Why Build Prototypes?

You might wonder why go through the effort of constructing a prototype when building a new piece? After all, it requires material, and at least as much time to create it, as the “official” piece. Why not just apply those resources to the finished piece?

So here’s my story…

I embarked on this Oeben re-creation, after much study, by constructing a three-dimensional SketchUp (SU) model. The thought being build it in SU to work out the bugs in the process. This may come as a surprise to you, but ALL of the aprons on this table exhibit a curve to some degree (fortunately they’re not compound!). There are very few flat exterior surfaces on the piece! At the time, I thought the easiest way to handle this would be to begin with thick material, then shape it appropriately. I drew it this way, and didn’t look back. Life was good… Or was it?

Now it comes time to cut wood. So, I choose an appropriately thick piece of stock for the “Main Box” front. The “Main Box” being the “drawer” that extends forward when the top is released and retracted. It contains all of the other “hidden” components. For several weeks, I continue working – yet, something keeps bothering me. Each time I look at the curve for the front, it seems out of place, like it’s inappropriate, and not representative of what I see in the original. Finally, I sit down, and look through the photos. I come across an image taken of the bottom. It’s available on the Getty Museum’s website. I’m showing an outline of its shape below. The two open spots at the bottom are where the legs protrude.

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From it, I calculate the extent of the curvature at its most extreme, and learn that my drawing is about 1.5 cm too shallow. Not necessarily a show stopper. But wait – I look closer. Not all of the aprons were created in this fashion! It appears that the board comprising the rear apron was bent. Looking further still, I come across this photo showing the underside of the “Main Box”.

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It shows a uniform thickness board bending in a gentle arc composing the front of the “Main Box”. So, it appears that my plan to work a thicker board to shape is incorrect. Now the question becomes how did Oeben’s craftsmen accomplish this?

There are several methods for creating curves in wood. The most basic is to cut perpendicular kerfs along its inside. This leaves voids allowing room for compression. However, these spaces are highly visible, and from the previous picture, not in evidence in this table. A second method involves slicing the board into thin planks along the grain then gluing them back together while clamped to a form. Once the glue dries, the board maintains the shape. I have witnessed instances where this was done and difficult to detect, but I don’t believe that this method was used either. A third procedure involved steaming the board and bending it. While this seems to be plausible, was this historically correct for the period in which the table was made?

Following a brief exchange with Patrick Edwards, this appears not to be the case. According to Patrick:

Oeben did not steam bend furniture.  French curves are sawn from solid wood.  Although Denis Papin (1647-1712) invented the steam pressure cooker, no one thought to use it to bend wood until the 19th century.  Complex two dimensional curves are “sculpted” as you suggest using chisels and scrapers.  Final truing of the surface was with toothing planes.

So, my initial methodology was correct, and the aprons were hewn from thicker stock.

If I hadn’t taken the time to construct the prototype, I may have missed out on this learning opportunity. Not only do prototypes help you work through unforeseen issues that weren’t caught during the modeling phase, and provide valuable practice for building the final version:

If you can’t get it right on the prototype, how do you expect to get it right for the final piece?

The process of creating a prototype gives insight into how the piece was originally made – living archeology, if you’re paying attention!

So, what becomes of the current prototype now that this issue has come to light? After all, good material and effort has already been expended to reach this point. Rather than discard it, I decided to laminate material and “thicken it up”, upon consideration. While this will allow me to obtain the appropriate external shape, the interior will remain as it is presently. It’s a compromise I will live with for now…

Table top marquetry drawing

One of the key elements of any marquetry embellishment is the drawing from which it is cut. A few weeks back I managed to make a full-scale photocopy of the table’s top.

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Pierre Ramond’s Masters of Marquetry, vol. III contains a detailed line drawing in reduced form. Luckily, it is printed on a fold-out page that doesn’t cross the spine of the book. Subject matter copied from near the spine always results in distortion! This drawing together with measurements of features in the original top allows computation of a scale (enlargement) factor. These full-scale copies are important because adhered to veneer packets they become the templates from which the marquetry is cut.

The table top marquetry is the only portion which will require a chevalet. Parquetry adorning the aprons and interior compartments can simply be cut with a chisel and straight-edge.

In school, I was always advised to come up with independent methods to validate any result. Having already constructed the prototype top, it should fit precisely on the bounds of the photocopy if my calculations (and drawings) were correct. Happily, they did! :-)

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On the importance of drawing…

 

 

One of the things that makes this project complex are the mechanisms on which the table operates. Although I have several pictures of them from my trip to the Getty, I still needed to draw everything out. This is necessary not only to have them manufactured, but also to ensure that everything fits together and operates smoothly. Despite these reasons, it’s a major pain in the posterior!

