tailieunhanh - Drilling and Associated Technologies Part 3

Tham khảo tài liệu 'drilling and associated technologies part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Drilling and Associated Technologies 107 ing considerable scope in diametric sizing of holes. These drills should be selected for the minimum overhang and need to have both the drill s and machine tool s centrelines parallel 23 to within mm or better. By utilising an indexable drill on a turning operation they can be beneficial when attempting to start drilling angled or uneven workpiece surfaces as illustrated by some of the surface faces depicted in Fig. 54. Indexable drills employed for such non-flat workpiece faces obviates the need for either a previous counter boring or spotfacing operation. The major advantage of indexable drills over either their HSS or carbide twist drill counterparts is their ability to run at much higher rates. For example and by way of simplistic comparison if a Ộ18 mm hole has to be drilled in free-cutting stainless steel then an HSS twist drill - can be run at m min-1 with a mm rev-1 feed would produce 280 rpm at 57 mm min-1 Solid carbide twist drill - can be run at 61 m min-1 with a mm rev-1 feed would produce 1 019 rpm at 103 mm min-1 Indexable drill - can be run at 170 m min-1 with a mm rev-1 feed would produce 2 852 rpm at 217 mm min-1. NB The indexable drill can be run up to mm rev-1 and if one were to attempt to utilise a twist drill with the cutting data shown above then it would either burn-out or catastrophically fail in the endeavour. An important safety note when using these indexable drills in turning operations is that the high penetration rates coupled to the exit of a through hole in the part creates a slug which is thrown clear from either the chuck or from the rear of the bore of the machine s headstock at tremendous force. If the machine tool is not guarded appropriately it could prove to be hazardous to any operator in the local vicinity. Counter-Boring Trepanning A counter-boring operation is an often utilised technique for the enlargement of previously manufactured holes normally