The introduction of twist drill

<2 .="">                       twist drill & hand tools 

        Drill bits are cutting tools used to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order to create holes drill bits are usually attached to a drill, which powers them to cut through the workpiece, typically by rotation. The drill will grasp the upper end of a bit called the shank in the chuck.

The above is the definition of twist drill, and then introduce the structure of twist drill.The standard twist drill consists of three parts.

Clamping part: the end of a drill bit, used for machine tool coupling and for transferring torque and axial force.According to the diameter of twist drills, they are divided into straight shank (d < 12mm) and taper shank (d > 12mm).

Neck: a transitional part between the working part and the rear part, used for grinding wheel cutter and printing mark.Straight shank drill has no neck.

Working part: it is the main part of the drill, the front part is the cutting part, and it undertakes the main cutting work.The back end is the guide part, which acts as the guide bit and the backup part of the cutting part.

Drilling is a very common step in hardware manufacturing, and drilling in the use of cutting manufacturing, the pressure can not be underestimated, according to understanding the global consumption of twist drills up to 6 billion a year, equivalent to every person on earth will use a drill in a year.This kind of data has to make us pay attention to the selection of drill bits.

In many people's eyes, a twist drill is a drill, it is just a drill.This understanding is correct, but not entirely right, in my understanding, twist drill can be divided into two types: 1, the standard commonly used type twist drill.2. Twist drills (left or left twist drills) specially used for machining holes in automatic lathes.But what's the difference?The biggest difference is that the direction of the spiral groove is not the same, the standard twist drill for the right, left drill for the left-handed.

The helix angle of the common twist drill mainly affects the size of the rake angle of the cutting edge, the strength of the blade and the performance of chip removal.The spiral groove can be machined by milling, grinding, hot rolling or hot extrusion, and the cutting part is formed after the front end of the drill is grinded.

The top angle of the cutting part of the standard common twist drill is 118, the cross edge angle is 40 ° ˜ 60 °, and the back angle is 8° ˜ 20 °.Due to the structural reasons, the front angle is large at the outer edge and gradually decreases toward the middle, and the cross edge is a negative front angle (up to -55 °).In order to improve the cutting performance of twist drills, the cutting parts can be modified into various shapes (such as group drills) according to the properties of the materials being machined.

Left-hand bits are almost always twist bits and are predominantly used in the repetition engineering industry on screw machines or drilling heads. Left-handed drill bits allow a machining operation to continue where either the spindle cannot be reversed or the design of the machine makes it more efficient to run left-handed. With the increased use of the more versatile CNC machines, their use is less common than when specialized machines were required for machining tasks.

Screw extractors are essentially left-hand bits of specialized shape, used to remove common right-hand screws whose heads are broken or too damaged to allow a screwdriver tip to engage, making use of a screwdriver impossible. The extractor is pressed against the damaged head and rotated counter-clockwise and will tend to jam in the damaged head and then turn the screw counter-clockwise, unscrewing it. For screws that break off deeper in the hole, an extractor set will often include left handed drill bits of the appropriate diameters so that grab holes can be drilled into the screws in a left handed direction, preventing further tightening of the broken piece.

Many different materials are used for or on drill bits, depending on the required application. 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.

Steels：

Soft low-carbon steel bits are inexpensive, but do not hold an edge well and require frequent sharpening. They are used only for drilling wood; even working with hardwoods rather than softwoods can noticeably shorten their lifespan.

Bits made from high-carbon steel are more durable than low-carbon steel bits due to the properties conferred by hardening and tempering the material. If they are overheated (e.g., by frictional heating while drilling) they lose their temper, resulting in a soft cutting edge. These bits can be used on wood or metal.

High-speed steel (HSS) is a form of tool steel; HSS bits are hard and much more resistant to heat than high-carbon steel. They can be used to drill metal, hardwood, and most other materials at greater cutting speeds than carbon-steel bits, and have largely replaced carbon steels.

Cobalt steel alloys are variations on high-speed steel that contain more cobalt. They hold their hardness at much higher temperatures and are used to drill stainless steel and other hard materials. The main disadvantage of cobalt steels is that they are more brittle than standard HSS.

Others：

Tungsten carbide and other carbides are extremely hard and can drill virtually all materials, while holding an edge longer than other bits. The material is expensive and much more brittle than steels; consequently they are mainly used for drill-bit tips, small pieces of hard material fixed or brazed onto the tip of a bit made of less hard metal. However, it is becoming common in job shops to use solid carbide bits. In very small sizes it is difficult to fit carbide tips; in some industries, most notably PCB manufacturing, requiring many holes with diameters less than 1 mm, solid carbide bits are used.

