I made this project mainly to learn using servos with arduino. Of course I could buy robot arm kit like MeArm and start programming, but making it from scratch teach you more. I wanted design the arm simple as possible and that's why I chose 4 axis build. Idea for pinch mechanism came from small tweezers. Arm accuracy is not best possible but good enough for practice purposes. Using better quality servos, advanced programming and more sturdier building components, accuracy can be improved significantly.
Step 2: Materials and Cutting Parts.
Main material is 3mm plastic sheet. I also used some thicker plastic for base and some MDF. But you can use what ever material you feel comfortable. Some brass spacers/stand off is used for attaching components together.Bunch of M3 bolts and nuts, small screws and 4 servos. Brass rod and sheet for pinch mechanism. For finished look I personally like to paint all my projects mainly with acrylics. Let's get started.
First print out blueprints. Then clue them directly to plastic sheet. Cut along with the outlines and drill holes. I used small bandsaw and handheld saw (last picture). For the base I don't have blueprints but I think you can improvise.
Step 3: Assembling the Frame
Assembly was quite easy task. Everything was planned ahead so I simply just screwed all parts together. Of course there was some minor tweaks to be done. I attached servo horns with screws that was included in a servo package. For the lower part of the arm I separated two pieces with one 12mm and two 6mm brass spacers. For upper part I used just one 12mm spacer. Servos are standard hobby servos. 3 Tower pro 9g micro and 1 tower pro 996.
Step 4: Making Base
Base is made from 10mm thick random plastic. First I cut square approx 100x100mm. With table saw I made those two grooves where I attached base of that arm. With 30mm forstner bit I made that embed area for servo horn. To minimize friction between base and MDF I sanded both surfaces with fine sandpapers. Then I painted them and added couple layers of silicone spray. To get best result I recommend using some kind ball bearing system here. But for practice purpose this is fine.
Step 5: Pinch Mechanism
Entire system is made from 0.8mm thick brass sheet and 2mm brass rod. Its very simple design compared to many other out there. I bend brass rod with nose pliers. Solder is same stuff that we use with electronics. I also added small foam pieces at the end of a "pinch". Foam adds more friction so you can grab round objects. Brass is very sturdy and easy to manipulate so its ideal building material for prototypes.
Step 6: Painting and Decals
Remove all dirt and grease with mineral spirit or similar product. Add primer. Small project like this I usually paint with airbrush. After painting I added some left over decals to make it look more interesting. Before final assembly I added clear coat of lacquer.
Step 7: Electronics
When using servos with arduino you need an external power source for them. Well you may power one micro servo direct from arduino but not multiple ones. You can connect all positive/negative leads together and signal wires should kept separated. Total of 6 cables are needed to control 4 servos. If you have two different power sources like me (USB for arduino and variable PSU for servos), you must connect all negative leads together.
To control arm you only need some buttons and a POT. First I made breadboard test setup and after that I made very simple expansion dock for arduino nano. Couple pulldown resistors for buttons and current limiting for leds. Leds indicates when you have saved pick and deposit places manually. This arm is not using any eeprom stuff so you need to write coordinates directly to arduino for permanent use.