Hardy Hole Tool Size Calculator
Calculate hardy hole dimensions and matching tool shank sizes for your anvil.
Covers hot cut, fuller, and swage tools.
The hardy hole is the square hole in the top face of a blacksmith’s anvil. It holds interchangeable bottom tools (called “hardy tools”) that sit shank-down in the hole. Getting the shank dimensions right is critical — too tight and the tool jams, too loose and it wobbles dangerously during use.
Common Anvil Hardy Hole Sizes
| Anvil Weight | Typical Hardy Hole | Metric Equivalent |
|---|---|---|
| 50–75 lbs | 5/8" (0.625") | 15.9 mm |
| 75–100 lbs | 3/4" (0.750") | 19.1 mm |
| 100–150 lbs | 7/8" (0.875") | 22.2 mm |
| 150–200 lbs | 1" (1.000") | 25.4 mm |
| 200–300 lbs | 1-1/8" (1.125") | 28.6 mm |
| 300+ lbs | 1-1/4" (1.250") | 31.8 mm |
Shank Sizing Formula
The tool shank must be slightly smaller than the hardy hole for easy insertion and removal:
Shank dimension = Hardy hole dimension - Clearance gap
Recommended clearance: 1/32" (0.8 mm) per side = 1/16" (1.6 mm) total.
For a 1" hardy hole: Shank = 1.000" - 0.0625" = 0.9375" (15/16").
Shank Length
The shank should extend deep enough into the hole for stability but not so deep that it bottoms out:
Shank length = Hardy hole depth × 0.75
Most hardy holes are 1.5"–2.5" deep. A typical shank length of 1.25"–1.75" works for most anvils.
Worked Example — Making a Hot Cut Hardy for a 150 lb Anvil
Hardy hole measures 1" × 1" (25.4 mm square), 2" deep. Shank: 15/16" × 15/16" (23.8 mm), 1.5" long (38 mm). Start with 1" square mild steel bar stock. Forge the shank portion by drawing down on two opposing sides to 15/16". Check fit frequently — the shank should drop in with a slight rattle and lift out easily.
The cutting edge above the shank should be forged from medium-carbon steel (1045–1075) or tool steel for durability. Harden and temper the cutting edge to straw yellow (approximately 220°C / 428°F).
Common Hardy Tools
| Tool | Purpose | Edge Angle |
|---|---|---|
| Hot cut | Cutting hot metal over the anvil | 30° included |
| Cold cut | Cutting cold metal (heavier build) | 60° included |
| Fuller (bottom) | Creating grooves and spreading metal | Rounded, various radii |
| Swage (bottom) | Shaping round/square stock | Matched to stock size |
| Bick iron | Small horn substitute | Tapered round bar |
| Hold-down | Clamping work to anvil face | Spring steel arm |
Safety Note
Hardy tools must sit firmly without rocking. A wobbling tool can eject under hammer impact. Always check the fit before each use, especially after the shank has been heated and may have deformed. Never hammer directly on a hardy tool’s cutting edge — always place the workpiece on the edge and strike the workpiece.