Blacksmithing Drift & Punch Sizing Calculator

The punch is sized at approximately 85% of the desired final hole diameter because the metal displaced during punching partially springs back and because the subsequent drifting operation is what brings the hole to its final dimension. Punching undersized creates a clean starter hole with less force and less distortion to the surrounding material, while the drift then smoothly opens and trues the hole to the exact finished size.

Drifting should be used whenever hole accuracy and surface quality matter. Punching alone tends to create tapered, rough holes with torn grain structure, especially in thicker stock. A drift produces a smooth, parallel-sided hole with burnished walls. For joinery such as mortise and tenon, eye holes in hammer and axe heads, or any hole that will receive a fitted component, always follow the punch with a drift.

H13 tool steel is the premium choice for hot-work punches and drifts because it maintains hardness at elevated temperatures and resists thermal fatigue cracking. S7 shock-resistant tool steel is excellent for punches that take heavy hammer blows. 4140 chromoly steel is a cost-effective option for occasional use that can be hardened and tempered in a typical blacksmith shop. Always temper punches to a spring temper (straw to dark straw color) to prevent chipping.

Hot punching is done with the workpiece at forging temperature (typically bright orange to yellow heat, 1800-2200 degrees Fahrenheit for steel), which dramatically reduces the force needed and allows the metal to flow around the punch. Cold punching requires a hydraulic press or heavy fly press and much higher force, but produces more precise holes. Most blacksmithing operations use hot punching because the metal is already at temperature during forging.

Apply a release agent to the punch before each use. Traditional blacksmiths use coal dust, powdered graphite, or beeswax. Dip the hot punch tip into the release agent just before driving it into the workpiece. Also ensure the punch has a slight taper (not perfectly cylindrical) and avoid driving it completely through from one side. Punch halfway, flip the workpiece, and punch through from the other side to meet in the middle.

For stock thicker than about 3/8 inch, always punch from both sides. Punch halfway through from one side until you see a dark spot appear on the opposite face, then flip the workpiece and punch through from the other side. This technique reduces the required force by roughly half, produces a cleaner hole with less distortion, and prevents the blowout crater that occurs when punching through from one side only.

Start by punching a round hole slightly smaller than the desired square dimension, then use a square drift to open and shape the hole to its final size. The round starter hole makes the initial punch much easier, and the square drift displaces material into the corners. Work the drift gradually with light blows, rotating it 90 degrees periodically to keep the hole centered. For precise mortises, finish with a fitting drift ground to exact dimensions.