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Unread 02-08-2013, 09:00 AM   #1
scarletonthetil
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Need help with LARGE arm prop!

I want to cosplay bacterial contamination miku, but I want to try and construct her giant claw arm...thing. Here are a couple of reference pictures:

http://oi47.tinypic.com/sor3i0.jpg
http://oi45.tinypic.com/2afh9b9.jpg

I am trying to come up with what materials would be best to make this thing so it is light enough to carry around on/over my arm. Paper mache, worbla, sculpey...I don't know what to use! I am not a very experienced prop maker and this by far will be the most complicated thing I construct. And if somehow this is possible I want to figure out a way to make the claw parts move. I am probably in over my head on this one...Any and all help is greatly appreciated!

Last edited by scarletonthetil : 02-08-2013 at 09:54 AM. Reason: broken links
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Unread 02-08-2013, 09:29 AM   #2
Chergnomebyl
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It might just be me, but the links aren't working.
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Unread 02-08-2013, 09:54 AM   #3
scarletonthetil
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I reuploaded them, here we go!

http://oi47.tinypic.com/sor3i0.jpg
http://oi45.tinypic.com/2afh9b9.jpg

Sorry about that
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Unread 02-08-2013, 10:39 AM   #4
nathancarter
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Some thoughts:

If you're just starting to get comfortable with your materials, you'll probably want to work on getting your construction techniques down before trying to make the claws move. Once you've honed your skills with basic construction, you can move on to more advanced tricks like articulating the claws.

If you're wanting to carry it around a con, you'll probably have to modify the scale just a little bit to make it practical to carry, and so you don't get worn out after half an hour of wearing it. You can still make the claw big and huge (if you use lightweight materials), but you'll probably want to scale down the first "shell" segment so it fits over the length of your forearm, with your hand inside the joint at the base of the claw. If you try to make that first forearm segment really long (as shown in the first reference picture), it's just going to make the weight of the claw that much more unbearable to carry around.

If I was approaching this, I would probably make it in individual pieces, maybe with a threaded rod in the base of each of the claw "fingers," and a reinforced cavity in the front underside of the claw "hand" where you could use a nut and washer to bolt the fingers to the hand. This would allow you to construct it in many pieces, instead of trying to do the whole giant thing in one go, and also allow you to disassemble it for transport and storage. The rear of the claw hand would be hollow, with an interior handle of some sort (maybe just a length of PVC pipe) to allow you to grip and maneuver the claw.

I'm no expert in pieces like this, I'd probably make each claw "finger" out of layered blue insulation foam, hollowing out each layer to reduce overall weight. Then cover with a thin later of paper mache, gesso and sand to finish the surface, use sculpey or Apoxie Sculpt to make the tiny details (the spines on the back of the claw). Then paint.

Fortunately, the wide surfaces of the claw shell are relatively large and smooth , so the "big" parts of the foam sculpt should be pretty straightforward.
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Unread 02-08-2013, 03:54 PM   #5
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The claw joint is one of the simpler joints out there, if you just want that to be opened and closed, you could potentially eyeball that joint. To make the other joints move, you'd need to carefully sculpt, mold, and then cast hollow pieces. In the traditional technique, you need to run a series of steel wires, pulleys, springs, and anchor points in such a way that the motion of your hand or arm causes the wires to retract or extend.

In Hollywood, when actors have wild animatronic appendages like this, they are very rarely operating them by themselves. They use camera tricks to hide the fact that there are a series of cables running down behind the actor connected to a box being run by one or more operators.

If you have money to burn, you could conceivably place a remote controlled electric actuator inside the claw. Since it would all be self-contained, you wouldn't need to run any cabling through the pieces. It would add weight, so the rest of the arm needs to be very rigid and strong and strapped to your shoulder, if not completely harnessed around your body.

