Abstract: A large dimension beam cutting, detailing and mitering attachment for removable connection to the base plate of a portable circular power saw, wherein the cutting blade has been removed therefrom, and which includes a flat base attachment plate having a flat cutter bar and endless saw chain arranged thereon, with the bar fixedly attached thereto and depending at right angles therefrom. A sprocket associated with the saw chain is mounted on the drive shaft of the saw and the base attachment plate flatly receives and has means for attaching the cicular saw base plate thereto so that beams may be smoothly cut or mitered when the attachment plate is flatly disposed thereon and moved thereover. This enables the saw chain to be used in the same manner as a circular saw.