Making Urethane Belts for Automaton Projects – Dug’s Tips 10
March 7, 2013 by Dug North
In the last article, we looked at how to make pulleys for automaton projects. We also made belts from O-ring gasket material. Custom-sized belts can also be made from urethane belting material. Joining the ends of this material is one of those jobs that seems to require three or four hands, but it doesn’t have to. Let’s take a look at an easy method that produces a strong welded joint.
A length of urethane belting suitable for your automaton project
Cork from a wine bottle
Butane micro torch
Abrasive cleaning pad such as Scotch-brite
Single-edged razor blade
Small flush cutting pliers (optional)
This project is all about the tools, materials, and preparation. With that in mind, let’s take a look the most important components.
Urethane belting material is sold by the foot, so you must connect ends of pieces to make the circle you need. Fractional inch diameters are clear or orange in color. The metric sizes come in green. The clear and orange varieties are smooth, while the green has a fine textured surface. For most automaton projects, either type will work fine. Unless you are making a large piece, you’ll probably want to use the smaller diameters offered, such as 3/32″ and 2mm. Both of these sizes are rated for a minimum pulley diameter of 3/4″ (about 19cm). Use a single-edge razor blade to cut the belting to length, making sure the ends are flat and square.
To weld the belt, the ends must be melted. To keep the ends flat as they melt, we will press them onto a hot piece of metal. I use an old kitchen knife that I’ve dedicate to this purpose. The knife should have a flat steel blade and a handle with flat sides that can be held securely in a vise. I’ve covered the tip of the blade with the cork from a wine bottle for safety. The tip could also be ground off. The knife works well, but any thin piece of steel that can be mounted in the vise will also suffice. The metal should be cleaned before each welding operation. Scrape off any residue left over from previous use, and scrub both sides with an abrasive cleaning pad.
To heat the knife, I use a small butane torch because it produces a clean, hot flame, and stands on its own at a suitable height. These torches are also handy for soldering metals. A candle or alcohol lamp can be used instead of the torch, but it will take longer to heat the metal to the proper temperature. The melting urethane will give off some fumes that are best not to breathe, so be sure to do this in a well-ventilated area or outside.
Any type of vise that can rest on or be mounted to a table will do. Mine is meant to be used in conjunction with a pillar drill (drill press). The vise will be one of those extra hands you need for the job.
Collect all of the tools and supplies and place them on a workbench or table. I recommend sitting down for the task. This allows you to rest your arms on the table for stability. Clamp the knife handle in the vise so that a portion of the blade is at the same height as the torch. With your arms or elbows resting on the table, position the vise so that the knife is located near your hands. Direct the micro torch away from you, centered on the edge of the knife blade. Place your soon-to-be belt on the table between the vise and the knife.
Start the butane torch and direct the flame straight at the edge of the knife blade. It only takes about 15 seconds for this part of the flame to heat up my knife sufficiently. If your knife is larger or smaller it may take more or less time. The metal is hot enough when the very edge of the blade starts to glow orange. Don’t expect the metal to turn orange if you are using a candle as the heat source. When the metal is hot, turn the torch off and place it well out of the way.
Grip the belt ends about 1/2 inch (or 12cm) from each end, one in each hand. Bring the two ends toward the blade so that their ends will touch the surface squarely on either side. The belt ends should be a mirror image of each other during this process. Apply a little pressure as if pressing the ends together through the metal. You will immediately see and feel the ends of the belt start to melt and splay out, forming a ring or a trumpet-like shape. This tells you the ends have melted all the way through to the core of the belting.
Immediately after you have seen the belt ends spread out, keep them pressed against the metal and carefully slide them in unison toward the edge of the blade. Continue to slide the ends until they are completely off the blade and touch end-to-end. You have just a second or two — no more — to slide the ends slightly so that they are in perfect alignment. Continue to lightly press the ends together. Now hold still and count to 100.
After you are done counting, inspect the weld. It may look pretty ugly with a lot of extra plastic around the joint. We will deal with that in moment. Check that the belt forms a smooth, continuous curved form. If there is a big offset step where one end meets the other, you may want to cut the belt apart and start again. If it looks good, place the belt on the table and leave it alone for 20 minutes.
The last thing to do is trim any extra melted belt material off of the newly welded joint. The easiest method I’ve found is to use a pair of flush cutting pliers, normally used to cut wire. Use the pliers to nip off the excess material, being careful not to accidentally cut into the belt itself.
If you are careful, you can also use a single-edged razor blade. Straighten out the belt on a protected flat surface. Hold the razor blade parallel to the belt and slice away the material that is outside of the belt’s true diameter.
One last approach advocated by some is to grind the excess material off with a coarse wheel on bench grinder. This method works, but it’s not without its risks. As a rule, you should keep any form of loose material such as clothing, hair, and jewelry away from spinning machinery. A wise person might consider belts to fall into this category too.
Dug’s Automata Tips, Techniques and Tricks
A quarterly column by automaton-maker and enthusiast Dug North
Copyright 2013 Dug North
Warning: The topics covered in this column include the use of tools and materials that have the potential to cause damage to property and/or bodily injury. Your safety is important and it is your sole responsibility. Always read and follow the safety instructions that come with tools and materials you use. Wear safety glasses, use guards and other forms of safety equipment, follow safety precautions, and use good judgment. Seek the guidance of experienced outside sources whenever required.
Osborne, Albert H. (1973) Welding Plastic Belts. Bulletin of the National Association of Watch and Clock Collectors, 15(165), 1347-1348.
Perkins, Archie B. (1982) How to Use the Modern Watchmaker’s Lathe Part III, Horological Times, November, 23 and 37.
Tope, John (Producer). (2005) Clockmaker Watchmaker Lathe Basics [DVD]. United States: Tick Tock Productions.
Urethane Round Belting, 1/8 inch Diameter, 10 ft Length, Clear
Urethane Round Belting, 2mm Diameter, 10 ft Length, Green