Drill, as the most common tool in hole processing, is widely used in mechanical manufacturing, especially for cooling device, power generation equipment tube plate and steam generator and other parts of hole processing, application is particularly extensive and important.
One, the characteristics of drilling
The drill usually has two main cutting edges. In machining, the bit is rotated while cutting.The front Angle of the drill becomes larger and larger from the center axis to the outer edge. The closer the drill is to the outer circle, the higher the cutting speed is. The cutting speed decreases to the center.The transverse edge of the drill is located near the axis of the rotary center, and the front Angle of the transverse edge is larger, with no chip space, and the cutting speed is lower, so there will be greater axial resistance.If the cutting edge of the cross edge is polished into type A or type C in DIN1414, and the cutting edge near the center axis is at the positive front Angle, the cutting resistance can be reduced and the cutting performance can be significantly improved.
According to the workpiece shape, material, structure, function, etc., the drill can be divided into many types, such as high speed steel drill (twist drill, group drill, flat drill), solid carbide drill, indexable shallow hole drill, deep hole drill, sleeve drill and convertible head drill.
Two, broken chip and chip
The cutting of the bit is carried out in the hole with narrow space, and the chip must be discharged through the groove of the bit blade. Therefore, the chip shape has a great influence on the cutting performance of the bit.The common chip shape has flake chip, tubular chip, needle chip, conical spiral chip, ribbon chip, fan chip, powder chip and so on.
Chip control is the key to drilling
When the chip shape is not suitable, the following problems will occur:
(1) small chip block edge groove, affect the drilling accuracy, reduce the life of the bit, and even break the bit (such as powder chip, fan chip, etc.).
(2) long chip winding bit, hinder the operation, cause the bit damage or block cutting fluid into the hole (such as spiral chip, ribbon chip, etc.).
How to solve the problem of improper chip shape:
(1) can be used separately or in combination with increasing feed, intermittent feed, grinding edge, chip breaker and other methods to improve the chip breaking and chip removal effect, eliminate the problem caused by chip.
(2) can use a professional chip breaking drill hole.For example, a chip breaker is added to the groove of the drill to break the chip into a chip that is easier to remove.Detritus is removed smoothly along the trench and does not clog in the trench.As a result, the new chip break drill achieves much smoother cutting effect than the traditional drill.
At the same time, the short scrap iron makes the coolant flow to the drill tip more easily, which further improves the heat dissipation effect and cutting performance during the processing.And because the new chip cutting edge penetrates the entire groove of the drill, it is able to maintain its shape and function after multiple refinements.In addition to the above functional improvements, it is worth mentioning that the design strengthens the rigidity of the drill body and significantly increases the number of boreholes before a single grinding.
Three, Drilling accuracy
The accuracy of the hole is mainly composed of pore size, position accuracy, coaxiality, roundness, surface roughness, burr and other factors.
Factors that affect the hole precision during drilling:
(1) Bit clamping accuracy and cutting conditions, such as tool clip, cutting speed, feed, cutting fluid, etc.
(2) The size and shape of the bit, such as the length of the bit, the shape of the blade, the shape of the drill core, etc.
(3) workpiece shape, such as orifice side shape, orifice shape, thickness, loading state, etc.
1, Reaming
The reaming is caused by the wobble of the bit during machining.The swing of the knife clip has a great influence on the locating accuracy of the aperture and hole, so when the knife clip is badly worn, it should be replaced in time.When drilling small holes, it is difficult to measure and adjust the swing, so it is better to use the thick shank small blade diameter drill with good coaxiality between the blade and the shank.When using the regrind drill bit, the reason that causes the hole precision to decline is mainly because of the back shape asymmetry.The cutting volume of the hole can be effectively controlled by controlling the edge height difference.
2, Roundness
Because of the vibration of the drill, the holes are easy to be polygonal, and the hole wall appears like a pattern of incoming lines.The most common polygonal holes are triangles or pentagons.Causes of triangle hole drilling bit has two rotary centers, them by once every 600 exchange between the frequency of vibration, the reason of vibration is cutting resistance was not balance, when the drill bit rotation after a turn, due to machining the hole of roundness is not good, cause the second turn cutting resistance unbalance, repeat the vibration of the last time, but has certain deviation vibration phase, created in hole wall rifling texture.When the drilling depth reaches a certain degree, the friction between the edge of the bit and the hole wall increases, the vibration decreases, the incoming line disappears and the roundness becomes better.The orifice of this kind of orifice is funnel shaped from the longitudinal section.The same reason, cutting may also appear in the pentagon, heptagonal hole.In order to eliminate this phenomenon, in addition to chuck vibration, cutting edge height difference, back and blade blade shape asymmetry and other factors to control, but also should be taken to increase the rigidity of the drill bit, increase the feed per revolution, reduce the back Angle, grinding edge and other measures.
3, Drill holes in bevels and curved surfaces
When the cutting surface or drilling surface of the bit is inclined, curved or stepped, the positioning accuracy is poor, because at this time the bit is radial single cutting tool, so that the tool life is reduced.
