Tuesday, February 20, 2007

Timberframe terminology - drawboring

Drawboring (aka as drawpinning) is the technique of intentionally offsetting the peg holes of a pair of mortise and tenons to be joined. When a tapered peg is driven into the hole, the peg will try to align the two holes and in the process, draw the joint together very tightly. As the wood seasons and shrinks with time (remember most imberframes are assembled with green lumber), the bowed peg will work as a spring to keep the joint tight.

The picture to the left (oak beam, ash brace) illustrates the intentional misalignment that I'm talking about. Because it takes more time, precision, and patience (all of which I find myself lacking) to create a drawbored joint, I only used this technique on the braces for my house (there are approximately 150 braces in my house, each with 2 pegs, so this was still a lot of work). The misalignment should be about 1/16th to 1/8th of an inch for hardwoods (I've read you can almost double that for softwoods). In reality, if you aim for 1/16th misalignment, you might find that you've achieved 1/8th misalignment, or no misalignment at all. Occasionally, I created holes that were off by as much as 1/4th" of an inch. It was nearly impossible to drive 1" pegs through these holes, so some of these got re-drilled and were therefore not drawbored.

It takes a lot of faith in your joinery to use drawboring because once joints are assembled in this fashion, there's not a lot of room for adjustment if you find your frame out-of-square. However, I can report that when you "get it right," drawboring really works. It's an awesome thing to pound on a peg and watch two pieces of wood spontaneously assemble themselves tighter than any mechanical means could have persuaded them together.

By the way, the typical alternative (shortcut) to drawboring is to temporarily assemble your undrilled pieces of wood as tightly as possible (using come-a-longs, ratchet straps, pry-bars, etc.), check your assembly for squareness, and then to drill the assemble in place. Then you drive the peg and remove the ratchet straps. The joints are tight, but they can't possibly stay as tight as drawbored joints will.

3 Comments:

Anonymous Anonymous said...

Hey Thomas,

Great blog! I really enjoy reading your posts. Have you been splitting any stone recently? Your stone splitter sounds great, I'd love to hear how that works out in the end.

Maybe sometime you could do a writeup on how you made all your pegs.

David in Montana

February 21, 2007 at 6:29 PM  
Anonymous Anonymous said...

Don't you just love the way the sound changes pitch as you're pounding in that drawbored peg? That's when you know you've got a REALLY tight drawbore.

Marc

February 22, 2007 at 1:45 AM  
Anonymous Anonymous said...

David,

Thanks! It's been to cold to split stone, and quite honestly, I've been busy on the aspects of the house that must be finished before I can move in. My wife and I have both decided that we are not too proud to live in a house with Tyvek on the outside, so the stone facing is probably at least 6 months away? But we might might need to fire up the splitter to make enough rock for our chimney. I'll definitely blog all about it! (We did revise the design once since the last blog - beefed it up even more).

Marc,

yeah, gotta love that sound. something like "thump thump whap whap doink doink pink pink tink tink tiiiink."

February 22, 2007 at 9:09 PM  

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