Uber cheap build idea, needing input

I'm looking to build an uber-cheap robot arm, with the purpose of just sitting on my desk and picking up whatever i want it to... Probably stick with a basic control (independent forward, off, reverse switches on each motor) setup. What i'm trying to figure out is, I'm planning on using a standard 3 wire (forward, ground, reverse) style motor similar to that found in a cheap rc car, yet i can't find any places to buy them, except in the cars. any sugestions? I figure i can create a kill switch using a metal conection piont (essitially a short in the circuit) that when the arm reaches the end of its movement it kills power to that circuit. The issue is, without an independent reverse wire, I can't...

Any ideas would be helpful...

**There is a reason hobbyists use servos **

Servos are geared motors whereas the motors you are talking about at best are geared down at the axle. Also, I believe cheap R/C cars contain typical 2 wire motors that are controlled via h-bridges that are built on the receiver board. Your best bet would be to find a cheap source of 3 or more standard servos or follow ChristheCarpenter’s build of an arm with the 9g mini servos. You may also want to consider OddBot’s or Chowmix12’s, same hack just worded differently, addition of a wire to standard servos to give feedback from servos. MarkusB’s hack that allows for a servo to be used in the typical 180 degree arc or continuous rotation, might be useful. I only point out the last one for completeness of what I have seen on this site.

I was lead into believing

I was lead into believing that servos wouldn’t be able to handle what i was going for… my design is set for 240 degrees of movement on all 3 joints of the arm. I wasn’t sure if a servo would give me that kind of freedom… thank you.

To get that much travel you will need continuous rotation

only because as you stated a typical servo will not give you 240 degrees of rotation. I would also use, as I mentioned earlier, OddBot’s or Chowmix12’s hack to give you an idea of where your joints are. In addition, home switches might be a thought if precision is required/requested. I wouldn’t guess that the arm would get too far out of position in the short term, but, over long periods you might notice that specified X,Y,Z coordinates are not what they were a week/month/6 months previous. A quick home routine would be able to reset the counters of the input gained from the servo hack.

DC motor and Potentiometer

I have used many different motorized satellite antenna dishes that consisted of a geared (or sometimes cable driven) dc motor with a potentiometer at each axis.  Usually the potentiometers were calibrated so that the mechanical zero of the axis and the electrical zero of the potentiometer were within a reasonable range of each other and the value stored at the antenna driver/controller.  Also this may allow the 240 degree range of motion that you desire with a cheaper dc motor source.

I am assuming you want to make a cardboard robotic arm?  This may be an excellent challenge to post for the group.

Does your design allow for 2 dc motors to help move each axis? Or is it restricted to one motor per axis?

It’s actually a balsa wood

It’s actually a balsa wood frame work, that i will be attempting to use lexan to cover. I’m building it just for when i’m bored sitting at my desk, i can rearange things. it’s all a part of my, “let’s overcomplicate everyday mundane tasks, in order to make them seem more interesting” goal in life.

The design i was actually toying with basically had me taking the standard motors, then gearing them down for torque, slightly combing the rotation of a standard motor, with the slow speeds and torque of a servo. I have a friend who designs and builds home home vehicle models to scale, and some of the companies he deals with sell the small gears, for me to use, yet after doing some research, i can not find anywhere that sells these, and am looking for a replacement idea.

the robot’s framework is all i have done at the moment. It stands about 3 foot when extended up. I’m planning on being able to lift roughly 3 lbs with the claw. so my motors are going to have to have the ability to lift around 6 to 8 lbs in order to move the arm and load. I figure the lexan, probably 1/8th inch, over the balsa framework should be able to provide the strength i need. i was planning on driving it using 5 different motors, 3 on the arm handling the rotation of the joints, 1 on the base to actually allow it to spin, and one after to last joint to rotate the 360 degrees. The one on the base and the one on the claw are assisted with a ball bearing setup in a carved wood dish with an alluminum faceplate to reduce resistance/ allow for a stronger joint.

once i get a new digital camera, i’ll try to get some pics up to show what i mean. I’m not well versed in explaining these things, everything i have built to this piont has been basic radio controlled characters with servos and basic components. on this i decided to go all out, and have thus far just ended up out.