When reworking, mostly only the circular edges are deburred ( only one in a blind hole ) . The ends of the holes are converted into low-height truncated cone surfaces. Because very little material is removed during the trimming, which can already with a hand-held countersink ( countersink with handle like a screwdriver ) take place. Its point angle and that of a corresponding machine-driven countersink is i. d. R. 90 °. Alternatively, a cylindrical drill (point angle ≥ 120 °) with a larger diameter or a center drill (point angle 60 °) can be used.
The mechanical processing of bores to adapt them to screw heads is also referred to as countersinking. Space is created for the heads of either standard countersunk screws (90 ° conical head) or for screws with a cylindrical head.
A 90 ° countersink (as for deburring) is used for the usual countersunk screws . The resulting truncated cone is just high enough that the top of the screw head is flush with the surface of the workpiece in which the hole is located.
A so-called flat countersink is used for screws with a cylindrical head ( cylinder head screw , hexagon socket screw, etc.) . It has a purely cylindrical shape (a drill usually has a conical tip) and transforms the upper part of the existing cylindrical bore into a larger diameter cylinder stub. Such a countersink is often provided with a cylindrical guide piece on its front side so that the bore and countersink are coaxial .
Flat countersinks or countersinks do not differ in principle from generally cylindrical end mills . Their specialty is the provision of tool sets with milling cutter diameters that correspond to the dimensions of the standardized screw head diameters in metal technology. Especially in the wood industry, for purposes other than screw connections i. d. Usually relatively large cylindrical countersinks ( Forstner bits ) are used. One example is the cylindrical recesses in doors of kitchen furniture to accommodate hinges ( pot hinges ).
Countersinks have a cutting tip in the shape of a cone . Usually the countersunk head diameter is larger than the shaft diameter. In order to achieve smoother running when countersinking, the number of cutting edges is usually uneven. Countersinks for holes often have 3 cutting edges or 5 cutting edges. Deburring tools for thin-walled tubes made of tough material such as stainless steel can, however, also have a much higher number of cutting edges.
The point angle is usually
- Countersinks are a common form of countersinks , which instead of several straight cutting edges are provided with a single hole at an angle of about 45 ° to the countersink axis. The resulting circular cutting edge with the conical surface forms two cutting edges - one engages clockwise, the other counterclockwise. The chips flow through the hole, which prevents the chips from jamming and the countersink from getting stuck in the workpiece. The countersink is free of burrs and chatter.
- Counterbores , even cutter as mentioned, have a straight cutting edge, and generate a planar depression (180 degrees). They usually have two, three or four cutting edges. Countersinks are used to make countersinks for screws with cylindrical heads (e.g. socket head screws). Flat countersinks or countersinks do not differ in principle from cylindrical end mills . However, their diameter is coordinated with the standardized screw head diameters . In timber construction , cylindrical depressions are generally used. d. Usually carried out with Forstner bits. Countersinking with a Forstner bit should be carried out before the actual drilling, as these drills run easily if their center point cannot be supported in the material.
- Countersinks are conical , flat or flat countersinks that have an additional, central pin in the rotationally symmetrical axis, which guides the tool in the bore and thus enables precise, chatter-free work. The end of the pin itself is provided with a bevel , which makes it easier to insert it into the borehole. In some designs, the pin can be removed to make it easier to regrind the cutting edges.
Counterbores or combination tools are used to drill and countersink in one operation:
- Countersinks are ring-shaped tools with flat or conical cutting edges that are attached to a twist drill and fixed with one or more screws (e.g. grub screws ).
Countersinks are only suitable for softer materials such as wood, wood-based materials and plastics, as not much force can usually be transmitted via the clamp on the shank of the drill.
- Combination tools usually have the shape of a twist drill with an integrally formed countersink. The diameter of the drill widens towards the end of the shank, so that the drilled hole can be provided with a chamfer at the same time.
- Counterbores , usually called burrs , are twist drills with three or four cutting edges. With them, pre-drilled holes are drilled. Since they are intended to expand an existing hole over its entire length, they are counted as drills rather than countersinks. In terms of the procedure, however, they are used to drill open and not lower, which is why they do not belong to the group of sinkers.
- Božina Perović: Machining and removing manufacturing processes. Expert Verlag, Renningen 2000, ISBN 3-8169-1911-1 .
- Erich Renner, Heinz Frisch, Erwin Lösch: Basic level metal technology. 14th edition, Bildungsverlag Eins, Cologne 2015, ISBN 978-3-8242-0423-6 .
- Manfred Kluge: Metal construction and manufacturing technology. 10th edition, Verlag Europa Lernmittel, Haan Gruiten 2014, ISBN 978-3-8085-1170-1 .
- List of machining processes
- Center punch (tool)
- List of tools
- Chip formation
- High speed machining
- Energy conversion and heat during machining
- Manufacturing technology VL 08: drilling, countersinking, planing, slotting, broaching, sawing (accessed on December 1, 2016)
- Lower technical knowledge (accessed December 1, 2016)
References and comments
- The structure and use of countersinks , October 18, 2018, IN: www.Ruko.de; accessed in April 2019
- The word part flat or plan does not refer directly to the generated geometric-cylindrical shape, but to its underside, the flat (and ring-shaped) surface for the contact between the underside of the screw head and the workpiece material. The cone of the countersink, on the other hand, refers to both, because the geometric-conical surface is also the force-transmitting contact surface with the countersunk screw.