Apparatus and method for accurately sizing pieces of insulation and for placing formed pieces of insulation into slots of a magnetic core. Dielectric strip material is fed to a sizing and forming station by a strip accumulating and urging device, and dielectric material pieces of preselected sizes are precisely formed. The core is precisely aligned relative to the forming member so that the insulation pieces may be inserted into a core slot which the insulation piece has been particularly formed to fit. Preferred form of apparatus includes constant force mechanisms for accurately returning strip material length determining, severing, and forming mechanisms to respective dwell positions; positive acting cams, e.g., conjugate and face cams, in selected mechanism drive trains; and positively controlled compensating means for selectively positioning a core support relative to a datum plane of the apparatus. Means in the form of relatively movable pressure bars are urged, by constant force mechanisms, toward forming devices having different sizes and shapes to form insulators there against. In another embodiment, a modified guide ring for forming and inserting devices is provided.