SketchUp to the rescue!

SketchUp is a fantastic design tool/laboratory. By determining proportions from the aforementioned pictures, I created scale drawings of each of the mechanism parts. While the urge to send these individual part drawings off for manufacture was great, better judgement prevailed. Luckily, this averted a “situation”. Instead, I methodically fit the pieces in place in the carcass model, and made several “discoveries” along the way!

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The top rack and guide are offset toward the outside of the carcass.

The upper racks are offset toward the outsides of the carcass least they interfere with the main box travel. Here’s surprise number 1: if I mount the upper racks and guides flush with the inside of the carcass, they will interfere with the “main box”. Checking my photos, sure enough, the original makers moved the racks and guides outward to make the room necessary.

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Initial model of the upper rack (it’s upside-down). It’s not tall enough!

Also, I needed to extend the height of these racks so that the top cleared the main box. My photos provided the multiple aspects necessary to deduce their dimensions. However, they can be unreliable sometimes. Small errors propagate. “Installing” the racks into the SketchUp model allowed their height to be tweaked, and the top to clear the main box to the desired level.

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Fixed the upper rack. Now the top will clear the main box!

This process also helped me discover a quirk with the lower rack installation. The lower guides mounted to the carcass sides extend beyond the inside face of the rear panel, that is, the back. Although I’d seen this before, I had never really considered the rationale for it. I figured that the makers were being lazy. Turns out it has a critical purpose. It accommodates the movement of the lower rack (the one fixed to the main box) when the table is closed. I didn’t realize it, but the back panel in the original is mortised just for this reason. The evidence was there in the photos the whole time. I just didn’t see it until my attention was drawn by this.

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Note how the lower rack projects into the rear panel? The original is mortised out for just this purpose.

The final parts to be modeled are the main spring barrel mechanisms. The chief consideration with these involves the force required to drive the main box/top assembly when the latch is released. Too great a force, and it could jolt the table possibly knocking it over. Too little force, and the assembly could get stuck. My solution is to build the prototype main box, I already have the prototype top, and get their combined weight/mass. From this, we should be able to determine the appropriate tension, or at least get close, for the spring drives. I’m betting that this is the same process that Oeben’s craftsmen used in their time too. Except for SketchUp! :-)

SketchUp modeling

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Finally, I’ve completed the 3-D SketchUp model for the carcass. Since I tend to use models like this in the studio as my “plans”, I wanted to create as accurate a representation of the piece as possible. Everything you see, even the parts you cannot, has dimensions associated with it.

I started from “scratch” by re-creating the top from the dimensions and drawings I made at the museum. The rest of the model proceeded from there. In the end, it contains nearly all of the details of the original table including (some) joinery.

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I find SketchUp to be an extremely useful program, and an important design tool. Representations like these help me plan out the piece, determine the sequence of steps I’ll follow as I’m building it, and aid in the purchase of materials.

Just for fun, I thought I’d include a picture of the original model I put together as I first contemplated this project, after seeing the Getty video for the first time…

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It’s a bit crude, isn’t it?! ;-)

Want to get a jump start on SketchUp? Popular Woodworking Executive Editor Bob Lang is teaching a five-day intensive 5-9 August. Check out the details and sign-up here.

A status report

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It’s been a blur since the Kickstarter campaign ended. Here’s what’s been happening:

• I’ve posted supporter acknowledgements from the surveys I’ve received so far. If you haven’t answered your survey yet, please send it in. Also, if you move, please let me know so that I can update your address to get your reward to you!

• I just finished up a portable workbench build based on a design highlighted by Chris Schwarz – the Moravian Workbench. The first and last images show the original.

• Our woodworking club just completed its first ever seminar, hence the need for the aforementioned workbench. Karen Wilson, and Donna Hill, both contemporaries from the Marc Adams School, joined us for the event.

• I’ve ordered, and received more parts for a mini-chevalet I’m building based on Don Williams’ design. This will be helpful when it comes time to cut the marquetry. More on this later…

• Finally, on the topic of the Oeben, I’ve been working on drawings, and dimensioning pictures to create the mechanisms (in between all of these other activities!). Since mid-February, I’ve been talking with David Lindow (David Lindow Clockmaker) about taking on this task.