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3D Printer high grip string drive

Posted by Bob Wood on
3D Printer high grip string drive

I have designed and built several CNC routers and added CNC to a Grizzly desktop mill. They all had one thing in common: that is ACME threaded rods for precise movements. Once I decided to build my first 3D printer I knew I would not need the force carrying acme screw, so I looked into belts and strings. Belts seemed to be an obvious choice, but they do have some issues. For me, they do not seem to be a cost effective way of moving the small mass of an extruder. I purchased some braided fishing string and started designing. At first, I used the common method of drilling a hole thru the steel or plastic coupler, then wrapping 8 to 10 turns for each direction. This works, but the sting takes up too much room axially. Fuel line or rubber hoses work well for a few printed parts but wear quickly as the string constantly is making small cuts into the rubber surface.  I have a 4th axis on my Bridgeport mill, so I make a “threaded” v-groove aluminum coupler. It worked better, but the grooves with the bit I had were too wide to grip well. This got me thinking… what if I used a threaded rod. What groove angle would I need for a given string diameter? From there, it was pretty simple. I settled on a 7/16-20 threaded rod with 0.54mm string diameter. It has an awesome grip with only 3 to 4 turns, and it is fairly simple to make on the lathe.  Recently, I received some customer feedback which stated “I was admittedly somewhat skeptical of the fishing line and screw ‘high grip’ drives. But holy s!#t, they work extremely well, and I find totally silent.”There might be other combinations that will work, and it would be easy to try a few just by taking your string to the hardware store and wrapping it around a few bolts.

The next issue to solve is how to keep the string tight? It makes sense that we should use a fishing knot since we are using fishing string. I watched several YouTube videos and found a knot that does not creep. It is named the Uni-knot. Ensuring that I kept the design simple and cost effective, I decided to try nylon zip ties to keep the string tight. They have several features that make them the right choice. First, they have a hole so that it is simple to tie a knot. Second, they lock into place and are designed to lock into position. Third, with 2 of these ties one can make an adjustable tensioning string drive for a 3D printer, Laser engraver,  or any other low mass CNC machine movement. And lastly, they are cost effective. I use the 40 lbs zip ties, and I have a video on YouTube that shows a working model, or you can download the manual.

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