Accurate drilling and tapping depend on matching the drill bit size to the desired thread size and pitch. This drill bit tap size chart helps you select the right hole size before cutting threads in metal, wood, or plastic.
Use the reference table and guidance below to reduce rework, improve thread fit, and speed up setup on the shop floor or in your home workshop.
| Material | Thread Size | Unthreaded Hole Size | Tap Drill Chart Reference |
|---|---|---|---|
| Steel | M6 x 1.0 | 5.0 mm | Coarse metric tap drill |
| Aluminum | M6 x 1.0 | 5.2 mm | Fine adjustment for softer metal |
| Steel | 1/4"-20 | 0.216 in | Standard UNC tap drill |
| Plastic | M4 x 0.7 | 3.6 mm | Slightly oversized for brittle materials |
Metric Drill Bit Sizes For Tapping
Metric tapping relies on a simple relationship between the thread pitch and the required hole size. For coarse pitches, subtract the pitch from the major diameter to find the drill size. Fine pitches often need a slightly larger hole to avoid splitting the bolt or strip threading in thin sections.
Use the drill bit tap size chart to look up common sizes from M2 up to M39. Always check the tap manufacturer’s recommendations for through holes and blind holes, since bottoming taps may require a slightly different approach.
Imperial Drill Bit Sizes For Tapping
In inch-based threading, you determine the drill size from the nominal fraction and the threads per inch. For UNC threads, use the standard tap drill column. For UNF threads, which have finer pitch, choose a slightly smaller drill to provide enough material for the extra thread form.
Decimal equivalents in the chart help you quickly switch between fractional and metric references. Verify hole size with calipers or a bore gauge before starting production runs.
How To Read A Drill Bit Tap Size Chart
Clear charts list thread size, pitch or TPI, recommended drill size in both metric and imperial, and the approximate depth for the tap. Some charts include notes for soft metals, hardwoods, and brittle plastics, so you can adjust the hole size for the specific workpiece.
Match the row for your thread, locate the correct column for your material, and choose the drill bit closest to the recommended size. When in doubt, test on a scrap piece to confirm thread engagement and torque.
Key Takeaways For Using A Drill Bit Tap Size Chart
- Confirm thread specification, pitch or TPI, and thread class before selecting a hole size.
- Use metric charts for M series fasteners and imperial charts for UNC/UNF or British standard threads.
- Adjust hole size based on material properties like hardness, brittleness, and thermal expansion.
- Verify through testing on representative scrap stock to prevent stripping or tool breakage.
- Keep an updated drill bit tap size chart in the shop and check tap manufacturer documentation for blind hole depth.
FAQ
Reader questions
What drill bit do I use for an M8 x 1.25 tap in structural steel?
Use a 6.8 mm drill bit as the primary recommendation for standard structural steel, and consider 6.9 mm for thinner sections to prevent bottoming out during tapping.
Can I reuse a tap drill size chart for hardwoods without changes?
No, reduce the hole size slightly for hardwoods and adjust for moisture content, because fibrous grains can bind and increase torque, leading to broken taps or split edges.
Is the drill bit tap size chart the same for brass and aluminum alloys?
Not exactly; increase the hole size by 0.1 to 0.2 mm for softer aluminum alloys, and follow chart notes or tap manufacturer guidance for brass to ensure proper thread fill and pull-out resistance. Choose a 4.5 mm drill for tapping M6 x 0.75 in many free-machining steels, but always verify with a certified chart for the exact grade and condition of the stock.
Do Imperial UNC and UNF taps share the same drill bit tap size chart row?
No, UNC and UNF have different threads per inch, so the recommended drill size differs; always select the column that matches the specified class and tolerance.