Tap Drill (Coarse) Spec Table

In-Depth Guide

What thread engagement the D−P formula really gives

The rule drill = nominal diameter − pitch produces roughly 75–77% thread engagement, not 100%. Going smaller to chase fuller engagement adds only about 5% more strength while sharply raising tapping torque and the risk of tap breakage. For ordinary steel, 60–75% engagement is the best balance of strength and machinability; in soft metals like aluminum and brass, or in blind holes, there is no benefit to forcing more than 75%. To size the drill for a target engagement, use drill = major diameter − (%engagement ÷ 76.98) × pitch. For M10×1.5, 75% engagement gives 8.54 mm and 60% gives 8.83 mm, so the standard 8.5 mm is about 77% engagement.

Material-based pilot-hole correction — ductile, stainless, cast iron

Table values are baselines that ignore how the material behaves. Ductile metals such as aluminum, mild steel and copper swell inward after the tap passes, narrowing the hole, so a drill 0.05–0.1 mm larger than the table value lowers tap load. Cast iron and brass break into powdery chips with almost no swell and need no correction. Stainless (especially 304/316) work-hardens badly, so reduce engagement (use the larger drill) and pair a sharp cut tap or a form tap with ample lubricant to avoid breakage.

Cut taps and form (roll) taps need different pilot holes

Applying the same table to cut taps and form taps is the most common shop mistake. A form (roll) tap shapes threads by plastic deformation instead of cutting, so it needs a larger pilot hole — roughly drill ≈ nominal diameter − 0.5 × pitch. Using the cut-tap value (D−P) with a form tap makes material overflow, jamming the tap or tearing the thread. Form taps produce no chips and suit blind holes, but only in ductile materials (aluminum, mild steel, stainless); they cannot be used in brittle materials like cast iron.

Blind holes: drill depth and tap geometry

For a blind hole, drill depth = required effective thread depth + the incomplete threads of a taper tap (2–4 threads) + room for chips. Depth alone is not enough — tap geometry decides whether it survives. Through holes favor a spiral-point (gun) tap that pushes chips forward; blind holes need a spiral-flute tap that pulls chips back out. Use a gun tap in a blind hole and chips pile up at the bottom and snap the tap.

When the exact drill is missing — substitution and engagement shift

When no drill matches the table value, substitute the nearest size but gauge the change in engagement: enlarging the hole by 0.1 mm drops engagement by roughly 5–9 percentage points. For strength-critical joints choose the smaller drill (higher engagement); for deep blind holes where tapping is the concern choose the larger drill (lower engagement and lower tap load). Never let engagement fall below about 50%, where there is too little thread and clamp strength collapses.