| 摘要 |
The hollow body (1) contains recesses (1c) in its forward end open in the drilling direction in which cutters (2) are inserted, these protruding in the axial direction and also beyond the body surface on the inside and outside. The body has portions (1d) of increased cross-section (1d) adjacent to the cutter recesses. The cutter recesses can protrude beyond the body wall at the inside and/or outside. The wall thickness (Q) at these points can be between 1.2 and 1.8 times that of the remainder, while their length (U) either side of the recesses can be between 0.3 and 1 times that (B) of the recesses. At the base of each recess, the thickened portion can extend away from the working direction for a distance (G) equal to between 0.5 and 1.4 times the width (B). |
