17th November 2016, 10:45
Hi everyone, so what does a chap, who has very limited mechanical engineering experience, no tools more advanced than a battery drill and jigsaw, who last built an antweight over 10 years ago and who has been bitten by the bug again since watching the last series do? Build a featherweight crusher of course!
I started generating some ideas a couple of months back and in hindsight, I should have started this build thread back then so you could have all told me about the mistakes I no doubt have made. I decided that I wanted to make a FW crusher. Crushers have always been my favourite type of robot and having a good read of the forum it doesn’t seem like there has been too many of them built, and I think it should push the limits of what I am capable of. I decided to aim (for no particular reason) to try to get around 1 tonne at the tip of a vertical crusher, as this seems reasonable.
I set about finding a ram and means to power it that would give me the force needed at a sensible ratio at as light a weight as possible.I’ll put some details of that in a later post, but once I had the sizes of this, I could set about designing the chassis and the rest of the bot around it. I have never really used much CAD software, but I downloaded Fusion 360 and after a bit of messing around I was able to get the hang of it. After about amonth of design and re-design, I came up with something I was fairly happy with. Again, I’ll put some pictures up of the CAD in another post when I get on the other PC. The thing that became apparent very quickly was that it was going to be difficult to get something in the weight limit that doesn’t tear itself apart if it bites on something it can’t crush.
Fusion 360 has some simulation capabilities built it, so after watching the tutorials I set it to model the chassis sides with the expected loads the ram and crushing arm will put on them. I still don’t really have much idea what I’m doing, but at least I could see if each iteration of my design was better or worse than the previous one. It seemed to make something strong enough and within the weight limit, that I was going to have to use titanium for the chassis. I spent a lot of time refining the design and then I had a look at the availability and cost of the titanium. Wow. I know it is expensive but I had no idea just how bad it was and I should have checked this before putting too much time into the design. After another read of the Riobotz manual and some more research into materials, I have changed the chassis to 7075 T6 ali. Still not as cheap as I would like, but much more reasonable than the titanium. Anyway, a couple of weeks back the designs went to waterjet cutters and yesterday I picked up this:
That is one of the chassis sides and it has come in on weight budget, yay! Its on the dining table as the shed isn’t ready yet. I have some more bits of the bot that I need to take pictures of and add to this diary, but I think that’s enough for the first post, comment and critique welcome!
P.S. As per the thread title, I don’t have name for the bot yet, any ideas on that welcome too.
17th November 2016, 21:18
I'm very much in the same boat as you... building / designing a fw crusher as well. Just wondering about your choice of material there for your chassi. How thick is it? And how much force will it have to take / can it take.... to achieve your 1tonn at tip.... 3 tonn from your ram maybe?
18th November 2016, 09:39
Hi Matt. The chassis is 8mm thick per side, which theoretically means I have 16mm total thickness to resist the forces of the ram and arm. However I think this would only work if you happened to crush everything perfectly vertically and in line which wont be the case. Therefore I have tried to design it so that one side could handle the forces on its own. I think the material for it will be ok from having a read of the Riobotz manual. Its one of the best aluminium alloys, especially with regards to yield strength which I think is was I need for the chassis and arm where the forces are. The arm is mainly made of the same stuff, but I plan to put a tool steel (or similar) tip on it. Titanium was way too expensive and the steels were too heavy so I don't think I had much choice.
The ram should give out 2.6 tonnes so I'm working on a 2.6:1 ration. Speaking of the ram......
18th November 2016, 09:44
Here is the business part of the design, 100mm stroke and weighing a shade under 3kg. I think that longer term there is scope for a smaller bore ram running at higher pressure to save some weight. However that would require a power pack that can produce greater pressure than any I have found so far.
Here is the power pack I’m using, it should make it as simple as running the motor one way to extend the ram and the other way to retract it. The pack comes with a heavy, fairly low power brushed DC motor on it. So I have removed that and I’m going to replace it with a smaller, lighter brushless motor, which should save on the order of a kilogram of weight and give me faster flow rates.
Lastly, a few spacers, one of the few turned items in the design, one of the guys at work was kind enough to make these for me, just some grade of aluminium and acetal plastic. There is another bit of turning to do, so I’m crossing my fingers he can do these for me too in a quiet time or I’ll have to look for alternatives.
18th November 2016, 10:21
I get what your saying about the steels... how much that chassi side cost then?
i was planning on making my arm chassi and arm out of 3mm stainless steel becuSe I have someone who will give me some and laser cut it for free... but I think it may come in too heavy.
i need to get myself that riobotz book I think and have a read.
i too am planning on using a different material for the tip.... I made a crude test of my weapon system and used a hss tip. It pierced through some 2mm mild steel. But found it difficult to contain sideways movement once the pressure really starts, but that may have been down to the crudeness of my test rig. Lol.. Going to be very tough in the weight limit for me I think. Lol
18th November 2016, 19:08
Looks like your off to a great start!, look forward to seeing how this progresses
Have alook at my build dairy on my featherweight crusher Cobalt i built in 2012 and message me if you have any questions
18th November 2016, 19:35
Just looked at your build diary kenny... looks amazing.,, no real info on the materials you used tho, chassi, tip, or real figures on what force your ram is producing and what downforce is at the tip....... i need these figures so i can aim higher ;-) lol
18th November 2016, 20:05
7075 Grade Main bulkheads and 6082 T6 Aluminium everything else, P20 Tool Steel Claw, 32mm Bore ram (1.6t at the pressure i run it at), i had different pivot points to adjust the pressure at the tip but limited the stroke of the claw depending on the opponent, the highest point was 1:1 Ratio and 1:2 at the lowest.
Originally Posted by MattLampitt
19th November 2016, 14:28
Hi Kenny, I've just had a read of your build diary, thanks for telling me about it, cobalt looks a very impressive machine! What happened to it? Did you get the new ram and self righter working and how did it get on after that?
19th November 2016, 14:43
Thanks for the compliments, It was split apart due to a redesign of the drive system and then sold the hydraulic system and ram which is in little nipper later that year, Over the years ive been designing and making bits for a new Cobalt which isn't so far away being ready for battle