When a pattern is imbedded in the sand, the latter is closely compressed all about it, and slightly adheres.
Fig. 182. Rapping and Draw Plate.
The molder is therefore in the habit of rapping the pattern gently in order to loosen it in the sand before attempting to draw it. If the pattern is not provided with a metal plate, the molder will drive the sharp point of a lifter into the wood and strike it alternately on opposite sides and at the same time use it to lift the pattern from the sand. This mars the pattern and will in time ruin it.
The rapping plate shown in Fig. 182 is a piece of thin metal 1/8 to 3/16 inch thick, inserted so that it is flush with the parting face of the pattern and is held by wood screws with countersunk heads. These plates are drilled and tapped for a 3/8-inch screw and should be the same for all patterns in the foundry so that one set of rods can be used interchangeably. The method of using is to screw the rod into the plate and rap it gently to and fro until the pattern has been loosened, when it may be lifted.
The Acme key rapping plates, Fig. 183, are quickly attached to the pattern, the mortise being bored out with a bit.
For small patterns, one rapping plate will be sufficient and this should be so placed that the hole for the lifting rod comes directly over the center of gravity of the piece. This prevents tilting of the pattern as it is lifted from the sand. However, if there is a portion of the pattern away to one side of the center of gravity, which by its nature is liable to resist drawing more than the other side, the rapping plate should be located away from the center of gravity toward this side of the pattern so that in drawing the lift will be nearly over the resultant center of resistance. For medium-sized patterns, two rapping plates should be provided so that the pattern can be raised from two opposite sides. For still larger patterns three or four rapping plates are used, the object being to give such perfect control when drawing that there can be no tearing away of the sand.
Fig. 183. Key Draw Plate.