![]() The larger gears attach to studding, which goes up and down when turned in a captive nut on the RepRap carriage. The gears described here are the ones used to raise and lower the turntable. As the plastic is not that strong, this needs big, chunky collars to hold big grub screws, or in the case of this example a recess to hold a nut.īack to Index Designing Cog Gears In Art Of Illusion So that sets the absolute minimum width of a gear tooth with no shaping whatsoever.įinally, we need to attach some of the gears to a shaft. Flexing leads to wear and noise.Īlthough the FDM processes available will produce a theoretical accuracy of 0.1mm, the smallest individual feature that can reliably be reproduced is 0.6mm. Everything is still kept nice and chunky to avoid flexing. To lower the mass of the plastic used, we put strategic cut-outs in the gear to create spokes. We use smooth curves rather than angular jumps, partly because the FDM machine can't be sufficiently accurate, and partly because they'd introduce points of wear. We make the gears fairly thick, as the larger contact area lowers the contact pressure and hence the wear. In short, your gears will be chunky and inaccurate. Wear & tear is a fact of life, and so plenty of extra material is needed. Little pointy bits will soon be broken off when the mechanism (inevitably) jams. Gears made from ABS or Polymorph need to be sturdily constructed in order to survive. A bump on the large gear (eventually) hits every tooth on the small gear, wearing them all equally.Īn even uniform gear wear is achieved by ensuring the tooth counts of the two gears meshing together are "relatively prime" to each other this occurs when the Greatest Common Divisor (GCD) of each gear tooth count equals 1. GCD(15,25)=5 then divide this result into the number of teeth in question. To calculate this first find the Greatest Common Divisor (GCD) of each gear tooth count. A bump on the large gear always hits the same 3 teeth on the small gear, creating uneven wear in this same example a bump of the small gear always hits the same 5 teeth on the large gear. evenly across all gear teeth involved.īad: 15 and 25 teeth. This is so that the same teeth do not always press against one another, so distributing wear, dirt, oil and squashed fingers etc. Gears typically have prime (or at least co-prime) numbers of teeth. A little gear driving a big gear will increase the torque and slow down the speed of rotation. To provide mechanical advantage, contacting gears have different numbers of teeth. These problems are made worse by teeth flexing around, so with a relativley soft plastic we're going to have interesting problems. The forces between the contacting teeth have to be even and consistent, or vibration builds up. If the height is the same on both gears, then the pitch is the same, and if the width, height, and pitch are the same, then don't they have to be the same diameter? Ideally, the pitch is 2.25 times the height of the teeth. You want the pitch on touching gears to be as close as you can get it. The distance between the centre of one tooth tip to the centre of the next is called the 'Pitch'. The angle of tilt on the faces of the teeth is called the 'Pressure Angle' and is normally about 20 degrees. The bit above the pitch line is called the 'Addendum', the bit below it the 'Dedendum'. ![]() ![]() Contact between teeth is notionally along a point just over half way (typically a 1:1.25 split) up the teeth, and this is called the 'Pitch Line'. If that happens the next incoming tooth tip smacks into the top of the opposing tooth. The tip of the gear teeth are not supposed to go all the way down into the opposing trough. The incoming gear teeth must contact at the right place, and only at the right place - which is why they're a funny "involute" shape and not square or little triangles. They need to mesh, and so need to be the same height. The number of teeth and their shape are more important than is initially apparent. If you're an engineer, this is probably old hat - but read it anyway so you can correct my mistakes. So, before we get stuck in to the AoI nitty-gritty, here's an Express Guide to Gear Design. When designing gears for FDM fabrication in ArtOfIllusion there are two essential things to bear in mind: The basics of conventional gear design, and the accuracy or lack thereof in the manufacturing process. ![]()
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