Drawboring is the traditional trick of offsetting the peg hole in the tenon slightly toward the shoulder, so driving the peg pulls the joint tight and holds it there mechanically, no clamps, no glue strictly required. The peg does real structural work in this joint, loaded in shear across the mortise walls and the tenon. Understanding that shear loading tells you how to size pegs, choose their wood, and offset the holes for maximum strength without splitting anything.
How the drawbore loads the peg
When the offset peg is driven through the mortise walls and the tenon, it bends slightly and bears against the holes, clamping the tenon shoulder hard against the mortise piece. In service, racking forces on the joint try to shear the peg at the planes where the tenon meets the mortise walls: two shear planes on a through-tenon, one on a blind tenon. The peg's shear strength at those planes is a hard limit on the joint's racking resistance.
The drawbore offset adds a permanent preload: the peg is already bent and bearing hard before any service load arrives. This preload is why drawbored joints stay tight for centuries, but it also means the peg starts life with stress in it. Size and material must handle preload plus service load together.
Sizing pegs for shear
Peg diameter follows the tenon, not fashion. A common rule sizes the peg at roughly one-third the tenon thickness, which keeps enough wood around the hole in both mortise and tenon to avoid splitting while giving the peg meaningful shear area. For a one-inch tenon, a peg around three-eighths of an inch is the traditional neighborhood; scale proportionally for other sizes.
Shear capacity grows with the square of peg diameter, so small increases in peg size buy large increases in strength, but the mortise and tenon pay for it in lost wood around the holes. The one-third guideline balances the two. For joints carrying serious racking loads, like chair stretchers or workbench legs, favor the upper end of the range and consider double pegs spaced along the tenon rather than one oversized peg.
Choosing peg wood
The peg should be stronger in shear than the surrounding wood is in splitting resistance, but not so hard and brittle that it snaps during driving. Traditional choices are straight-grained hardwoods: white oak, hickory, and locust are classics, each tough, shear-resistant, and willing to bend slightly without breaking. Avoid brittle species and any peg with grain runout; a peg that follows diagonal grain will shear along the grain during driving.
Make pegs slightly tapered and faceted rather than perfectly round and parallel. An octagonal, gently tapered peg drives with less force, compresses into the hole, and resists rotation in service. Round pegs from dowel stock work but benefit from the same slight taper; a peg that must be hammered brutally is a peg about to split the mortise.
Setting the drawbore offset
The offset, typically around one-sixteenth of an inch toward the shoulder, determines the clamping force. Too little offset and the joint does not draw up; too much and the driving force splits the tenon or the mortise cheek. The right amount also depends on peg stiffness: a stout oak peg tolerates more offset than a slender one, and dry wood splits more readily than wood with some moisture in it.
Drill the mortise holes first, assemble the joint dry, and mark the tenon hole location through the mortise holes. Then offset the tenon mark toward the shoulder by the chosen amount before drilling. Test on scrap with your actual pegs and wood; the test joint should draw up tight with firm mallet blows, not require heroic force and not close with finger pressure.
Assembly and long-term behavior
Drive pegs with the grain of the mortise piece considered: orient any peg grain to resist the expected shear, and chamfer the peg's leading end to start it without brooming. A touch of glue on the peg is traditional and harmless, but the joint's strength should never depend on it; the mechanical draw is the joint.
Drawbored joints tighten their reputation over time because the preload keeps shoulders seated through seasonal movement. Inspect old work and you will find the pegs doing exactly what they were sized to do, carrying shear quietly for generations. Size them thoughtfully, offset carefully, and drive them home; the joint will outlast the maker.
Drawboring turns a peg into a clamp and a shear pin in one. Size the peg to the tenon, choose tough straight-grained wood, offset just enough to draw the joint home, and prove it on scrap first. The shear planes will carry the load, and the joint will stay tight long after the glue in lesser joints has given up.
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