With the tops flattened, I will return to the base, which is currently just fitted together, but not fastened permanently. There are some small tasks remaining.
“Small tasks” is misleading, actually. It’s not that they’re huge tasks; there just seem to be a lot of them, and each has hidden complications. Many of the operations here involve drilling large diameter, deep holes. I have a small drill press, which while better than nothing, is not well suited to this scale of work. A good brace and bit would be the preferred tool for much of this, but my bits are no good. Alas. I make do with the drill press and cordless drill.
The overall plan for the base is to permanently attach the front and back legs on each side via the short stretchers. I’ll glue the tenons and reinforce the joints with drawbore pins. Then, the left and right assemblies will be attached with a de-mountable mortise and tenon joints with the long stretchers, using barrel nuts to secure the joint mechanically. This will allow for relatively easy transport of the bench – each top will come off, then the base can be broken down into the two assemblies and the two long stretchers. Each section can be carried single-handed, up and down stairs and narrow corridors if necessary.
I will also need to cut tenons on the tops of the legs, and drill oblong through-holes in the top stretchers for the lag bolts that will secure the tops to the base.
Here we go:
First I disassemble the base, and bring the legs over to the bench to begin preparing the drawbores.
Each joint will get a pair of drawbore pins. Here I’m beginning to mark for drilling by determining the placement of the holes. I make the initial marks on the edge of the mortise I’ll be pinning through. Since I’ll be drilling across the mortise , I need to transfer the marks around the corner.
Once I’ve done this for both short-stretcher mortises on the first leg, I can just transfer the positions to the other three legs:
Then I mark the other dimension of the hole location. I need to drill holes where the lines intersect:
Set up on the drill press with one of my adjusting stands as a support:
I’m drilling 1/2″ holes for 1/2″ red oak dowels. The holes will not go all the way through the leg- there is not enough travel on the drill press quill. I could finish the hole with a cordless drill, but I think there is enough material on either side of these mortises for a blind hole to be sufficient. The holes extend about 1″ past the far mortise wall.
Ideally, one would drill for drawbores prior to creating the mortise, in order to avoid this sort of tear-out inside the mortise:
However there isn’t much you can do to avoid the tearout if you forget, and that’s what happened here. I just clean the splinters out with a chisel and move on- there will be plenty of glue surface area left, plus a couple of stout drawbore pegs to make double-sure the joint is solid.
Now all the holes are drilled in the legs…
…and the short stretchers can be installed to transfer the hole positions onto the tenons:
Above, I’m using a 1/2″ transfer punch, shown here:
In general, transfer punches are fantastic and much better than using a drill bit to transfer a mark. However, the point on this 1/2″ punch is quite fat, and in soft wood like doug fir it makes a huge depression. This would be fine if I just wanted to drill on the center of the mark, but when you need to offset by a slight amount, the new offset mark ends up inside the “cone” of the indention created by the punch. It’s awkward to make the offset mark, and when you go to drill on the offset mark it’s difficult to predict where the bit is actually going to go.
Of course, I went and marked all of the tenons before I realized this. I would have rather used a 1/2″ bradpoint drill bit, some other kind of transfer punch with a finer point, or this same punch with a much lighter touch.
Anyways, here is how the offsetting works:
I set a pair of sharp dividers to my offset distance, 1/32″. I can probably get away with much more, but since I’m using store-bought dowels, I’m being conservative. I’ve had success with this offset before.
I set one tip of the dividers in the transfer mark, then lean the dividers over to make the offset mark. The offset is always towards the tenon shoulder, in order to force the pin to draw the shoulder up tight.
You can see how the large divot from the transfer punch makes this awkward. The result, after deepening the offset mark with a scratch awl:
The smaller mark centered in the divot is the original mark. Now I just go back to the drill press and drill on the offset marks as best as I can.
Now I’ll make the actual drawbore pegs. The raw material:
As I said before, these are 1/2″ red oak dowels. It is typically recommended to avoid commercial dowels, and use riven (split) wood shaped into a dowel (by whitling or driving through a dowel plate). That recommendation is based on considerations of grain strength, and the fact that 9 out of 10 commercial dowels will have significant grain runout. Grain runout could lead to the pin breaking while being driven in and bending through the offset holes.
