Flatness of a surface on a part is the deviation from a true plane or straight edge.

Rubber Product (Unground)

Molded Surfaces (unground) will be flat within 0.25mm (.010in).

Fig 7 On a cup as illustrated, the bottom can be concaved or convexed by no more than 0.25mm (.010in).

Rubber Product with Metal Insert

Surfaces that are ground after molding will be flat within 0.12mm (.005 in). (Allowance must be made for removal of stock during grinding operation.)

In Sketch 1 Fig 8 after molding, deviation from the true plane can be held to 0.25mm (.010in).

In Sketch 2 Fig 8 after grinding, deviation can be held to 0.12mm (.055in) but dimension "H" will necessarily be affected.