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Cnc Drilling

As the most common tool in hole processing, drill bits are widely used in mechanical manufacturing, especially for the processing of cooling devices, tube sheets of power generation equipment, steam generators and other parts. The application is particularly extensive and important.

1. Drilling accuracy

Drilling is a basic method of hole processing. Drilling is often carried out on drilling machines and lathes, but can also be carried out on boring machines or milling machines. The machining accuracy of drilling is low, generally only reaching IT10, and the surface roughness is generally 12.5-6.3μm. After drilling, reaming and reaming are often used for semi-finishing and finishing.

The accuracy of the hole 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 processed hole during drilling:

  • ① The clamping accuracy and cutting conditions of the drill, such as tool holder, cutting speed, feed rate, 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, and the clamping state.

1. Reaming

Reaming is caused by the swing of the drill bit during processing. The swing of the tool holder has a great influence on the hole diameter and the positioning accuracy of the hole, so when the tool holder is severely worn, a new tool holder should be replaced in time. When drilling small holes, it is difficult to measure and adjust the swing, so it is best to use a coarse-shank and small-diameter drill with a good coaxiality between the blade and the shank. When using a regrind drill, the reason for the decrease in hole accuracy is mostly due to the asymmetry of the back shape. Controlling the blade height difference can effectively suppress the cutting and expanding amount of the hole.

2. The roundness of the hole

Due to the vibration of the drill bit, the drilled hole is easy to be polygonal, and rifle-like lines appear on the hole wall. Common polygonal holes are mostly triangles or pentagons. The reason for the triangular hole is that the drill has two rotation centers when drilling. They vibrate at an interval of 600. The main reason for the vibration is the unbalanced cutting resistance. When the drill rotates one revolution, the roundness of the processed hole is not Well, the resistance force is unbalanced during the second revolution of cutting, and the last vibration is repeated again, but the vibration phase is shifted to a certain extent, resulting in rifle lines appearing on the hole wall. When the drilling depth reaches a certain level, the friction between the edge of the drill bit and the hole wall increases, the vibration is attenuated, the rifle line disappears, and the roundness becomes better. This type of hole has a funnel-shaped orifice when viewed in longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear during 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 back and the shape of the blade, etc., the rigidity of the drill should also be increased, the feed per revolution, the clearance angle, and the grinding should be improved. Measures such as cutting edge.

3. Drill holes on inclined and curved surfaces

When the cutting surface or the drilling surface of the drill bit is inclined, curved or stepped, the positioning accuracy is poor. At this time, the drill bit is radial single-sided cutting, which reduces the tool life.

In order to improve the positioning accuracy, the following measures can be taken:

  • ①Drill the center hole first;
  • ② Mill the hole seat with an end mill;
  • ③Choose drills with good penetration and rigidity;
  • ④ Reduce the feed speed.

4. Treatment of burrs

During drilling, burrs will appear at the entrance and exit of the hole, especially when processing tough materials and thin plates. The reason is that when the drill bit is about to drill through, plastic deformation of the processed material occurs. At this time, the triangular part that should be cut by the cutting edge of the drill bit near the outer edge is deformed to the outside after being affected by the axial cutting force, and is bent on the outer edge of the drill bit. Under the action of chamfering and edge surface of blade, it curls further to form curl or burr.

The Characteristics Of Drilling

The drill bit usually has two main cutting edges, and during machining, the drill bit performs cutting while rotating. The rake angle of the drill is getting larger and larger from the central axis to the outer edge. The cutting speed of the drill is higher the closer to the outer circle, and the cutting speed is gradually reduced toward the center, and the cutting speed of the rotation center of the drill is zero. The chisel edge of the drill is located near the central axis of rotation, the secondary rake angle of the chisel edge is large, there is no chip space, and the cutting speed is low, which will produce a large axial resistance. If the chisel edge is ground to A-type or C-type in DIN1414, and the cutting edge near the center axis has a positive rake angle, the cutting resistance can be reduced and the cutting performance can be significantly improved.

According to different workpiece shapes, materials, structures, functions, etc., drill bits can be divided into many types, such as high-speed steel drill bits (twist drill, group drill, flat drill), solid carbide drill bit, indexable shallow hole drill, deep hole Drills, nesting drills and interchangeable bit drills, etc.

Chip Breaking And Chip Removal

The cutting of the drill is carried out in a hole with a narrow space, and the chips must be discharged through the groove of the drill. Therefore, the shape of the chip has a great influence on the cutting performance of the drill. Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, band chips, fan chips, powder chips and so on.

The key to drilling process-chip control

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

  • Fine chips block the cutting 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.);
  • Long chips wrap around the drill bit, hinder the operation, cause the drill bit to break or prevent the cutting fluid from entering the hole (such as spiral chips, banded chips, etc.).

