抄録
The drilling conditions to obtain a good shape and fine accuracy in dimension of drilled holes in metallic sheets of mild steel (S35C), brass (6/4 brass) and duralumin (2017) as well as in nonmetallic sheets of bakelite, vinyl and acryl with conventional standard twist drills or a fishtail point special drill were examined. The results obtained are as follows:
In the case of drilling metallic sheets with conventional standard drills of different point angles, the drillled hole shows more enlarged hole diameter, more distorted shape and larger pile-up at the hole-inlet as the drill point angle becomes wider.
In the case of drilling nonmetallic sheets, the influence of the point angle is little and a circular hole is easily obtainable. And if the point angle is obtuse, the burr at the hole-outlet is little even in metallic sheets, but the cracks tend to appear at the hole-outlet in nonmetallic sheets.
In the case of drilling with a special drill, the enlargement of hole diameter is smaller compared with the case with a standard drill and a circular hole is obtainable. But, in drilling nonmetallic sheets, large cracks tend to appear at the hole-outlet and the burr at the hole-outlet is smaller compared with the case of standard drill.
For the drilling of bakelite, it has been proved that a good drilled holes is obtainable by drilling after placing bakelite on top of aluminium sheet. The drilled hole shows no cracks, little enlargement of hole diameter and little pile-up or burr.
