Taliesin Desk Build - How's and Why's #7: Mortises Three Ways

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Blog entry by EarlS posted 08-21-2016 01:41 PM 1384 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 6: Big Mortises Part 7 of Taliesin Desk Build - How's and Why's series Part 8: Intermission »

As I mentioned in the last entry I broke the mortising into 3 groups. The big mortises were made using templates and a router.

Most of the smaller mortises were ¾” which were made using a ¾” Forstner bit and squaring up the holes with a corner chisel. Some what tedious, but a sharp chisel makes quick work of it.

Any square holes that were ½” or smaller were made with a mortising attachment on my drill press.

There are some serious shortcomings in the Delta Mortising attachment used with the Delta Drill press. Most of them relate to the sloppy 12” Delta Drill press.

The most significant is the amount of vertical deflection in the drill press shaft that occurs when you start to apply force to the chisel to make the sides of the mortise. From what I have been able to figure out, the drill press shaft has enough play in it that the force causes it to deflect both toward the front and left or right.

Similarly, there is also some play in the head of the drill press that can cause the bit to twist out of square relative to the fence. I can feel the play in the drill press shaft by twisting the attachment body after it is securely tightened to the drill press shaft.

The table will also flex out of level from the force of the chisel on the wood. Generally, I don’t use the fence that comes with the mortising attachment since it is so primitive that set up is cumbersome and typically winds up being a trial and error effort.

Lastly, a ½” chisel bit is about as big as the drill press can handle. Trying to string several of them together to make a larger mortise results in a sloppy outside edge that requires a lot of clean up and generally I wind up with a larger overall opening in order to square and clean everything up. For these reasons, I typically cut ½” or smaller mortises with the Delta mortising attachment.

With all my chisel bits freshly sharpened, and all of the mortises laid out on the boards I started setting up for the various cuts. Typically, there are several pieces that need the same cuts so I can group them together. The same holds if there are several mortises that share a common line along a board.

I start by setting the spacing from the fence to the chisel, using space bars, or ripping a 12” piece of wood to the appropriate width and checking it with my calipers. The chisel should be tight up against the spacer, but not pushing into it. With my fence, I adjust either side and keep making slight adjustments until both sides of the chisel are tight against the spacer. During the process, I also raise and lower the chisel to make sure it is not being deflected by squaring the fence against the spacer and chisel.

Once the chisel and fence are square, I set the depth of the cut by setting the drill press depth stop to stop the chisel at the correct depth in the wood. From there, I cut a spacer to set the side stop, again, being careful not to deflect the chisel. The last step is to set a piece in place that has the cut marked out and carefully lower the chisel and confirm that it matches up with the pencil marks. I mark every cut and check each time since I have been known to get the wrong piece or mark the wrong spot.

From there it is a matter of cutting the mortises in batches and making sure to get all of the same cuts finished before moving to the next set up. Generally you find one you missed right after you’ve moved to the next set up.

While I had the mortising attachment set up I also cut the square holes for the square G&G style pins I decided to add (One of my favorite details). They use the same set up but the attachment does much better with the ¼” and 3/8” chisels, especially when they are freshly sharpened.

Coming soon: To the Tenons and Beyond!!!

-- Earl "I'm a pessamist - generally that increases the chance that things will turn out better than expected"

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