Countersink
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In
The basic geometry of a countersink (cutter) inherently can be applied to the plunging applications described above (axial feed only) and also to other
Types
| ||
| Normal countersunk angle | Type of thread | |
|---|---|---|
| 60° | ||
| 82° | American inch screws (UNC, UNF, UNS) | |
| 90° | ISO metric | |
| 100° | British imperial inch screws (BA, BSF, BSW, etc.), Unified National (i.e. inch size) Aviation fasteners[2] | |
| 110° | ||
| 120° | ||
Machining
A countersink may be used in many tools, such as
Cross-hole countersink cutter
A cross-hole, "Weldon style" or "zero flute" countersink is a cone-shaped tool with a cutting edge provided by a hole that goes through the side of the cone. The intersection of the hole and cone form the cutting edge on the tool. The cone is not truly symmetrical as it is essential that the cone retreats away from the cutting edge as the tool rotates providing clearance. If this does not occur the cutting edge will lack clearance and rub rather than bite into the material. This clearance is referred to as cutting relief.
These tools are best used as deburring tools, where the
Fluted countersink cutter
The
Back countersink
A back countersink, also known as an inserted countersink, is a two piece countersink used on tough to reach areas. One component is a rod that is inserted into the existing hole in the workpieces; the other component is the cutter, which is attached to the rod, or extends out of it, after it is in position.
Speeds, feeds, and avoiding chatter
It can often be difficult to avoid chatter when cutting with countersink cutters. As usual in machining, the shorter and more rigid the setup, the better. Better-quality fluted countersink cutters sometimes have the flutes (or at least one flute) at an irregular pitching. This variation in pitching reduces the chance of the cutting edges setting up a harmonic action and leaving an undulated surface. This surface ripple is also dependent on the surface speed of the cutting edges, material type, and applied pressure (or feed rate); once started it is hard to remove. Too light a feed tends to increase chatter risk. As in many other machining operations, an appropriate response to the chatter may be to decrease speed and increase feed. On a drill press, the slowest available spindle speed is usually best. With a variable-speed handheld power drill, the trigger is best squeezed lightly to yield a low spindle speed.
Good chatter-free results can usually be had by countersinking by hand (as opposed to running the tool in a powered spindle). The slow speed and sensitive feed tend to prevent chatter. With a quarter-inch-hex shank, the countersink cutter can be held with a screwdriver handle of the indexable-bit type.
Form countersinking
Form countersinking, also known as dimpling, is a countersink that is formed into sheet metal to increase the strength of a structure as the countersinks of multiple pieces nest together. There are two processes for producing formed countersinks: coin dimpling and modified radius dimpling.[4] Such dimples in fairly thick sheet can even be tapped to yield a threaded hardpoint on the sheet without the bother and expense of welding a nut to the sheet. This style of construction is often seen in modern household appliance design, because it allows the product to be lower-priced, and the quality can still be good as long as the sheet is thick enough.
See also
References
- ISBN 978-0-7680-7646-2.
- ^ MIL-STD-1515A, MILITARY STANDARD: FASTENER SYSTEMS FOR AEROSPACE APPLICATIONS
- ^ Reithmaier 1999, p. 90.
- ^ Reithmaier 1999, p. 91.
Bibliography
- Reithmaier, Lawrence W. (1999), Standard aircraft handbook for mechanics and technicians (6th ed.), McGraw-Hill Professional, ISBN 978-0-07-134836-2.