An apparatus includes a base, a cover and a rotating shaft. The base includes a platform formed on the base for placing a workpiece, a supporting post protruding from the platform, and a pair of opposite connecting seats protruding from an end of the base. Each connecting seat defines a pin hole. The cover includes a board portion defining a positioning hole corresponding to the supporting post, a hollow rotating portion defining a through hole, and a connecting portion interconnecting the board portion and the rotating portion. The rotating shaft extends through the rotating portion with two ends received in the pin holes, rotatably interconnecting the cover to the base. The radius of the through hole is larger than that of the rotating shaft. When the board portion presses on the workpiece, the supporting post lifts the board portion.