousands of dollars wasted for each one single hour.

JS's Innovative Solution

Faced with the customer's dilemma, JS precision machining created a combined approach of "high-feed milling + large-diameter square shoulder finishing" and clamping and parameter optimization.

Clamping Optimization:

We used eight hydraulic clamps to clamp the bottom ring firmly onto the machine table with 50kN pressure per clamp to avoid zero displacement during machining. We also made use of blocks of the same height underneath the bottom ring to prevent workpiece deformation.

Roughing Stage:

We selected a φ250mm high-feed milling cutter having ultra-fine-grain carbide inserts. We employed a shallow depth of cut approach, extremely high feed, full-edge cutting, cutting velocity of Vc = 180m/min, and spindle speed of S = 229rpm.

Finishing Srage:

A φ315mm square shoulder face milling cutter was used with a fine grind wiper blade. Using parameters of small cutting depth, small step pitch, and high-speed medium feed, specifically Vc=200m/min, S=202r/min, and fz=0.2mm/tooth.

Final Results

The entire machining process completed only 64 hours, handed over eight hours in advance, saving the customer hundreds of thousands of dollars in lost time. Life of high-feed milling cutter during machining was 12 hours, life of square shoulder milling cutter was 8 hours, both higher than customers' requirement of 5 hours.

This case demonstrates clearly that "maximum depth of cut" is not the goal, "optimal metal removal rate" and "process reliability" are keys to success for CNC milling.

FAQs

Q1: Can the depth of cut in milling aluminum alloys be raised?

Yes. The cutting force of aluminum alloys is about one-third that of steel, and consequently, machine power is less. Aluminum tools like wave-edge milling cutters may be employed. They possess broad chip flutes that do not pack up with chips, and 1-1.5 times tool diameter maximum depth of cut is achievable (e.g., 100-150mm for a φ100mm tool).

Q2: Does machine power limit depth of cut?

Yes, the machine power directly limits the maximum cutting depth. The power determines the ability to withstand cutting forces, and forcibly increasing the cutting depth when insufficient can lead to overload, stoppage, and damage to the spindle.Before determining the cutting depth, it is necessary to confirm the rated power of the machine tool.

Q3: How would I know if my depth of cut is excessive?

Note four significant signs:

• A dull rumbling or metal-to-metal vibration of the machine indicates that the cutting forces are overpowering the machine capacity.
• Insert chipping (abnormal wear) before its anticipated service time.
• Chatter marks on the work surface of the machined component.
• Dark-colored chip (overheating). If any of these phenomena occur, reduce Ap or re-optimize other parameters.

Q4: Do I employ single-pass depth of cut or layered milling?

It depends on workpiece material hardness and the machining allowance. When dealing with very hard materials (e.g., HRC40 + alloy steel) or machining allowances in excess of 15mm, layered milling will be better. For soft materials such as copper and aluminum alloys with a stock removal of ≤5mm, one cut depth is more effective, and can also reduce CNC milling prices.

Summary

Face mill cut depth is far more than a simple numeric answer. It's a complex balancing act among tool technology, machine capability, material properties, and process knowledge. Only by understanding the science behind it and mastering the art of balancing parameters can you turn your machining center into a true production tool.

JS has extensive experience in CNC milling for decades and can provide professional support for you.whether selecting a suitable face mill, determining optimal cutting parameters, or pushing through the challenge of machining large parts.

Our easy online CNC milling service process and transparent CNC milling price make us capable of helping you create effective solutions to many face milling depth and machining issues for safe and efficient CNC milling parts.