This research aims to achieve a new grinding robot system that can grind an object into desired shape with force-sensorless feed-forward control. In order to grind the target object into desired shape with sufficient accuracy, the hand of the robot arm have to generate desired constrained force immediately after the grindstone being contacted with the metal object to be ground. However, a time delay arises in the transition of constrained force while the hand of a robot arm generates desired constrained force stemming from a first-order time delay caused by dynamics in motors. This paper suggests some methods how to improve transient response of constrained force that influences shape-grinding accuracy directly. Results observed by real grinding experiment have confirmed how our proposed method effectively improved to shorten the time delay of constrained force.