Drawbore Mortise and Tenon

Mortise and Tenon:
The mortise and tenon joint is one of the strongest joints a woodworker can use. It also resists movement in any direction. I am in the process of building a new workbench and want it to be the most solid, sturdy, functional bench I can make (Currently the wood for my bench is sitting on stickers acclimating). So when it came to which joint I should use, the mortise and tenon joint won hands down. For the front rails I will use drawbored through tenons. The end assemblies will be drawbored but the tenons will not be through; I want the front posts smooth and flush with the bench top. After reading many magazines articles, online articles and forums these are the general guidelines I will use:

• Wide: 1/3 the width of the thinnest part of the joint.
• Deep: Over 1/2 of the mortised piece. A through tenon is the strongest.
• Tall: The tenon should be 1/2 the height of the board leaving both shoulders at 1/4 the height.

 

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Drawbore:
1. Drill a hole through the mortise close to the edge where the tenon will be inserted.

2. Drill a hole in the tenon just offset of the hole that was drilled through the mortise – toward the shoulder of the tenon about 1/32.

One way to achieve this is to use a forstner bit to drill the hole in the mortise. Then insert the tenon, insert the forstner bit in the hole and tap it with a hammer. This should leave a mark on the tenon. Then measure up 1/32 from the mark toward the shoulder of the tenon and drill a hole through the tenon using the forstner bit.

 

3. Insert the tenon into the mortise.

 

4. Pound the peg (dowel) into the hole which will draw the tenon deeper into the mortise.

 

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Tips:

One source recommends applying glue to just the peg. I plan on applying glue to both the joint and the peg.

From what I have read it is best to limit the number and size of the pegs so you don’t weaken the joint. I could not find any hard and fast rules on this but I would think the combined size of the pegs should be around 1/3 of the width of the tenon. I will give an update with the size I use when I complete my bench.

Put a small bevel or chamfer on the end of the peg by rolling the end over sandpaper at a 45° angle.

One tip from David Charlesworth is to drill completely through mortise. This helps prevent the surface grain from splitting out on the far side and removes the need to put saw kerfs in the peg. Once the long peg is driven home a short dummy peg is inserted in the far end to cover the hole.

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References:
The Pegged Joint, Exposed – from Fine WoodWorking.com – via Matthew Teague
Drawbore Joinery — via WoodworkingTips.com

Dowel plates used to make hardwood pegs — by David Charlesworth
-See “My bench frame” section.

Woodworking Magazine Weblog – via Christopher Schwarz
-See “Wedged Tenon v. Drawboring”

Drawboring a Mortise & Tenon Joint – via Loren Hutchinson
A Treatise on The Haunched and Drawbored Mortise & Tenon Joint — by Bob Smalser

Lumber Preparation

Below are the steps in preparing lumber.

1. Start by purchasing the straightest, flattest, dry lumber you can from the lumber mill.

2. When you get your lumber home stack it with stickers between the boards off the ground and let it acclimate to your shop. The boards will lose or gain moisture; normally lose moisture. To tell if you lumber is acclimated test the moisture content against a board that has been in your shop for a long time with a moisture meter. If you don’t have a moisture meter, then wait a week or two.

3. Take a wire brush and then a regular bristle brush to each side of your lumber to remove any grit from the lumber mill. I would rather spend a little time cleaning lumber than changing and sharpening blades.

4. Rough cut your lumber to size. Cut the boards to length leaving them a little long. If you rough cut your boards to length before you work with the jointer and thickness planner the boards will be more manageable and most importantly you will have to remove less material to get the boards flat (See Fig1).

5. Flatten one face and then joint one edge square to that face on the jointer. Using a jointer properly deserves it’s own article but here are a few general tips:

    Tune up the jointer before you attempt to flatten a board.

- Make sure the knives are parallel and level to the out feed table.
- Make sure the in feed table is parallel to the out feed table.
- Make sure the fence is square to the in feed and out feed tables.

     If the board is cupped place the concave side down. If you place the concave side up the board will rock and it is difficult to make it flat. (see Fig 2).

6. Plane the opposite face parallel on the thickness planner leaving the boards a little thick.

7. Cut the board to width at the table saw leaving the boards a little wide.

8. Re-stack the boards with stickers since fresh wood is now exposed and wait a few days for the boards to loose moisture.

9. Finish milling your boards to finial dimensions using the steps above when you are ready to build your project.

 

Stickers: Stickers are thin strips of dry wood placed between boards that allow air to circulate around lumber so the wood dries evenly.

Dry lumber: For indoor projects you want Kiln-dried wood which has a moisture content of around 6-8 percent.