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Five key issues in drilling and processing

As the most common tool in hole processing, drill bits are widely used in machinery manufacturing, especially for the processing of holes in parts such as cooling devices, power generation equipment, pipe plates, and steam generators. The application surface is particularly extensive and important.
1. The characteristics of drilling. Drill bits usually have two main cutting edges. During processing, the drill bits are rotated and cut at the same time.The front angle of the drill bit is getting larger and larger from the central axis to the outer edge. The closer it is to the outer circle, the higher the cutting speed of the drill bit, and the cutting speed decreases to the center, and the cutting speed of the drill bit at the center of rotation is zero.The horizontal edge of the drill bit is located near the central axis of rotation. The sub-front angle of the horizontal edge is large, there is no chip space, and the cutting speed is low, which will produce greater axial resistance.If the edge of the horizontal edge is sharpened into type A or type C in DIN1414, and the cutting edge near the central axis is a positive front angle, the cutting resistance can be reduced and the cutting performance can be significantly improved.
Depending on the shape, material, structure, function, etc. of the workpiece, drill bits can be divided into many types, such as high-speed steel drill bits (twist drills, group drills, flat drills), integral cemented carbide drill bits, indexable shallow hole drills, deep hole drills, nesting drills and interchangeable head drill bits.

2. The cutting of the chip breaking and chip removal drill bit is carried out in a hole with a narrow space, and the chips must be discharged through the edge groove of the drill bit, so the shape of the chips has a great impact on the cutting performance of the drill bit.Common chip shapes are flaky chip, tubular chip, needle chip, tapered spiral chip, ribbon chip, fan-shaped chip, powdery chip and so on.The key to drilling-chip control
When the shape of the chips is not appropriate, the following problems will arise: ① Fine chips block the edge groove, affect the drilling accuracy, reduce the life of the drill bit, and even break the drill bit (such as powdery chips, fan-shaped chips, etc.).②The long chips wrap around the drill bit, which hinders the operation, causes the drill bit to break or prevents the cutting fluid from entering the hole (such as spiral chips, ribbon chips, etc.).
How to solve the problem of improper chip shape:
①the methods of increasing the feed volume, intermittent feed, sharpening the horizontal edge, and installing a chip breaker can be used separately or in combination to improve the chip breaking and chip removal effect, and eliminate the problems caused by chips.
② You can use a professional chip breaking drill bit to punch holes.For example: adding a designed chip breaking blade to the groove of the drill bit will break the chips into debris that is easier to remove.Debris is removed smoothly along the groove, and no clogging in the groove will occur.Therefore, the new type of chip breaking drill has obtained a much smoother cutting effect than the traditional drill bit.At the same time, the short-broken iron shavings make it easier for the coolant to flow to the drill tip, further improving the heat dissipation effect and cutting performance during processing.Moreover, because the new chip-breaking blade penetrates the entire groove of the drill bit, its shape and function can still be maintained after multiple grinding.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 drilling holes before a single grinding.

3. Drilling accuracy

The accuracy of the hole is mainly composed of factors such as aperture size, position accuracy, coaxiality, roundness, surface roughness, and orifice burrs.
Factors that affect the accuracy of the hole to be machined during drilling:
①the clamping accuracy and cutting conditions of the drill bit, such as tool clamp, cutting speed, feed volume, cutting fluid, etc.
②The size and shape of the drill bit, such as the length of the drill bit, the shape of the blade, the shape of the drill core, etc.③The shape of the workpiece, such as the shape of the side of the orifice, the shape of the orifice, the thickness, the state of the card, etc.

Reaming
Reaming is caused by the swing of the drill bit during processing.The swing of the cutter clamp has a great impact on the aperture and the positioning accuracy of the hole, so when the cutter clamp is severely worn, a new cutter clamp should be replaced in time.When drilling small holes, it is more difficult to measure and adjust the swing, so it is best to use a coarse-shank small-blade drill bit with a good coaxiality between the blade and the shank.When using a re-grinding drill bit for processing, the reason for the decrease in hole accuracy is mostly due to the asymmetry of the shape behind it.Controlling the blade height difference can effectively suppress the cutting and expansion of the hole.
Roundness of the hole

Due to the vibration of the drill bit, the drilled hole shape can easily be polygonal, and lines like repeating lines appear on the hole wall.Common polygonal holes are mostly triangles or pentagons.The reason for the triangular hole is that the drill bit has two centers of rotation when drilling, and they vibrate at a frequency of 600 exchanges every interval. The main reason for the vibration is the unbalanced cutting resistance. When the drill bit rotates for one turn, due to the poor roundness of the processed hole, the resistance is unbalanced during the second turn of cutting, and the last vibration is repeated again, but the vibration phase is offset to a certain extent, resulting in the appearance of repeating lines on the hole wall.When the drilling depth reaches a certain level, the friction between the edge surface of the drill bit and the hole wall increases, the vibration attenuates, the reciprocating line disappears, and the roundness becomes better.This hole type is funnel-shaped from the longitudinal profile.For the same reason, pentagonal and heptagonal holes may also appear in cutting.In order to eliminate this phenomenon, in addition to controlling the vibration of the chuck, the height difference of the cutting edge, the asymmetry of the shape of the back and the blade, measures such as improving the rigidity of the drill bit, increasing the feed per revolution, reducing the rear angle, and sharpening the horizontal edge should also be taken.

Drilling holes on bevels and curved surfaces

When the knife-eating surface or drill-through surface of the drill bit is inclined, curved or stepped, the positioning accuracy is poor. Since the drill bit is radial and single-sided at this time, the tool life is reduced.In order to improve the positioning accuracy, the following measures can be taken:
① Drill the center hole first.
②.Use an end mill to mill the hole seat.③Choose a drill bit with good cut-in and rigidity.④Reduce the feed speed.

