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Tools & Craft #103: How Drawbore Pins Work

Since we introduced Ray Iles’s draw borepins last fall, we’ve gotten a ton of questions about why real ones such as these work better than concentric tapered pins - that is, pins that are sometimes misconstrued as drawbore pins but really are just tapered drift pins. Drift pins are just tapered rods, which from any angle look like cones.

Traditional drawbore pins, on the other hand, look like drift pins in one dimension, but if you rotated the pin 90 degrees, one side would becomes straight and the other more tapered. A section of the drawbore pin at any point is a circle, but geometrically the shape is what’s known as an “oblique cone.”

Now, why is this shape better? When you put a drawbore pin into a partially assembled mortise, it will contact the joint at some point shown in the first drawing. Where the point of contact would be exactly would depend on the taper, the thickness of the joint, etc., but the pin will engage somewhere. If it engages with the joint with the vertical side of the cone touching the left side of the joint (as the joint is drawn in Fig 1) and isn’t symmetrical, turning the drawbore pin and letting it sink down will make it go deeper into the joint. At some point it will be engaged, and then when you turn it, the oblique part of the taper forces the joint tighter. You can actually see this in use. 

Now you may say, “Well, doesn’t a regular pin do this, except you have to pull it towards the joint to push it out?” That’s a good question. And the answer is yes - but not so much. Since you don’t know the actual geometry of engagement, it’s hard to make sure to tilt to the right degree, neither too much nor too little. The real issue is leverage. There is some mechanical advantage of the fulcrum between where a tapered pin will hit the top of the mortise and the inside of the tenon against the length of the entire drift pin. But with a real drawbore pin, the leverage you get by tuning an oblique cone is significantly greater. So when you use real drawbore pins, you can actually sometimes see joints being pulled together, squishing bits of detritus of the joint out of the way.

Drawbore pins are also hardened so they don’t bend and have welded bolsters for added strength.

In the old days, their common use was in doors, although they make pulling any mortise and tenon joint together easier. It’s also easy to use them for test-fitting joints. A critical use for them was on wooden buildings,to align beams and pull wooden joints together. To this day, a modern version of a drawbore pin with even more offset is used on steel structures to pull steel beams into alignment. (Our product description of the drawbore pins shows an example of this). They work better on hardwood; with very soft woods, there is so much force in turning the pin you can distort the hole in the tenon.

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This “Tools & Craft” section is provided courtesy of Joel Moskowitz, founder of Tools for Working Wood, the Brooklyn-based catalog retailer of everything from hand tools to Festool; check out their online shop here. Joel also founded Gramercy Tools, the award-winning boutique manufacturer of hand tools made the old-fashioned way: Built to work and built to last.