I didn't found anyone yet who purchase Modular ARM design. There are various ways to make robotic arm. see this link https://www.societyofrobots.com/robot_arm_tutorial.shtml
You should design one for your self and then give it to a machine-man to make it for you.
The first thing i have to do is make a design and give it to someone who can prepare it, right? or can i make it myself?
What is the shop called? those who make the robotic arm (external part). It must be hallow so i can fill it inside with a wires and servos connected to my microcontroller?
Yes you got it.
You can use Aluminum tubes available at hardware shops. like trunk manufacturers. For machining you may need grinder/ cutter, Drill and lathe work may be. You can also concern Engineering University Project Reports available at libraries. They will give you complete guide from scratch.
I guess aluminium tubes will be a little heavy. Do i have any alternative for that?
How much will the servo cost? It must have high torque to lift the arm.
I am not so clear about how i have to connect the servo near the joint of the arm, little explanation on that will be helpful.
Thanks.
Firstly you need to decide what extand of load your arm is supposed to lift.
For heavy load, Aluminum structure and high torque servos will be used.
For light weight load, you can also use Acrelic Sheet for arm, much lighter. It all depends on the application. Acrelic Sheets can not be cut easily, So i make a design in CAD and give it to LASER cutting machinist. Then Joint them using elfi (plastic joining solution).
Lot of servos are available in market. Lighter to Heavier and Cheep to Expensive.
China market may give you less expensive motors visit aliexpress website.
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Also servo are mounted using "servo mount" they generally comes will motor.
Good luck