Rawbots Club Forum

Off-Topic => Robotics => Topic started by: MarvinMan on March 10, 2016, 10:55:02 pm

Title: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 10, 2016, 10:55:02 pm
This is a place to share and discuss your own hardware and software projects.

Feel free to post or ask about anything vaguely related, from your first attempts at controlling servos to building your own terminator army.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 10, 2016, 11:05:23 pm
Here's a child size prosthetic hand I built for the group design project part of my degree.

The fingers can spread the load between all of the finger sections, and the two pairs of fingers can be independently driven. The fingers are actuated by a pair of screws directly driven by brushless motors, and the thumb is driven by a micro servo. We had a few problems with the screws sticking, but once it's moving it can grab things with a surprising amount of strength. It's meant to be controlled by an EMG sensor, but that stopped working a week before the final demonstration.

Almost all of the parts are hand machined, and the outer shell is moulded in fibreglass. I originally wanted to 3D print most of it, but the printers we have at uni are pretty poor quality despite being obscenely expensive.

(http://i.imgur.com/cIr0b5j.jpg)
(http://i.imgur.com/wXR2cMb.jpg)
(http://i.imgur.com/O0mWOwd.jpg)
Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 11, 2016, 01:59:59 am
That's pretty awesome.  Sucks about the 3d printers, tho.  You could always have gotten something from shapeways or the such, but I guess that would have been even more expensive than machining.  I'm curious, why did you went with fiberglass?  I must admit the rest of the hand looks much better relatively speaking.

I haven't done anything this cool (in fact not even sure these things qualify for this thread) but I have made a crude vacuum chamber (rather than buying the 150$ one from ebay)  Turns out Walmart do sell big, conveniently shaped vacuum proof jars at 30$, even with a convenient hole for the hose.  I agonized over a cheap yet stiff cover material (small pieces of thick cell cast acrylic aren't obtainable without costly shipping)  But finally went with a floor ceramic tile, since the jar is transparent anyway.  for the seal I cut some rubber mat thing that was made for chimneys.  The plumbing however was complicated.  Hose clamps were very useful, but I didn't have standarized parts so I had to fit a bunch of components not meant to join together.

In the end, it does work alright, which is very surprising.  Keeps reasonnable vacuum overnight.  With all the mistakes I made however, it cost me as much as a real one...  At least its pretty big.

(http://i84.servimg.com/u/f84/19/43/45/55/vacuum11.jpg)


Also, After I had to bring a plumber to defrost my water pipes (200$ a time) i ripped off the technique for myself with a tea boiler, a bucket, a hand pump meant to siphon gas (15$, wasn't going to wait a week without water for an electric one) and some silicon tubing.  While the pump did eventually fail, it did save me two calls, so this time I definitely saved money.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 11, 2016, 08:40:35 am
The fibreglass is mostly the result of not being able to draw something like that in solidworks. I had a few attempts at modelling it, but it was easier just to sculpt a form out of wax.

What do you use the vacuum chamber for? If you want to hold a vacuum for longer, greasing the sealing surfaces a bit should help.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 11, 2016, 10:19:32 pm
The reason I wanted to make a vacuum chamber was for resin casting, which can make much stronger and lower tolerance parts than your typical 3d printer can.  This guide http://lcamtuf.coredump.cx/rstory/07-planetary-parts2.jpg (http://lcamtuf.coredump.cx/rstory/07-planetary-parts2.jpg) is what gave me the taste of doing this.  With a good cnc mill you can do stuff like this.

(http://lcamtuf.coredump.cx/rstory/07-planetary-parts2.jpg)



Whats stopping me is that a decent cnc costs around 1500-3000$ and as you can see with my other post I'm strigny with money.  But maybe I'll do the leap one day...
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 11, 2016, 11:33:03 pm
I had a go at resin casting like that to make parts for the hand. Turns out fibreglass resin eats through foam moulds, so I ended up machining the parts out of some board.

If you're thinking of getting a CNC machine, it's worth getting something with a decently solid build. If you go too cheap and get something too weak or underpowered, you'll never get a decent finish, especially for larger cuts.

I got one of the ~1000 4-axis routers that are all over ebay a couple of years back. I've upgraded it with a decent spindle motor, and the thing can handle pushing a 6mm cutter through nylon. Even with a more typical spindle motor, the machine itself is pretty solid with an all aluminium structure and ball screws throughout. IIRC, the smaller 3-axis machines were going for around 750 when I was looking.

Building your own CNC is always an option, although doing it well could cost more than just buying from china.

That tiny gearbox is pretty cool, makes me want to get some smaller cutters and have a go at making something like that myself.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 12, 2016, 12:29:25 am
If you do (try resin casting again one day, you might just end up machining instead) I really advise you to go take a look at the link, because the author seems to have a knack for chemistry and tried different resin formulations.  According to him, resin from industrial suppliers makes the stuff from smooth on, aluminite etc look bad as far as mechanical properties are concerned.

As far as cncs are concerned the last time I checked I was looking at brands like sherline or taig but maybe that was overkill?  I've also looked at the chinese routers, either way I appreciate the advise.  I've heard good 4 axis cam software is expensive so I think I'll stick to 3 axises if I ever try to do this.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 12, 2016, 08:48:22 am
I'll definitely keep that in mind for the next time I need to make some really small parts.

