When it comes to machining, precision is everything. Whether you’re working on a CNC machine in a professional shop or running a hobby mill in your garage, the choice of tooling directly affects the outcome of your project. Among the many tools used in milling, end mills are some of the most versatile. They can cut slots, shape surfaces, carve intricate details, and remove material quickly. But to get the best results, you need to use the correct size.
This is where an end mill size chart becomes essential. Instead of guessing or wasting time measuring every tool manually, a chart gives you an instant reference for both imperial and metric sizes, their decimal equivalents, and common shank diameters. In this article, we’ll dive deep into the world of end mills, explain why size matters, provide a detailed size chart, and show you how to apply it in real machining situations.
What Is an End Mill?
At its core, an end mill is a type of milling cutter designed for cutting metal, wood, or plastic. Unlike drill bits, which are designed to plunge straight down, end mills can cut in multiple directions: down (axially), sideways (radially), and sometimes even at angles depending on the geometry.
End mills come in many forms: flat end mills, ball nose end mills, corner radius end mills, and more. Each has a unique role. Flat end mills are great for cutting sharp edges and slots. Ball nose end mills are perfect for 3D contouring, especially in molds or aerospace parts. Corner radius mills extend tool life when sharp edges aren’t required.
Because of this versatility, machinists rely on them daily. But versatility also means variety—sizes can range from micro tools thinner than a sewing needle to heavy-duty cutters over two inches wide. That’s why a reference chart is so valuable: it organizes all of these sizes in one place.
Why End Mill Size Matters
Choosing the right end mill isn’t just about convenience—it has a direct impact on performance, accuracy, and even tool life. Here’s why:
1- Cutting Accuracy:
Small tools allow you to cut fine details, engrave text, or create sharp internal corners. For example, if you’re machining a mold cavity with tight radii, only a small-diameter end mill can reach those areas. On the other hand, using a tool that’s too large may leave excess material that requires extra passes.
2- Material Removal Rate (MRR):
In roughing operations where you need to remove lots of material quickly, larger tools are preferred. A 1/2″ or 1″ end mill can clear stock much faster than a 1/8″ cutter. If you try to rough out with a small tool, you’ll waste time and increase wear on the tool.
3- Tool Strength and Rigidity:
Larger diameters can withstand more cutting forces without deflecting. A thin 1/16″ cutter will flex under load, potentially breaking if pushed too hard. A 3/4″ tool, however, can plow through tough steel with less risk of failure.
4- Surface Finish Quality:
End mill size also affects surface smoothness. Larger tools tend to leave flatter, smoother surfaces because they cover more area per pass. Smaller tools may leave more tool marks and require finishing passes.
5- Machine Compatibility:
The shank size of an end mill must match the collet or tool holder of your machine. Using the wrong size can cause chatter, tool slippage, or even dangerous ejection of the cutter.
Simply put: the size of your end mill determines what’s possible in your machining project. Without the right size, you risk inaccuracy, inefficiency, or tool damage.
Standard End Mill Sizes
End mills are produced in two main measurement systems: imperial (inch-based) and metric (millimeter-based). Depending on where you are in the world or what industry you’re in, you might encounter one or both.
Imperial Sizes:
These are common in the U.S. and often come in fractional increments, such as 1/16″, 1/8″, 1/4″, and so on. Decimal equivalents are also used in precision settings. For example, machinists often say “point one-two-five” instead of “one-eighth inch” because decimal values eliminate ambiguity.
Metric Sizes:
Widely used in Europe, Asia, and in international industries like automotive and aerospace. Metric tools usually range from 1 mm up to 40 mm or more, with common sizes being 3 mm, 6 mm, 10 mm, etc.
In practice, shops often stock both types because many blueprints and CNC programs mix imperial and metric units. That’s why charts include both systems side by side.
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End Mill Size Chart (Fractional, Decimal, and Metric)
Below is a reference chart listing common end mill sizes in both imperial and metric measurements, along with typical shank diameters. This makes it easier to choose a tool that fits your collet or tool holder.