Polycrystalline diamond (PCD) is among the hardest of all tool materials and is therefore extremely resistant to wear. It consists of a layer of diamond particles, typically about 0.5 mm (0.020 in) thick, bonded as a sintered mass to a tungsten-carbide support. Bits are fabricated using this material by either brazing small segments to the tip of the tool to form the cutting edges or by sintering PCD into a vein in the tungsten-carbide "nib". The nib can later be brazed to a carbide shaft; it can then be ground to complex geometries that would otherwise cause braze failure in the smaller "segments". PCD bits are typically used in the automotive, aerospace, and other industries to drill abrasive aluminum alloys, carbon-fiber reinforced plastics, and other abrasive materials, and in applications where machine downtime to replace or sharpen worn bits is exceptionally costly. It should be noted that PCD is not used on ferrous metals due to excess wear resulting from a reaction between the carbon in the PCD and the iron in the metal.

coatings：

Black oxide is an inexpensive black coating. A black oxide coating provides heat resistance and lubricity, as well as corrosion resistance. Coating increases the life of high-speed steel bits.

Titanium nitride (TiN) is a very hard ceramic material that can be used to coat a high-speed steel bit (usually a twist bit), extending the cutting life by three or more times. Even after sharpening, the leading edge of coating still provides improved cutting and lifetime.

Titanium aluminum nitride (TiAlN) is a similar coating that can extend tool life five or more times.

Titanium carbon nitride (TiCN) is another coating also superior to TiN.

Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard materials. Large amounts of heat are generated by friction, and diamond-coated bits often have to be water-cooled to prevent damage to the bit or the workpiece.

Zirconium nitride has been used as a drill-bit coating for some tools under the Craftsman brand name.

Problem of content：

The aperture increases. The error is large

The reasons causing：

1. The left and right cutting edges of the drill bit are not symmetrical and the pendulum difference is large. 2. The cross edge of the drill bit is too long. 3. Bit edge is broken. 4. There are chip deposits on the drill edge belt. 6. Too much feed. 7, drilling machine spindle pendulum or loose.

The solution：

1. Ensure that the cutting edges of the drill bit are symmetrical and the pendulum difference is within the allowed range when grinding. 2. Grinding the transverse edge to reduce the length of the transverse edge. 3. Timely find the broken edge and replace the drill bit. 4. Trim the product nodules on the edge belt with oilstone to be qualified. 5. Straighten or replace. 7. Timely adjust and maintain the drilling machine.

Problem of content：Aperture small

The reasons causing：The bit edge belt has been severely worn out 2 drilling hole is not circular aperture small.

The solution: Replace the qualified bit.

Problem of content：Vibration or unroundness during drilling.

The reasons causing:1. The back Angle of the drill bit is too large. 2. No guide sleeve or excessive clearance between guide sleeve and drill bit. 3. The left and right cutting edges of the drill bit are not symmetrical and the pendulum difference is large. 4. Loose spindle bearing. 5. The workpiece is not clamped tightly. 6, the workpiece surface is not smooth, there are holes sand. 7. There are products and cross holes inside the workpiece.

The solution: 1. Reduce the back Angle of the drill bit. 2. the drill pipe extended too long must have a guide sleeve, the use of suitable clearance of the guide sleeve or first hit the center hole and then drilling. 3. Ensure that the cutting edges of the drill bit are symmetrical and the pendulum difference is within the allowable range. 4. Adjust or replace the bearing. 5. Improve fixtures or positioning devices. 6. Replace qualified blank. 7. Change the process sequence or the structure of the workpiece.

Problem of content: The hole position is out of tolerance and the hole is askew.

The reasons causing: 1. The drill tip of the bit has been blunt. 2. The left and right cutting edges of the drill bit are not symmetrical and the pendulum difference is large. 3. The cross edge of the drill bit is too long. 4. Excessive clearance between bit and guide sleeve. 5, spindle and guide sleeve axis is not coaxial, spindle and table is not vertical. 6. The drill bit vibrates during cutting. 7, the workpiece surface is not smooth, there are holes sand. 8. There are products and cross holes inside the workpiece. 9. The distance between the bottom end face of the guide sleeve and the working surface is far, and the length of the ambition sleeve is short. 10. The workpiece is not clamped tightly. 11. The workpiece surface is inclined. 12. Uneven feed.

