This article is part of my CNC & Manufacturing Engineering knowledge base — a growing collection of practical CNC machining guides, engineering tools, and programming resources.

Material Removal Rate (MRR) Explained: Why Faster Feed Doesn't Always Mean Higher Productivity

Imagine two CNC milling processes.

Process A

• Feed Rate = 1,000 mm/min

Process B

• Feed Rate = 600 mm/min

Which process removes material faster?

At first glance, the answer seems obvious: Process A. After all, the tool is moving almost twice as fast.

But now let's add two more pieces of information.

Process A

• Feed Rate = 1,000 mm/min
• Axial Depth of Cut = 1 mm
• Radial Width of Cut = 5 mm

Process B

• Feed Rate = 600 mm/min
• Axial Depth of Cut = 4 mm
• Radial Width of Cut = 10 mm

Now which process removes more material per minute? The answer changes completely.

This is why feed rate alone is not a good measure of machining productivity. To understand how much material we are actually removing, we need another parameter: Material Removal Rate — MRR.

MRR connects three important dimensions of a milling process:

How deep are we cutting? × How wide are we cutting? × How fast are we feeding?

This article will explain MRR from the basics, then go one step further: why the highest MRR is not always the best machining process.

1. What Is Material Removal Rate?

Material Removal Rate describes the volume of material removed during a given amount of time.

For metric milling calculations, MRR is commonly expressed as:

• mm³/min
• or sometimes cm³/min

Conceptually:

MRR = Volume Removed / Time

If a process removes 20,000 mm³ of material every minute, then:

MRR = 20,000 mm³/min

MRR gives us a much better view of roughing productivity than feed rate alone.

2. The Basic Milling MRR Formula

For a straightforward milling operation, MRR can be calculated as:

MRR = ap × ae × Vf

Where:

The idea is surprisingly simple. Imagine the cutter moving forward through the material. The cut has a cross-sectional area:

ap × ae

Then that cutting area moves forward at:

Vf

So:

Area × Distance per Minute = Volume per Minute

Or:

mm × mm × mm/min = mm³/min

That's MRR.

3. Think of MRR as a Tunnel

Here is another way to visualize it. Imagine the cutting tool is digging a rectangular tunnel through the material.

The tunnel has:

• Height = ap
• Width = ae

So its cross-sectional area is:

ap × ae

Now the tool moves forward at:

Vf

The larger the tunnel and the faster you move through it, the more material you remove every minute.

That is essentially what the MRR formula describes. This visualization also reveals something important:

Feed rate is only one of three variables controlling MRR.

4. Our First MRR Calculation

Suppose a milling operation uses:

• ap = 2 mm
• ae = 10 mm
• Vf = 800 mm/min

Then:

MRR = 2 × 10 × 800
MRR = 16,000 mm³/min

Convert that to cubic centimeters:

16,000 mm³/min = 16 cm³/min

So the process removes approximately 16 cm³ of material every minute.

Now we have a productivity measure that says much more than:

Feed Rate = 800 mm/min

5. Back to Our Opening Question

Let's calculate the two processes from the beginning.

Process A

• Vf = 1,000 mm/min
• ap = 1 mm
• ae = 5 mm

MRR = 1 × 5 × 1000
MRR = 5,000 mm³/min

Process B

• Vf = 600 mm/min
• ap = 4 mm
• ae = 10 mm

MRR = 4 × 10 × 600
MRR = 24,000 mm³/min

Now compare them:

Process B has the lower feed rate. But it removes almost 5× more material per minute.

This is the first major lesson:

A faster feed does not automatically mean higher machining productivity.

6. Feed Rate Is Speed. MRR Is Volume.

This distinction is extremely useful.

Feed rate tells us: How fast the tool moves along the programmed path.

MRR tells us: How much material is removed per unit time.

Those are not the same thing.

A tool can move very quickly while taking a light cut. Another tool can move more slowly while removing a much larger cross-section of material.

So when comparing roughing strategies, asking:

"Which one has the higher feed rate?"

may be less useful than asking:

"Which one removes more material per minute, and at what cost?"

The second half of that question becomes very important later.

7. Understanding ap — Axial Depth of Cut

Let's look at the first MRR variable: ap.