In fact, I looked through every single 1/2″ dowel at Lowe’s (more than 50 I would guess) and managed to find 4 that weren’t terrible in this respect. Of the ones I chose, only about half of the length is usable on each. So I cut my 16 pegs from the good sections, and I reckon these will do just fine, especially with a modest offset like I’m using.
For reference, this is what grain runout looks like on an oak dowel:
The “V”s in the grain (often called cathedrals by woodworkers) signify where the grain direction is running off the edge of the dowel, instead of straight up and down its length.
The good sections are cut into lengths to become pegs. They are cut to be about 3/4” longer than the hole depth. I need to taper the leading end of the peg to assist it in snaking through the offset holes, and I need to chamfer the back end to resist splitting while being driven in. I’ve found that chucking the dowel into a cordless drill and using a belt sander is a fast method for this:
In the last photo you can see a block of paraffin – I like to wax the pegs to help them drive in easier. Spinning the bit in the drill is a good way to apply the wax. I can finish shaping and waxing a peg in perhaps 30 seconds this way.
Before I started shaping, I had to consider how far back to taper the end of the peg. This is not as critical with a through-hole, but since my holes are blind, too much taper means not enough bearing area for the peg to do its job. Since I have about 1″ of hole depth past the far wall of the mortise, I chose to taper 1/2″ of the peg, leaving 1/2″ of bearing surface on the far side of the mortise.
All 16 done:
But before I can drive these into the joints, I have some other tasks to complete.
These are the barrel nuts (Bench bolts) I ordered from Benchcrafted:
Pretty nice stuff, and not too badly priced, either.
These are a little bit tricky to install, as they require you to bore two intersecting holes, one of which is very long and into the end of a board. Drilling an accurate hole of any length into the end of something has always been a challenge for me.
I start by finding the center of the mortise for the long stretchers:
Then I transfer that to the back (outer face), where I’ll be drilling. Where the transferred lines intersect, I make make an indention with a scribe, which will give the brad-point drill bit a definitive place to start.
I then drill as far as I can with the drill press. But it doesn’t have enough quill travel to get to the backside of the mortise, so I have to continue with a cordless drill until the bit comes through the bottom of the mortise.
That’s the hole through the leg done. Now I insert the long stretchers into the legs and use the existing hole through the leg as a guide for drilling into the long stretcher.
I think I started getting a little frustrated at this point, and I didn’t take pictures for most of this process. I’ll try to describe it with my sparkling prose instead:
The 1/2″ drill bit isn’t long enough to go more than a couple of inches into the stretcher while the leg is in place, so I can only get the hole started. To finish, I needed to remove the stretcher form the leg. The results of drilling into the stretchers were not very impressive – placing the long 1/2″ bolt into the hole I just created showed it to be out of plumb in both dimensions by a considerable amount.
*I did all of the initial drilling with a standard-length 1/2″ brad point bit. I think it would be worth buying a longer bit just for this job, to enable the stretcher to be drilled as far as necessary without removing the leg (which is serving as a guide). That was at least part of the problem.
The big problem with the out of plumb hole is that it’s hard to know where to drill the intersecting hole for the barrel nut.
I ended up marking a pencil line down the length of two adjacent faces of the long stretcher. The pencil marks are centered on the hole at the end of the stretcher’s tenon, and parallel to the edges of the stretcher (plumb, in other words). I would use the lines to drill for the barrel nuts, so I needed to “adjust” the existing holes to match the plumb pencil lines.
The adjustments were made by enlarging the existing 1/2″ hole first with a 9/16 twist bit, then a 5/8 twist bit. Whereas I would normally try to follow the existing hole’s path when enlarging, here I consciously tried to steer the bits to match the pencil lines. It sort of works.
I also went back to the legs and enlarged the 1/2″ holes to 9/16″, to give some wiggle room. Then I chucked a forstner bit into the drill press and drilled for the barrel nuts.
And I’ll be damned, it went together drama free:
The barrel nuts really work well, drawing the joint together solidly.
