An extruder or other tool head of a three-dimensional printer is instrumented to detect contact force against the extruder, such as by a build platform or an object being fabricated. One frame period has m different sub-frame periods SF1, SF2,. Replacing means is provided to prepare a plurality of master media, and replacing the master medium after magnetic transfer is performed a predetermined number of times. The second delay element also outputs a second clock signal that is delayed relative to the first clock signal by the first time period.