ok heres whats up im trying to build a robotic arm for my science fair
i need a list a parts that i can build a basic one for like 70-80 dollars i was thinking about buying the atom bot board and the bsp2p or watever off ebay and some servos then making the brackets myself let me no if u can help
Hmm…
I’m not sure that $80 dollars is realistic.
The botboard and BS2px (if that’s the one you’re talking about) is about 80 dollars, themselves.
If you’re not factoring that into your available money, you still won’t get very far.
The max that your going to get is a gripper arm, 2 weak servos, a “C” bracket, and the arm base.
And, that’s being very optimistic.
If you’re planning on continuing this project, after the fair, I’d suggest taking one of these two options.
(1) Invest more money into the project (if possible).
(2) Chose another project for your science fair, and save your money until you can invest more in this robotic project.
I suggest these options, because you’ll end up being frustrated, later on, when you try to expand on your project.
While weak servos might work for a small 2 DOF arm, they aren’t going to be able to pick up much, nor are they going to be able to pull their weight when you build a larger, greater DOF, arm.
Don’t feel obligated to follow the above…
I just wanted to point out the spikey pit that’s around the bend.
Hope this helps.
Yep, no way you can build any kind of reasonable arm for 80 bucks. Maybe the electronics, cheapest gripper in the world and a wooden stick connecting it to a servo. 
with that in mind you are deeply underestimating a machine shop sheet metal and ebay…
servos arent really the problem its just the electronics which would be the best cost effective componenets to get so i can control it with the ps2 controler
one more thing to interject what servos should i use?
That depends.
If money isn’t a problem, I’d go with the 5645’s.
They’re cost effective, standard sized (which means they’ll fit all the brackets), and they’re the second most powerful of the Hitec line.
However, they go for around $55 a piece, so that might be out of the ballpark.
Again, if you’re looking for a 20 dollar servo that will be able to actually have your hand pick something up, I highly doubt that you’re going to find it.
It’s like the old saying goes… you get what you pay for.
As to what servos will best suit your needs and price-range?
I’d say, price out how many you need, find out what the most expensive one you can afford that number of them is, and go with that one.
In general, torque goes up as price goes up.
The best way to decide would be to go to hitecrobotics.com, download their pdf catalogue, and compare the servos that they list their, since they have a price/torque chart.
how much would it run for all the parts on the custom arm example for the erector sets with out the brackets just the electronics and servos
You will need the SSC-32 servo controller and some software to make it easy to create sequences of motion. If your robot is a 3 axis planar arm, (has a shoulder, elbow, and wrist all on the same plane) then you can use RIOS. If you’re building something different than I recomend the Sequencer program. Both programs can export Basic Atom or BS2 code for simplifying the programming of these processors for use in a stand alone (no PC required) operation. As a general rule 422 servos can be used if the weight and distance between each axis is minimized. 475 servos will allow a bit more distance between each axis, but should still be kept around 4". The use of 645 servos will allow distances of around 6" between each axis. Be aware that with all of these, a means to load balance the elbow is recommended. The prices for these items are on the website. Hope this helps.
PS the SSC-32 has a group move function that makes controlling multi axis assemblies really easy. You simply send the new position data and the time the move should take, and all servos will start and stop at the same time, even if they are moving different distances. Very powerful to say the least.