Quick Answer: 8-32 Tap Drill Size
| 多维数据监测 | Tap Drill (Threading) | 间隙孔 |
| Drill bit number | #29 展位 | #18 (close) / #16 (free) |
| 直径（英寸） | 0.1360“ | 0.1695英寸/0.1770英寸 |
| 直径（mm） | 3.45 毫米 | 4.31 mm / 4.50 mm |
| 目的 | Hole to be tapped with threads | Hole for screw to pass through |
If you do not have a #29 bit on hand, a 9/64" (0.1406") drill is a close substitute, though it produces slightly shallower threads. For maximum strength in soft metals, the #29 remains the recommended choice.
What Does "8-32" Actually Mean?
Understanding the numbering helps you read any tap drill chart with confidence.
• The "8" is the screw gauge (a #8 machine screw has a major diameter of about 0.164 inch).
• The "32" is the number of threads per inch (TPI).
• Together, 8-32 describes a #8 screw with 32 threads per inch in the UNC (coarse) thread standard.
The tap drill must be slightly larger than the screw’s minor diameter but smaller than its major diameter. This leaves just enough material for the tap to cut threads without binding.
Tap Drill vs. Clearance Hole: Know the Difference
Choosing the wrong hole type is the most common tapping mistake. There are 2 completely different holes:
| 孔类型 | 目的 | 8-32 Drill Size |
| Tap drill | Drilled smaller so the tap can cut threads into the wall | #29 (0.1360" / 3.45 mm) |
| 间隙孔 | Drilled larger so the screw passes through freely without threading | #18 close fit, #16 free fit |
Rule of thumb: tap the smaller hole, pass the screw through the larger one. A through-hole assembly uses a clearance hole on the top piece and a tapped hole on the bottom piece.
Why Thread Engagement Percentage Matters
You may notice tap charts mention a thread percentage, usually 75%. This refers to how deeply the threads bite into the hole wall, and it directly affects strength and ease of tapping.
A higher thread percentage sounds stronger, but it adds surprisingly little holding power while dramatically increasing the force needed to turn the tap. Going from 75% to 100% thread can more than double the tapping torque, which is why nearly every shop targets the 60% to 75% range.
• 50 - 60% 绪: Easiest to tap, lowest tap breakage risk, still holds most of the strength. Good for hard materials.
• 75% thread (standard): The sweet spot used on most charts, including the #29 drill for 8-32. Strong threads with manageable torque.
• 100% 绪: Rarely needed. Very high torque and a high chance of snapping the tap for little extra strength.
This is exactly why the #29 drill (0.1360") is recommended for 8-32 rather than a smaller bit. It produces clean 75% threads while keeping the tap easy to turn, even by hand.
Drill Size and Your Material
The #29 tap drill works across most materials, but the way you drill and tap should change with the workpiece.
• Aluminum and brass: Soft and forgiving. The #29 drill gives clean threads; use light cutting fluid to prevent the tap from grabbing.
• 低碳钢: Use cutting oil and back the tap off frequently to clear chips and avoid heat buildup.
• 不锈钢: Work-hardens easily. Consider a slightly larger drill (toward 60% thread) and use a sharp tap with plenty of lubricant.
• 塑料和复合材料: The #29 drill works well; reduce speed to avoid melting and use a slow, steady tapping motion.
Imperial Tap Drill Size Chart (Common Sizes)
Here are the standard tap drill sizes for the most common imperial machine screws at 75% thread engagement. Keep this chart near your bench.
| 丝锥尺寸 | 攻丝钻 | 英寸 | mm |
| 4-40 北卡罗来纳州 | #43 展位 | 0.0890“ | 2.26 毫米 |
| 6-32 北卡罗来纳州 | #36 展位 | 0.1065“ | 2.70 毫米 |
| 8-32 北卡罗来纳州 | #29 展位 | 0.1360“ | 3.45 毫米 |
| 10-24 北卡罗来纳州 | #25 展位 | 0.1495“ | 3.80 毫米 |
| 10-32 联合国 | #21 展位 | 0.1590“ | 4.04 毫米 |
| 1/4-20 UNC | #7 | 0.2010“ | 5.11 毫米 |
| 1/4-28 UNF | #3 | 0.2130“ | 5.41 毫米 |
| 3 / 8-16 UNC | 5 / 16“ | 0.3125“ | 7.94 毫米 |
| 3 / 8-24 UNF | Q | 0.3320“ | 8.43 毫米 |
Metric Tap Drill Size Chart (mm)
For metric taps, the tap drill size is calculated as the major diameter minus the thread pitch. Here are the common values, including the frequently searched M8 x 1.25.
