Camwheel for sewing machine OpenScad

thingiverse

At "gered gereedschap" in Groningen we refurbish machinery tools and sewing machines for a second life in developing countries. This often reveals design flaws of machines built many decades earlier. While sewing machines made before the fifties are generally virtually indestructible because all parts are made from sturdy metal, later models are constructed using plastic for gears and camwheels. An old Husqvarna machine was sent to us for refurbishment. It contained a plastic camwheel that defined different stitches. The plastic was injection molded onto an iron center with a diameter of 3 cm. The plastic was between 3 and 5 mm thick and featured 7 distinct layers for the various stitches. The camwheel was made from PVC and due to aging and differing expansion characteristics of metal and plastic it had broken with a 0.5 mm gap, rendering it unstable when fixed to the metal core. Normally we discard these machines because a replacement camwheel is not easily available and if available the costs are prohibitively expensive. A good quality second-hand sewing machine costs between 10 and 30 euros and original parts are generally more costly. I wondered if such machines could be rescued by creating a 3-D printed copy of the wheel. This required reverse engineering of the cams. The camwheel rotates once for every 18 stitches. Each needle position requires space on the wheel to shift into position and space without movement during the period that the needle is down in the fabric. The 3-D CAD-CAM program OPENSCAD is ideally suited to design such regular structures. The cams are defined by extruding a contour around a circle with 36 subdivisions. When all the cams are created, the core part is subtracted.