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« Last post by fester225 on April 29, 2025, 09:19:45 PM »
I have a 3D printer capable of printing 10" parts. I'm working on a design for a two-legged robot that will need leg sections around 24" long.
How do I attach 3D printed sections to each other in a way which will allow a total leg 24" long and give as much strength as the original sections?
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« Last post by artbyrobot1 on April 29, 2025, 08:14:34 PM »
Here is the tension spring in question from the previous post. I want this spring inside the tubing though which is not shown in the drawing of it. 
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« Last post by artbyrobot1 on April 29, 2025, 07:46:19 PM »
After further consideration, I'm scrapping using the elastic cord for a bracelet idea (as a tension spring for the winch in place pulley). The point of that was to use as little space as possible but I just don't trust it. I am not sure what material it is made of and my experience with rubber bands has always been dry rot issues. I am going with 2mm OD tension spring instead. It has to stretch 12.5" and so I'm using a 12.7" strip of it to start. That feels like a snug stretch but does comfortably reach the 12.5" of stretch needed. This brings its stretched total length to 25.2". I bought 3mm ID 4mm OD TPFE tubing to be its guidance tube for this. That arrives tomorrow and then I can begin assembly.
This 4mm OD guidance tube is a bit bulky and long for the arm IMO so I will relocate it to the torso since if I use this method for other motors these 4mm OD tubes will add up in space taken up fast. The arm can't house them - it's just too much space taken at that point for these. But the torso can house them in the back or sides I think. For now, since the torso is not yet attached, I'm going to place this tube ON the string suspended from my ceiling and treat that string as though it were to torso for now.
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« Last post by artbyrobot1 on April 24, 2025, 08:45:54 PM »
Update: I bought several sizes of kevlar string to use to replace the PE fishing line particularly in the high tension areas that may face the highest durability challenges. I found it on amazon in many sizes. I just bought one of each size: 0.5mm diameter 50lb test, 0.8mm diameter 100lb test, 1.1mm diameter 200lb test and 1.3mm diameter 300lb test. This is probably a game changer for my project IMO.
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« Last post by artbyrobot1 on April 15, 2025, 02:01:34 AM »
A note on fishing line durability. I think this was mentioned in passing quite a few times but at some point, a mention of durability concerns becomes that final mention that makes you really start to question it more and so I finally did some research on chatgpt and found out a human finger joint probably will actuate like 1-2k times per day which blew my mind since that would add up to like 10-30k times per month and so basically, once a month the main finger fishing lines will likely need replacement. I looked into alternative materials but didn't have much luck. So what to do?
Well after thinking about this a fair bit, my conclusion is to just shrug and move forward as it stands with the fishing line approach. I'll treat them as a consumable. My plan now is to just expect 1 hour of maintenance for every 20 hours of runtime. Or maybe to be more conservative, lets bump that to 1 hour of maintenance to every 10 hours of runtime? Maintenance will involve redoing pulley systems with fresh fishing line, or swapping in full new pulley systems to replace older ones every so often. It can have a pre-emptive maintenance schedule. My intention is that one robot will maintain his neighbor and the two will have a buddy system of maintaining eachother. Once I have expected time to failure of fishing lines established, they can swap in new ones automatically to prevent failures from happening during work times.
I don't think this is too bad of a deal or a deal breaker. Yes, hopefully materials advances will give me a better string one day, but for now, I'm okay with this maintenance scheduling thing. As long as its all automated, I think this is fine.
After all, our bodies muscles constantly need repairs and they grow from the repair process. So what do you expect for artificial muscles that can't self heal? Maintenance has to be a regular thing IMO.
