Scratchbuilding a Bascule Lift Bridge: The Tower and the Leaf

Scratchbuilding a Bascule Lift Bridge: The Tower and the Leaf

Part 1:

Part 2:

Part 3:

Part 4:

Just to recap, so far I’ve shown how I’ve completed the base structure of the leaf and the rocking structure. Photo 15 below shows all that’s done so far, and the links above will tell you how I’ve reached till this point.

Photo 15: Front view of the bridge with the basic structure, minus the fixed tower.

The Tower

This part of the build is important from a structural standpoint since all of the moving components witll be attached to this structure. As mentioned before, the base of the tower is 60mm X 40mm X 10mm (2.36” X 1.57” X 0.4”). Looking at the drawing you can see that the vertical side of the tower needs to be 70mm (2.75”) in height. My first job was to cut two strips of 80mm (3.15”) in length and fasten them in place.  10 mm (0.4”) of these beams are glued to the base and secured with pins as shown in Photo 16. I also added a 40 mm (1.57”) stabilizing cross support beam in between these two vertical beams towards the top (10 mm/0.4” below the zenith). This makes the basic structure of the tower. One important note is that since the tower will act as the rigid frame of the structure and the frame of movement of the moving parts, it is important to test the assembly in each major step. Photo 17 shows the testing of clearance of the tower and the rocking truss.

Photo 16: Starting point for building the tower – overall height 80 mm (3.15”).
Photo 17: Quick testing of the tower and rocking truss assembly. Notice the paper pins protruding on top of the tower. I tested the clearance and how freely the rocking truss moves at this stage.

In Photo 18, you can see the diagonal end posts being installed. Note that it is extended beyond the top face of the base to the entire side face of the block to ensure the stability of the structure.

Photo 18: The diagonal support posts after installation.

Notice that the structure has started taking solid shape now and all basic references are in place. As you see, the actual construction looks slightly different from what you see in the drawings. These deviations are to ensure better strength, rigidity and reliability of the structure, parameters that unfolded only during the building phase. Since we are dealing with wood at 1/160 scale, it is permitted to make slight deviations from your drawings to ensure a better long lasting bridge. However, make sure that the basic and critical dimensions match with your initial design; or you might end up having alignment issues in the future.

Now, with the basic structural components in place, I will demonstrate (through photos) an accelerated process for completing the rest of the details without going back to the drawing for every step. If you have maintained your dimensions accurately to this point, the rest of the sides will fall into place automatically. You will see this in depth as you complete the tower as shown in the step-by-step photos 19 – 39.

Photo 19: Checking the distance from the top surface of the base to the top surface of the top cross support beam, 60 mm (2.36”). Mark the mid-point.
Photo 20: Mark 30 mm (1.18”) from the top surface of the base on the other face of the vertical beam.
Photo 21: Mark 30 mm (1.18”) from top of the base on the slanted/diagonal end-post.
Photo 22: Draw a straight line joining the two and extend over the full side faces of both the beams. This will be the centerline of the side cross support beam.
Photo 23: Draw 2 lines, 2mm (0.078”) apart from the centerline of the cross support beam on both posts. Put some stick-glue on the area between the lines.
Photo 24: Temporarily paste a wood strip longer than the overall length on the area between the lines, perfectly aligning with the upper and lower lines.
Photo 25: Reverse the structure and mark the inside edge location of the vertical beam on the new cross support beam with a sharp pencil.
Photo 26: Repeat the same method from Photo 25 on the slanted/diagonal end post side of the cross support beam.
Photo 27: Take out the wood strip and you see the markings on the cross support beam.
Photo 28: Cut them precisely with a hobby knife.
Photo 29: You will get a piece like this.
Photo 30: Test it in its designated position – it should fit perfectly and should align with the lines drawn on the side faces of the posts.
Photo 31: Use the new cross support beam as a template and cut another new one from another piece of the stripwood for the other side. Glue them in position; they should look like those in Photo 30.
Photo 32: Once the glue is dried, mark the intersection point of the lower line in the marking and the inside end point of the vertical beam (Marked in red in the picture for demonstration).
Photo 33: Take a long wood strip and join the two points using one edge of the strip. Use stick glue to secure it in position temporarily.
Photo 34: As shown in Photos 25-29 for the cross support beam, mark the edges and cut the new beam precisely; then glue it in position permanently.
Photo 35: Repeat the steps in Photos 32-34 on the other side; glue the completed beam in position permanently.
Photo 36: Use same measurement and marking techniques to complete the top cross supports for both the vertical and diagonal beams.
Photo 37: Test fit the rocking truss and make sure the movement is free. If not, use fine emery paper/wood file/nail file to smooth the contact surfaces until the movement is free.
Photo 38: Mark and make the base cross support beam. This piece does not have to be 4 mm (0.16”) in width; it can be anything between 4 – 10 mm (0.16”- 0.4”) wide. Use the width that best suits your supply from your cut out wood strips.
Photo 39: As a final step, use the methods mentioned before (or simply use a ruler and a pencil) to measure the height of the vertical cross support beams for both sides, cut them from the supply of stripwood and glue them in position. This completes your tower.

Completing the Leaf (Deck)

If you’ve following along, you’d know that I took a little deviation from completing the leaf and started working on the more critical structures.  The reason being, they are that critical.  The basic structure of the leaf was done previously as you have seen in Part 3.  I wanted to make sure the complicated components were complete before returning to the leaf. This ensured that I could mock up the overall project as I progressed, and cross-reference it with the drawing from time to time. The joints in the leaf also got time to get seasoned and securely held by the glue and the pins.

I used the same accelerated method that I used for the tower to measure, cut and affix the diagonal support beams of the leaf. As long as the square joints are proper, this is not a problem. The diagonal supports on the sides were easy, however, the ones at the top and bottom had to be individually beveled to achieve a snug fit. Photo 40 gives you a good idea of how this looks upon completion.

Photo 40: Completed ‘Leaf’ (deck) of the bascule bridge with all the diagonal support beams in place.

Here are the some other photos of the completed components so far.

All 3 major components are now ready for assembly.

Now that the components are ready, it’s time to star assemblying. If you subscribe to the blog via email, the articles will be delivered directly to your mailbox.

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