paraphrase
Twist drill is a tool for drilling round holes in workpieces by rotating the drill relative to the fixed axis.It is named for the spiral shape of its chip - holding groove, which resembles twist.Spiral grooves have 2 grooves, 3 grooves or more, but 2 grooves are the most common.Twist drill can be clamped on manual and electric hand-held drilling tools or drilling machines, milling machines, lathes and processing centers.The drill material is usually high speed tool steel or hard alloy.
constitute
Standard twist drill.The twist drill consists of a shank, a neck and a working part.
basic angle
（1）helical angle β the spiral Angle is the Angle between the outermost circular spiral line on the spiral groove of the drill bit and the axis of the drill bit when it is expanded into a straight line.Since the lead distance of each point on the spiral groove is the same, the spiral Angle at different diameters of the drill bit is different. The spiral Angle at the outer diameter is the largest, and the closer to the central spiral Angle, the smaller.Increasing the spiral Angle will increase the front Angle, which is conducive to chip removal, but the drill stiffness decreases.Standard twist drill helix Angle is 18 ° ~ 38 °.For bits with smaller diameter, the helical Angle should be smaller to ensure the rigidity of the bit.
（2）angulus parietalis γOm Because the front cutter face of twist drill is spiral, the front Angle of each point on the main cutting edge is different.From the outer circle to the center, the front Angle decreases.Blade Angle is about 30 ° before, near the chisel edge is about 30 °.Chisel edge of anterior horn for - 50 ° ~ 60 °.
（3）relief angle αOm The back Angle of the selected point on the main cutting edge of a twist drill，It passes through the feed back Angle in the point column section aOm expressive.The column section Select a point across the main cutting edge M，Make a straight line parallel to the axis of the bit, which rotates around the axis of the bit to form a cylinder. αOm It also varies along the main cutting edge，Closer to the center αOm bigger. Twist drill the back corner of the outer circle a， Usually take 8 ° ~ 10 °, chisel edge after in the Angle of 20 ° ~ 25 °.This can make up for the effect of reducing the back Angle of each point on the main cutting edge due to the axial feed movement of the drill bit, and adapt to the change of the front Angle.
（4）tool cutting edge angle κrm The main offset Angle is the Angle between the projection of tangent line of the selected point m of the main cutting edge on the base plane and the feeding direction.The base surface of a twist drill is a plane containing the drill axis through the selected point of the main cutting edge.Since the drill bit's main cutting edge does not pass through the axis line, the base plane of each point on the main cutting edge is different, and the main deflection Angle of each point is also different.When the top Angle is grinded out, the main declination Angle of each point is also determined accordingly.The principal declination Angle and the top Angle are two different concepts.
（5）Front Angle 2φ Edge Angle is the Angle between two main cutting edges projected on a plane parallel to them.Smaller blade Angle is easy to cut into the workpiece, axial resistance is small, and the cutting edge working length increases, the nominal cutting layer thickness decreases, is conducive to heat dissipation and improve tool durability;If the front Angle is too small, the strength of the bit will weaken, deformation will increase, torque will increase, and the bit will break easily.Therefore, we should according to the strength and hardness of the workpiece material grinding reasonable front Angle, standard twist drill front Angle 2φ is 118 °
（6）Horizontal edge Angle Ψ The oblique Angle of the transverse edge is the projection Angle between the main cutting edge and the transverse edge on the plane perpendicular to the drill axis.When the twist drill after the cutter surface grinding,ψ form naturally.Horizontal edge Angle ψ The transverse blade length and axial resistance decrease.The horizontal edge Angle of the standard twist drill is about 50 ° ~ 55 °.
process influence
(1) the diameter of twist drill is limited by the aperture, spiral groove makes the drill core thinner, and the drill bit stiffness is low;Only two ribbed belt guide, the axis of the hole is easy to deflect;Transverse edge makes centering difficult, axial resistance increases, and the drill bit is easy to swing.Therefore, the shape and position error of drilling hole is large.
(2) of the twist drill knife before and after the blade surface is curved, at various points along the cutting edge of rake Angle, Angle of each are not identical, the chisel edge rake Angle of 55 °.Poor cutting conditions;Cutting speed along the cutting edge distribution is not reasonable, the lowest strength of the tip cutting speed is the largest, so serious wear.Therefore, the machining hole precision is low.
(3) the whole cutting edge of the main cutting edge of the drill takes part in cutting, and the cutting speed of each point on the edge is not equal, which is easy to form spiral chip and difficult to remove chip.Therefore, the chip and the hole wall extrusion friction, often scratches the hole wall, the surface roughness after processing is very low.
operational approach
"Less grinding" is the first "no grinding", get the drill in a hurry to grinding, must be blind grinding.Only by placing the edge in a good position before grinding, can we lay a solid foundation for the next step of "grinding well", which is quite important.
