Boggs Side Chair Build #15: Slat Mortising, Part 3

Now that I have finished building the slat mortising jig I am ready to cut the slat mortises into the rear legs. I prefer cutting the slat mortises using a router and this jig. The process is very fast, taking a half hour or less and it produces a series of  mortises in each leg that are identical even if the profile of each leg varies slightly. I also prefer the look of the rounded corners on each end of the mortise. Of course if you like hand chopping mortises there is no reason not to — layout the mortises directly on each leg using the method I describe here, then chop as you usually do.

If you remember from the blog post on post-bend shaping the last step was to determine the inside and outside faces of each leg and cut a taper on the inside face. The slat mortises will be cut into the tapered inside face. Just looking at a single leg it’s sometimes difficult to see which is the tapered face and which is the flat, so it can be easy to cut the mortises into the wrong face. I have a simple method to avoid this. Hold the two legs together so that the tapered faces are on the inside as shown on the left. Then put some blue tape on the outside face roughly where the top slat mortise will be. This way when I put the leg into jig, if I see blue tape through the template openings, I know I have the leg in upside-down. 

I need one reference mark on the leg in order to orient it correctly in the jig. This mark is 4-1/4″ from the top on the tapered inside face. This will put the top of the mortise 4-1/2″ from the top of the leg because of the offset of the bushing.

After drawing the mark 4-1/4″ from the top I place the leg in the jig with the tapered side facing up. If I see blue tape through any of the template openings the leg is in upside down. Next I place the long wedge under the leg until the leg is flush against the top of the jig. There are three points on the leg that need to align to points on the jig. First I align the 4-1/4″ mark with the top edge of the top opening (1). Next I align the front edge of the leg with the long edge of the top opening (2). And finally I align the front edge of the leg with the bottom corner of the bottom opening (3). When the leg is aligned correctly there will most likely be a gap between the leg and the backstop. I simply place a wedge between the leg and the backstop and clamp, making sure that I keep the leg in alignment. 

Here I’ve placed the first leg in the jig and aligned it at the three points. The problem that I have is that there is a small gap between the edge of the leg and long edge of the middle opening. If you look closely you’ll see this gap at the top of the middle opening. The bushing locates the router bit roughly 1/4″ from the edge of the opening. My goal is to have all the mortises 1/4″ from the edge of the leg. As it is now the top and bottom mortises will be correct but the middle mortise will be a little closer to the edge. To fix this I am going to have to move the leg until the gap in the middle opening disappears. To do this I loosen the clamps and tap the wedges in further until gap is gone. This will have the effect of moving the top the bottom mortises a little further away from the edge, while the middle mortise will be exactly 1/4″ from the edge. I would rather do this than have a mortise that is too close to the edge. The nice thing about this system is that the relationship of the mortises is independent of the profile of the leg. So if the legs vary a little bit, the relationship of the mortises will still be identical on each leg.

Here is a top view of the leg aligned within the jig and ready for mortising.

And another view of jig ready for mortising. 

Before mortising I want to describe the movement of the bushing within each template opening. As I explained in the last post the bushing rides against three bearing surfaces — the top edge, the long front edge, and bottom edge. The length of the mortise is determined by the distance between the top and bottom edges of each opening minus the offset of the bushing. The width of the mortise is determined by the diameter of the router bit, in this case 1/4″. So, it is very important to start the cut in the left corner and always keep the bushing tight against the long front edge as you move the router to the end of the cut at the right corner. Never move the bushing away from the long front edge.

The bushing I use has a 51/64″ outside diameter. Although this sounds like an odd dimension it is a standard size available in most bushing sets. You could also use a bushing that has a 3/4″ outside diameter. The router bit is a 1/4″ straight cutting bit. It needs to be long enough to plunge through the template and the 3/4″ depth into the leg. The bit I use is the Whiteside SC16 which has a 1″ cutting length and a 3″ overall length. This is a 2 flute, 1/4″, solid carbide bit. Brian does not like 1/4″ spiral upcut bits because he says they vibrate too much.

Here is a step-by-step description of the process.

  • Set the plunge depth. Place the bushing inside one opening and lower the bit until it just touches the surface of the wood and lock it in place. Move the depth stop turret to it’s highest position. Loosen the depth rod and let it touch the highest step on the turret, then tighten the rod. Raise the bit back to it’s raised position. My router has 1/8″ steps on the turret, so six turns of the turret will yield a 3/4″ cutting depth.
  • Make a very shallow first cut. Set the turret to the stop that will allow for a 1/4″ depth cut. Turn on the router. Place the bushing tight against the left corner. Plunge to about a 1/16″ depth or less. Move the router to the right always keeping the bushing against the long bearing surface until you reach the right corner. Raise the bit to it’s raised position. I explain the reason for this very shallow cut below. Repeat for each mortise.
  • Complete the first cut to 1/4 depth. With the bushing tight against the left corner plunge to the full 1/4″ depth. Move the router to the right always keeping the bushing against the long bearing surface until you reach the right corner. Raise the bit out of the cut. Repeat for each mortise. I like to vacuum out any chips before making the next cut.
  • Cut to 1/2 depth. Reset the turret stop for a 1/2″ depth cut. Make one cut to full 1/2″ depth in each mortise. Raise the bit out of the cut. Vacuum out the chips.
  • Complete the mortise to full 3/4 depth. Reset the turret for a 3/4″ depth cut. First make the initial cut to full depth moving from left to right. Keeping the bit in the cut, next make a cleaning cut moving the router right to left until you reach the starting point in the left corner. Raise the bit out of the cut. As always be careful to keep the bushing against the long bearing surface. Repeat for each mortise and vacuum out the chips.
  • Check the mortise depth. Before taking the leg out of the jig I always check the depth of each mortise to confirm that they are all 3/4″ deep or a little more. If any of the mortises are not full depth, check the depth setting on your router, and cut the mortise again to full depth. Once you remove the leg from jig it will be impossible to make the mortise any deeper.

Here I am checking the depth of the middle mortise.

And here is a top view of the jig with the three mortises cut.

And finally the finished leg. For the opposite leg, flip the jig over and repeat the entire process.

One of the things that I always had problems with in cutting the mortises is that I usually ended up with a small dimple at the beginning of the cut at the point of the initial plunge. You can see an example of this in the left, bottom corner of the top mortise. No matter how carefully I plunged I always seemed to have this same result. When I was in Asheville recently I pointed this out to Brian and asked him if I was doing something wrong. He explained that router bits are designed to cut while moving within a piece of wood. They are not designed for boring holes as a drill bit is. So the small dimple you see is a result of the router bit not be able to accurately bore the initial hole on the first plunge. I thought about this for a while and decided to try a very shallow initial plunge, say 1/16″ or less, then move the router to the right completing an entire pass at this 1/16″ depth. This is followed by subsequent plunges and cuts of 1/4″ each until I reach the full depth of 3/4″. The bottom mortise was done this way and you can see that the mortise is nice and straight with no dimple on the left, bottom corner. I’m not sure why this works, but I’m very glad to have figured out a work-around.

Now that the slat mortises are complete the next step is to fit the slats to the mortises. I’ll begin describing that process in the next post.

Jeff Lefkowitz


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