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« Last post by artbyrobot1 on April 12, 2025, 09:31:51 AM »
Ok so I did a big refactor of my pulleys and ran a test again and it still is not working. The first set of archimedes pulleys tops out and can't move anymore while the finger still hasn't moved. This is because of slack in the lines. I did not calculate slack in the lines into my calculations at all and am shocked by how much there is... Rather than do a major new overhaul with new math and new draw distances on every pulley AGAIN, I'm going to just drop the final pulley of the system so instead of 44:1 it will be 22:1 now. While we cut half the grip strength with this move, this might be okay after all. It still gives us 20lb of of burst grip strength I believe and 11lb of casual easy sustainable grip strength. Most common tasks should only require 8lb of grip strength anyways for a single joint tops. Because remember, I'm not doing a single motor for all 3 finger joints but one motor per joint which helps alot in the strength department and control department. So anyways, this hack I think is okay also because it hit me lately that I highly doubt I'd use the full beast mode burst strength of a 44:1 downgear anyways. I'd be too worried about the wear and tear on the fishing lines and pulleys and maintenance times getting too short between maintenance overhauls if the robot is using that level of grip strength for tasks. In reality, I am now imagining I will only let the robot do VERY minimal strength stuff to reduce the maintenance to a minimum. Like sewing, cutting, and delicately picking up small loads. I will treat it like it has the strength of my 4 year old just to baby it and make it last longer between repairs. Kind of like having a old beater car you don't trust and never throttling the engine hard but just gradually easing on the gas pedal to avoid blowing a gasket so to speak and avoid a trip to the mechanic. So that said I think 22:1 might actually be okay. And with that final pulley out of the way, I'll have WAY more than enough draw distance to bend the finger 90 degrees and account for string slack AND account for string stretch over time without any issues at all. Much better. Not to mention we do pick up speed this way and that might be a VERY nice feature when all is said and done. A faster moving finger can speed up its work I think. Like notice how 3d printers go way faster and that speeds up prints. So speed might be king over grip strength in the end perhaps. It's a tradeoff.
Another update is I realized I can wind a second very fine 0.08mm fishing line on the output portion of the winch in place pulley mounted next to the motor and this second line coming off that pulley will be attached to a tension spring consisting of a bracelet jewelery making cord for jewelry for kids. This line will maintain tension on that winch in place pulley and the motor output shaft at all times to prevent derailments. The metal tension spring that extends the finger will then have the help it needs to keep the whole system taught. That's the plan anyways. The runout of this line will need to be 12.48" of tensioned draw. To achieve that I need a length of this cord of about 15" I think which stretches to 27.48" at full extension. This would occur each time the motor causes the grasping actuation and it would be playing tug of war with the motor so that when motor relaxes or reverses direction, the winch in place is remaining tautly in opposition.
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« Last post by artbyrobot1 on March 20, 2025, 02:47:21 AM »
Ok so a quick couple updates.
First, since the ideas for downgearing with pulleys have been coming in fast and furious, ways to do it easier or ways to fit it here or there or what have you, it's getting a bit scattered and I'm now starting to tear down my work too much for my comfort. It's like I'm chasing the next shiny new approach a bit overly now. So I decided to stick to the current approach as long as it is viable enough to be "good enough" so as to not waste my hard work anymore as I was starting to do. For example, the pulley system I was testing with a 10lb dumbbell did not need to be torn down and rebuilt I don't think. Stuff like that is starting to cripple progress in some sense. So my new approach is when I come up with a idea for a possibly better downgear implementation, I will just write it down and put it in a queue. Then on the next joint actuation I will use these. This way I can have like 10 different downgearing approaches over 10 joints and I can compare and contrast them, note the pros and cons of each, and over long term testing I can find the clear winners. This will also give me a greater understanding and experience and take more out of so much guesswork and into more concrete and tested territory on this stuff.
A side benefit is that people tend to think I've progressed zero with pulleys since I keep building them then taking them apart and starting over. At least under this new approach, I get joints done and over with and working before building the next downgear iteration so the progress feels more tangible and the robot gets done rather than just being in iteration and tear-down cycle hell where it appears from the outside like I am not actually accomplishing anything. So that part will be nice.
Another cool development is that I realized I can put a pulley downgear inside a tube. Normally up to now I was exiting the guide tubing to do a downgear and then afterward the string goes back into tubing to go to wherever. But I realized particularly if doing a fishing hook eye downgear that the entire downgear phase of that can fit into a tiny tube and that has some nice perks. For example, if the 2:1 downgear is the first downgear right off the motor, and the motor is reeling in 32" of string, that 2:1 will be 16" long. Well now that I can do my first 2:1 downgear all within tubes, I can run the downgear from the shoulder to the wrist, giving me PLENTY of room to deal with that amount of runout. This is quite exciting and just gives me more freedom and flexibility. I might do something with this for the first couple downgears so a 2:1 downgear pulley #1 and a 4:1 downgear pulley #2 but then do the rest in the forearm as initially planned and most likely using ball bearing based pulleys for the more heavily downgeared higher force phases of the downgearing process.
