CNC Knowledge: What do you know about drill presses and drilling?

As the most common tool in hole processing, drill bits are widely used in machine manufacturing, especially for hole processing in parts such as cooling devices, tube sheets of production equipment. electricity and steam generators. The areas of application are particularly extensive and important. .

1. Drilling characteristics

Drill bits usually have two main cutting edges. When processing, the drill bit cuts while rotating. The cutting angle of the drill becomes larger and larger from the central axis to the outer edge. The closer the outer circle is, the more the cutting speed of the drill bit decreases towards the center and the cutting speed of the drill bit. the center of rotation of the drill is zero. The chisel edge of the drill bit is located near the central axis of rotation. The secondary cutting angle of the chisel edge is large, there is no chip space, and the cutting speed is low, resulting in greater axial resistance. If the chisel edge is sharpened according to type A or type C according to DIN1414 and the cutting edge near the central axis has a positive cutting angle, the cutting resistance can be reduced and the cutting performance can be significantly improved. 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, cluster drills, flat drills), solid carbide drills, indexable shallow hole drills, deep hole drills, drill bits, interlocking drill bits and replaceable head drill bits, etc.

2. Chip breaker and chip removal

The cutting of the drill bit is carried out in a hole with a narrow gap, and the chips should be discharged through the groove of the drill bit. Therefore, the shape of the chips has a great influence on the cutting performance of the drill bit. Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, powder chips, etc.

The key to drilling: chip control

When the chip shape is inappropriate, the following problems will occur:

① Fine chips block the edge groove, affecting the drilling accuracy, reducing the life of the drill bit, and even causing the drill bit to break (such as powdery chips, fan-shaped chips, etc.);

② Long chips wrap around the drill bit, hindering operation, causing the drill bit to break or preventing cutting fluid from entering the hole (such as spiral chips, tape chips, etc.).

How to solve the problem of incorrect chip shape:

① Methods such as increasing the feeding amount, intermittent feeding, chisel edge grinding, and installing a chip breaker can be used separately or in combination to improve the breaking effects and chip removal and eliminate problems caused by chips.

②A professional chip breaker drill bit can be used to drill holes. For example: adding a chip breaker edge designed into the drill groove breaks the chips into chips that are easier to remove. Debris is discharged smoothly along the groove without clogging in the groove. As a result, the new chipbreaker drill bit achieves a much smoother cutting effect than traditional drill bits.

At the same time, the short crushed iron chips facilitate the flow of coolant to the tip of the drill bit, thereby improving the heat dissipation effect and cutting performance during the machining process. And because the new chipbreaking edge penetrates the entire groove of the drill bit, it can still maintain its shape and function after repeated sharpening. In addition to the above functional improvements, it is worth mentioning that this design strengthens the rigidity of the drill body and significantly increases the number of holes drilled before a single grinding.

3. Drilling accuracy

Hole accuracy is mainly composed of factors such as aperture size, position accuracy, coaxiality, roundness, surface roughness and hole burrs.

Factors that affect the accuracy of the hole being processed during drilling:

①Drill clamping accuracy and cutting conditions, such as tool holder, cutting speed, feed amount, cutting fluid, etc. ;

②Drill bit size and shape, such as drill bit length, blade shape, sprue shape, etc. ;

③ Workpiece shape, such as hole side shape, hole shape, thickness, clamping state, etc.

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1. Expansion

The enlargement of the hole is caused by the oscillation of the drill bit during processing. The oscillation of the tool holder has a great influence on the hole diameter and hole positioning accuracy. Therefore, when the tool holder is seriously worn, a new tool holder should be replaced in time. When drilling small holes, it is difficult to measure and adjust the wobble, so it is better to use a drill bit with a thick handle and a small blade diameter that has good coaxiality between the blade and the shank . When using resharpening drills, the decrease in hole accuracy is mainly due to the asymmetry of the subsequent shape. Controlling the edge height difference can effectively suppress the cutting expansion of the hole.

2. Hole roundness

Due to the vibration of the drill bit, the drilled hole can easily become polygonal, with lines like scratched lines appearing on the wall of the hole. 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. They vibrate at a frequency that is exchanged every 600. The main reason for the vibration is unbalanced cutting resistance as the bit rotates for one revolution. the roundness of the treated hole will be uneven. Well, the resistance is unbalanced in the second round of cutting and the last vibration is repeated again, but the vibration phase is shifted to a certain extent, causing scratch lines to appear on the hole wall. . When the drilling depth reaches a certain level, the friction between the surface of the drill edge and the hole wall increases, the vibration becomes attenuated, the scratched line disappears, and the roundness improves. This type of hole has a funnel-shaped opening seen in longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear when cutting. In order to eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting height difference, asymmetry of the back surface and blade shape, etc., measurements should also be taken. taken to improve the rigidity of the drill, increase the feed per revolution, reduce the relief. angle and grinding of chisel blade and other measurements.

3. Drilling holes on inclined and curved surfaces

When the cutting surface or drilling surface of the drill bit is a beveled, curved or stepping surface, the positioning accuracy is poor. Since the bit cuts on one side in a radial direction, tool life is reduced.