Here with four formula to guide the cutting edge grinding process, the effect is better.
Formula 1: "edge balance wheel face."
This is the first step in the relative position of the drill bit and the grinding wheel. Often, some students lean on the grinding wheel and start to sharpen the edge before they even balance the edge.This must not wear well.Here "edge" is the main cutting edge, "balance" refers to the grinding part of the main cutting edge in a horizontal position."Wheel surface" means the surface of the grinding wheel."Lean" means to draw close slowly.At this point, the bit cannot touch the grinding wheel.
Formula 2: "drill shaft Angle Angle Angle."
This refers to the position relationship between the drill bit axis line and the grinding wheel surface."Edge horn" namely apex Angle 118 ° half of plus or minus 2 o, approximately 60 ° the location is very important, directly affect the bit apex Angle and shape of the main cutting edge and chisel edge Angle.Remember to prompt students to the commonly used a piece of 30 °, 60 °, 90 °, 60 ° Angle in the triangle students easy to master.Formula one and formula two refer to the relative position of the bit edge before grinding. They should be taken into consideration as a whole. Do not ignore the bevel Angle in order to balance the edge, or ignore the edge in order to balance the oblique axis.These errors are common in practice.At this point the bit is ready to contact the grinding wheel in the correct position.
Formula 3: "grind the back of the blade."
This refers to the slow grinding of the edge of the drill bit along the entire back surface starting from the edge of the drill bit.This facilitates heat dissipation and sharpening.On the basis of stabilizing and consolidating the formula one and two, at this time, the drill bit can gently contact the grinding wheel for a small amount of edge grinding. During the edge grinding, the uniformity of spark should be observed, the pressure should be adjusted in time, and the cooling of the drill bit should be paid attention to.When the cooling start sharpening again, to continue to put the formula one, two position, this point is often difficult to master at the beginning of learning, often can not help but change the correctness of its position.
Formula 4: "up and down swing tail don't become warped."
This action is also very important in the process of bit sharpening. Often, some students change "up and down swing" into "up and down rotation" during the sharpening process, so that the other main blade of the drill bit is destroyed.At the same time, the tail of the drill bit should not be high above the horizontal center line of the grinding wheel, otherwise the edge will be dull and unable to be cut.
Defect
Although the geometric shape of twist drill is more reasonable than that of flat drill, it still has the following disadvantages:
(1) the front Angle values at each point on the main cutting edge of standard twist drill differ too much from each other inside and outside.Bit before the outer edge of the main cutting edge Angle is about + 30 °;Towards linger-sometimes constantly, rake Angle is about 30 °, near the linger-sometimes constantly rake Angle is too small, cause big chip deformation, cutting resistance;And near the outer edge of the front Angle is too large, in the processing of hard materials, cutting edge strength is often insufficient.
(2) the chisel edge is too long, the chisel edge negative rake Angle is very big, up to 4-5 ° ~ 6 0 °, which will produce very big axial force.
(3) compared with other types of cutting tools, the main cutting edge of standard twist drill is very long, which is not conducive to chip splitting and chip breaking.
(4) the sub-back Angle of the sub-cutting edge at the edge belt is zero, resulting in increased friction between the sub-cutting surface and the hole wall, increased cutting temperature, greater wear at the corner of the outer edge of the drill bit, and deterioration of the roughness of the processed surface.
The above defects often make twist drill wear quickly and seriously affect the drilling efficiency and the quality of the processed surface improvement.
Analysis of frequently asked questions
The aperture increases. The error is large
Cause
1.The left and right cutting edges of the bit are not symmetrical and the pendulum difference is large.
2. The cross edge of the drill bit is too long.
3.The bit blade is chipped.
4.Chipping is on the blade.
5.drill curve.
6.The supply is too large.
7.Drilling machine spindle pendulum or loose.
solution
1.When grinding the edge, ensure that the left and right cutting edges of the bit are symmetrical, and the pendulum difference is within the allowable range when grinding the edge, ensure that the left and right cutting edges of the bit are symmetrical, and the pendulum difference is within the allowable range.
2.Repair and grind the transverse edge to reduce the length of the transverse edge.
3.Find the gap and replace it in time.
4.Clean up Chipping.
5.Correction or replacement.
6.Reduce the supply.
7.Adjust and repair drilling machine in time.
The aperture is too small
Cause
1.The bit edge band has been badly worn out.
2.The hole is not round.
solution
Replace the qualified bit.
