How are drill bits made? What problems will be encountered in drill processing? About the drill material and its properties? What do you do when your drill bit fails?
As the most common tool in hole machining, drill bits are widely used in mechanical manufacturing, especially for the machining of holes in parts such as cooling devices, tube sheets of power generation equipment, and steam generators. The application is particularly extensive and important. Today, the Professor of Mechanical Engineering found this drill bit collection for everyone on the WeChat platform. Everything you need is here!
Drilling Features
Drills usually have two main cutting edges. During machining, the drill cuts while rotating. The rake angle of the drill bit increases from the central axis to the outer edge. The cutting speed of the drill bit increases as it gets closer to the outer circle, and the cutting speed decreases towards the center. The cutting speed of the drill bit’s rotation center is zero. The chisel edge of the drill bit is located near the axis of the rotation center, the chisel edge has a large auxiliary rake angle, no chip space, and the cutting speed is low, which will generate a large axial resistance. If the chisel edge is ground to type A or type C in DIN1414, and the cutting edge near the central 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., drills can be divided into many types, such as high-speed steel drills (twist drills, group drills, flat drills), solid carbide drills, indexable shallow hole drills, deep hole drills, etc. Drills, trepanning drills and replaceable head drills, etc.
1. Process/processing
1.1 Process
❶ According to the diameter and total length of the designed drill bit, you can choose alloy bar cutting machine or use wire cutting equipment for fixed-length processing.
❷ For the fixed-length cut bar, the two ends of the bar are flat-ended, which can be realized on a manual tool grinder.
❸ Chamfering or drilling the end face of the alloy bar that has been ground, in preparation for grinding the outer diameter and shank of the drill bit, depending on whether the cylindrical grinding fixture is a male tip or a female tip.
❹ On the high-precision cylindrical grinding machine, the outer diameter of the drill bit, the hollow part and the outer diameter of the shank are processed to ensure the design requirements such as outer diameter cylindricity, circular runout, and surface finish.
❺ In order to improve the processing efficiency on the CNC grinding machine, before the alloy bar is put on the CNC grinding machine, the drill tip part can be chamfered, for example, the drill tip angle is 140°, and the chamfer can be roughly ground to 142°.
❻ After the chamfered alloy bar is cleaned, it is transferred to the CNC grinding machine process, and each part of the drill bit is processed on the five-axis CNC grinding machine.
❼ If it is necessary to improve the flute of the drill bit and the smoothness of the outer circle, it can also be ground and polished with wool wheels and abrasives before or after the fifth step. Of course, in this case, the drill bit needs to be processed in more steps.
❽ For the drill bits that have been processed and qualified, they will be laser marked, and the content may be the company’s brand LOGO and drill size and other information.
❾ Pack the marked drill bits and ship them to a professional tool coating company for coating.
1. If the flute of the drill bit is opened, or spiral or straight flute, this step also includes negative chamfering of the peripheral edge; then process the cutting edge of the drill point, including the backlash part of the drill point and the back corner of the drill point; then proceed The back part of the peripheral edge of the drill bit is processed, and a certain amount of drop is ground to ensure that the outer diameter part of the peripheral edge of the drill bit and the contact surface of the workpiece hole wall are controlled in a certain proportion.
2. For the processing of the negative chamfer of the drill tip edge, it is divided into CNC grinding machine processing or manual processing, which is different due to the different processes of each factory.
1.2 Processing issues
❶ When processing the outer circle part of the drill on the cylindrical grinding machine, it is necessary to pay attention to whether the fixture is invalid and to fully cool the alloy bar during processing, and to maintain a good habit of measuring the outer diameter of the drill tip.
❷ When processing drills on CNC grinding machines, try to separate rough and fine processing into two steps when programming, so as to avoid potential thermal cracks caused by too much grinding, which will affect the service life of the tool.
❸ Use a well-designed material tray for the handling of the knives to avoid damage to the cutting edge caused by the collision between the knives.
❹ For the diamond grinding wheel that has become black after grinding, use the oil stone to sharpen the edge in time.
Note: According to the processed materials/equipment/working conditions, the processing technology is not the same. The above process arrangement only represents the author’s personal opinion and is only for technical communication.
2. Drill material
2.1 High speed steel
High-speed steel (HSS) is a tool steel with high hardness, high wear resistance and high heat resistance, also known as high-speed tool steel or front steel, commonly known as white steel.
