PURPOSE: To bore a stepped but near to tapered bore in one boring and to reduce the number of production processes by roughly boring it with a drill furnished with a point cutting blade the rotational locus of which makes a circle and a diametrically extensional cutting blade provided on the base end side rather than this point cutting blade and gradually extending its cutting blade outer diameter toward the base end side.
CONSTITUTION: A prepared hole is roughly bored by a drill, and thereafter, it is finished and bored as a tapered bore with a tapered tool. This rough boring is carried out with the drill furnished with point cutting blades 20a, 21a the rotational locus of which makes a circle and diametrically extensional cutting blades 21b, 20c, 21d provided on the base end side rather than these point cutting blades 20a, 21a and gradually extending their cutting blade outer diameter toward the base end side. Consequently, it is possible to bore a stepped but near to tapered bore in one boring and to reduce the number of production processes.
HIRAI ISAO
IWATA YOSHIKAZU
KASUYA HIROSHI
MITSUBISHI MATERIALS CORP