| Metric Tap | Tap Drill (mm) | Tap Drill (inch equiv.) |
| M3 x 0.5 | 2.5 毫米 | 〜0.0984英寸 |
| M4 x 0.7 | 3.3 毫米 | 〜0.1299英寸 |
| M5 x 0.8 | 4.2 毫米 | 〜0.1654英寸 |
| M6 x 1.0 | 5.0 毫米 | 〜0.1969英寸 |
| M8 x 1.25 | 6.8 毫米 | 〜0.2677英寸 |
| M10 x 1.5 | 8.5 毫米 | 〜0.3346英寸 |
| M12 x 1.75 | 10.2 毫米 | 〜0.4016英寸 |
The popular 8 mm x 1.25 tap (M8 x 1.25) uses a 6.8 mm drill. This is the standard coarse-pitch M8 size found on most general fasteners.
Cutting Taps vs. Forming (Roll) Taps
Most charts list sizes for standard cutting taps, which remove material. Forming taps (also called roll taps) displace material instead and require a larger drill.
• Cutting tap (8-32): Use a #29 drill (0.1360"). The standard choice for most shops.
• Forming / roll tap (8-32): Use a slightly larger drill, around 0.1405–0.1440", because no material is cut away.
• 为何重要: Using a cutting-tap drill size with a forming tap creates excessive torque and can break the tap.
Forming taps work best in ductile metals like aluminum and mild steel, producing stronger threads with no chips. Always check the tap manufacturer’s recommended drill size when using roll taps.
How to Drill and Tap an 8-32 Hole
Whether you are working by hand or on a数控machine, the sequence is the same:
• Mark and center-punch the hole location to keep the drill from wandering.
• Drill with a #29 bit (0.1360") at the correct speed for your material.
• Add cutting fluid to reduce friction and extend tap life, especially in steel.
• Run the 8-32 tap squarely into the hole, turning forward a half-turn, then back a quarter-turn to clear chips.
• Clean and test by threading an 8-32 screw to confirm a smooth fit.
On a数控machine, drilling and tapping are programmed operations. For precision threaded holes in metal parts, a 数控铣床 or CNC加工中心 performs drilling and tapping in a single setup with repeatable accuracy.
避免常见的敲击错误
• Using a clearance drill to tap: A hole that is too large will not hold threads. Always use the #29 tap drill for 8-32, not the clearance drill.
• Skipping cutting fluid: Dry tapping in metal dulls the tap fast and risks breakage.
• Tapping at an angle: A crooked tap creates weak, misaligned threads. Keep the tap perpendicular to the surface.
• Not clearing chips: Forcing the tap without backing off packs chips and snaps the tool.
• Wrong drill for material thickness: Match drilling and tapping speeds to your material to avoid burning or work-hardening.
Choosing the right bit and tool also matters. The STYLECNC指导 数控router bits and end mills explains how to select cutting tools for clean holes in aluminum, steel, and composites.
常見問題解答
What size drill do I use for an 8-32 tap?
Use a #29 drill bit, which measures 0.1360 inch (3.45 mm). This is the standard tap drill size for 8-32 UNC threads at about 75% thread engagement.
What is the clearance hole size for an 8-32 screw?
Use a #18 drill (0.1695") for a close fit or a #16 drill (0.1770") for a free fit. The clearance hole lets the screw pass through without threading.
Can I use a metric drill instead of a #29 bit for 8-32?
Yes. A 3.5 mm drill (0.1378") is very close to a #29 and works well for 8-32 in most materials, producing slightly under 75% thread.
What drill size is needed for an M8 x 1.25 tap?
Use a 6.8 mm drill for a standard M8 x 1.25 coarse tap. This is the metric equivalent commonly searched as the 8 mm x 1.25 tap drill size.
How do I calculate any metric tap drill size?
Subtract the thread pitch from the major diameter. For example, M8 x 1.25 = 8 minus 1.25 = 6.75 mm, rounded to a standard 6.8 mm drill.
Is the 8-32 tap drill the same as 8-36?
Yes. Both 8-32 and 8-36 taps commonly use a #29 drill bit (0.1360"), since they share the same #8 screw gauge and similar minor diameters.
Precision Holes Start with the Right Machine
For consistent, repeatable drilling and tapping across many parts, a数控machine removes the guesswork. Explore STYLECNC金属数控机床 并学习 how to use a数控milling machine to produce accurate threaded holes in metal, aluminum, and composites at scale.
