end and the same procedure is done to the section "B". This is repeated as required with the operator sighting along the length of the timber from time to time to check on straightness of the timber. When the timber is almost straight, the guard is replaced and the last cut is made in the normal way. Twisted material is treated in a similar way. The operator lays the timber on the bed of the machine and rocks it slowly from side to side to estimate the amount of twist. If there is, say, 20mm of twist in the board, he holds the board level and takes 10mm off one end, then repeats it for the other end. Jointers are also used for making rebates (also known as rabbets in North America) in finished timber. The fence is set to the width of the rebate and the infeed table is set to the depth. A jointer that is used for rebating has the outside ends of its blades also sharpened and set with a small clearance from the cutter head. A jointer cannot be used to create a board of even thickness along its length. For this task, after jointing one face, a thickness planer is used. Thickness planers and jointers are often combined into one machine, with the work piece passing underneath the same rotating blade for thicknessing, but in the opposite direction. In the US this is called a planer–thicknesser or over-and-under. See also combination machine Thickness planer References Woodworking machines pdc:Howwel