|Driling, Countersinking and Counterboring - Course: Technique for manual working of materials. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 18 p.)|
The removal of chips is effected by the combination of the feed and the rotary motion. The rotary motion can be made by the tool or by the workpiece (the latter being the case when drilling by turning machines). The chips are removed by the principal cutting edges only. This is done at the bottom of the drilled hole. For this purpose, the material of the drill roust be harder than that of the workpiece to be drilled.
By the friction in the bore hole, tool and workpiece are heated up. Too strong heating up leads to a loss of hardness Of the drill. Therefore, a suitable lubricating and cooling agent must be used. The best suitable agents are hydrated fluids with soap or oil constitutents.
When the water evaporates, it absorbs great quantities of heat, the rest of the constituents enables a reduction of the friction and smooth bore hole walls.
Figure 11 - Operation of drilling
By drilling, only roughing quality of the wall of the bore hole and a dimensional accuracy of 0.05 to 0.1 mm is achieved, more accurate bore holes must be reamed afterwards.
4.1. Materials of the drills
In general, drills consist of tool steel, high-speed steel and super high-speed steel. In addition, the cutting edges can be tipped with cemented carbide tips.
Modern drills, as to their hardness, are influenced further by techniques such as nitrogen freezing hardening and various coating techniques (evaporation coating with titanium nit ride).
Also, drills of a combination of metallic and ceramic materials are being developed.
All these changes result in a considerably wider field of application and longer service life of the drills under the special conditions of modern production.
Even after resharpening of the cutting edges (due to abrasion), these tools are still more efficient than traditional drills.