On Saturday 30 May 2020 14:12:28 Chris Albertson wrote: > [image: image.jpeg] > > On Sat, May 30, 2020 at 7:31 AM cogoman via Emc-users < > > [email protected]> wrote: > > On 5/29/20 8:44 PM, Gene Heskett wrote: > > > In this case, openscad gives me the tools I want right now, and I > > > could even do a set of gears for stage 2 of the reduction since > > > that spacing is almost too close for a belt coupling. So by using > > > the correct tooth profiles it could make a very usefull gearset, > > > no belt at all, and a higher reduction at the same time. But would > > > those teeth be strong enough. Worth a try IMO. And a swarf cover > > > from the printer should be doable. > > Two points: > > 1) can you (you, not someone else) actually modify a downloaded gear > design using openscad? Try it Go get any random gear from SPD/SI or > Grainger and change the hub and face width and save as STL. I bet > this is not easy. It is nearly trivial in Fusion or Onshape or any > that work like them. Seriously, go through the steps using each > system. Downloading and installing 3 or 4 CAD systems takes days but > doing the same test project in each is the only way to see what each > can do. Downloading a gear and making a trivial modification like > (boring out the hub) then saving as STL is a good test case. > > 2) Yes, the TEETH would be strong enough to carry the force from any > reasonable size motor. Remember that a 1NM motor can only generate > 1NM of torque. What is the radius of the gear? lets guess at 0.1 > meter. If so then the force n the tooth is 10 Newtons (or in > barbarian units about 2.5 pounds) and this is in the worst case where > the motor is stalled at full current against a hard stop. > > When you slice the gear you can specify the wall thickness and the > infill percent and style. For gears of this size in PLA I'd go with > 1.6mm walls and skin and 50% infill density. Cutting those numbers > in half to 0.8 and 25% would still make a serviceable gear for a small > < 2NM motor. PLA is the most rigid plastic. It is not the toughest > against impacts but for driving a CNC machine rigidity matters more > than impact resistance > > I've been doing experiments with plastic gears. I can print small > ones down to module 0.5 but the percent error in tooth shape is very > high for small gears. For my self-drive car project I am using > module 2.0 spur gears with 24mm face width. These gears are huge but > the front tire will be hitting rocks and the computer will be making > fast steering corrections. These gears are very strong so that > "something else" will break first. > > But notice I said the TEETH are very strong. In a gear the stres is > greatest near the hub and minimal on the teeth. Larger diameter gears > have less force on the teeth then do smaller diameter gears. So you > always want to use the largest gears possible. If the gear fails, > it fails at the hub. It slips on the shaft after the keywhy shears > off of the threads holding the setscrew strip out. With PLA gear > slipping on the hub is a "death spiral" because slipping make heat and > heat softens the PLA and makes it slip more. And the hub to shaft > interfce is the highest stressed place. It fails if not > over-engineered. > > My solution is to make my plastic gears with a 24mm (or larger) > diameter borehole. Then turn a 24mm steel bushing that fits the > shaft and has set screws. he gear is press fit with epoxy to the > bushing. This way the hub is steel and will never fail and the > teeth never do fail. (Later I might try making hexagonal hub.) > > That said. When you design the gear, just figure that PLA s about > 1/3rd as strong as metal and give the plastic gear a wider face and > try to use module 1.0 size at least if you can. > > Finally one more thing: With plastic gears, you really can benefit > from using *helical* gears as more tooth area is in contact and they > slide better. It costs no more to print a helix than a spur gear. > Next, you think you don't want to deal with axial loads (side force) > so you use TWO helix gears to make herringbone gears. These have zero > side force, are self-aligning, and cost no more than spur gears. > MANY people doing gears with printers use this kind of gear. They are > expensive to cut in steel but cost no more in plastic. > > If you have a CAD system herringbone gears are easy. (1) find a helix > gear some place like Grainger or McMaster Carr, Download both the > left and right-hand versions. (2) in the CAD system stick them > together, side by side, (3) cut the face width as required, (3) bore > the hub to 24mm, (4) slice and print. Try doing the full exercise > using OnShape then Oopenscad then with fusion and see which works best > for you. (If you are a hard-core Linux user I bet onshape works best) > > As a further exercise try getting you CAD system to make g-code for a > 4-axis mill to make the same herringbone gears. Try using a tiny > ball-end cutter. > > Here is a photo. > > [image: images.jpeg] > > > Strength of 3d printed parts is a deep subject. PLA if done right > > can be pretty strong, but other filaments done right are stronger. > > > > I like this video showing how a veteran at 3d printing goes about > > refining his process for printing in nylon. > > > > https://www.youtube.com/watch?v=5MFX1whwjGg > > > > Here is a video where some guys test out different filaments on a 15 > > horsepower outboard motor. > > > > https://www.youtube.com/watch?v=scUEEQyC0GI > > > > > > Keep in mind, the odor will be much worse using any filament besides > > PLA, and most will need a hot end that can do the higher > > temperatures. > > > > Also with higher temperatures you might want to put a box around the > > printer to keep drafts from messing up a print. One guy uses grow > > tents around his printers. > >
All of which are very good points. Funny you should mention Onshape though, formerly Solidworks. I made the mistake years ago of downloading their so called freebie that had a 30 day free trial, then found it had zero help unless you had paid for it, and I was totally lost, never having dealt with any CAD like program before. So I deleted it, then spent the next year feeding their legal threats to spamassassin and replying to them by denying that a copy of it existed on any system I owned. To say they were insistent assholes about it is being way nicer than they deserved. They did every threat in the book short of sending it to a bill collector. I half expected to get a registered letter at anytime, demanding I cough up the $600 they wanted, but eventually they gave up. So you can guess how fat the chance is that I'll ever deal with those people again. > > > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