Fractional & Decimal Inch End Mills
| Fractional Size | Decimal (Inches) | Common Shank Size (Inch) |
|---|---|---|
| 1/32″ | 0.0313 | 1/8″ |
| 1/16″ | 0.0625 | 1/8″ |
| 3/32″ | 0.0938 | 1/8″ |
| 1/8″ | 0.1250 | 1/8″ or 1/4″ |
| 3/16″ | 0.1875 | 3/16″ or 1/4″ |
| 1/4″ | 0.2500 | 1/4″ |
| 5/16″ | 0.3125 | 3/8″ |
| 3/8″ | 0.3750 | 3/8″ |
| 7/16″ | 0.4375 | 1/2″ |
| 1/2″ | 0.5000 | 1/2″ |
| 5/8″ | 0.6250 | 5/8″ |
| 3/4″ | 0.7500 | 3/4″ |
| 1″ | 1.0000 | 1″ |
| 1-1/2″ | 1.5000 | 1-1/2″ |
| 2″ | 2.0000 | 2″ |
Metric End Mills
| Metric Size (mm) | Decimal Equivalent (Inch) | Common Shank Size (mm) |
|---|---|---|
| 1 mm | 0.0394 | 3 mm |
| 2 mm | 0.0787 | 4 mm |
| 3 mm | 0.1181 | 3 mm or 6 mm |
| 4 mm | 0.1575 | 4 mm or 6 mm |
| 5 mm | 0.1969 | 6 mm |
| 6 mm | 0.2362 | 6 mm |
| 8 mm | 0.3150 | 8 mm |
| 10 mm | 0.3937 | 10 mm |
| 12 mm | 0.4724 | 12 mm |
| 16 mm | 0.6299 | 16 mm |
| 20 mm | 0.7874 | 20 mm |
| 25 mm | 0.9843 | 25 mm |
| 32 mm | 1.2598 | 32 mm |
| 40 mm | 1.5748 | 40 mm |
This chart isn’t exhaustive—specialty mills can fall outside these ranges—but it covers the most common diameters found in workshops.
End Mill Size Chart PDF:
The End Mill Size Chart PDF provides a clear reference for both imperial and metric dimensions, including fractional, decimal, and common shank sizes. Designed in landscape format for easy readability, this chart helps machinists, engineers, and hobbyists quickly select the correct end mill size. It saves time, reduces errors, and ensures precision in machining projects.
Flute Length and Cutting Depth
When selecting an end mill, diameter isn’t the only factor. The flute length—the part of the tool that actually cuts—determines how deep you can go in a single pass.
Short Flute End Mills: Shorter flutes increase rigidity, reduce deflection, and are ideal for shallow cuts. For example, if you only need to cut a 3 mm-deep slot, a short flute tool will give you better stability and surface finish.
Long Flute End Mills:Longer flutes allow deeper cutting, such as pocketing or cutting through thicker material. However, the trade-off is that long tools are more prone to chatter and vibration. This can leave poor surface finishes or even break the cutter if pushed too aggressively.
A good rule of thumb is to avoid cutting deeper than 3 times the diameter of the tool in one pass. For instance, with a 6 mm cutter, try not to exceed an 18 mm cutting depth without using special tooling or strategies.
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Choosing the Right End Mill Size
With so many options, how do you choose the right one? Here are key considerations:
Workpiece Material
Softer materials like aluminum allow for larger cutters because they can handle higher removal rates.
Harder materials like stainless steel often require smaller, more rigid tools to avoid breakage.
Machine Power and Rigidity:
A lightweight CNC router cannot handle a 1″ carbide end mill without chatter. Match tool size to your machine’s spindle horsepower and rigidity.
Detail vs. Speed:
If you are engraving fine lettering, a 1 mm or 1/32″ tool is necessary. But if you’re roughing out a block of stock, a 1/2″ or larger tool will save hours of machining time.
Tool Holder Compatibility:
Check your collet sizes. It’s common for beginners to buy a 3/8″ tool only to realize their ER11 collet maxes out at 1/4″. Always match the shank to your holder.
By considering these factors, you’ll select a tool that balances speed, precision, and safety.