High-speed steel cutter is a kind of cutter that is tougher and easier to cut than ordinary cutters. High-speed steel has better toughness, strength and heat resistance than carbon tool steel, and its cutting speed is higher than carbon tool steel (iron-carbon alloy). There are many, so it is named high-speed steel; and cemented carbide has better performance than high-speed steel, and the cutting speed can be increased by 2-3 times.
Features: The red hardness of high-speed steel can reach 650 degrees. High-speed steel has good strength and toughness. After sharpening, the cutting edge is sharp and the quality is stable. It is generally used to manufacture small and complex-shaped knives.
2.2 Carbide
The main components of cemented carbide drill bits are tungsten carbide and cobalt, which account for 99% of all components, and 1% is other metals, so it is called tungsten carbide (tungsten carbide). Tungsten carbide is composed of at least one metal carbide Sintered composite materials. Tungsten carbide, cobalt carbide, niobium carbide, titanium carbide, and tantalum carbide are common components of tungsten steel. The grain size of the carbide component (or phase) is usually between 0.2-10 microns, and the carbide grains are held together using a metal binder. Binder metals are generally iron group metals, commonly used are cobalt and nickel. Therefore, there are tungsten-cobalt alloys, tungsten-nickel alloys and tungsten-titanium-cobalt alloys. The sintering molding of tungsten steel drill bit material is to press the powder into a billet, then heat it to a certain temperature (sintering temperature) in a sintering furnace, keep it for a certain time (holding time), and then cool it down to obtain the tungsten steel material with the required properties.
Features:
The red hardness of cemented carbide can reach 800-1000 degrees.
The cutting speed of cemented carbide is 4-7 times higher than that of high-speed steel. High cutting efficiency.
The disadvantages are low bending strength, poor impact toughness, high brittleness, and low impact and vibration resistance.
3. Application issues/measures
3.1 Drill point wear
reason:
1. The workpiece will move downward under the action of the drilling force of the drill bit, and the drill bit will bounce back after drilling through.
2. The rigidity of the machine tool is insufficient.
3. The material of the drill bit is not strong enough.
4. The drill bit jumps too much.
5. The clamping rigidity is not enough, and the drill bit slides.
measure:
1. Reduce the cutting speed.
2. Increase the feed rate
3. Adjust the cooling direction (internal cooling)
4. Add a chamfer
5. Check and adjust the coaxiality of the drill bit.
6. Check whether the back angle is reasonable.
3.2 Ligament collapse
reason:
1. The workpiece will move downward under the action of the drilling force of the drill bit, and the drill bit will bounce back after drilling through.
2. The rigidity of the machine tool is insufficient.
3. The material of the drill bit is not strong enough.
4. The drill bit jumps too much.
5. The clamping rigidity is not enough, and the drill bit slides.
measure:
1. Choose a drill with a larger back cone.
2. Check the runout range of the spindle drill bit (<0.02mm)
3. Drill the top hole with a pre-centered drill.
4. Use a more rigid drill, a hydraulic chuck with a neck sleeve or a heat shrink kit.
3.3 Accumulated tumor
reason:
1. Caused by the chemical reaction between the cutting material and the workpiece material (low carbon steel with high carbon content)
measure:
1. Improve lubricant, increase oil or additive content.
2. Increase the cutting speed, reduce the feed rate and reduce the contact time.
3. If you drill aluminum, you can use a drill with a polished surface and no coating.
3.4 Broken knife
reason:
1. The spiral groove of the drill bit is blocked by the cutting, and the cutting is not discharged in time.
2. When the hole is drilled quickly, the feed rate is not reduced or the maneuver is changed to manual feed.
3. When drilling soft metals such as brass, the rear angle of the drill bit is too large, and the front angle is not ground, so that the drill bit will automatically screw in.
4. The grinding of the drill edge is too sharp, resulting in chipping, but the knife cannot be withdrawn quickly.
measure:
1. Shorten the cycle of tool replacement.
2. Improve the installation and fixation, such as increasing the supporting area and increasing the clamping force.
3. Check the spindle bearing and slide groove.
4. Use high-precision tool holders, such as hydraulic tool holders.
5. Use tougher materials.
Post time: Apr-18-2023
