According to Helmuth von Moltke, a Prussian military strategist, “No battle plan survives contact with the enemy.” When your plan meets the real world, the real world wins. Nothing goes as planned. Errors pile up. Mistaken suppositions come back to bite you. The most brilliant plan loses touch with reality (Lexician).
The mechanisms, now installed in the carcass, are stiff. They don’t move freely which is a necessity. Otherwise, the table will bind and won’t open. I began the investigation by checking each rack/guide individually outside of the table to make sure it moves freely. Ideally, they should slide by gravity alone when held vertically although sometimes, they require a push. My first “discovery” during this process was that the drive barrel was making contact with the lower guide. It left a “rub ring” around the barrel very near the gear (as shown in the above photo), and a shiny spot on the lower guide brass. After a quick consultation with Jon the machinist, I addressed these areas with a file to relieve material, and while it did prevent further contact, it did not rectify the problem.
Bright spot from contact with drive barrel
Another thing to consider…
Even slight differences between the aprons and legs can be enough to cause the mechanics to “hang”. If the legs protrude even less than 1/32 inch out from the side apron the lower rack gets caught. I gently used a chisel with a swiping motion to bring the two surfaces flush. Depending on the severity, however, it still leaves marks.
My current thinking is that the rack is too closely fitted to the drive gear. That is, the rack and drive gear are so tightly positioned that it’s causing excessive friction in the mechanism. I’ll test this out by relocating the rack and guide slightly lower than it currently is, and report back.
While contemplating this issue a thought occurred. It would seem that the position of the lower racks/guides with relation to one another is not critical. In other words, they don’t really need to be aligned at identical heights in the aprons. So long as the main box is mounted squarely to them the height shouldn’t be a problem. We’ll soon see how this turns out.
Before we can proceed to shape the carcass, the mechanisms must be completely installed and operating properly. With the mechanisms in place, the challenge becomes mounting the main box and tabletop to them. The tabletop is relatively straightforward: center it on the carcass, then mark the top racks. The main box, however, isn’t so simple. Not only does it have to be located left/right, front/back within its opening, but it also needs to be situated in height above the carcass floor. If it’s too high, the bolt on the latch won’t catch; too low, and the bolt will drag on the main box as it opens. To complicate matters, it is impossible to directly mark any of these components. They’re buried deep in the carcass interior!
Being the case, I’ve devised the following plan…
- With the tabletop removed, center the main box between the lower racks in the carcass. This dictates the thickness of the shims that will be needed.
- Remove the top racks such that the main box can be lifted out versus slid through the front.
- Coat the latch bolt with graphite.
- Retract the latch bolt by turning the key, and place the main box into the carcass. Align it appropriately, then release the latch bolt. This should imprint a line on the underside of the main box where the latch makes contact.
- Align the back of the hole in the metal latch plate with this line.
While I’m certain some level of adjustment will be necessary, this methodology provides a plan to move forward. Such is the nature of product development or, to put it another way, if we knew what we were doing, it wouldn’t be called research!
The main box latch is the trigger for the table’s entire mechanical system. When the key is turned within the right side apron, it actuates a lever releasing a bolt from the bottom of the main box. Under the tension from the drive mechanisms, the main box is propelled forward while, simultaneously, the table top slides back. Before proceeding with the mechanisms, it’s necessary to mortise in the main box latch.
This is a bit of a delicate operation. The latch mechanism has a 90° bend where the key connects which has to be mortised behind the lower rack guide. It also has a sleeve into which the key is inserted. This needs to be contained within the apron, not protruding from it.
After a bit of drilling, sawing, and paring things finally slide neatly into place!
Next, the long portion of the mechanism must be mortised into the floor of the carcass.
Up to this point, we’ve seen the top racks and the drive mechanisms installed in the side aprons. The final pieces to address are the lower racks. While the tabletop attaches to and rides on the top racks, the lower racks mount and support the main box. Like the upper racks, the lower racks engage the drive mechanisms and span not only the apron, but also both legs. So, we need to be careful about their alignment. An additional complication is that the rear end of the lower guide is blind mortised into the rear leg, while the front end is grooved. The main box is supposed to be removed through this groove for assembly and maintenance. To hide them, the grooves get covered with dovetailed keys veneered to match the rest of the carcass, then covered with guilt bronze mounts. No one will ever know…
While making the prototype, I’ve had to keep the aprons rough to facilitate installation of the mechanisms. No sense in making them curvy when I’ll need to stabilize them for mortising. This week, however, the project is starting to turn a corner.
To install the lower racks, there need to be defined boundaries. Unlike the upper racks which extend beyond the carcass, the lower racks are contained within the carcass, and are unseen. This means knowing the extents of aprons once they’re shaped. I created a template from photos of the original, and referenced it from the center line on each side block. Repeat the process on the undersides. These lines can now be carried down the verticals on each leg for reference during final shaping.