Drill bit
Many hard materials, such as carbides, are much more brittle than steel, and are far more subject to breaking, particularly if the drill is not held at a very constant angle to the workpiece; e.g., when hand-held.
This style of bit requires a starter hole, but does not bind or suffer premature wear so long as a suitable feed rate is used.
However, they are not the best tool for routinely drilling deep holes, as they require frequent withdrawal to clear the flutes of swarf and to prevent breakage of the bit.
Unibits are ideal for use in electrical work where thin steel, aluminum or plastic boxes and chassis are encountered.
The short length of the unibit and ability to vary the diameter of the finished hole is an advantage in chassis or front panel work.
An additional use of unibits is deburring holes left by other bits, as the sharp increase to the next step size allows the cutting edge to scrape burrs off the entry surface of the workpiece.
However, the straight flute is poor at chip ejection, and can cause a burr to be formed on the exit side of the hole, more so than a spiral twist drill bit turning at high speed.
Although it is claimed that the stepped drill was invented by Harry C. Oakes it was in fact conceived by George Godbold and first produced by Bradley Engineering, Wandsworth, London in the 1960s and named the Bradrad.
A workpiece machined between centers can be safely removed from one process (perhaps turning in a lathe) and set up in a later process (perhaps a grinding operation) with a negligible loss in the co-axiality of features (usually total indicator reading (TIR) less than 0.002 in (0.05 mm); and TIR < 0.0001 in (0.003 mm) is held in cylindrical grinding operations, as long as conditions are correct).
These core drill bits are similar in appearance to reamers as they have no cutting point or means of starting a hole.
A common use is to allow the head of a bolt or screw, with a shape exactly matching the countersunk hole, to sit flush with or below the surface of the surrounding material.
Almost all indexable drill bits have multiple coolant channels for prolonged tool life under heavy usage.
They are also readily available in odd configurations, such as straight flute, fast spiral, multiflute, and a variety of cutting face geometries.
Trepans usually have multiple carbide inserts and rely on water to cool the cutting tips and to flush chips out of the hole.
Metals are typically isotropic, so even an ordinary twist drill bit will shear the edges of the hole cleanly.
Modern spoon bits are made of hand-forged carbon steel, carefully heat-treated and then hand ground to a fine edge.
The bit includes a center brad point which guides it throughout the cut (and incidentally spoils the otherwise flat bottom of the hole).
The cylindrical cutter around the perimeter shears the wood fibers at the edge of the bore, and also helps guide the bit into the material more precisely.
The radial cutting edges remove a slice of wood of thickness equal to the pitch of the central screw for each rotation of the bit.
The Irwin or solid-center auger bit is similar, the only difference being that one of the cutting edges has only a "vestigal flute" supporting it, which extends only about 1⁄2 in (13 mm) up the shank before ending.
The diameter of auger bits for hand braces is commonly expressed by a single number, indicating the size in 16ths of an inch.
Many European kitchen cabinets are made from particle board or medium-density fiberboard (MDF) with a laminated melamine resin veneer.
A Forstner bit could bore the mounting hole for the hinge, but particle board and MDF are very abrasive materials, and steel cutting edges soon wear.
While the shape of the cutting edges is different, and one uses screw threads and the other a twist bit for the pilot, the method of adjusting them remains the same.
Rotating the bit also brings the cutting edges onto a fresh portion of the hole bottom with every hammer blow.
Masonry bits up to 1,000 mm (39 in) long can be used with hand-portable power tools, and are very effective for installing wiring and plumbing in existing buildings.
A great number of holes with small diameters of about 1 mm or less must be drilled in printed circuit boards (PCBs) used by electronic equipment with through-hole components.
Most PCBs are made of highly abrasive fiberglass, which quickly wears steel bits, especially given the hundreds or thousands of holes on most circuit boards.
To solve this problem, solid tungsten carbide twist bits, which drill quickly through the board while providing a moderately long life, are almost always used.
Very high rotational speeds—30,000 to 100,000 RPM or even higher—are used; this translates to a reasonably fast linear speed of the cutting tip in these very small diameters.