OBLONG HOLES FOR LAG BOLTS
In the split top Roubo design, the tops are attached to the base primarily by tenons on the ends of the legs, but these joints aren’t secured with glue, wedges, or pegs, which enables the tops to be removed for transport. Instead, the tops are secured in use by lag bolts which fasten the tops down to the upper stretchers.
It makes sense to do this prior to assembling the legs to the stretchers, so that I can use the drill press. The plan is to drill two 3/8″ holes side by side, then take out the waste between the holes to create a single oblong hole. The hole is oblong in order to allow the top to expand and contract – the head of the bolt can move back and forth a little bit along the stretcher.
I calculated that I would need no more than 1/4″ of allowance for movement, so 1/4″ was the offset I used between the two holes on each stretcher. This turned out to be a mistake – not because 1/4″ was an inappropriate allowance, but because it is not feasible to drill long 3/8″ holes 1/4″ apart without the bits wandering towards the other hole. At least with the equipment I have. I think I would have been alright with a 1/2″ offset.
Again the hole needed was deeper than the drill press could go, so I had to drill from both sides. With the wandering issue, I ended up creating a waisted hole (narrower in the center of the stretcher than at the either end). This would not permit the lag bolt to move enough.
I did what I could to fix this with a gouge, but the depth of the hole meant I couldn’t get all the way to the middle, where most of the unwanted material was. A 3/8 twist bit got it as close as I think I can do, although it wallows out the sides of the hole, which makes the shape look out of whack but doesn’t affect anything of substance. Lastly I cleaned up the counterbore on the underside of the surface, leaving this:
The primary attachment for the tops are through four tenons, one on each leg. The shoulders of the leg tenons should form a co-planar surface with the tops of the upper stretchers. To achieve this, I’ve waited to cut the tenons until I had the stretchers installed and could nail down their final positions. So, with the short stretchers and legs mated I can take a marking knife and make a small mark on each leg, indicating the upper surface of the stretchers:
Then after hammering the assembly apart I take a square and extend the little mark all the way around the leg, defining the future tenon shoulder:
These tenons will be a bit simpler than those on the stretchers- having a shoulder on only two sides instead of four. The exception is the front right leg, which will have a shoulder on three sides- the third shoulder is to allow some material (1″) between the edge of the mortise in the top and the recess for the tail-vise. This should become clearer as the build continues.
Otherwise the process for making these tenons is the same as you’ve seen here before.
All four done! Notice the tenon on the top left- this will be the front right leg with the 3-shouldered tenon.
ASSEMBLING THE SIDES & FINISH PLANING
After finish planing the short stretchers and inside surfaces of the legs, I can return to the drawbore pegs and assemble the short stretchers into the legs. I’m using liquid hide glue for extra assurance, although glue isn’t technically necessary with drawboring.
With the stretchers inserted and prior to knocking the pegs in, you can get a picture of how the offset in the drawboring process works:
The offsets looked closer to 1/16″ at this stage…the variance from my intended dimension is probably a function of the awkwardness involved marking the offset off the huge divot created by the transfer punch. But I am actually thinking that 1/16″ should be the preferred dimension for this sort of large scale joinery, especially in a soft wood. 1/16″ is pretty modest in fact, compared to what I’ve seen online. Anyways, it’ll work.
The pegs are all driven in with my rosewood mallet. No drama with this, thankfully. I am sure the straight grained dowels and wax helped this along.
With both assemblies pegged, I can go ahead and trim the pegs flush…
…and finish plane the inside and outside faces of the legs:
I’ve not planed the front and back surfaces yet, because I want to flush them with the long stretchers. Before assembling each side frame with the long stretchers, I plane the top, back, and bottom of both long stretchers, leaving the front face to be flushed with the side frames. They are very close to flush as-is, but the legs are proud of the stretchers by 1/64″ or so.
Now I assemble the whole base unit, and tip it on its side for planing flush:
One side done:
With both sides done, I can flip the base upright and reveal the (substantially) completed base.
Most of the remaining work will be done to the tops, beginning with the next entry in this series.
Hope you’ve enjoyed. Thanks for reading.