A Bridge Too Far
by W. Ambrose "TheBugKing" Coddington

Rob at Privateer Press contacted me about building a working drawbridge. At first it sounded like it was going to be something built into a board section or other piece. He came back with a "We need it modular to make it on a flat base OK?" statement. Well this increased the difficulty of the piece considerably.

After some brainstorming on how to cram all the necessary mechanisms on to a flat board with nowhere to hide things I set to work.

Materials Needed:
Square brass tubing
Rectangular brass box tubing

A section of round brass tubing sizes that nest well
5/32" brass rod
Lead free solder and flux
A selection of gears. Preferably ones that have the "Iron Kingdoms" look
Plastic "I" beams of various sizes
Diamond plate plasticard (an 8-1/2" x 11" sheet will be enough)
1/16" plasticard
A selection of plastic angle pieces
A selection of plastic box tubes
1/32" plasticard "metal roofing"
1/8" masonite or hard board
Plaster of Paris
A medicine bottle or small bottle
1/16" x 1/2" balsa wood
1/16" x 1/16" balsa wood
P3 modeling putty
Plastic glue
A two-part mold making medium to cast copies of your gears if needed
A two-part casting material to cast copies of your gears if needed
A battery bracket that will hold 6 AA batteries
Low voltage wire
A 12v DC motor from a model train turntable power source

2 double pole single throw micro switches
1 double pole double throw switch
Thin card

General Safety Notice:

Always wear eye protection and operate power tools in the safe and recommended fashion. When using knives cut away from yourself. When using noxious chemicals be sure that there is adequate ventilation.

Begin by casting copies of your gears if you do not have enough. There are several companies that supply mold making material as well as casting material. Be sure to purchase casting material that is relatively hard as the gears will see a decent amount of wear. (1A, 1B & 1C)

Once you have 6 small gears, 5 medium gears and 2 large gears plot out how the gears will mesh with each other. Reduce the gearing from medium to small, then medium to small to large. This will give the bridge enough mechanical advantage to allow the bridge to work.

Once the gears are laid out draw a rectangle around the gearing that will be the brass box tube frame. (1D)

Measure and cut the rectangular brass box tube to build the frame. (1E & 1F)

Once all the pieces are cut, sand them so that they are the same length then scuff up the ends where the pieces will be soldered together. Use clamps to hold the pieces in place being sure that they are square and solder them together. (1G, 1H, 1I, 1J, 1K & 1L)

Once the rectangles are assembled measure sections of the square brass box tube to fit in inside the rectangle. (1M & 1N)

Cover two opposite sides of the box tube with black marker and scribe a line down the center of the piece. Then lay out where the hole will need to go. Drill a hole with the power drill being sure to be exact on each side of the box tube. Do not try to go all the way through as the holes will not line up properly. (1O, 1P & 1Q)

Slide several pieces of square tube over a piece of brass tube that will fit your solid bras rod piece. I chose to do six at once. This will keep the brass box tube section square when they get soldered. Once the pieces are soldered but them off and sand the brass tube flush. (1R, 1S & 1T)

Lay the brass box tube sections and the rectangle piece over the drawing to locate where the pieces need to be. Solder them in place. Complete four sections. (1U & 1V)

Cut four sections of brass rod with two about 1-1/2" long and two about 2" long. Then glue a medium gear in the center of the 2" long pieces and a small and medium gear to the center of the 1-1/2" pieces (1W).

Cut the large gears so that only 40% of the full radius remains. Lay one leg over a piece of large plastic "I" beam and use a small piece of "I" beam to figure out the height of the bridge rail (1X).

Clamp the large "I" beam to the bench and lay out and glue the smaller "I" beam sections in using 45 degree angles. Check that the large hear lines up well. Then cut a strip of plasticard about 1/2" wide and lay it over the bridge sections. Glue the pieces in place.

Use a leather punch to make rivets and glue four to each angle brace so that two each are over the small "I" beams. (1Y, 1Z, 2A & 2B)

Cut four pieces of large plastic box tube for the bottom bridge braces. Glue them between the two bridge rails evenly with two pieces making up the ends. (2C & 2D)

Cut a piece of diamond plate plastic card and glue it onto the surface of the bridge. Turn the bridge over and glue in small "I" beams between each square box tube to function as additional bracing and support. (2E & 2F)

Add small angle bracing along the bed of the bridge to mast the join with the large "I" beam. Then use superglue to adhere the large gears to a large piece of brass tube. Then affix the assembly to the rear of the bridge. Use plastic glue to add a piece of plastic card to the bottom of the large "I" beam under each gear and use superglue to adhere the gear to the plasticard piece. This will ad necessary support to the hinge mechanism. (2G, 2H & 2I)

Insert a brass tube that fits just inside of the gear assembly. Be sure that about 1/2" sticks out on each side. Cut two pieces of the larger brass tube. Carefully solder the larger pieces onto the smaller tube. It is extremely important that you do not let the solder touch the hear assembly or the bridge will not work (2J).

