The forward kinematic setup works differently from inverse kinematic setups. Whilst IK is designed to invert the movement of the hierarchy, FK moves down the hierarchy so that each movement affects the next joint down the chain. This FK arm setup is made up of three joints. A shoulder joint, elbow joint, and wrist joint.
Make sure the joints are oriented to world so X is pointing down the bone. Y has a tendency to point upwards. Orientation is important. If the joints are not oriented properly, the rig may flip or break. Next, some controls need to be created to control the arm. Create three identical controls at the centre of the grid and freeze their transformations. Each control is going to work with a joint. Therefore one control is called the shoulder_FK_Control and is associated with the shoulder joint. Another is called the elbow_FK_Control and is associated with the elbow joint. The final control is called the wrist_FK_Control and is associated with the wrist joint.
Buffer each of these controls and name these control buffers correctly (eg. shoulder_FK_ControlBuffer, elbow_FK_ControlBuffer, wrist_FK_ControlBuffer).
Parent constrain these buffers to the joints they are associated with (eg. Shoulder_FK_ControlBuffer and shoulder_Joint). Select the buffer first then the joint and parent constrain (with ‘maintain offset’ off). This will snap each of the controls into place and force the buffer to take in all the translate information, but leave the control inside the buffer untouched.
Now delete these constraints in the outliner as they will not be needed anymore. Their purpose was to put the buffers and controls in place and will not be needed any further.
Orient constrain the joints to the controls so that the control is the master and the joint is the slave. Select the shoulder_FK_Control and then the shoulder_Joint and orient select orient contraint. Repeat this with the other two controls and joints.
Parent the left wrist buffer (child) to the elbow control (parent) by dragging the wrist buffer into the elbow control in the outliner or select the buffer then the control and parent them. Repeat this step with the elbow buffer as the child and the shoulder control as the parent.
The FK arm rig is now complete.