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This is one of several Instructables that cover the building of a 1:160 (N) Scale Model of a Bascule Style Railroad Draw Bridge. This Instructable addresses printing and assembly of the drawbridge lift mechanism and it's integration with the bridge component. There are two other Instructables with one addressing the printing and assembly of the bridge and the other covering the electronics. There are also two articles on my blog that are worth reviewing (first article and second article).

The irony of my designing this bridge is my total lack of an N-Scale Model Railroad Layout! That has not stopped me from enjoying the project though and hope to see others build the bridge and put it to use. One person is doing so and has shared this gallery of photographs from his build. They are all worth a look but the ones that are most salient will be linked into the appropriate page of this Instructable.

Step 1: Assemble the Needed Tools

In no particular order relative to the photograph here are the tools that you will be needing:

  1. Super glue, and optionally, some accelerant. I find the latter to be really handy stuff though I know that it detracts from bond strength. Your call!
  2. Small pair of needle nose pliers.
  3. Small pair of wire clippers.
  4. Philips and regular screw drivers.
  5. Small detail files, at a minimum a flat one and a round one (maybe some fine sand paper as well).
  6. Tweezers.
  7. Torch style lighter (or kitchen torch or shop torch or soldering iron with blade tip though may be too hot)
  8. Some kind of tool that can be heated with the above and then used to melt the end of a retaining pin.
  9. High temperature oil to be used with the above to help plastic to not adhere.
  10. Craft knife, known to some as an Xacto Knife

Step 2: Printing Parts for the Drawbridge Lift Mechanism

Picture TBD. Model files are on Thingiverse here.

If you are using a 3D Printer for N-Scale features you should have a smaller nozzle than the standard 400 microns given the improved detail that smaller nozzle will deliver. That said the design of the draw bridge mechanism is such that all parts can be printed with a "standard" nozzle though there is one where an alternate design is available for the larger nozzle (AFrameBrace).

Unfortunately the bridge component is not printable without a smaller nozzle (ideally 250 microns). More about this elsewhere.

Note that I have assembled the bridge without painting first. I suspect that this is not the optimal approach but I was in a hurry to get this written.

Step 3: Install A-Frames

There are two of these with features facing outwards. There is only one way they fit so go for it!

Step 4: Build the Counter Weight

The counter weight in this model is actually functional and needs some additional mass to balance the bridge and track. I used GBP two pence coins. Glue the top to the bottom once filled. If I was doing this for the best possible modeling result I would use some putty at the join at this point.

Step 5: Attach Links to the Counter Balance Cars

There are two cars that ride up and down the rail as the counter balance and bridge move. There are two links that need to be attached to each of these cars using a technique where the end of the pin is melted to secure the pin.

I have tried three different techniques for these pins:

  1. 1.75mm filament - Requires a melt at both ends so is not as cosmetically appealing - though practice helps!
  2. Printed pin - Vertical - Has a nice end but the shaft will likely be irregular and strength will be less given orientation of layers
  3. Printed pin - Horizontal - Shaft will be smooth but the end will not be a complete circle.

I think that 1.75mm filament, and practice doing a nice melt, are the best answer but I am not sure!

The technique that I use is as follows:

  1. Heat the instrument that you will be using until it glows. I would not use a craft knife as it does not retain heat well. A screw driver will work nicely.
  2. Wipe the heated instrument on a folded paper towel soaked in the high temperature oil.
  3. Melt the end of the pin.

I would recommend some practice before you do this on parts that you have spent hours printing!

Anyway...apply the above technique, using the pin style of your choice, to attach two links to each car! The link should be attached on the long side and the contoured side of the link should face the shorter side as shown in the pictures.

Step 6: Attach Counter Weight to Top of A-Frame

First glue the two mounting arms to the counter weight with the pin shown being loosely inserted as it will be temporary. Once the glue as set you can hang the counter weight using pins and melting them as previously discussed.

Adding weight to the counter balance will help with the bridges operation!

Note the orientation of the mounting arms relative to the weight and how they will be mounted on the A-Frame!

Step 7: Load the Cars on the Inclined Rails and Install on A-Frame

Slide the cars onto the rail noting the orientation of the car relative to the angle of the end of the rail and also noting where the hole is for the push bar (below the rail). Do a dry fit to ensure that everything mates well. The end of the rail should fit snuggly into the slot provided on the base and the top of the rail should clip neatly onto the far leg of the A-Frame.

It is crucial that the cars slide smoothly and freely up and down the inclined rail. If they do not then make sure they do!