Treatment of burrs
During drilling, burrs will appear at the inlet and outlet of the hole, especially when processing materials and thin plates with high toughness.The reason is that when the drill bit is about to drill through, the processed material is plasticly deformed. At this time, the triangular part that should have been cut by the edge of the drill bit near the outer edge is deformed and bent to the outside by the axial cutting force, and it is further curled under the action of the chamfering of the outer edge of the drill bit and the edge surface of the edge belt, forming crimping or burrs.
Fourth, the processing conditions of drilling
In the general drill bit product sample book catalog, there is a “Basic Cutting Dosage Reference Table” arranged by the processing material. Users can refer to the cutting dosage provided to select the cutting conditions for drilling.Whether the selection of cutting conditions is appropriate, trial cutting should be passed, and a comprehensive judgment based on factors such as machining accuracy, processing efficiency, and drill bit life should be made.
1 Drill bit life and processing efficiency

Under the premise of meeting the technical requirements of the workpiece to be processed, whether the drill bit is used properly should be comprehensively measured based on the service life and processing efficiency of the drill bit.The evaluation index of the service life of the drill bit can choose the cutting distance; the evaluation index of the processing efficiency can choose the feed speed.For high-speed steel drill bits, the service life of the drill bit is greatly affected by the rotation speed and less affected by the feed per revolution. Therefore, the processing efficiency can be improved by increasing the feed per revolution, while ensuring a longer drill life.

However, it should be noted: if the feed per revolution is too large, the chips will thicken, making it difficult to break the chips. Therefore, the feed range per revolution that can break the chips smoothly must be determined through trial cutting.For cemented carbide drill bits, the cutting edge has a larger chamfer in the direction of the negative front angle, and the optional range of feed per revolution is smaller than that of high-speed steel drill bits. If the feed per revolution during processing exceeds this range, the service life of the drill bit will be reduced.Since the heat resistance of cemented carbide drill bits is higher than that of high-speed steel drill bits, the rotation speed has little effect on the life of the drill bit, so the method of increasing the rotation speed can be used to improve the processing efficiency of cemented carbide drill bits, while ensuring the life of the drill bit.

2 Rational use of cutting fluid
The cutting of the drill bit is carried out in a hole with a narrow space, so the type of cutting fluid and the injection method have a great impact on the life of the drill bit and the machining accuracy of the hole.Cutting fluid can be divided into two categories: water-soluble and non-water-soluble.The non-water-soluble cutting fluid has good lubricity, wettability and adhesion resistance, and it also has anti-rust effect.The water-soluble cutting fluid has good cooling properties, non-fuming and non-flammable properties.Due to the consideration of environmental protection, the use of water-soluble cutting fluid has increased in recent years.However, if the dilution magnification of the water-soluble cutting fluid is improper or the cutting fluid deteriorates, the service life of the tool will be greatly shortened, so attention must be paid in use.Whether it is a water-soluble or non-water-soluble cutting fluid, the cutting fluid must be fully reached at the cutting point in use, and the flow rate, pressure, number of nozzles, and cooling method (internal cooling or external cooling) of the cutting fluid must be strictly controlled.
5. Re-sharpening of the drill bit
Drill bit re-grinding discrimination
The criteria for determining whether the drill bit needs to be re-sharpened are:
①The amount of wear on the edge, horizontal edge, and edge surface of the cutting edge;
②The dimensional accuracy and surface roughness of the processed hole;

③The color and shape of the chips;

④Cutting resistance (indirect values such as spindle current, noise, vibration, etc.) ;

⑤Processing quantity, etc.In actual use, according to specific circumstances, accurate and convenient discrimination criteria should be determined from the above indicators.

Using the amount of wear as the criterion for determining, the best re-wear period with the best economy should be found.Since the main sharpening parts are the back of the head and the horizontal edge, if the drill bit wears too much, the sharpening time is too much, and the grinding amount is large, the number of times it can be re-ground is reduced (the total service life of the tool = the tool life after re-grinding × the number of times it can be re-ground), on the contrary, the total service life of the drill bit will be shortened; the dimensional accuracy of the processed hole is used as the discrimination standard, and the column gauge or limit gauge is used to check the hole’s cutting expansion, non-straightness, etc., once the control value is exceeded, it should be re-sharpened immediately; the cutting resistance is used as the discrimination standard, and the set limit value (such as spindle current) can be used immediately. Automatic shutdown and other methods; when using the management of processing quantity limits, the above-mentioned discrimination content should be integrated and the discrimination criteria should be set.
How to sharpen the drill bit
When re-sharpening the drill bit, it is best to use a special machine tool for sharpening the drill bit or a universal tool grinder, which is very important to ensure the service life and machining accuracy of the drill bit.If the original drill type is in good processing condition, it can be re-ground according to the original drill type; if the original drill type is defective, the back shape and horizontal edge grinding can be appropriately improved according to the purpose of use.
The following points should be paid attention to when sharpening:

①.Prevent overheating and avoid reducing the hardness of the drill bit.② All the damage on the drill bit (especially the damage to the edge of the blade) should be removed.③The drill type should be symmetrical.④Be careful not to touch the edge during sharpening, and the burrs after sharpening should be removed.⑤ For cemented carbide drill bits, the sharpening shape has a great impact on the performance of the drill bit. The drill type at the factory is the best drill type obtained by scientific design and trial and error. Therefore, the original blade type should generally be maintained when re-sharpening.

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