When creating internal threads, choosing the correct drill bit is essential for strength, accuracy, and tool life. Many machinists—beginners and professionals alike—ask the same question: How do you know which drill bit to use when tapping threads?
Selecting the right drill size is one of the most crucial steps in threading because the wrong size can lead to oversized threads, tool breakage, low-quality finishes, or damaged workpieces.
In this comprehensive guide, you will learn how to determine which drill bit to use when tapping threads, the formulas behind drill size selection, standard tap-drill charts, material considerations, and the role of high-performance taps in improving thread quality.
We also introduce MisolGroup — a leading global manufacturer of premium threading tools — and explain how their products ensure maximum accuracy and performance.
Table of Contents
- Why Drill Bit Selection Matters for Thread Tapping
- Understanding Tap Types and Hole Requirements
- How to Know Which Drill Bit to Use When Tapping Threads
- Tap Drill Size Formulas (Metric & Imperial)
- Universal Tap Drill Size Chart
- How Material Affects Drill Bit Choice
- Company Spotlight: MisolGroup — High-Performance Tapping Tools
- Featured Products from MisolGroup
- Frequently Asked Questions
- References
Why Choosing the Right Drill Bit Matters
Before tapping, the drilled hole must be the correct size so that the tap can cut enough material to form strong internal threads.
Using the wrong drill bit can cause:
- Too-small drill bit → tap breakage, excessive torque, and overheating
- Too-large drill bit → weak, loose, or incomplete threads
- Poor surface finish → thread failure under load
- Reduced tool life → especially with hardened materials
This is why understanding which drill bit to use when tapping threads is essential for machinists, fabricators, engineers, and DIY users working with metal, plastics, or composites.
Understanding Tap Types & Their Hole Requirements
Before choosing a drill bit, you must know the tap style, thread standard, and whether you are tapping a blind hole or through hole. Different taps have different chip loads and cutting characteristics.
Common Tap Types
- Spiral Point Tap – Best for through holes; pushes chips forward.
- Spiral Flute Tap – Best for blind holes; pulls chips upward.
- Straight Flute Tap – Standard tap for general-purpose threading.
- Forming (Roll) Tap – Creates threads without cutting; requires larger drill size.
Cutting vs. Forming Taps
Cutting taps remove material. Forming taps displace material.
Because of this, form taps require larger hole diameters.
This makes knowing which drill bit to use when tapping threads dependent on the tap type.
How Do You Know Which Drill Bit to Use When Tapping Threads?
There are three primary ways to determine the correct drill bit size:
1. Use the Tap Drill Formula (Metric or Imperial)
Simple formulas allow machinists to calculate drill bit size manually.
2. Use a Tap Drill Chart
This is the most common and reliable method. Charts list tap sizes and their corresponding drill bit sizes.
3. Use Manufacturer Recommendations
Premium tap manufacturers like MisolGroup provide exact drill size recommendations based on material, tap geometry, and application.
Below are the industry-standard formulas and charts to help you know which drill bit to use when tapping threads.
Tap Drill Size Formulas
Metric Tap Drill Formula
Example: For M10 × 1.5
10 – 1.5 = 8.5 mm drill bit
Imperial Tap Drill Formula
Example: 1/4″-20
0.250 – (1/20 = 0.050) = 0.200 in (≈ #7 drill bit)
Universal Tap Drill Size Chart (Metric & Imperial)
| Thread Size | Pitch | Type | Recommended Drill Bit |
|---|---|---|---|
| M6 × 1.0 | 1.0 mm | Cutting Tap | 5.0 mm |
| M8 × 1.25 | 1.25 mm | Cutting Tap | 6.8 mm |
| M10 × 1.5 | 1.5 mm | Cutting Tap | 8.5 mm |
| 1/4″-20 | 20 TPI | Cutting Tap | #7 (0.201″) |
| 3/8″-16 | 16 TPI | Cutting Tap | 5/16″ |
This chart makes it easier to choose which drill bit to use when tapping threads across multiple standards and applications.
How Material Affects Drill Bit Selection
Different materials require different allowances to prevent tool damage.
Harder materials need slightly bigger drill sizes; softer materials can maintain tighter tolerances.
Material Considerations
- Stainless Steel: Increase drill size slightly to reduce torque.
- Carbon Steel: Standard drill sizes are usually sufficient.
- Aluminum: Maintain tighter hole tolerance for stronger threads.
- Copper/Brass: Use standard drill sizes for clean thread formation.
- Cast Iron: Brittle material; maintain standard drill sizes.
Always use cutting fluid when tapping metals to protect the tap and improve finish.
Company Spotlight: MisolGroup — Your Trusted Threading Tool Manufacturer
Advanced Manufacturing & Uncompromising Quality
- Full In-House Production: CNC automation, vacuum heat treatment, full testing control.
- Premium HSS Materials: M35, DV3, M2 for durability and tool longevity.
- Strong R&D Team: Nearly 20 years of tapping tool development expertise.
Industries Served
- Automotive
- Aerospace
- Defense
- Electronics
- Mold & Die Manufacturing
Material Expertise
MisolGroup taps are engineered for superior performance in:
- Cast iron
- Carbon steel
- Stainless steel
- Non-ferrous alloys
Why Customers Trust MisolGroup
- ISO-certified quality management
- Reliable delivery and contract compliance
- Global customer support
Featured MisolGroup Products
1. (10S-529) Standard Series Tapping Threads Drill Bit
A versatile general-purpose tapping drill bit designed for consistent performance across a wide range of steels and alloys.
2. DIN371 German Industrial Standard Tapping Threads Drill Bit
Built to strict German standards, ideal for precision manufacturing environments requiring tighter tolerances.
3. JIS Spiral Groove Tap Series Tapping Threads Drill Bit
Optimized for blind hole tapping, offering excellent chip evacuation and reduced torque.
These premium products make it easier to determine which drill bit to use when tapping threads because each specification is engineered for accuracy and reliability.
Frequently Asked Questions
1. What happens if I use the wrong drill bit size for tapping?
Using the wrong size may cause tap breakage, weak threads, or improper fit. Always follow a tap drill chart.
2. Do cutting taps and forming taps use the same drill size?
No. Form taps require larger drill sizes because they form threads by displacement, not cutting.
3. How do I know which drill bit to use when tapping threads in stainless steel?
Increase the drill size slightly (0.05–0.1 mm for metric) to prevent excessive torque on the tap.
4. Is lubrication necessary during tapping?
Yes. Lubrication reduces friction, prevents galling, and increases tool life.
5. Should I drill deeper than the tap depth?
Yes—typically 1.5× deeper for blind holes to allow chip clearance.
References
