Drive Mechanisms

Layout Prep With the top guides and racks “installed” (…and by that I mean in place in the carcass. At this point, they are not actually fastened to the carcass…), the spring barrel drive mechanisms are the next components to be addressed. These two devices, mounted on each side of the main box, provide the power to make the table open. 

The pinnacle of high technology during the eighteenth century, the drive mechanisms are based on clock mainspring barrels. Holes in the exterior of the aprons admit a key to tension them, just as one winds a clock. Since they engage both the upper and lower racks, their placement within the carcass aprons is critical. The image shows the layout lines defining where the mechanisms will go. But it’s not just a matter of simply excavating the material. The depth of each component is important since the racks, and drive gear teeth must align.

 I will be excavating the material, and installing these mechanisms over the next week. I anticipate there will be a number of adjustments required. Holes will need to be bored in several stages, mirroring the various diameters of the mechanism followed by mortising. 

Fitting the Top Racks

It’s time to install the racks and guides into the prototype carcass. I’m following a methodical process to make sure everything aligns and functions. As with the carcass construction, start at the top and work down. So, the first components are the top racks and guides. These are easy. Simply rabbet the top edges of the legs and aprons to be flush with both the top surface, and the interior apron side.

Test fitting these, however, exposes an issue. The hinges for the flaps on main box interfere…


I need about 3 mm more clearance on each side for main box to fit. Two options: either make the main box smaller, or recess the racks further into the aprons. However, if the latter option is exercised the rabbets for the racks will cut into the mortises and tenons for the aprons and legs! So, I have to make the main box smaller. But, in order to take the main box down to the required dimensions, I’ll have to un-glue it! (Don’t recall doing that in the first place, but oh well…). Good thing I used hide glue!

Since I have only a small kettle, it’s a slow process, but after about 6 or 7 iterations, the dovetail joints holding the sides on are free.

Ungluing the main box 1Ungluing the main box 2

With the main box disassembled, I remove 3 mm from each side. This keeps the front profile uniform.

Just 3 mm…
Note that after drying out for a night, the pieces are totally usable again! Reassembled main box

Now resized, the main box slides within the top racks.

While this addresses the immediate problem, it causes others. In particular, future (foreseeable) issues include:

  • The flaps covering the two side compartments in the main box will need to be resized. No problem. I used hide glue on those too!
  • Also, the lower racks may need to be built out to remain in-line with the drive assemblies (and top racks).