The two items of the half-lap joint have to be flush on the high and backside. Sometimes sawdust in your work space can elevate the board you are reducing off the floor of the noticed desk. This will provide you with a nasty half-lip joint. To remedy this drawback, shim out the fence so the entrance desk leaves a spot for sawdust to fall into, away from the desk.
Draw a line on the sacrificial fence to point the place the fence meets the desk, then take away the fence.