Camwheel for sewing machine OpenScad

thingiverse

At 'gered gereedschap' in Groningen we fix machinery tools and sewing machines for a second life in developing countries. This exposes sometimes design flaws of machines made many decennia earlier. While sewing machines made before the fifties are generally virtually indestructable because all parts are made from sturdy metal, later machines are built using plastic for gears and camwheels. An old Husqvarna machine was given to us for refurbishment. It contained a plastic camwheel to define the different stitches. The plastic was injection moulded on an iron center with a diameter of 3 cm. The plastic was between 3 and 5 mm thick and contained 7 different layers for the different stitches. The camwheel was made from PVC and due to ageing and different expansion characteristics of metal and plastic it was broken with a 0.5 mm gap and therefore not anymore stable fixed to the metal core. Usually we discard these machines because a new camwheel is not easily available and if available the costs are prohibitive. A good quality second hand sewing machine costs between 10 and 30 euro and original parts are generally more expensive. I wondered if such machines could be rescued by making a 3-d printed copy of the wheel. This required reverse engineering of the cams. The camwheel rotates once for every 18 stitches. Every needle position needs space on the wheel to shift position and space without movement for the period that the needle is down into the fabric. The 3-D cad-cam program OPENSCAD is ideally suited to design such regular structures. The cams are defined by extrusion of a contour around a circle with 36 subdivisions. When all the cams are made, the core part is subtracted.