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Drill bit shank
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The shank is the part of a drill bit grasped by the chuck of a drill. The cutting edges of the drill bit are at one end, and the shank is at the other. Different styles of shank/chuck combination deliver different performance, such as allowing higher torque or greater centering accuracy.
shank was common before 1850, and is still produced in 2009.
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Encyclopedia
The shank is the part of a drill bit grasped by the chuck of a drill. The cutting edges of the drill bit are at one end, and the shank is at the other. Different styles of shank/chuck combination deliver different performance, such as allowing higher torque or greater centering accuracy.
Brace shank
This shank was common before 1850, and is still produced in 2009. At first, the tapered shank was just rammed into a square hole in the end of the drill. Over time, various chuck designs have been invented, and modern chucks can grasp and drive this shank effectively.
- Easy to make in a forge
- Very wide tolerances allowable (not very precise)
- Moderate torque transmission but without the slippage common to round shanks
- Hard to grasp with any precision without the proper chuck
Straight shank
The straight shank is the most usual style on modern drill bits, by number manufactured. It is almost always made the same diameter as the drill bit, for economy. It's then held in a 3-jaw drill chuck. Very small bits can have straight shanks larger than the drill diameter, often for holding in a standard size collet. Large drill bits can have straight shanks smaller than their drill diameter, so that medium-size chucks can be used to drill large holes. Such a drill bit is called reduced-shank or a blacksmith's drill. One particular type of reduced-shank drill bits are Silver & Deming (S&D) bits, which have a 1/2" reduced shank, and are 6" long with a 3" flute length which are designed for slow-speed (350rpm or less) large-scale boring.
- Easy to turn on a lathe
- Minimum of turning or grinding needed if the drill bit is made from appropriately sized round bar stock
- Can be held in a collet chuck
- Can be held in a drill chuck, the commonest sort
- very accurate centering
- low torque transmission
Hex shank
The flats of a hex shank can either be machined on a round shank, as in the photograph, or can be the natural flats of hex bar stock. A hex shank can be grasped by a 3-jaw drill chuck, or can be held in a custom chuck specifically for hex shanks. 1/4 inch hex shanks are common for machine screwdriver bits, and have spread from that application to be used for drill bits that are compatible with screwdriver machinery.
- Zero manufacturing if the drill bit is made from hex bar stock
- Can be held in a drill chuck
- Can be held in a screwdriver bit chuck
- High torque transmission
- moderately accurate centring
- Cannot be held in a collet
SDS shank
The SDS shank has the advantage of a simple spring-loaded chuck, so that bits can be chucked with a simple and quick hand action. Further, the shank and chuck are uniquely suited to hammer drilling in stone and concrete. The drill bit is not held solidly in the chuck, but can slide back and forth like a piston. The hammer of the drill acts to accelerate only the drill bit itself, and not the large mass of the chuck, which makes hammer drilling with an SDS shank drill bit much more productive than with other types of shank. So, SDS shanks are most often seen on masonry drills, for which hammer drilling action is most helpful.
Rotational drive uses the sliding keyways that open to the end of the shank, which mate with keys in the chuck. The smaller indentations that do not open to the end are grasped by the chuck to prevent the drill bit falling out. The hammer of the drill hits the flat end of the shank. To allow the bit shank to slide in the chuck, the shank must be lubricated with grease.
SDS is available in four sizes, SDS, SDSplus (or SDS-plus or SDS+), SDS-Top and SDS-max. SDS-plus is the most common by count of tools manufactured, with masonry drills from 5 mm diameter to 30 mm diameter ordinarily available. The shortest SDS-plus masonry drill bits are about 110 mm overall length, and the longest 1000 mm. This long drill bit is entirely practical for use in a portable power drill, provided a 400 mm long bit is first used to start the bore off. It's very useful for installing wiring and plumbing in existing brick and stone buildings.
The SDS bit was developed by Bosch in 1975 and the name comes from the German "Steck – Dreh – Sitz" (Insert – Twist – Stay). German-speaking countries may use "Spannen durch System" (Clamping System), though Bosch uses "Special Direct System" for international purposes.
- Relatively complex to manufacture
- Superb hammer drilling performance
- Drills with a "rotation-stop" mode can use chiselling bits
- One-handed quick chucking operation
- Can only be held in an SDS chuck
- Not very accurate centring
- High torque transmission
Triangle shank
The triangle shank is almost always made by machining three flats on round bar stock. It is intended as a minor modification of a straight shank, still allowing it to be held in a 3-jaw drill chuck, but allowing higher torque transmission and limited slipping.
- Can be held in a drill chuck
- High torque transmission
- Moderately accurate centering
- Cannot be held in a collet
Morse taper shank
The morse taper twist drill bits pictured right are used in metalworking. The full range of tapers is from 0 to 7.
The morse taper allows the bit to be mounted directly into the spindle of a drill, lathe tailstock or (with the use of adapters) into the spindle of milling machines. It is a self locking (or self holding) taper of approximately 5/8" per foot that allows the torque to be transferred to the drill bit by the friction between the taper shank and the socket. The tang at the end of the taper is only for ejecting the drill bit from the spindle, with the aid of a drift.
The arbor of a drill chuck is often a morse taper and this allows the chuck assembly to be removed and directly replaced with the shank of a morse taper drill bit. A range of sleeves may be used to bring the size of the smaller morse tapers up to the size of the drive spindle's larger taper. Sockets are also available to extend the effective length of the drill as well as offering a variety of taper combinations.
The detail image shows a morse taper shank on a 16 mm diameter drill bit.
- Simple to manufacture on a lathe
- Cannot be held in a chuck or collet
- High torque transmission provided the bit is driven hard into the workpiece
- Very accurate centering
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