In many milling operations, ap represents the axial depth of cut. Think of it as the amount of cutting engagement along the tool axis.

For a typical side-milling example:

• ap = 1 mm → relatively shallow axial engagement
• ap = 10 mm → much deeper axial engagement

If everything else remains constant:

Increase ap → Increase MRR

For example:

Case 1

• ap = 2 mm, ae = 10 mm, Vf = 500 mm/min
• MRR: 10,000 mm³/min

Case 2 (increase only ap)

• ap = 4 mm, ae = 10 mm, Vf = 500 mm/min
• MRR becomes: 20,000 mm³/min

Doubling ap doubled the theoretical MRR. Simple mathematically.

But physically, the cutting process has changed too. More cutting edge is now engaged with the material. That can change:

• Cutting force
• Tool deflection
• Heat distribution
• Spindle load
• Tool wear
• Vibration behavior

So increasing ap is not "free" productivity.

8. Understanding ae — Radial Width of Cut

The second variable is: ae.

In milling, ae commonly represents radial engagement or width of cut.

For example, with a 10 mm diameter end mill:

• ae = 10 mm → full-width engagement in a simple slotting case
• ae = 1 mm → only 10% of the tool diameter

Again, if everything else remains constant:

Increase ae → Increase MRR

But increasing radial engagement also changes how the cutter interacts with the material. It can affect:

• Engagement angle
• Chip thickness
• Cutting force
• Heat
• Tool deflection
• Chip evacuation
• Stability

This is why two processes with the same MRR may behave very differently. We'll come back to that.

9. The Third Variable — Feed Rate

The final variable is: Vf, which we covered in the previous article.

For milling:

Vf = fz × z × n

So MRR can also be written conceptually as:

MRR = ap × ae × fz × z × n

Now something interesting happens. MRR is no longer an isolated calculation. It connects directly back to everything we have already learned:

Cutting Speed
↓
Spindle Speed
↓
Feed per Tooth
↓
Feed Rate
↓
Material Removal Rate

This is the first point in this series where all the earlier parameters begin to describe machining productivity.

10. Building the Entire Calculation Chain

Let's work through one complete example.

Suppose we have:

• Tool Diameter = 10 mm
• Cutting Speed = 120 m/min
• Feed per Tooth = 0.05 mm/tooth
• Number of Teeth = 4
• ap = 3 mm
• ae = 5 mm

Step 1 — Calculate Spindle Speed

n = (1000 × Vc) / (π × D)
n = (1000 × 120) / (π × 10)
n ≈ 3,820 RPM

Step 2 — Calculate Feed Rate

Vf = fz × z × n
Vf = 0.05 × 4 × 3820
Vf ≈ 764 mm/min

Step 3 — Calculate MRR

MRR = ap × ae × Vf
MRR = 3 × 5 × 764
MRR ≈ 11,460 mm³/min

Or approximately: 11.46 cm³/min.

Now look at the complete chain:

Vc = 120 m/min
↓
D = 10 mm
↓
n ≈ 3,820 RPM
↓
fz = 0.05 mm/tooth
↓
z = 4
↓
Vf ≈ 764 mm/min
↓
ap = 3 mm, ae = 5 mm
↓
MRR ≈ 11,460 mm³/min

This is why machining parameters should not be learned as isolated formulas. They are part of one connected process.

11. Here's Where MRR Gets More Interesting

Imagine two milling strategies. Both achieve:

MRR = 20,000 mm³/min

Are they automatically equivalent? No.

For example:

Strategy A

• ap = 2 mm, ae = 10 mm, Vf = 1,000 mm/min
• MRR: 20,000 mm³/min

Strategy B

• ap = 10 mm, ae = 2 mm, Vf = 1,000 mm/min
• MRR: 20,000 mm³/min

Same MRR. Same feed rate. But the cutter engagement is completely different.

• Strategy A: Shallow axial engagement + wide radial engagement
• Strategy B: Deep axial engagement + narrow radial engagement

The theoretical volume removed per minute is identical. But tool loading, heat distribution, chip thickness behavior, deflection and stability may be very different.

This leads to our second major lesson:

Same MRR does not mean same machining process.

12. Why Modern Roughing Often Uses Different Engagement Strategies

Traditional roughing often used relatively heavy radial engagement with shallower axial depths.

