Drilling
Various cutting processes of drilling, reaming or countersinking are done using different types of drill bits.
Drilling is a cutting process that produces holes using twist drills, flat drills, or center drills in solid materials to create through holes or blind holes.
Reaming increases the diameter of a pre-existing hole in a workpiece using a countersink bit.
Countersinking is performed using a countersinking drill bit at one end of the pre-existing hole to produce countersinks, tapered holes, partial planes, or spherical shapes, which are used to install fasteners.
During the drilling process, a twist drill has two primary cutting edges and a transverse edge, commonly referred to as “a tip (center of the drill) and three blades”, which participate in cutting.
The twist drill works in a semi-closed state, where the transverse edge is strongly compressed and chip removal is difficult. Therefore, the processing conditions are more complex and challenging than turning or other cutting methods, resulting in lower processing precision and rougher surfaces.
The drilling accuracy of steel materials is generally IT13-10, with a surface roughness of Ra20-1.25μm, while the reaming accuracy can reach IT10-9, with a surface roughness of Ra10-0.63μm.
The quality and efficiency of the drilling process largely depends on the shape of the drill's cutting edge.
In production, the shape and angle of the cutting edge of a twist drill are often changed by sharpening to reduce cutting resistance and improve drilling performance. The China group drill is an example of a twist drill produced using this method.
When the ratio of depth (l) to diameter (d) of a drilled hole is greater than six, it is generally considered deep drilling. The drill bit used for deep drilling is thin and has low rigidity. During drilling, the drill is subject to deflection and friction with the hole wall, making cooling and chip removal difficult.
Therefore, when the l/d ratio is greater than 20, a specially designed deep hole drill is required, and a cutting fluid with a certain flow rate and pressure is used for chip cooling and washing to obtain high-quality drilling results with high efficiency. .
The cutting part of the flat drill is spade-shaped and its structure is simple and has low manufacturing costs. Cutting fluid can be easily introduced into the hole, but its cutting and chip removal performance is poor. Flat drills can be divided into two types: integral and mounted.
The integral type is mainly used to drill micro holes with a diameter of 0.03mm to 0.5mm. Mounted flat drills have replaceable blades and can be internally cooled. They are mainly used to drill large holes with a diameter of 25 mm to 500 mm.
Deep drills generally refer to tools that have a depth-to-hole diameter ratio greater than 6. Commonly used deep drills include gun drills, BTA deep drills, jet drills, DF deep drills, etc. used for deep hole processing.
Reamers have 3 to 4 teeth and are stiffer than twist drills. They are used to enlarge existing holes and improve processing accuracy and smoothness.
Countersunk drill bits have multiple teeth and are used to shape the end of holes, as countersunk holes for various types of countersunk screws, or to level the surface of the outer end of holes.
Center drills are used to drill center holes in shaft-type parts. Essentially, they are composed of twist drills and countersunk drills with very small helix angles and are also called compound center drills.
The application of parabolic drill in deep hole machining
When mechanical processing personnel choose a drill for a specific hole processing task, the depth of the processed hole needs to be considered first. The deeper the processed hole, the more chips will need to be discharged during the machining process.
If the chips generated during processing cannot be discharged in a timely and effective manner, it may block the chip removal groove of the drill, thereby delaying the machining process and ultimately affecting the quality of hole processing.
Therefore, effective chip removal is a key factor in successfully completing the hole processing task of any material.
The length-diameter relationship of drills
When processing personnel choose the most suitable drill type for a specific hole processing task, they need to calculate the drill length/diameter ratio.
The length-to-diameter ratio is the relationship between the depth of the processed hole and the diameter of the drill. For example, if the diameter of the drill is 12.7 mm and the depth of the hole to be machined is 38.1 mm, then its length to diameter ratio will be 3:1.
When the length-to-diameter ratio is about 4:1 or less, most standard twist drills can smoothly discharge the cut chips from the drill tip.