Applications of Different End Mill Sizes
Let’s break down the real-world uses of different size categories:
Micro End Mills (1 mm – 3 mm / 1/32” – 1/8”)
Used for engraving, circuit boards, jewelry making, and fine details in molds. They’re delicate but essential when accuracy matters most.Medium End Mills (4 mm – 12 mm / 3/16” – 1/2”)
The workhorses of most shops. Perfect for pocketing, profiling, and general-purpose machining. If you only stock a few tools, these are the sizes to keep.Large End Mills (16 mm – 40 mm / 5/8” – 2”)
Best for heavy roughing, surfacing large areas, and machining thick plates. They require rigid machines but can remove material at incredible rates.
Each size group has its place, and experienced machinists often switch between multiple diameters during a single job for efficiency.
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Practical Tips for Using End Mill Size Charts
A size chart is only useful if you integrate it into your workflow. Here are practical tips:
Keep a Laminated Chart Nearby – Post it near your CNC machine or toolbox for quick reference.
Use Digital Calipers – Verify the actual diameter before running a job, especially if the cutter has been rehappened.
Match Feeds and Speeds – Larger cutters can handle higher feeds, while smaller ones need gentler settings.
Check Shank Sizes Twice – A mismatch between shank and collet is a common beginner mistake that can lead to tool slippage.
These habits save time and prevent costly mistakes.
Benefits of Having a Reference Chart
You might wonder: why not just measure tools as needed? The answer is efficiency. Having a chart brings multiple benefits:
Saves Time – Instead of measuring each cutter, you instantly know the size.
Improves Accuracy – Reduces human error in conversions between fractions, decimals, and metric.
Speeds Up Setup – Faster tool changes mean more productive machining.
Training Tool – New machinists can learn size relationships faster by referencing the chart.
In professional environments, these small improvements add up to significant cost savings.
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Conclusion
End mills may seem simple, but their sizes play a huge role in machining success. From micro cutters for detailed engraving to large tools for heavy stock removal, every diameter has a purpose. By using an end mill size chart, you can quickly match the right tool to your project, saving time and avoiding mistakes.
Whether you are a hobbyist working on aluminum parts or a professional machinist in a production shop, having both imperial and metric charts on hand ensures precision and efficiency. Pair this knowledge with smart choices about flute length, tool material, and feeds and speeds, and you’ll be well-equipped for any milling challenge.
Keep this chart handy, and remember: the right size tool is the foundation of accurate, efficient machining.
FAQs Related to End mills Size
What is the purpose of an end mill size chart?
An end mill size chart helps machinists quickly identify the correct tool dimensions for their projects. It provides imperial, metric, fractional, and decimal equivalents along with common shank sizes. This saves time, reduces measurement errors, and ensures compatibility with machine collets or holders, making machining more efficient and precise.
How do I choose the right end mill size?
Selecting the right end mill size depends on your project needs, material, and machine. Small diameters work best for fine details and engraving, while larger tools are better for heavy material removal. Always consider tool rigidity, depth of cut, machine capability, and shank compatibility before deciding on the size.
Are metric and imperial end mill sizes interchangeable?
Metric and imperial sizes serve the same purpose but aren’t always interchangeable. Some jobs require strict adherence to blueprint measurements, which may be in inches or millimeters. While you can sometimes use close equivalents, precision machining often demands the exact size. A size chart makes conversions easier when both systems are used.
What happens if I use the wrong end mill size?
Using the wrong end mill size can lead to poor accuracy, surface finish issues, excessive tool wear, or even tool breakage. For instance, an oversized tool may not fit the pocket dimensions, while an undersized tool can require extra passes. Correct sizing ensures optimal machining performance, efficiency, and long tool life.
Why do shank sizes matter in end mills?
Shank size determines whether an end mill fits securely in your machine’s collet or tool holder. A mismatch can cause vibration, chatter, or tool slippage, leading to poor results and potential hazards. The size chart includes shank dimensions to help machinists quickly verify compatibility before selecting and installing a tool.
Can end mill size charts help beginners?
Yes, end mill size charts are especially valuable for beginners. They simplify tool selection by showing clear conversions between fractions, decimals, and metric sizes. Instead of memorizing measurements, new machinists can use the chart as a reference. It reduces mistakes, speeds up learning, and builds confidence in CNC or manual machining.