Standard Drill Bit Sizes for CNC Machining: A Practical Guide (2025)

1. Introduction

Drill bits are very important tools for CNC machining. When you pick the right size, your parts will be better. When you pick the wrong size, your tools can break. 

Making holes with CNC machines needs the right drill bits. Some bits are for metric systems. Some are for imperial systems. Picking the wrong one can make big problems. 

The biggest problems when drilling are:

• Bits that get too hot
• Chips that get stuck
• Bits that break
• Holes that are not the right size

Let’s learn about standard drill bit sizes so you can make better parts with your CNC machine.

2. Standard Sizing Systems Explained

There are two main ways to measure drill bits:

Metric vs. Imperial (Fractional/Decimal)

Metric bits are sized in millimeters (mm). Imperial bits are sized in inches (in). 

Here is how to change from one to the other:

• To change inches to mm: multiply by 25.4
• To change mm to inches: divide by 25.4 

For example:

• 1/16 inch = 1.5875 mm
• 10 mm = 0.3937 inches

Different industries use different systems:

• Aerospace companies often use inches
• Automotive companies often use mm
• Medical parts often use mm

ANSI/ASME and ISO Standards

The ANSI/ASME B94.11M standard is used in the USA. The ISO 286-1 standard is used in many other countries. 

These standards tell you how close to perfect your holes must be. For very good parts like in airplane parts, the holes must be very, very close to perfect. 

Remember these key entities in CNC drilling: Sandvik Coromant, Haas Automation, ISO 13399, ER collets. They all help make better parts with your CNC machine.

By using the right drill bit size and following these tips, you can make better parts with your CNC machine every time!

3. Drill Bit Selection Factors

Material-Specific Choices

Different materials need different drill bits:

For soft metals like aluminum:

• Use High-Speed Steel (HSS) bits
• Look for bits with smooth sides
• Use faster speeds 

For hard metals like stainless steel:

• Use carbide bits
• Use titanium-coated bits
• Use slower speeds
• Use more coolant 

Bit Geometry

The shape of the bit matters a lot:

• Helix angle: This helps chips come out. A bigger angle is good for soft metals.
• Point angle: Most bits have a 118° point. For hard metals, use 135°. 

You can learn more about CNC milling tools at iStar Machining.

4. CNC-Specific Considerations

Feeds & Speeds

The right speed makes good holes and keeps your bits working longer:

To find the right RPM (how fast the bit turns), use this formula:

• RPM = (Cutting Speed × 12) / (π × Diameter)

For example:

• For a 1/4″ bit in aluminum with 300 SFM: RPM = (300 × 12) / (3.14 × 0.25) = 4,584 RPM

When using coolant:

• You can go a little faster
• Your bits will last longer
• You will get better holes

When cutting dry (no coolant):

• Go slower
• Be careful about heat
• Check for chips more often

Toolpath Optimization

Peck drilling is when you go in and out while drilling. This:

• Keeps the bit cooler
• Helps chips come out
• Works well for deep holes

Trochoidal milling is a special way to make holes that:

• Makes less heat
• Is good for hard materials
• Makes the bit last longer

For complex operations like thread milling, the right speeds are even more important.

5. Drill Bit Types & Applications

There are many kinds of drill bits for CNC machines:

Twist Drills

• Most common type
• Good for most jobs
• Not too expensive
• Come in many sizes

Carbide Inserts

• Last longer
• Good for making lots of parts
• More expensive
• Better for hard materials

Micro-Drills

• Very small (less than 1mm)
• For tiny holes
• Used in electronics
• Need special care

Countersinks/Chamfer Bits

• Make holes for screw heads
• Make the top of a hole bigger
• Help screws sit flat 

For special jobs like CNC drilling of deep holes, you need special bits.

6. Conversion Tables & Resources

Quick-Reference Charts

Here are some common drill bit sizes:

For tap drill sizes (for making threads):

• For 1/4″-20 thread, use a #7 (0.201″) drill
• For M6 thread, use a 5mm drill

Software Tools

• G-Wizard can help you pick speeds
• Fusion 360 has tools to help with drilling
• Many CNC machines have built-in help

7. Pro Tips to Avoid Common Mistakes

Prevent Breakage

To keep your bits from breaking:

• Make sure they are set up right
• Don’t go too deep at once
• For steel, don’t drill deeper than 4 times the bit width
• Use the right speed

Heat Management

Heat is a big enemy of drill bits:

• Use coolant when you can
• MQL (Minimum Quantity Lubrication) uses less fluid
• Flood coolant is good for hard materials
• Take breaks to let the bit cool down

For precision work like 5 axis drilling, managing heat is very important.

8. Case Studies and Real-World Applications

Case Study: Aerospace Part

A company making airplane parts switched from HSS to carbide bits for stainless steel:

• Tools lasted 3 times longer
• Work was done 25% faster
• Fewer bad parts
• Saved money even though bits cost more

Case Study: Electronics Manufacturing

A company making circuit boards used tiny drill bits:

• Used peck drilling and broke 30% fewer bits
• Used special holders to keep bits straight
• Slowed down the speed for better holes

9. FAQs

10. Conclusion

Picking the right drill bit sizes for your CNC machine makes a big difference. Remember these key points:

1. Match the bit to the material
2. Use the right speed for your bit and material
3. Keep bits cool with good coolant
4. Follow standards for best results
5. Replace bits before they get too dull

The right bit makes better parts, saves money, and keeps your machine running well.