The so-called deep hole is a hole whose length to hole diameter ratio is greater than 10. In most cases, the depth-to-diameter ratio of general deep holes is L/d≥100. Such as cylinder holes, shaft axial oil holes, hollow spindle holes and hydraulic valve holes, etc. Some of these holes require high machining accuracy and surface quality, and some of the materials to be processed have poor machinability, which often becomes a major difficulty in production. What methods can you think of for deep hole processing? I believe that many people will first think of gun drilling - the mainstay of deep hole processing. Today let's take a look at how gun drills solve the problem of deep hole processing:
Gun drilling has a wide range of processing: from plastics such as fiberglass and Teflon to carbon steel, alloy steel, non-ferrous metals, and high-strength alloy steel (such as high-temperature heat-resistant alloys and titanium alloys), it can be used for deep hole processing.
Gun drilling was first used in gun barrel processing. Due to its excellent hole processing performance, it is now widely used in industries such as shipbuilding, automobiles, engines, locomotives, military industry, chemical machinery, oil nozzles and oil pumps, mining machinery and hydraulic parts.
The precision of gun drilling that must be mentioned
Gun drilling requires forced chip removal. High-pressure cutting oil enters the inner hole of the gun drill from the drill shank and reaches the cutting area to cool and lubricate the drill edge. Iron chips and cutting oil are discharged along the "V" groove of the drill pipe, which is also called external chip discharge.
Using appropriate deep hole processing machine tools and cutting parameters, the following processing effects can be achieved:
Aperture size: IT6～IT611
Inner hole roughness: Ra0.2～Ra6.3
Deflection: 1/1000 × hole depth, workpiece fixed, tool rotating
0.5/1000×hole depth, workpiece rotates, tool rotates in reverse direction
Gun drilling is especially suitable for processing small-diameter holes. Under normal circumstances, holes with a diameter less than 10mm are more suitable for gun drilling. The most advantageous thing is that it can discharge iron chips smoothly.
The lever for deep hole machining efficiency
Because of the special tool structure of gun drilling, there is no need to remove chips when processing deep holes, and the depth can be processed in one go. The deeper the hole depth, the more processing efficiency can be reflected. Depending on the material and hole diameter, the processing speed can reach more than 30-100mm/min. Gun drilling is not only suitable for deep hole drilling machines, but also can be used for other traditional machine tools, such as machining centers and CNC lathes.
(Reminder: Unlike special gun drilling machines, machining centers rarely use drill sleeves for guidance. Therefore, to use gun drilling on a machining center, you must first pre-drill a pilot hole on the workpiece to be processed. In order to achieve better processing Effect, the pilot hole must meet the requirements of hole diameter tolerance, hole depth, etc.).
Issues that must be paid attention to during processing
1. The clamping of the workpiece must be safe and reliable, and coaxial with the center of the machine tool. The outer circle and end face of the workpiece can be processed, and at least the positioning surface must be turned out.
2. When processing long workpieces, the workpiece and gun drill should use a fixed center frame. In addition, the drill rod also needs to have 1-3 movable support frames.
3. The center hole on the workpiece must be smaller than the diameter of the gun drill. If the size of the center hole cannot be reduced, a special guide sleeve can be used.
4. When starting drilling, a pilot drill sleeve must be used. The inner diameter of the drill sleeve matches the diameter of the drill bit. The inner diameter of the drill sleeve must be ground to IT6 level. When the workpiece is short or the hole straightness is not required, it can also be used on the workpiece. Pre-drill pilot holes for guidance.
5. The drill bushing is a wearing part. When the wear amount of the inner diameter of the drill bushing is greater than 0.02mm, the drill bushing should be replaced. It is best to use a carbide drill bushing.
Common problems encountered in gun drilling and their causes are shown in the table below.
The drill cannot drill into the workpiece
1. The inner edge is higher than the center
2. The rear angle is too small
3. The blade is too small off-center
4. Excessive wear or chipping
5. Cutting speed is too low or feed rate is too large
The tool bar vibrates or bends violently
1. The feed amount is too large
2. The center frame or positioning guide sleeve is loose and cannot support the tool holder.
3. Chip clogging
4. Severe wear or chipping
Chips easily clog
1. The coolant pressure is not enough, the pump leaks, and the oil inlet pipe is blocked.
2. Coolant viscosity is too high and cleanliness is poor
3. The oil inlet hole in the cutter head is blocked or the oil inlet hole is too small.
4. Chip shape is not conducive to chip removal
Coaxiality difference
1. Improper selection of drill tip eccentricity and internal and external edge angles
2. The guide sleeve is excessively worn or deflected
3. The position error of the machine tool-tool-fixture axis is too large
4. The workpiece is too bent
Drilling hole diameter out of tolerance
1. The outer diameter of the drill bit swings too much
2. Excessive drill bit wear
3. The blade material has an affinity with the material to be processed and is prone to causing knife tumors.
4. The machine tool has poor rigidity and poor accuracy.
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How to resharpen carbide gun drill
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The most common problem encountered by gun drills is wear. When it comes to wear and tear, it is nothing more than re-grinding, which is currently the only and easiest way. Since carbide gun drills are the most commonly used, let's first talk in detail about how to put carbide gun drills into better use after regrinding.
Regrinding of carbide gun drills:
The grinding of the gun drill should be forced grinding at regular intervals. When sharpening, the gun drill needs to be clamped and indexed by a sharpening fixture, and the tool end edge must be sharpened on a special sharpening machine.
Standard for dulling carbide gun drills
When the flank width of the outer edge of the gun drill exceeds the value in the table below, the gun drill must be ground to prevent the cutter head from breaking due to increased cutting force.
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Things to note when sharpening carbide gun drills
1. The grinding of the gun drill should be forced grinding at regular intervals. That is, when the wear amount on the back of the tool reaches the standard of blunt grinding, it should be regrinded.
2. When grinding, you need to use a sharpening fixture and a special sharpener. Do not sharpen with a hand-held drill.
3. When grinding, the force direction of the drill bit should be toward the tool pad to prevent damage to the tool due to rocking of the drill rod. The amount of feed each time should not be too large to prevent the carbide drill bit from cracking and breakage, causing personal injury.
4. After the five cutting surfaces of the drill bit are ground, the edges and corners between the drill bit flank surface and the drill bit guide part should be rounded manually on the grinding wheel.
5. The gun drill needs to be reground on a special diamond grinding wheel.
6. The dust generated by sharpening the carbide part of the gun drill is harmful to the body. Please wear protective glasses and masks.