The solution: 1. Regrind the drill bit. 2. Ensure the left and right symmetry of the drill bit and the pendulum difference is within the allowed range when grinding the edge. 3. Grinding the transverse edge to reduce the length of the transverse edge. 4. Adopt appropriate clearance guide sleeve. 5. Correct the fixture position of the machine tool and check the perpendicularity of the spindle of the drilling machine. 6, first hit the center hole and then drilling, using guide sleeve or changed to the workpiece rotation. 7. Replace qualified blank. 8. Change the process sequence or the structure of the workpiece. 9. Lengthen the guide sleeve length. 10. Improve fixtures and positioning devices. 11. Correct positioning device. 12, to make uniform feed.

Problem of content: Bit broken

The reasons causing: 1. Improper selection of cutting parameters. 2. The drill bit breaks the edge. 3. The cross edge of the drill bit is too long. 4. The drill bit is blunt, and the blade belt is severely worn and conical. 5. The distance between the end face of the guide sleeve bottom and the surface of the workpiece is too close, and it is difficult to discharge. 6, cutting fluid supply is insufficient. 7. The cutting blocks the spiral groove of the drill bit, or the cutting rolls on the drill bit, so that the cut blood cannot enter the hole. 8. The guide sleeve is worn into an inverted cone, and the drill bits are sandwiched between the drill bit and the guide sleeve when the tool is withdrawn. 9. The position at the end of the fast stroke is too close to the workpiece, and the error is large when the fast stroke turns to feed the workpiece. 10, when drilling through the hole, due to the rapid decline in feed resistance and feed suddenly increased. 11, workpiece or fixture rigidity is insufficient, drilling through the elastic recovery, so that the feed suddenly increased. 12, feed screw wear, power head hammer weight is insufficient. 13. The reverse pressure of the power hydraulic cylinder is insufficient. When the hole is drilled, the power head will automatically fall down to increase the feed. 14, cone handle flat tail broken.

The solution: 1, reduce the feed and cutting speed. 2, timely detection of edge breakage, when the processing of harder steel pieces, the back Angle should be appropriately reduced. 3. Grinding the transverse edge to reduce the length of the transverse edge. 4. Replace the drill bit in time, and all the worn parts will be removed when the edge is worn. 5. Increase the distance between the guide sleeve and the workpiece. 6. When the blood cutting nozzle is aligned with the machining hole, increase the cutting fluid flow. 7. Reduce cutting speed and feed, adopt chip breaking measures or hierarchical feed mode to make drill bit exit times. 8. Change the guide sleeve in time. 9. Increase working distance. 10. Fix and grind the top Angle of the drill bit, reduce the axial force of the drill hole as much as possible, except when inertia is about to be drilled through, change to manual feed, and control the feed quantity. 11. Reduce the elastic deformation of machine tools, workpieces and clamps, improve clamping positioning, increase the rigidity of workpieces and clamps and increase the secondary feed. 12, timely maintenance of the machine tool, increase the weight of the power head hammer is shown by the addition of secondary feed. 13, the estimated shrinkage of the casting to reduce the feed. 14. Replace the drill bit and pay attention to oil stains on the taper handle.

Problem of content: Low bit life

The reasons causing: 1. Same as the reason why the drill bit broke. 2, drilling concerns about cutting part of the geometry and material processing is not suitable for the low life of the bit.

The solution: 1. Same as the solution of bit breaking. 2, when processing copper pieces, the bit should choose a small back Angle, to avoid the bit automatically into the workpiece, so that the feed suddenly increased: when processing low carbon steel, can appropriately increase the back Angle, to increase the life of the bit processing harder steel, can use double bit top Angle, separate chip groove or grinding transverse edge, etc., to increase the life of the bit. 3. Use new and applicable high speed steel (aluminum high speed steel, cobalt high speed steel) drill bits or coated tools to eliminate abnormal conditions such as sand inclusion and hard spots in the workpiece.

Problem of content: The hole wall has a rough surface.

The reasons causing: 1. The drill bit is not sharp. The back corner is too big. 3. Too much supply. 4, cutting fluid supply is insufficient, cutting fluid performance is poor. 5. Chip blocks the spiral groove of the drill bit. 6, fixture rigidity is not enough workpiece material hard too low.

The solution: 1. Sharpen the drill bit. 2, use the appropriate back Angle. 3. Reduce the supply. 4, increase the cutting fluid flow to select the cutting fluid with good performance. 5. Reduce cutting speed and feed, adopt chip breaking measures or hierarchical feed mode to make drill bit exit times. 6. Improve fixtures. 7, increase the heat treatment process, appropriately improve the hardness of the workpiece.


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