Modern toolpaths may use a different strategy:

• Smaller radial engagement
• Larger axial engagement
• Controlled tool engagement
• Higher feed rates where appropriate

You may see this idea in strategies described as:

• High-Efficiency Milling
• Dynamic Milling
• Adaptive Clearing
• Trochoidal Milling

The exact implementation differs between CAM systems and applications, but the underlying idea is important:

Productivity is not only about pushing the tool harder. It can also come from using the cutting edge more effectively and controlling engagement more consistently.

That is a much more interesting engineering problem than simply asking:

"How high can I make the feed rate?"

13. MRR vs. Tool Life

Suppose we increase MRR by 30%. Great. But what if tool life falls by 60%? Is the process better?

Maybe. Maybe not. We need more information.

Imagine:

Process A

• MRR: 50 cm³/min
• Tool life: 60 minutes

Process B

• MRR: 70 cm³/min
• Tool life: 20 minutes

Process B removes material faster. But it also requires tool changes much more frequently.

Now we need to consider:

• Tool cost
• Tool-change time
• Machine downtime
• Operator intervention
• Scrap risk
• Process consistency
• Production volume

This is where machining optimization becomes an engineering and economic problem — not just a formula problem.

14. The Highest MRR Is Not Always the Best Process

This deserves its own section.

A common optimization mindset is:

Higher MRR = Better Process

But a production process has more than one objective. We may also care about:

• Tool life
• Machine utilization
• Surface quality
• Dimensional stability
• Reliability
• Cycle time
• Tool cost
• Scrap rate
• Machine power
• Workholding stability
• Operator intervention
• Repeatability

So a better engineering question is:

What MRR gives us the best overall process performance?

Sometimes the answer is not the maximum possible MRR. It may be a slightly lower MRR that produces:

• Longer tool life
• Fewer tool changes
• Lower process variation
• Better reliability
• Less risk

In real manufacturing, repeatable productivity often matters more than impressive peak numbers.

15. MRR and Spindle Power

Removing more material generally requires more cutting energy. As MRR increases, the power required for cutting also tends to increase.

A simplified conceptual relationship is:

Cutting Power ≈ MRR × Specific Cutting Energy

The exact calculation depends on material and cutting conditions, but the important idea is:

MRR cannot increase indefinitely without demanding something from the machine.

At some point you may encounter limitations from:

• Spindle power
• Spindle torque
• Tool strength
• Machine rigidity
• Workholding
• Axis capability

A calculator may tell you:

MRR = 100,000 mm³/min

But the calculator does not know whether your machine can physically support that cutting condition. That is the engineering part.

16. Power and Torque Are Not the Same Thing

This is also worth understanding early.

A machine may have sufficient spindle power at one RPM range but limited torque or available power at another. For example, heavy cutting at relatively low spindle speed can place very different demands on the spindle compared with high-speed light cutting.

So when increasing MRR, don't look only at:

• Maximum machine power

Also consider:

• Spindle speed
• Available torque
• Machine power curve
• Tool diameter
• Cutting force

This topic deserves its own future article:

Spindle Power vs. Torque: Why the Same kW Doesn't Mean the Same Cutting Capability

That will connect machine specifications directly to machining parameters.

17. MRR and Chip Evacuation

Here's another limit that is easy to overlook.

If you remove more material every minute, you also create more chips every minute. Those chips have to go somewhere.

Poor chip evacuation can cause:

• Chip recutting
• Tool damage
• Surface damage
• Heat buildup
• Chip packing
• Tool breakage

This is especially important in:

• Deep pockets
• Slots
• Cavities
• Deep-hole machining
• Materials producing long or difficult chips

So sometimes the limit is not:

Can the cutting edge remove this much material?

The real limit is:

Can we get the chips out of the cutting zone fast enough?

18. MRR and Workholding

Imagine you find a cutting condition capable of extremely high MRR. The tool can handle it. The spindle can handle it. But can the fixture?

Increasing material removal often increases cutting forces. Those forces travel through:

Cutting Edge → Tool → Toolholder → Spindle

and also through:

Workpiece → Fixture → Machine Table

A weak link anywhere in that system can become the actual process limit. This is why machining optimization should consider the entire mechanical chain.

