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Craftsman Extra Board - May 2012

Spar and Rib Benchwork

With the module tipped on its backdrop, we get a good view of what spar-and-rib benchwork is all about. Not only is this construction lightweight and sturdy, but also economical because it uses less material. The centered holes allow you to pass through wiring.

Easy and lightweight spar and rib bench work

by George Riley/photos as noted

Small portable, sectional and modular layouts have grown in popularity both at shows and among space restricted railroad layout builders. Being one of these hobbyists, I am always on the look out for a better way to built bench work that meets the needs of this select, yet growing group. It has been over thirty years since I built my first module in N scale. That was a number of layouts, sections and modules ago constructed in seemingly as many scales and gauges. Since then I have learned a lot about wood working, however, the quest for strong, light weight portable bench work has remained an illusive goal. Most wooden bench work construction that I have built to various published and invented specifications has been vastly overbuilt making it heavier than it has to be. An early answer to the weight issue was the substitution of Styrofoam components to attempt to save some of the weight. While lighter these variations of construction were not as durable as is needed for portability, resulting in frequent rebuilding during their service lives.

At first glance "spar and rib" bench work looks more like the construction used on aircraft than a base board for a model railroad. While it does borrow some of the stressed skin techniques used in aviation, the concept evolved from the popular L-girder method of bench work assembly. L-girder has proven to be both strong and flexible. However, this material thrifty bench work method lacks the portability needed for modules and traveling layouts. By substituting an internal spar that is inserted through the lateral cross members (Ribs) for the supporting L-girder weight is saved and strength is maintained.

As with any project, the materials used are crucial to success. Even using premium materials the cost of building a 2’ x 4’ module with back scene and fascia will be less than $100.00, so use the best grades of wood available. If lower grade materials are used, the construction will be more difficult, the level of finish lower and the results less than satisfactory. The list of materials should provide all that will be needed to construct the project. By adding or subtracting to the list one can adjust the quantities needed for their specific needs.

Spar and Rib Construction

Before beginning construction there are two critical steps to assure that every thing lines up square and true. The first is making certain that the holes for the spars are all located correctly in the ribs. Begin by cutting the rib stock (1/2” x 3” poplar) to length. Stack them in alignment and mark the top rib for pilot holes. When stacking up all the ribs make certain that you align them towards what will be the top or side to which the track base will be secured. I have found that not all boards claiming to be a certain dimension are true and one will surely find several ribs that may be either narrower or wider in dimension. This is also the case when purchasing partial sheets of plywood. These usually measure slightly smaller than billed since no allowance has been made to compensate for the kerf of the saw used to cut them.

Using a slightly smaller bit than the pilot bit of the hole saw drill pilot holes down through the stack of ribs. If your stack is taller than the bit is long, drill the pilot holes in one shorter stack and then use the top master rib as a guide to drill pilot holes in the additional ribs. In addition to the spar holes, plan to cut out one or more additional holes to allow a pass through for the wiring.

Spar and Rib Benchwork

With a hole saw, cut out the spar and wiring holes on each rib using the pilot hole to align the saw. This technique will yield very accurate results. To minimize tear out cut from one side, cut about half way through the rib and then finish the hole from the other side, this will also make it easier to remove the plug from the hole saw. Once each rib is complete the actual assembly of the base board can begin.

Spar and rib bench work is constructed upside down. Clamp the top sheet of plywood to the strong back. This is the 2” x 6” board specified in the materials list. This board needs to be true and warp free to assure that the finished base is true. Next, mark and square the rib locations. For ¼” or 6 mm thick plywood the spacing should be approximately every eight inches apart. Glue and clamp the ribs to the plywood sheeting using ‘Gorilla Glue’ making certain that on each rib that the side designated as the top is in contact with the plywood. Allow the glue to completely harden.

With the ribs in place and the glue fully hardened run the spars through their respective holes and secure with glue. For the spars, 1 ¼” hardwood dowel were used. An easy way to make sure that the dowels are straight is to roll them on a level floor. If they do not run true find one that does. Check that the plywood surface is true. If there is a slight warp or bow this can be corrected by flexing the assembly before the glue on the spars has set.

Spar and Rib Benchwork

The fascia of 1/8” thick x 3” high plywood is now added to the front and the back drop made from the same thickness stock 12” high is attached to the rear. Both are glued and clamped in place. Additional scenic risers and end panels made using 1/8” plywood can also be installed. These items are all glued and clamped into place and allowed to dry completely.

If a coved back drop is desired use 1/32” birch plywood bent and laminated to the 1/8” plywood back drop. Glue these sheets of plywood with yellow carpenters glue and clamp in place over night until the glue has fully cured. Fill any joints and holes with wood filler and sand smooth. A final coat of sanding sealer and finish sanding with 220 grit sand paper will complete the bench work of the module. At this point the base board is ready for priming, painting and the addition of the railroad.

Legs have been left up to discretion of the builder. As a rule when building a stand alone mini-layout I add leveling bolts to the ribs at either end and dispense with adding legs. The ‘layout’ is simply set up on a conference table, however, when setting up a larger sectional or modular display having supporting legs is a requirement. 2 x 2 wooden legs can easily be bolted to ribs near the ends of the bench work or adjustable metal out feed stands with their rollers removed can be used as well. These are an easy, flexible and inexpensive option available from the home improvement store. The choice of adding legs or setting up on a table top is totally up to the builder’s preferences and requirements.

Weighing half as much as a conventionally framed table style module due in large part to the use of lighter materials, Spar and rib construction does sacrifices neither rigidity nor durability, while still remaining easy to assemble with common house hold tools and easily sourced materials.

Spar and Rib Benchwork

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