How to solve the problem of inappropriate chip shape:

  • Methods such as increasing feed rate, intermittent feed, grinding chisel edge, and installing chip breaker can be used separately or in combination to improve the effect of chip breaking and chip removal, and eliminate the problems caused by chips.
  • Professional chip-breaking drills can be used to make holes. For example: adding a designed chip breaker in the groove of the drill bit will break the chips into more easily cleaned chips. Debris is smoothly removed along the groove and will not block in the groove. Therefore, the new chipbreaker drill has a much smoother cutting effect than the traditional drill bit.

At the same time, the short-crushed iron chips make it easier for the coolant to flow to the drill tip, which further improves the heat dissipation effect and cutting performance during the machining process. And because the newly added chipbreaker penetrates the entire groove of the drill bit, it can still maintain its shape and function after repeated grinding. 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 dressing.

The Machining Conditions Of Drilling

In the general drill product catalogue, there is a “reference table of basic cutting parameters” arranged by processing materials. Users can refer to the cutting parameters provided to select the cutting conditions for drilling. Whether the selection of cutting conditions is appropriate should be judged comprehensively by trial cutting, based on factors such as machining accuracy, machining efficiency, and drill life.

1. Drill bit life and processing efficiency

On the premise of meeting the technical requirements of the workpiece to be processed, whether the drill is used properly or not should be comprehensively measured according to the service life of the drill and the processing efficiency. The evaluation index of the service life of the drill can choose the cutting distance; the evaluation index of the machining 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. But it should be noted that if the feed per revolution is too large, the chips will thicken and cause chip breaking difficulties. Therefore, it is necessary to determine the range of feed per revolution that can smoothly break the chips through trial cutting.

For cemented carbide drills, the cutting edge has a larger chamfer in the negative rake angle direction, and the optional range of feed per revolution is smaller than that of high-speed steel drills. If the feed per revolution exceeds this range during processing, the use of the drill will be reduced. life. As the heat resistance of cemented carbide drills is higher than that of high-speed steel drills, the rotation speed has little effect on the life of the drill. Therefore, the method of increasing the rotation speed can be used to improve the processing efficiency of the cemented carbide drills while ensuring the life of the drill.

2. Reasonable use of cutting fluid

The cutting of the drill is carried out in a hole with a narrow space, so the type of cutting fluid and the injection method have a great influence on the life of the drill and the machining accuracy of the hole.

Cutting fluids can be divided into two categories: water-soluble and non-water-soluble. The non-water-soluble cutting fluid has good lubricity, wettability and anti-adhesion properties, and also has anti-rust effect. The water-soluble cutting fluid has good cooling properties, non-smoke and non-flammability. Out of consideration for environmental protection, the use of water-soluble cutting fluids has been relatively large in recent years.

However, if the dilution ratio of the water-soluble cutting fluid is improper or the cutting fluid deteriorates, it will greatly shorten the tool life, so care must be taken during use.

Regardless of whether it is water-soluble or non-water-soluble cutting fluid, the cutting fluid must fully reach the cutting point during use. At the same time, the flow rate, pressure, number of nozzles, cooling method (internal cooling or external cooling) of the cutting fluid must be strictly controlled.

Re-Sharpening Of The Drill

1. Judgment of drill bit regrind

The criteria for judging the drill bit to be re-sharpened are:

  • Abrasion of cutting edge, chisel edge and land edge;
  • The dimensional accuracy and surface roughness of the processed hole;
  • The color and shape of the chips;
  • Cutting resistance (indirect values ​​of spindle current, noise, vibration, etc.);
  • Processing quantity, etc.

In actual use, accurate and convenient criteria should be determined from the above-mentioned indicators according to specific conditions. When using the amount of wear as the criterion, the most economical and best regrind period should be found.

Since the main sharpening parts are the back of the head and the chisel edge, if the amount of drill bit wear is too large, the cutting time will be longer, the amount of grinding will be large, and the number of regrinds will be reduced (total tool life = tool life after regrind × available Re-grinding times), it will shorten the total service life of the drill bit; when using the dimensional accuracy of the machined hole as the criterion, use the column gauge or the limit gauge to check the cut and expansion of the hole, the non-straightness, etc., once the control value is exceeded, it should be immediately Re-sharpening; when cutting resistance is used as the criterion, methods such as automatic shutdown immediately beyond the set limit value (such as spindle current) can be adopted; when the processing quantity limit management is adopted, the above-mentioned discrimination content should be integrated to set the criterion.

2. How to sharpen the drill

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 reground as the original drill type; if the original drill type is defective, the back shape can be appropriately improved and the chisel edge sharpened according to the purpose of use.

Pay attention to the following points when sharpening:

  • Prevent overheating and reduce the hardness of the drill bit;
  • All the damage on the drill bit (especially the damage on the edge of the cutting edge) should be removed;
  • The drill shape should be symmetrical;
  • Be careful not to damage the cutting edge during sharpening, and remove the burrs after sharpening;
  • For cemented carbide drills, the sharpening shape has a great influence on the performance of the drill. The drill shape at the factory is the best drill shape obtained through scientific design and repeated tests. Therefore, the original cutting edge shape should generally be maintained when re-sharpening.

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