[WIP] Motorized stabbing weapon

AlecMoody

I wanted to build a first person weapon for my portfolio but also do something that would stand out a little. The idea is inspired by the kinds of things kids draw in their notebooks in school, and also battlebots (but with people!). Also, if you ever saw that Simpsons episode where Bart and Lisa are swinging their arms around and walking into each other – I imagine it like that but more disturbing. Obviously the design is super impractical. Nobody would ever build this for any serious use.

In contrast to the far fetched idea, I intend to make sure it is fully functional in terms of construction and engineering. I also plan to build it out of simple materials and parts found on websites like mcmaster-carr. Basically the way its engineered should be completely believable and the construction will be fully articulated. In all 3 blockouts the design is centered around a rectangular steel tube, a large battery, a dc motor, a simple transmission, and a cam and linear bearing.

I'm also testing to see how many gears vs belts I want to use. I would want at least an 1:8 gear reduction which would equate to something like a 10 tooth and 80 tooth gear interfacing. My tests show a simple 80 tooth gear with teeth modeled would be about 1100 triangles (10 tris per tooth * 80 + cylinder caps for two 160 segment faces) so unless its very up front it might not be worth doing.


I'm looking for feedback on which design sketch I should go with. I have some simple blockout geometry for 3 variations in this video file:


http://www.alecmoody.com/stab/




Right now I am leaning towards the bottom one that uses the spring and follower design but I would be interested to hear other people's opinions.

Also this was my original drawing (which matches the top variant):




Any other feedback would be great since this is early in the design process. In the event I go with belts to do the reduction, I am looking at ways to integrate a belt tensioner. It could also add some asymmetry.

Lord McMutton

An Automatic Stabbin' Flabber?

blackwaffle

I read the title of the post and thought, "awesome"

Good luck with the model. Might i suggest...more knives?

lbigbirdl

Lookin' good. I think you should add another arm so you have two knives alternating. Also i love how the knife is only tied on.

bbob

More knives seconded.. Im imagining a small array of knives on sticks asyncronised for optimal sustained stabbage. That might also give you more mechanical thingies for you to make to add to the visual interest.

Also, do you have any specific style in mind for this? Or is this purely an engineering extravaganza?

Nistrum

heheh. have a group of knives set on a cam driven system so they stab alternately

blackwaffle

i like the cam driven system, but that just sounds like a really big chainsaw. Maybe thats a good thing.

Also, you should model a gas-powered lawnmower engine or something instead of just having gears connected to supposedly a hand crank.

AlecMoody

I did a blockout sketch of multiple cams on a shaft and there are some limitations in terms of packaging. Basically the length of each cam is limited by the clearance from the main body tube.



In that image there is only about 2 inches of throw per knife. I could continue to move the camshaft upwards but it would become progressively more awkward. If anyone has any other ideas on how to get that to fit together better I would be interested.

Probably the biggest issue with this design is that there wasn't room to reverse the direction of the spring. This means the forward stabbing motion is working against the spring instead of with it. It also means I couldn't use the cutout profile cam for a more rapid forward stab.


I tweaked the last design to include gears and sorted out a few small mechanical issues:
http://www.alecmoody.com/stab_mk2/stab_mk2.html

obviously this would run much quicker - I also have the two gears running much slower than belt would need to in order to operate the teardrop cam at that speed. I think my target speed for the final model will be something like 3 stabs per second.

again, if I am missing a huge design possibility with the multiple cam shafts let me know.

bonds0097

You should look into the reciprocating saw, like the sawzall. Check out this wiki page that shows you how it converts rotary motion into linear motion, there's a great little AVI at the bottom of the page that illustrates this:

http://gicl.cs.drexel.edu/wiki/Reciprocating_Saw

Quasar

I don't know how complicated you want to make this thing, but for maximum stabbing, you can try making a similar mechanism that's used to operate the bolt in a chaingun:



Basically, it's like the animation bonds0097 was referring to, only instead of a wheel with a peg attached to it driving the blade, you can make it a chain that wraps around two gears and the chain has the peg on one of its links. Sorta' like this:



One chain can drive two blades by having pegs on both sides (offset, of course), and a second chain can drive the third blade.

EarthQuake

I think you're getting much too caught up in the engineering side of this. I mean sure, it would be cool if it actually works, but I would try and work on a concept that is aesthetically pleasing before you go all CAD and such, as right now, this isn't looking like something that would make a good asset.