You'd make the base of the claw hollow with a slot. You'd make the "finger" such that it has a wooden bar at the end. the wooden bar would go into the slot, and you'd use an axle to anchor it. The end of the bar is connected to the actuator. Assuming it's a linear actuator (a bar that moves back and forth), you just fasten the actuator housing to the far side of the claw and you're all set. If it's a stepper motor, then you need to set up a cable, a spring, and a pulley or two.

You can get RC actuators to do small things often by just tearing apart a cheap RC car toy. If clever, you could use the main motor of an RC car to do this. Otherwise, buying the component you need will easily run you $100 or more.

Oh, and one fun trick to make a hollow piece is to carve it from polystyrene insulation foam, paint that with gesso so resin won't dissolve it, then coat the foam in fiberglass+resin. Once the resin has fully hardened, drill a hole and pour acetone into the foam. The acetone will eat the foam leaving you with a hollow fiberglass shell. Naturally, this is a messy process and should be done outside with all sorts of proper safety equipment, and away from sparks and flames.
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Unread 02-11-2013, 08:22 AM   #6
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So insulation foam seems to be the most popular material for me to at least start making it out of. I have never worked with fiberglass or resin before (plan to try out resin for another piece of this costume). Do you have any suggestions on where I could purchase these materials?

and verdatum, i'm going to show my boyfriend what you wrote because he is better at understanding mechanics of things than I am :P thank you both so much for your replies!
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Unread 02-11-2013, 10:29 AM   #7
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Insulation foam can be found at your local hardware/building store - Ace, Home Depot, Lowe's. It's pastel pink or baby blue in color, and comes in big sheets that are 8 feet by 4 feet and about an inch thick. If that's too big to put in your car, they'll probably slice it into sections if you ask politely.
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Unread 02-11-2013, 12:02 PM   #8
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You want to do your research before/if you open your first can of resin. Watch videos of people using it (sorry, too lazy to pick out the better videos...I really should do that some time). read plenty of tutorials. Cut out a random chunk of foam about the size of your hand and try to go through the entire process. of prepping, fiberglassing, surfacing and painting it.

Adding to what nathancarter said, last time I purchased insulation foam, I just had a swiss army knife on hand and scored and split the thing in half in the parking lot. So if you gotta work to find someone competent in the store, you might not want to sweat getting them to do it.

General-purpose fiberglass & resin can be purchased from most large hardware stores. It's usually near the adhesives section. More exotic products like colloidal silica or gelcoat often need to be purchased from marine supply stores or ordered online. For online orders, tapplastics.com is pretty good for fiberglassing supplies.
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Unread 05-31-2013, 10:43 PM   #9
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You guys have been such a great help, and I have been doing a lot of research since then! I started to buy my materials, and wanted to make sure this would work out. I bought polystyrene insulation foam board at Lowe's (it was green, i didn't see any blue or pink but i'm pretty sure its the same thing!). I saw this video from xrobots and youtube http://www.youtube.com/watch?v=MDwQg6CX25A and wanted to try a method like that for the claws. After more research I have found that the claws I want to make are actually quite flat, and I am no longer going to try to make the individual claws move. Instead I want to figure out a way to make the claw light and flexible enough, since it will go up my entire arm, so that i can bend at my elbow and move the entire arm/claw around instead of the individual "fingers". I am trying to find out what kind of resin I should use, and wondering if i should use the same materials in the video. This was kind of hard to explain, i hope it makes sense!!
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Unread 07-09-2013, 05:53 PM   #10
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In case anyone is interested, here is what I have so far after carving out the foam
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Unread 07-10-2013, 06:43 PM   #11
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Nice! You've got a good eye for proportions and shape. You should definitely take some sandpaper to the edges of your foam to round them out a bit. It will help your claws look more organic. Keep up the progress pics!
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Unread 07-10-2013, 06:58 PM   #12
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Thank you for asking this! I was planning on cosplaying her but could not figure out how to do the arm!
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Unread 07-11-2013, 11:01 AM   #13
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I love it when people post project updates. It looks like you're off to a fine start.

Now I've gotta know, what is that impressive-looking piece of equipment behind you in that photo??
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