19. MRR and Thin-Wall Parts

This becomes even more obvious with thin-wall components.

Suppose the machine and cutter are capable of very aggressive roughing. But the component itself becomes flexible as material is removed.

A high-MRR strategy may create:

• Part deflection
• Vibration
• Dimensional error
• Wall distortion

In this case, the workpiece — not the tool or spindle — may define the practical MRR limit.

Again:

Maximum theoretical productivity ≠ Maximum practical productivity

20. MRR and Surface Finish

MRR is especially useful when thinking about roughing. But roughing and finishing usually have different priorities.

During roughing, the goal may emphasize:

• Efficient material removal

During finishing, the priority may shift toward:

• Surface finish
• Dimensional accuracy
• Geometric accuracy
• Tool marks
• Part stability

Trying to maximize MRR during every operation makes little sense. The machining objective changes throughout the process. A good process uses the right strategy for each stage.

21. The Hidden Metric: Stable MRR

Let's introduce an idea that I think is more useful than simply chasing:

Maximum MRR

Consider two processes.

Process A

• Peak MRR: 100 cm³/min
• But it frequently experiences: chatter, tool chipping, feed overrides, operator adjustments

Process B

• MRR: 80 cm³/min
• But it runs continuously and predictably

Which process produces more parts during an eight-hour shift? The answer may easily be Process B.

This suggests another useful concept:

Stable MRR

Not just:

How much material can we remove at one moment?

But:

How much material can we remove consistently, predictably and economically over time?

That is much closer to how manufacturing productivity should be evaluated.

22. Peak Performance vs. Production Performance

This distinction appears throughout manufacturing.

A process may demonstrate very high peak performance during a short test. But production requires:

• Repeatability
• Tool life
• Machine uptime
• Predictable quality
• Stable dimensions
• Low intervention

A process that works perfectly for 30 seconds is not necessarily a good production process. That is why parameter optimization should not stop when we find the fastest cut. We need to ask:

Can this process keep working?

23. A Better Way to Think About Optimization

Instead of asking only:

How can I maximize MRR?

Try asking:

1. What is limiting the process?
Is it: Tool life? Machine power? Torque? Chatter? Workholding? Chip evacuation? Part rigidity? Surface finish?

2. Which parameter can I change?
Cutting speed? Feed per tooth? ap? ae? Tool geometry? Toolpath?

3. What happens elsewhere when I change it?

For example:

• Increase ae → Higher engagement → Potentially higher force and heat
• Increase ap → More cutting edge engagement → Different tool loading and wear distribution
• Increase feed → Higher fz → Higher cutting load per tooth

This is how machining optimization becomes systematic rather than trial and error.

24. A Practical MRR Comparison

Suppose we need to remove a large amount of material from a component. Let's compare three hypothetical strategies.

At first glance: Strategy C wins. It has the highest MRR.

But now suppose:

• Strategy C produces chatter and requires frequent tool changes.
• Strategy B runs reliably for an entire batch.

Suddenly:

22,500 mm³/min may be more valuable than 24,000 mm³/min

The calculator gives us the number. The machining process gives us the answer.

25. MRR Is a KPI, Not a Target by Itself

This is an important distinction.

MRR is extremely useful as a Key Performance Indicator. It helps us compare:

• Cutting strategies
• Tools
• Programs
• Process changes
• Roughing approaches

But a KPI should help us understand a process. It should not become the process objective by itself.

A high MRR with:

• Poor tool life
• High scrap
• Unstable cutting
• Frequent alarms

is not necessarily an improvement.

The goal is not:

Maximum MRR

The goal is closer to:

Maximum sustainable manufacturing performance

MRR is one part of that picture.

26. Engineer's Note — Ask What Is Actually Limiting You

When a machining process feels slow, increasing feed rate is an obvious reaction. But before changing the program, I prefer asking:

What is actually limiting this operation?

Maybe feed rate is the limitation. But maybe it is:

• Radial engagement
• Axial engagement
• Spindle power
• Tool overhang
• Fixture rigidity
• Chip evacuation
• Tool life
• Machine acceleration
• Part rigidity

If the real limitation is workholding, increasing feed may simply create more problems. If the real limitation is chip evacuation, increasing MRR may make the process less reliable. If the tool has unused axial cutting length, changing the engagement strategy might create more productivity than simply increasing feed.