To improve positioning accuracy, the following measures can be taken:

1. Drill the center hole first;

2. Use an end mill to countersink the hole seat;

3. Choose a drill bit with good penetration and rigidity;

4. Reduce the feed speed.

4. Burr treatment

When drilling, burrs will appear at the entrance and exit of the hole, especially when processing hard materials and thin plates. The reason is that when the drill bit is about to drill, the processed material undergoes plastic deformation. At this time, the triangular part that needs to be cut by the edge of the drill bit near the outer edge is deformed and bent outward. the axial cutting force, and is curved outward at the outer edge of the drill bit. It is then curved under the action of chamfers and edge surfaces to form curved edges or burrs.

4. Processing conditions for drilling

The general drill bit product catalog contains a “Basic Cutting Usage Reference Table” organized by processing materials. Users can refer to the provided cutting quantities to select cutting conditions for drilling. The suitability of the selection of cutting conditions should be comprehensively evaluated through cutting tests and based on factors such as machining accuracy, machining efficiency and drill life. .

1. Drill bit life and processing efficiency

In order to meet the technical requirements of the workpiece, whether the drill bit is used appropriately should be comprehensively measured based on the life of the drill bit and the processing efficiency. The cutting distance can be used as an index for evaluating the life of the drill bit; the feed speed can be used as an index for evaluating the effectiveness of the treatment. For high speed steel drills, drill life is strongly affected by rotational speed and less affected by feed per revolution. Therefore, the processing efficiency can be improved by increasing the feed per revolution while ensuring a long drill bit. life. However, it should be noted that if the feed per revolution is too large, the chips will thicken, making chip breaking difficult. Therefore, the range of feed per revolution to successfully break chips should be determined by test cutting. For carbide drills, the cutting edge is ground with a large chamfer in the direction of the negative cutting angle, and the optional feed per revolution range is smaller than that of high speed steel drills if the feed per revolution exceeds this range during. treatment, the life of the drill bit will be reduced. Since the heat resistance of carbide drills is higher than that of high-speed steel drills, the rotational speed has little impact on the life of the drill. Therefore, the method of increasing the rotation speed can be used to improve the treatment. efficiency of the carbide drill bit while ensuring the life of the drill bit.

2. Reasonable use of cutting fluid

The cutting of the drill is carried out in a hole with a narrow space, so the type and injection method of the cutting fluid have a great impact on the life of the drill and the machining accuracy of the hole. Cutting fluids can be divided into two categories: water-soluble and water-insoluble. The water-insoluble cutting fluid has good lubricating, wettable and anti-stick properties, and also has an anti-rust effect. Water-soluble cutting fluid has good cooling properties, does not emit smoke and is non-flammable. For environmental protection reasons, water-soluble cutting fluids have been used in large quantities in recent years. However, if the dilution rate of water-soluble cutting fluid is inappropriate or the cutting fluid deteriorates, the tool life will be greatly shortened, so caution should be exercised when using it . Whether it is water-soluble cutting fluid or not, the cutting fluid should completely reach the cutting point when using, at the same time, the flow rate, pressure, number of nozzles and the cooling method (internal or external cooling). cooling) of the cutting fluid must be strictly controlled.

5. Re-sharpening drill bits

Judgment of resharpening the drill bit

The criteria for determining whether a drill bit needs to be resharpened are:

1. The degree of wear of the cutting edge, chisel edge and floor surface;

2. Dimensional accuracy and surface roughness of the processed hole;

3. Color and shape of the chips;

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

5. Processing quantity, etc.

In actual use, precise and practical discrimination standards should be determined from the above indicators according to specific circumstances. Using the degree of wear as a criterion, the best resharpening period with the best economic efficiency should be found. Since the main sharpening parts are the back of the head and the edge of the chisel, if the bit is worn too much, sharpening will take a long time, the sharpening volume will be large, and the number of resharpening will be reduced (the total tool life = tool life after resharpening × number of times it can be resharpened) The number of resharpenings) will reduce the total life of the bit using dimensional accuracy of the hole treated as a criterion; , it should be Use a column gauge or limit gauge to check the cutting expansion, straightness, etc. of the hole. Once the control value is exceeded, it must be resharpened immediately when the cutting resistance is used as a criterion, if the limit value is set. (such as spindle current) exceeds the set limit value (such as spindle current), it can be automatically shut down and other measures when using processing quantity limit management, the content of judgment above must be combined to define standards of judgment.

Drill sharpening method

When resharpening a drill bit, it is better to use a special machine tool for sharpening drill bits or a universal tool grinder, which is very important to ensure the service life and processing accuracy of the drill bit. forest. If the original diamond shape is in good processing condition, it can be ground to the original diamond shape; if the original shape of the diamond is defective, the shape of the back can be improved appropriately, and the edge of the chisel can be sharpened according to the purpose of use. .

The following points should be considered when sharpening:

1. Avoid overheating and reduce the hardness of the drill bit;

2. All damage on the drill bit (especially damage on the blade edge) must be removed;

3. The shape of the diamond should be symmetrical;

4. Be careful not to damage the cutting edge when sharpening, and remove the burrs after sharpening;

5. For carbide drill bits, the sharpening shape has a great influence on the performance of the drill bit. The shape of the drill bit before leaving the factory is the best shape achieved through scientific design and repeated testing. Therefore, the original shape of the blade must generally be respected. be maintained during resharpening.