Why is my linear feed rate in mm/sec but joint moves are in percent?

Joint motion feed rate is a percentage of the robot's maximum because all axes are coordinated as a group. A linear move controls the TCP on a straight line, so it's specified in linear speed: mm/sec, cm/min, or inch/min (or as a time in sec/msec). Different quantity, different unit.

Does linear motion keep the tool pointing the same way the whole move?

It controls attitude by interpolating between the start orientation and the target orientation, so the pointing changes smoothly if those two differ. On a 5-axis robot, exact attitude control during a linear move only holds when the flange faces vertically, otherwise the robot uses a reachable attitude while still keeping the TCP position exact.

What happens to my program if I switch a circular move to linear?

The single circular instruction becomes two motion instructions: one that moves the tool to the former passing point and one that moves it to the end point. Going the other way, from linear to circular, cancels the taught end-point data for the arc.

Motion

Linear Interpolation

Linear interpolation is the motion mode (L) that drives the tool center point in a straight line from a start point to an end point while controlling tool orientation across the move.

What it is

Linear interpolation is one of the four FANUC motion formats you pick at the pendant: joint (J), linear (L), circular (C), and circle arc (A). When you program an L move, the controller commits to holding the tool center point (TCP) on a straight line between the taught start point and the taught end point.

Unlike joint motion, linear motion controls tool attitude during the move by blending the orientation at the start point toward the orientation at the target point. So both the position of the TCP and the way the tool is pointing are managed, not left to fall out of whatever the joints happen to do.

How it actually works

You teach an end point and the controller holds the TCP on a straight path to it. Feed rate for an L move is specified in linear units rather than a percentage: mm/sec (1 to 2000), cm/min (1 to 12000), inch/min (0.1 to 4724.4), or as a time in sec or msec. A line like "4: L P[3] 100mm/sec FINE" moves to P[3] at 100 mm/sec and stops exactly on the point.

Rotary operation is a special case of linear motion: the tool rotates about the TCP from the start orientation to the end orientation, with feed rate given in deg/sec (1 to 272). A 5-axis robot can hold exact tool attitude during a linear move only when the flange faces vertically; otherwise it substitutes a reachable attitude while keeping the TCP position exact.

How it differs

Joint motion · Joint motion (J) moves all axes to accelerate, run, and stop together, so the TCP path is usually non-linear and tool attitude is not controlled. Linear motion (L) forces the TCP onto a straight line and controls attitude across the move. Use J when only the end point matters, L when the path itself matters.
Circular motion · Circular motion (C) curves the TCP through a taught passing point to an end point in a single instruction. Linear motion has no passing point, just a straight run to the end point. Converting a C move to L splits it into two instructions: one to the former passing point, one to the end point.

Where you meet it in the field

Arc welding · The weld bead follows a straight programmed line, which is exactly what an L move holds the TCP to.
Laser welding · The beam path has to stay on a straight line, so linear motion between taught points does the work.
Spot welding · A controlled straight approach into each spot is a linear move to the end point.

Common questions

Why is my linear feed rate in mm/sec but joint moves are in percent?: Joint motion feed rate is a percentage of the robot's maximum because all axes are coordinated as a group. A linear move controls the TCP on a straight line, so it's specified in linear speed: mm/sec, cm/min, or inch/min (or as a time in sec/msec). Different quantity, different unit.
Does linear motion keep the tool pointing the same way the whole move?: It controls attitude by interpolating between the start orientation and the target orientation, so the pointing changes smoothly if those two differ. On a 5-axis robot, exact attitude control during a linear move only holds when the flange faces vertically, otherwise the robot uses a reachable attitude while still keeping the TCP position exact.
What happens to my program if I switch a circular move to linear?: The single circular instruction becomes two motion instructions: one that moves the tool to the former passing point and one that moves it to the end point. Going the other way, from linear to circular, cancels the taught end-point data for the arc.