A skeleton you can take apart
Picture your own arm reaching for a cup. Your bones do not bend — the upper arm and forearm stay stiff — and all the movement comes from the joints between them: the shoulder, the elbow, the wrist. A robot arm works in exactly the same way. It is a chain of rigid pieces, and the cleverness lives in the small hinges that connect them.
So a robot really has just two kinds of part. The stiff bars are the bones, and the movable hinges are the knuckles. The bars are called links, and the hinges are called joints. Almost everything you will ever read about robot arms is a story about how links and joints are chosen and arranged.
The link: a bone that refuses to bend
A link is one rigid piece of the robot — a metal bar, a casting, a section of the arm between two hinges. The word that matters most is rigid: we assume a link does not stretch, twist, or sag, so the distance between any two points on it never changes. A real aluminum bar does flex by a hair under load, but for understanding motion we treat that as zero.
Why insist on this? Because a stiff bar carries its shape with it. If you know where one end of the link goes, you instantly know where the whole link goes — every point on it moves together as one solid body. That single assumption is what makes a robot arm predictable enough to do math on. A floppy noodle would have infinitely many shapes; a rigid link has exactly one.
The joint: the only place motion lives
If links never bend, where does all the movement come from? Entirely from the joint. A joint is the connection between two links that deliberately lets them move relative to each other in one controlled way. Bend your elbow: the upper-arm bone holds still, the forearm bone swings, and the elbow is the joint that allows it. Every motion a robot makes is the sum of motions at its joints.
One rule keeps the whole picture tidy: a joint connects exactly two links — never three, never one. This is why an arm reads cleanly as bone, knuckle, bone, knuckle, all the way down. The two most common joints are the revolute joint, which rotates like a door hinge or your elbow, and the prismatic joint, which slides straight in and out like a drawer or a telescope. Almost every industrial robot is built from just these two.
Walking the arm from base to hand
Now put the pieces in a row. Starting from the floor, a typical industrial robot arm is nothing but links and joints taking turns. This alternating pattern — link, joint, link, joint — is called a kinematic chain, and it is the backbone of the whole machine.
- Start at the base: the link bolted to the floor. It never moves — it is the fixed bone everything else is measured from.
- Add the first joint: usually a revolute joint that lets the whole arm swivel left and right, like turning at the waist.
- Then a link (the 'shoulder' bone), then another joint, then a link (the 'upper arm'), and so on — bone, knuckle, bone, knuckle, climbing outward.
- Stop at the far tip, where you bolt on the tool — the gripper, welder, or suction cup. That last fitting is the end-effector, the 'hand' that actually touches the world.
Read an arm this way once and it stops looking like a tangle of metal. It becomes a simple sentence: a fixed bone, then a handful of knuckles and bones in turn, ending in a hand. The famous six-axis factory robot is just this chain with six joints in it.
Why these two pieces decide everything
It can feel surprising that a whole field rests on two ideas. But the power is in the arrangement. How long you make the links sets how far the robot can reach. How many joints you add, and whether they rotate or slide, decides how nimbly the hand can twist into awkward positions.
The cloud of space the hand can actually touch is called the workspace, and it is set entirely by the lengths of the links and the limits of the joints. Even bigger choices follow the same logic: stringing all the joints in one open line gives a flexible serial arm, while looping several chains back to a shared platform gives a stiff, fast parallel robot. Same two ingredients, very different machines.