I've got a second hand sherline spindle motor on the router, and it seems to be good quality (although it has to be almost completely disassembled to change the position of the belt). As for whether it's overkill, it really depends on what you're planing to use it for.

As for software, nealy all of the stuff I do is 2D/2.5D, which most CAM packages should be able to handle (IIRC, mach 3 has some capacity to import .dxf files and generate basic code). I'd recommend getting a USB based CNC controller as parallel ports are becoming rarer, the software won't run on an x64 OS, and the standard version of mach 4 won't support it at all.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 23, 2016, 07:30:49 pm
Here's my latest project, a vaguely mantis inspired robotic arm for one of my courses at uni.

Most of it was meant to be done on the laser cutter with some 3D printed parts, but I've ended up having to re-make a lot of it on my CNC router and modify the 3D printed parts.
(http://i.imgur.com/ykEsQx9.jpg)
Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 23, 2016, 08:20:01 pm
I see 2d parts (is that acrylic?  I'm especially curious about the pink stuff), rc servos and screws.

so the black blocks are the 3d printed parts you're talking about?  Did they needed to be a custom thickness?

Also what was wrong with the first attempt?  Once I ordered some laser cut parts and completely screwed up the scale.

What is the course about? prototyping? controlling motors?

Am I asking too many questions?
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 23, 2016, 08:32:45 pm
All the 2D parts are acrylic, with a couple of PVC and polystyrene parts where I needed a thickness other than 3 or 5mm.

Yeah, the black blocks are 3D printed, but the printers at uni aren't great to start with, and the workshop was backed up with orders and they didn't print the parts solid, and now it's the easter break so I can't go in to try and print more.

For stuff where overall dimensions aren't especially important, the laser cutter does a reasonable job, but it has no way to compensate for the ~0.2mm beam diameter, which is useless when you're trying to push fit bearings where 0.05mm/0.002" is the difference between cracking the plastic and the bearing falling though the hole. I had to re-design a lot of the parts too, so only the pink stuff is left from the original version.

I've also found that the thickness of plastic sheet is all over the place, to the point where I've got a bit of ~10mm plate where one end is 1mm thicker than the other.

Tomorrow I've got to do a bit of woodwork to build a stand for the arm so it doesn't have to clamped to the edge of the bench.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 23, 2016, 09:00:47 pm
Well that stuff is good to know.

I wonder if you could machine the sheets flat or if that would leave too bad of a finish.

Also can't you just assume you're drilling with a 0.2mm circle and compensate for it?  Mills have much wider kerf after all...  Unless the shape of the laser kerf is just too irregular to compensate for.  Then you need to rapidly spin the laser to even the kerf out :p
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 23, 2016, 11:13:20 pm
I could skim over the surface of the parts if I had to, but it was easier to sand down the 3D printed blocks that go between them in this case. You'd loose the perfectly smooth finish, but it shouldn't look to bad.

The problem with the laser is that the control software has no way to compensate for the beam diameter (which varies with focus and material thickness), so any compensation would have to be made in the drawings. The other thing is that the laser is pulsed, so it leaves a rough edge like its been cut on a bandsaw.

Title: Re: Things you've built/invented/programmed (or want to)
Post by: z26 on March 23, 2016, 11:25:21 pm
laser cutters use pulsed beams?  I guess you learn new stuff every day.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 23, 2016, 11:43:39 pm
I think the laser at uni tops out at about 5kHz. Seeing the different finish on different sides of the parts, I suspect vibration from the steppers contributes to the rough finish.

Most high power lasers are pulsed. In the case of the laser cutter, an electric arc discharges in the tube, which excites the gas. The gas then de-excites, creating the laser beam. It's quite interesting to read up on how lasers work, the lasing material acts like an optical amplifier, creating more and more identical photons as the beam bounces back and forth inside the tube.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: Schicko on March 26, 2016, 10:04:03 am
Not mine, but here is a transforming hexapod: http://zentasrobots.com/robot-projects/morphex-mkiii/

Someone should try and make this in Rawbots.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 26, 2016, 12:31:59 pm
Building a folding hexapod should be possible, but getting things to walk in rawbots tends to be very tricky.
Title: Re: Things you've built/invented/programmed (or want to)
Post by: cupid_the_conqueror on March 27, 2016, 04:36:58 pm
Building a folding hexapod should be possible, but getting things to walk in rawbots tends to be very tricky.

LOL, I'm thinking to myself "well that would be practically impossible! it could never be done!"

Then there is marvin "eh, it would be kinda tricky"
Title: Re: Things you've built/invented/programmed (or want to)
Post by: MarvinMan on March 27, 2016, 09:29:50 pm
Just getting the thing to walk is certainly possible, but making it even remotely fast or smooth is an entirely different story.

Look at the spider tank- that was about the right size and could walk slowly. All it would need is a bit of modification to the end leg sections to turn them into sphere segments so it could fold into something resembling a ball. If anything, 6 legs would be less code to wrangle and less lag than 8.