Working with Thick Fibers

Thick fibers are some of the most interesting and useful materials for robotics. They include tubes, thick elastics, plastic optical fibers, and resistive stretch sensors. A fiber that is too thick to pass through the needle can still be used as bobbin thread; others have developed a sewing-based method to cast wires and tubes into polymers using them as bobbin thread in a regular sewing machine, with water-soluble thread in the needle. When the bobbin thread is much thicker than the needle thread, the thicker bobbin thread stays on one side of the material, which is an advantage for systems like fiber optics that need to avoid sharp bends.

This fiber has a diameter of 0.5 mm. It was dispensed from the bobbin and is too thick to get pulled through the material by the white thread, so it stays on one side in an embroidered pattern.

Couching Example

However, fibers thicker than about 1mm diameter begin to get caught in the bobbin case. For scale-up, these thick fiber patterns can be transferred to textiles or water-soluble stabilizers using industrial machinery, for example, the large-scale specialized embroidery systems that install heater wires in car seats. There is a path to scale-up, but robotics labs need rapid prototyping alternatives. To capture these fibers using sewing, we can run a zig-zag stitch over them. The zig-zag can be made from a permanent thread or a water-soluble one; soluble threads allow you to transfer your thick fiber into a resin or silicone. We illustrate two methods: sewing with a couching foot and templated embroidery.

1. Sewing a thick fiber with a couching foot

 Most regular non-embroidery sewing machines can be equipped with a couching foot. This accessory usually costs less than $10 USD and has a guide for one or more fibers. You set the machine to do a zig-zag stitch wider than your thick fiber, and the couching foot takes care of putting your fiber(s) at the center of the stitch so they don't get pierced. The disadvantage of this method is that it does not work with an embroidery machine's X-Y stage, so if you want to install the fiber along a complex path you have to steer the needle along the path by hand.

Couching foot

Couching example

 

2. Trapping guided fibers under a zig-zag stitch using an embroidery machine

 In this section, we describe how to attach a thick fiber along a path in the embroidery machine. This method allows the fiber to take computer-controlled curving paths. The Inkscape plugin can generate a zig-zag stitch along your path, so the problem becomes aligning the fiber’s temporary pattern to a pair of registration marks.

 A simple way to do it is to print out the path and marks on a transparency. Then, use adhesive (an Elmer’s double sided tape roller works well) to lay down the fiber along the path manually. If it’s being transferred into a silicone, it may not be necessary to capture it in a zig zag stitch; just pour the silicone on top. The double sided tape was used to manually route a polyurethane optical fiber (1.8 mm Stretch Magic) along a path traced on a styrene sheet.

Elmers

The tape is clear so you can see the guide lines underneath, and sticks well to urethane. Silicone was poured on the urethane fiber to encapsulate it; the final device can be used as-is or peeled off. The tape did not interfere with curing of Wacker M4641 silicone.

However, laying down a fiber on traced or printed lines can lead to a slow and hard to reproduce alignment process. And, the sticky tape won’t work as a permanent part of a stretchable device. We developed small 3D printed cleats to guide the fiber along with a process to transfer the aligned fiber to other materials. These guides stick through laser-cut holes in a water-soluble adhesive sheet as shown:

Sticky1

Top: StickySolvy laser cut with alignment marks. The backing is partially peeled off. Bottom: a 3D printed guide that works with the sticky sheet. Both the 3D printing file and laser cutting file are available in the Downloads section.

Sticky2

The StickySolvy sheet is laid on the template sticky-side up so fiber can be guided on a curving path in registration with the alignment marks.

Sticky3

Sticky4

When the StickySolvy is removed from the template, the fiber and alignment marks go together. The sticker can be attached to a soft material for capture by an aligned zig-zag embroidery pattern following the procedure in the previous Alignment Process section. 

Sticky5

 

3. Modular method for making large-area fiber alignment guides

What if you want to use the guide method for thick fibers, but do not have a large enough 3D printer (or enough time) to print a base the same size as your embroidered pattern? We are developing a modular method that works with a laser-cut base and drop-in 3D printed octagonal pegs to hold the fiber as it is being attached to the sticky embroidery backer. The advantages here are that it is very fast to laser-cut a wood or acrylic template, and the small 3D printed pegs are reusable from pattern to pattern.  Then the backer can be placed in a hoop and embroidered using the methods above.

3D pegs

pegs1