Taliesin Desk Build - How's and Why's #6: Big Mortises

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Blog entry by EarlS posted 08-14-2016 01:15 PM 1303 reads 0 times favorited 2 comments Add to Favorites Watch
« Part 5: Making Legs Part 6 of Taliesin Desk Build - How's and Why's series Part 7: Mortises Three Ways »

All of the leg pieces have been cut to length and marked for the multitude of mortises and tenons. Before I get into the details of what I did, I wanted to talk about options for cutting mortises and tenons.

I spent a lot of time researching some of the new options on the market. I really wish I lived near a wood working store that had demonstration tools to look at and maybe even try out.

The Festool Domino looks like a biscuit jointer on steroids, with a special router bit mounted horizontally that oscillates inside the cutting area to produce a rounded rectangular mortise. It looks really interesting but the price (>$1500 for one that has enough bells and whistles to cut more than one size) was prohibitive. Maybe someone that has one can comment on how well they like it. The other down side is that the mortises would still need to be squared up for this project since it is a Prairie style which means things need to have sharp, square corners and edges, mortises and tenons included.

I also looked at the Leigh Mortise and Tenon jig. The standard version was $1000 or so. There was a less costly, lower quality version for $500 though it was widely criticized as being flimsy and cheap. Again, the extras necessary for cutting a variety of sizes was costly and the jig required specialized bits as well. The mortises and tenons also have rounded edges unless you spend more $$ to get the square tenon adapter.

Lastly, I looked at a dedicated mortising machine, ~$500. The Powermatic 701 had good reviews but was $500 plus chisel bits were extra.

I have a Delta mortising attachment for my drill press. Not the best quality tool I’ve used but with some TLC and attention to set up it produces a decent mortises up to ½” and 1.75” deep. Trying to make a decent 1.5”x4” mortise all the way through a 3” piece of wood would take forever and look terrible using this approach (past experience).

Having exhausted all of these options, I was left with using a template to router the mortises using a 1/2” spiral upcut bit and mdf templates. This approach also has its hazards. Keep in mind that template routing uses a guide sleeve with a diameter that is ~1/8” larger than the bit. So layout is a bit more complicated to include the extra 1/16” on the edges (1/16” is the difference in the radius which is the important thing to remember). I always measure the OD of the guide sleeve with my digital calipers so I know the exact diameter.

I clearly label the top, bottom, front, back, left and right on the pieces to help keep things straight. When setting up multiple mortises, using the same front edge keeps things lined up. The same holds for left and right, top and bottom. No matter how careful you are, if you don’t pay keep track of a piece’s orientation you will wind up with an upside down, backwards piece that will have to be discarded and re-made. (again – past experience)

I started with the legs, setting up the through mortise for the bottom apron and the 1-1/2” deep mortise for the top apron.

Here is picture of a typical mortise template set up. The boxes and wood are there to keep chips from flying all over the shop. I also vacuumed out the hole after each pass to get the chips out.

I used the plunge router base and utilized the 1/8” preset stops to work my way through the legs for the through mortises. I use the stops to keep from getting too aggressive on the depth of the cut which causes all kinds of issues, including chattering, rough cuts, breaking the guide sleeve, or gouging it, and maybe even breaking or damaging the bit. It is faster, easier, and a lot less dangerous to use the steps. (you guessed it – past experience)

Here is a picture. Each set of steps is 1”. After each 1/8” step, twist the preset to the next lower step and repeat. When you reach the bottom step adjust the depth rod so it is just touching the top step and keep going. When you get close to the final depth of the mortise, check with calipers and use the fine adjustment knob to dial in the final cuts.

Even with all of the templates and good set up you can get a ledge when you have to cut a mortise from both sides. After the mortise is cut I used my router table and a 3” straight bit to clean up the ledge.

The last thing to do on the through mortise is to add 1/16” on the top and bottom cuts on the outside face to allow for the wedge flare. I used the table saw to take 1/16” off the top and bottom of the mdf portion of the template then used the plunge router to make the step. It is important to do this AFTER cleaning up any ledges so you don’t accidentally “clean up” the ledge for the wedge flare.

The other large mortises were completed using the template router approach. It took several days to set up the templates and router out the mortises. No need to rush things and make a mistake that has to be “fixed” or start over. Another reason to make sure the design and layout is what you want and all of the dimensions are figured out.

In preparation for squaring up the mortise corners I made sure all my chisels were sharp using a Work Sharp 3000. It is a nice machine for sharpening tools. The angles are preset and 2 sets of chisels plus a couple of ½” mortising chisel bits and a corner chisel took about 30 minutes. I will need to invest in a water stone to sharpen the inside of the corner chisel. Anyone else want to comment on how their preferences for sharpening corner chisels? Cleaning up the corners with a sharp chisel is like cutting butter with a warm knife. In short order all of the big mortises were square.

The small mortises will be covered in the next entry since this one is rather long.

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

2 comments so far

View CaptainSkully's profile


1615 posts in 4634 days

#1 posted 08-14-2016 02:35 PM

I’m loving your blog! There are pattern following bits that have a bearing the same diameter as the bit so you can make the template the exact size you want the finished mortise. My Delta mortising machine was under $300 and works well. MDF templates are gradually dented by the bearing on the router bit, which eventually will translate to the modifying (in a bad way) the finished template.

-- You can't control the wind, but you can trim your sails

View EarlS's profile


4447 posts in 3424 days

#2 posted 08-14-2016 11:34 PM

I’m leaning towards the Leigh FMT Pro mortise and tenon jig at some point rather than a mortising machine if I ever get to the point where I buy something new. I actually prefer the look of the rounded edge in a through tenon, kind of looks like a Greene and Greene accent if you use a roundover bit on the edges.

I also considered the flush pattern bits but the larger ones were 3/4” diameter and that made the rounded corners even wider. You also mentioned the other down side with the denting.

I went with the approach I know best, which seems to be a common theme. There are so many different approaches that you can use. It comes down to deciding which one you prefer and/or are familiar with.

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

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