It is important to get the right positioning of the inclined rail on the A-Frame by doing a dry fit and measuring or marking prior to using any adhesive!

Once the car and rail combo are mounted on the A-Frame the top link on each car can be attached to the counter weight as shown in the picture. The link needs to go inside the arm coming from the top of the A-Frame. My retaining pins fit into the counter weight with friction and did not require glue. I got lucky.

There are three strategies for accomplishing this connection. Print a pin that fits with the right amount of friction. glue a piece of filament into the hole before attaching the counter weight to the A-Frame. Attempt to glue the pin during the assembly process. Your call!

Step 8: Attach A-Frame Brace and Platform

As shown!

Step 9: Attach the Lift Mechanism to the Sub Base

The sub base will house the stepper motor that will move the bridge and will be attached to the bottom of your table. Secure them as shown.

Step 10: Attach the Bridge to the Lift

There is not much to say about this step other than it is very fiddly. Attach things as shown. Be careful not to break any of the delicate bits on the bridge! The two remaining arms from the cars should be attached to the bridge as shown.

Note that the bottom of the bridge is secured with 1.75mm filament and the top with 3D Printed pins that were done in the horizontal orientation.

Step 11: Attach Stepper Motor and Open End Stop

This is one of the most important steps of the project...and one of them that is also quite fiddly!

First you need a drive shaft that is 80mm of 4mm threaded rod. Then you need to attach that drive shaft to the stepper motor in one of two ways:

  1. Thread two nuts onto the bottom of the drive such that they lock. The best way to do this is to use a counter rotating nut but you can do it with force as well. Maybe with a dab of epoxy. Then cut a piece of shrink wrap that will fit over the nuts and your drive on the stepper motor. Note that most stepper motors in the form factor we need have a gear attached. Finally, shrink the heat wrap. Now pull the shaft off the gear. The final tightening with the cable tie will come later.
  2. Use the 3D Printed mount that I designed. Insert two M2 nuts into the mount and then screw in two M2 bolts and slide over servo gear and tighten the appropriate bolt.

Once the above is done get the pusher bar and insert an M4 nut into the space provided. Insert the pusher bars through the bottom of the base with the bolt holder oriented down. You should have done a dry fit of this to ensure that everything is smooth. If not, do so, and if not smooth, make it so!

Now the fiddly bit. With the pusher assembly as far up as it can go insert the drive shaft and turn it until the nut engages and the shaft has moved enough that you can slide it into the coupler for option 2, or over the gear for option 1, from above.

If you are doing option 1 you can now use the cable tie on the part of the shrink wrap over the gear to secure the drive shaft to the stepper motor gear. If you are doing option 2 then tighten the two set screws.

At some point the ends of the push bars should be glued into the holes provided for them on the cars.

Finally, attach the open end stop as shown in the picture. The mounting holes allow for adjustment so please use them such that the switch closes just before the cars hit the base!

Step 12: Attach the Closed End Stop

Now we can attach the end stop that is triggered when the bridge is fully closed. There is a little button that can be inserted into the sub base that will extend up and engage the bridge as it descends. The bottom of this button depresses the switch and should do so as the bridge makes contact with the sub base. Adjust the position of the switch to make it so.

Step 13: Attach Bridge Open Guides

There are two small guides that should be attached as shown in the picture to ensure that the bridge descends into the right place...to trigger the closed switch and to make sure that tracks align.

Step 14: Last Things

The bridge should now be ready to connect to the electronics for testing and final adjustments. Once this is complete it can be mounted to the underside of your layout using the mounts you can print. They will need to be attached to the sub base of the bridge at the height that is appropriate for the thickness of the board you have used for your layout.

Note that the bridge shown is short enough that I was able to print an extension with it that joins to the rest of the sub base. This is neither needed nor possible with the larger bridges. You will need to attach the landing area for the bridge to the underside of your layout such that it is lined up precisely.

Brilliant! I love your ingenuity! I am an O gauge railroading enthusiast and would welcome your thoughts on applying these principles to a 1:48 model. Thank you and once again great work!
Thanks for the thumbs up...it was a fun project. O gauge, however, is big! Way bigger than possible for anything but a special purpose printer! Technically not an issue just one of logistics. The other problem is one of detail. My models would look crap scaled up as I have not designed in the kind of detail you would expect as they would not print at N-Scale sizes.

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Bio: I design, and occaisionally even implement, solutions that exploit single board micro-processors crossing the physical interface between the computer and the real world. Chosen platforms ... More »
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