Solder two gear towers to the brass tube that was just assembled. This is a very difficult step. It is imperative that the gears on the bridge line up with a small gear on the top shaft. It may make sense to trial fit the pieces before you take the step to solder the joint. It is also important that the bottom shaft clears the tube or the far hear tower will not work. Once everything is lined up, solder the joint (2K).

Paint the bridge. Start with a coat of Pig Iron. Then mix Khador Red Base, a touch of Sulfuric Yellow, Umbral Umber and a touch of Quick Silver and stipple the mix onto the bridge and over the gear towers. Begin to add more Sulfuric Yellow to bring out a nice good mottled look to the piece. End with a bit of Armor Wash to add some depth and shadow in a few places (2L).

Paint the gears in the same fashion as the bridge. Then fit them to the bridge. The lower shaft should stick out from the bridge about 3" o the near side and 1" on the far side. Slide and glue small gears in place on the lower shaft being careful to not get any glue the gear towers. Slide and glue small gears onto the top shaft so that they lineup with the large gears on the bridge. Finish by painting and then sliding the two remaining gear towers into place. (2M & 2N) 

Solder the lead wires onto the nominally closed poles of one of the micro switches. Glue two large plastic "I" beams to the gear towers so that they contact each tower evenly. Then tack the micro switch into place in the gap between the two "I" beams so that the switch is struck where you want the bridge to be in its lowered position. (Try for level) Be sure to use very little superglue as the fumes will oxidize the micro switch in a permanent fashion once you are sure that the switch still works. (2O, 2P & 2Q)

Remove the plastic shed from the turntable gear and motor. Then cut off the extra plastic. Finally mount it and the bridge to a piece of hard board so that there is a small gap between the worm gear on the motor unit and the medium sized gear on the bridge. Add two large plastic "I" beam pieces to further stabilize the motor unit. (2R, 2S & 2T)

Solder lead lines to the other micro switch and mount it so that the large gear strikes it when the bridge is in the up position. Again, use P3 modeling putty to adhere the switch in a permanent fashion as superglue will damage the switch. Then take two medium sized brass tubes, bend them 90 degrees and thread the lead wires through the tubes. Finally, mount the tubes to the base. (2U & 2V)

Cut the threads off the medicine bottle and cut a slot for the double pole double throw switch in the top. Then cut slots for the battery leads and the leads to the switches. Solder the circuit together so that one pole has the positive going through the down switch, to the motor and back to the negative pole. Solder the other side in reverse. Test the circuit to be sure it works. Insert the switch into the slot in the medicine bottle and mount the bottle so that the battery pack and the switch leads are not obstructed. Finally add some P3 modeling putty to the top of the bottle and some detail to the sides as well to mask the bottle look. (2W, 2X, 2Y, 2Z & 3A)

Cut several pieces of balsa wood so that they are longer then the battery pack as well as several pieces of 1/16" balsa that are taller than the pack. Scrub the pieces with a wire brush to bring out the grain and weather them. Glue them together. Finish the top with a larger piece of balsa. (3B, 3C, 3D, 3E & 3F)

To fabricate the door, use a thin piece of card, edge it in 1/16" balsa, add to thin strip of plasticard across the middle and a few pieces of brass pinning material for the hinges and door handle (3G).

To fabricate the roof, take thin strips of plasticard and glue them vertically to another piece of plasticard (3H).

Cut balsa to create the pith to the roof and add some plastic pins so that the roof will attach to the building. (3I & 3J)

To build the ramps, measure the bridge to get an elevation and then build a frame out of plastic "I" beams. (3K, 3L & 3M)

To make the tiles on the base, cover the base with about a 1/8" thick layer of plaster of paris. Wait 'til the plaster sets to the touch. You will be able to tell by putting your finger on the surface and it will fell wet but firm. Scribe in a tiled pattern with a P3 sculpting tool and allow the plaster to dry. Once it is dry re-scribe the tile lines and sand the surface smooth. (3N, 3O, 3P & 3Q)

To color the base, wash it with a 50% diluted mix of Thamar Black. While that is wet drop in some diluted Umbral Umber. Allow that to dry completely. Then mix Thamar Black, Umbral Umber and progressively more Morrow White and drybrush the surface tiles.

To finish the piece add rivets where they look good. Add diamond plate plasticard to the ramps and mount them. Add some angle plastic to the tops of the gear towers to finish them and add a card and brass tube chimney to the switch to mask it. Add some ballast around the base of the bridge and anywhere that needs a bit of ground cover for interest.

To paint the verdigris copper, base the piece in Solid Gold, then wash it with Blighted Gold. Then dribble Green Ink, Turquoise Ink and Armor Wash from the top and down the sides. Keep adding layers until you get a desirable result.

To paint the shack, base it in Thamar Black and drybrush progressive shades of Umbral Umber and Morrow White to highlight it up. Paint all other metal parts with the same process as the bridge.

Have Fun and Good Luck!

Ambrose "TheBugKing" Coddington