The useful question is not:

"How can I make this number bigger?"

It is:

"Where is the bottleneck in the complete machining system?"

That question is much closer to manufacturing engineering.

27. Try the MRR Calculator

I built a free online CNC Machining Calculator that includes Material Removal Rate:

👉 https://lilu1626.github.io/cnc-machining-calculator/

For example, enter:

• Axial Depth of Cut = 3 mm
• Radial Width of Cut = 5 mm
• Feed Rate = 764 mm/min

The result is approximately:

MRR = 11,460 mm³/min

You can use this to compare different machining strategies. But remember:

The calculator answers:

"What is the theoretical MRR?"

It does not answer:

"Is this cutting condition appropriate for my machine, tool, workpiece and setup?"

That still requires engineering judgment.

28. Common MRR Mistakes

Mistake #1 — Using Feed Rate as the Only Productivity Measure
High feed does not automatically mean high MRR.

Mistake #2 — Maximizing All Three Variables
Increasing ap + ae + Vf simultaneously may produce a large theoretical MRR, but also dramatically increase cutting load.

Mistake #3 — Ignoring Machine Power and Torque
The spindle must be capable of supporting the cutting condition.

Mistake #4 — Ignoring Tool Engagement
Two processes with the same MRR can load the cutter very differently.

Mistake #5 — Ignoring Chip Evacuation
Removing material quickly is useless if chips cannot leave the cutting zone reliably.

Mistake #6 — Ignoring Tool Life
A faster process may not be economically faster if tooling consumption increases dramatically.

Mistake #7 — Comparing MRR Without Looking at Stability
Peak performance is not the same as production performance.

29. The Complete Machining Chain So Far

We have now built a complete parameter chain:

Cutting Speed (Vc)
  ↓ (with diameter)
Spindle Speed (n)
  ↓ (with fz and z)
Feed Rate (Vf)
  ↓ (with ap and ae)
Material Removal Rate (MRR)

Now the formulas are no longer isolated. They describe a machining system.

30. Final Takeaway

The basic MRR formula is simple:

MRR = ap × ae × Vf

But the real lesson is much more important.

Feed rate tells us how fast the tool moves. MRR tells us how much material we remove. And neither one, by itself, tells us whether the machining process is good.

A productive machining process must balance:

• Material Removal
• Tool Life
• Machine Capability
• Process Stability
• Workholding
• Chip Control
• Quality
• Cost

The highest theoretical MRR may look impressive. But in production, the better process is often the one that removes material:

quickly, predictably, repeatedly and economically.

That is the difference between:

Making the machine move faster

and:

Improving the manufacturing process.

What's Next?

We have now connected:

Vc → RPM → fz → Feed Rate → MRR

At this point, there is another very practical question:

If I know how much material must be removed, can I estimate how long the machining operation will take?

This brings us to the next topic: Machining Time.

But instead of simply learning:

Time = Distance / Feed

we will look at a more realistic question:

Why Is the Actual CNC Cycle Time Always Longer Than the Simple Calculation?

Because a CNC machine does much more than cut in a straight line. The next article will look at:

• Cutting time
• Rapid movement
• Acceleration and deceleration
• Tool changes
• Spindle acceleration
• Positioning
• Approach and retract moves
• CAM toolpaths
• Machine dynamics
• Non-cutting time

This will take us from:

Machining Parameters

into:

Cycle Time and Manufacturing Productivity

Free CNC Machining Calculator

👉 https://lilu1626.github.io/cnc-machining-calculator/

About Li Lu | CNC & Manufacturing Engineering

Li Lu is a Machining Development Engineer at GE Vernova Advanced Manufacturing & Repair Technology (AMRT) in Singapore, with 10+ years in CNC machining, process development, and machine tool integration.

• Engineering Knowledge Base: https://lilu1626.github.io/cnc-manufacturing-engineering/
• CNC Machining Calculator: https://lilu1626.github.io/cnc-machining-calculator/
• GitHub: https://github.com/LiLu1626
• LinkedIn: https://www.linkedin.com/in/lu-li-29b464163/