To improve positioning accuracy, the following measures can be taken:
(1) Drill the center hole first.
(2) Milling hole seat with end mill.
(3) choose a good cutting, good rigidity of the drill.
(4) reduce the feed speed.
4, Treatment of burrs
During drilling, burrs may appear at the inlet and outlet of the hole, especially in the processing of tough materials and sheets.The reason is that when the drill bit is about to drill through, the processed material appears plastic deformation. At this point, the triangular part cut by the cutting edge of the part near the outer edge of the drill should be deformed to the lateral bending under the action of the axial cutting force, and further curl under the action of the chamfering of the outer edge of the drill bit and the edge of the blade, forming a rolled edge or rough edge.
Four, drilling processing conditions
In the general sample catalog of drill bits, there is a reference table of basic cutting parameters arranged according to the processing materials.The selection of cutting conditions is appropriate, should be through the cutting test, according to the machining accuracy, machining efficiency, bit life and other factors of comprehensive judgment.
1, Bit life and machining efficiency
On the premise of satisfying the technical requirements of the workpiece to be machined, whether the use of the drill is appropriate or not should be comprehensively measured according to the service life and machining efficiency of the drill.The evaluation index of bit service life can choose cutting distance;The evaluation index of machining efficiency can choose the feed speed.For HSS bits, the service life of the bits is greatly affected by the rotary speed and is less affected by the feed per revolution. Therefore, the machining efficiency can be improved by increasing the feed per revolution and the long life of the bits can be guaranteed at the same time.However, it should be noted that if the feed per turn is too large, the chip will thicken, causing chip breaking difficulties, so it is necessary to determine the chip breaking per turn feed range through trial cutting.For carbide bits, there is a large chamfering Angle in the negative front Angle of the cutting edge. The optional range of feed per revolution is smaller than that of HSS bits. If the feed per revolution exceeds this range, the service life of the bits will be reduced.Because the heat resistance of the carbide bit is higher than that of the high speed steel bit, the rotary speed has little effect on the life of the bit.
2, Reasonable use of cutting fluid
The cutting of the bit is carried out in the hole with narrow space, so the type of cutting fluid and the way of injection have a great influence on the life of the bit and the machining accuracy of the hole.Cutting fluids can be divided into water-soluble and water-insoluble two categories.The non-water-soluble cutting fluid has good lubricity, wettability and adhesion resistance as well as anti-rust effect.Water - soluble cutting fluid has good cooling, no smoke and no flammability.In order to protect the environment, water-soluble cutting fluid has been used a lot in recent years.However, if the water-soluble cutting fluid dilution rate is not appropriate or the cutting fluid deterioration, will greatly shorten the tool life, so the use must be paid attention to.Whether water soluble or insoluble cutting fluid, the use of the cutting fluid must make the cutting fluid to the cutting point, while the flow of cutting fluid, pressure, nozzle number, cooling mode (internal or external cold) must be strictly controlled.
Five, The resharpening of the bit
Drill regrind discrimination
The discriminant criteria for the drill to be resharpened are:
(1) cutting edge, cross edge, edge edge edge edge wear;
(2) size accuracy and surface roughness of the hole being processed;
The color and shape of the chips;
(4) cutting resistance (spindle current, noise, vibration and other indirect values);
Processing quantity, etc.
In actual use, the accurate and convenient discriminant standard should be determined from the above indicators according to the specific situation.When the wear quantity is used as the criterion, the best economic rewear period should be found out.Because the main parts of the cutting edge are the back of the head and the transverse edge, such as the bit wear is too large, the grinding time is too much, the grinding amount is large, the number of regrinding is reduced (the total service life of the tool = the tool life after regrinding × the number of regrinding), but will shorten the total service life of the bit;When the dimensional accuracy of the hole to be machined is used as the criterion for judging, the column gauge or limit gauge should be used to check the cutting and expanding capacity and irregularity of the hole.When the cutting resistance is used as the criterion, it can be used to exceed the set limit value (such as spindle current) and stop automatically.When using the limit management of processing quantity, the above discriminant content should be integrated and the discriminant standard should be set.
Method of grinding the edge of a drill
When regrinding the bit, it is better to use the special machine tool or universal tool grinder, which is very important to ensure the service life and machining accuracy of the bit.If the original drilling machine is in good working condition, it can be remounted according to the rough drilling machine;If the rough drill is defective, the back shape and the cross edge can be modified according to the purpose of use.
The following points should be paid attention to when sharpening:
. Prevent overheating, avoid reducing the hardness of the bit.
The damage on the drill (especially the damage on the face of edge edge) should be completely removed.
The drill type should be symmetrical.
(4) in the cutting edge grinding pay attention not to hurt the cutting edge, and should remove the burr after the grinding.
(5) for the carbide drill, the shape of the grinding has a great impact on the performance of the drill bit, the delivery of the drill is the best through scientific design, repeated tests, so the re-grinding should generally maintain the original blade.