That all said, I have the downgear system of 44:1 downgear now done and attached to the finger fully and the extension spring attached to the extension side of that joint fully. So I am ready to begin testing and see how much that spring fails to extend the joint due to friction and motor magnetic cogging issues. I will then add more and more springs until it works. That is my solution. Yes, those springs collectively are fighting the motor when the motor goes to actuate grasping, however, that is just a concession we have to make with this design. Other downgearing designs that don't involve springs for that aspect but involve bidirectional motor actuation with pulley systems for either motor direction are coming next. But I'm finishing the spring based design I was talking about for some time now rather than scrapping it as I was planning of late. It is not THAT bad and it deserves to be at least tested and shown the light of day. It would be a shame to waste that work. It was good work. Also, I realize it MIGHT be the best solution. My theory says no but I can be wrong. Testing is the only way to know 100%. So it's worth keeping it as one of the downgearing methods I'll be testing out.
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« Last post by artbyrobot1 on March 14, 2025, 03:32:56 AM »
So the idea to move a portion of the pulley system stuff over to the torso is now out because I've been kind of talked out of it so I'm putting that aside for now. Going to actually try to do that stuff within the forearm. Also instead of a fishing sinker I'm going to try to use an elastic cord made for making bracelets for kids. I think that will be enough force just to keep tension on the line that is being unreeled. Doesn't have to be much I don't think.
I'm also considering just hand testing my pulley systems for now. So disconnecting them from the motor shaft entirely so I can just do testing to see how things feel and can observe things easier way quicker and with less hassle. And when I do go to test by way of motor, I'm just going to use a brushed motor and connect a lab power supply by hand with alligator clips so I can avoid messing around with microcontrollers and firmware and custom motor controllers entirely which is a bunch of rabbit holes I want to avoid as I just secure testing my pulley designs for now. I don't want to get hung up in a year or two of electronics stuff just so I can test my pulleys which would be so stupid and annoying. I need to get my testing iterations done as soon as possible without distractions and longer delays. Once I am happy with the pulley's performance and they pass all my tests and everything seems solid then we'll go ahead and connect it back up to the BLDC motor and then will worry about the custom microcontroller and custom motor controller and all the firmware or whatever at that time and will be doing that with the confidence of a big win with the pulley systems giving us momentum as we enter into those rabbit holes of electronics.
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« Last post by joeywilson on March 03, 2025, 03:18:18 PM »
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« Last post by artbyrobot1 on March 01, 2025, 02:31:24 AM »
Also, I recently stumbled upon a VERY much simplified version of my miniature pulleys. So up to now I've been using 1x3x1mm ball bearings to make tiny pulleys and been variously perfecting this approach but it is still not THAT small and is a bit complex to make and we have to make literally THOUSANDS of these to do the whole robot. That presents a bit of an issue due to the large work that requires. At least until mass manufacture of them comes in one day perhaps. But while DIYing that, it's alot to deal with making SO MANY somewhat challenging to make things. That said, my proposed EVEN MORE miniature and WAY WAY WAY simplified to make pulley is to just use a single fishing hook eye. Literally, that's it. I can use a tiny fishing hook eye and use that as my very first pulley for the 2:1 16" long Archimedes downgearing systems in the torso. This will cut down on size taken dramatically and complexity of its build. It will make the pulley basically failure proof too. The way it will EVENTUALLY fail is by the rope rubbing it enough to cut it in half. But I think the rope would fail before the pulley would fail and so that doesn't matter then. You'd replace them both at once on routine maintenance. No need then to worry about that eventuality. And the ridiculous ease of manufacture of such a simple pulley makes replacing it trivial. I also think that using this just in low load, high speed, low force early pulley downgearing stages is a non-issue since the friction with such a low load on the first downgear or two will be so trivial that the string itself would fail WAY before it would slice through the metal (acting like a saw over time). I think it would take literally MANY years due to the super low friction at these low forces. Now I'll still use the ball bearing style for later stages of downgearing where the loads go way up, but for the first stage or two I think this will work just fine. 
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