However, when the length-to-diameter ratio exceeds the above range, specially designed deep hole drills are required to achieve effective machining.
Since the length/diameter ratio of the processed hole exceeds 4:1, it is difficult for standard twist drills to remove chips from the cutting area and discharge them out of the hole. Chips will quickly block the chip removal groove of the drill.
At this point, it is necessary to stop drilling, remove the drill from the hole, remove the chips from the chip removal groove and then resume drilling to continue cutting.
The above operation needs to be repeated several times to achieve the required hole depth. This drilling method is often called “deep drilling”. Using “deep drilling” to machine deep holes will reduce tool life, reduce machining efficiency, and affect the quality of the processed hole.
Each time the drill is retracted from the hole to clear chips and reinserted into the hole, it may deviate from the centerline of the hole, causing the hole diameter to increase beyond the specified size tolerance range.
In order to solve the problem of deep hole machining, drill manufacturers have developed two new types of drills for deep hole machining in recent years – ordinary parabolic drills and wide-blade parabolic drills.
Common parabolic drill
The chip removal groove of a parabolic drill is parabolic in shape and is specifically used for continuous drilling of deep holes with a length-to-diameter ratio of up to 15:1 and material hardness not exceeding 25-26 HRC (including low-grade steel). carbon, various aluminum alloys, copper alloys, etc.).
For example, a parabolic drill with a diameter of 12.7 mm can successfully machine a hole depth of up to 190 mm.
Due to its large chip removal space, an ordinary parabolic drill can quickly discharge chips to the cutting edge, allowing more cutting fluid to enter the cutting area, greatly reducing the possibility of cutting friction and chip welding.
Furthermore, it also reduces energy consumption, torque load and cutting impact during machining.
The helix angle of a parabolic drill is 36°-38°, which is greater than the helix angle of a standard twist drill (28°-30°). The helix angle can indicate the degree of “twist” of the drill, and the greater the helix angle, the faster the drill speed and chip removal.
Another feature of common parabolic drills suitable for deep hole machining is that the drill core is thicker (the drill core refers to the central part of the drill that has not been ground after forming the chip removal groove).
The core of a standard twist drill accounts for about 20% of the entire finished drill, while the core of a parabolic drill can account for about 40% of the entire drill.
In deep drilling, a thicker drill core can increase the rigidity of the drill bit and improve the stability of the drilling process. The tip of the parabolic drill has a groove, so a larger drill core diameter can be used. Furthermore, it can prevent the bit from shifting during the initial drilling phase.
Parabolic drills are made of high-speed steel and can be surface coated to improve their cutting performance.
Wide blade parabolic drill
To meet the needs for deep drilling of difficult-to-machine materials (such as cold-worked, hardened materials), some tool manufacturers have developed wide-blade parabolic drills.
Many features of this type of drill are similar to those of common parabolic drills, such as a larger helix angle (36°-38°) for easy chip removal and a thicker drill core for better rigidity and stability when machining deep holes. .
The difference between it and common parabolic drills is in the shape of the groove for chip removal and the edge of the blade. The blade edge of the wide blade parabolic drill makes a smooth transition to the chip removal groove, making the drill tip stronger and stiffer. At the same time, chips can be discharged smoothly through the chip removal groove.
When deep drilling, the high temperature caused by friction can cause slight softening or annealing of the drill's cutting edge, which accelerates wear. The ability of the drill tip to maintain hardness during processing can be expressed as “red hardness”.
Wide blade parabolic drills are generally made of high speed steel and cobalt high speed steel materials. Due to the higher red hardness of cobalt high-speed steel, the tool life is longer and the wear resistance is higher.
Drill Surface Coatings
The following surface coatings are commonly used for common parabolic drills and wide blade parabolic drills:
① Titanium nitride (TiN) coating: This coating can significantly improve the service life of drill bits and the quality of processed holes. Compared with uncoated drill bits, TiN coated drill bits are more suitable for high-speed drilling of various materials (especially various steel parts).
② Titanium carbonitride (TiCN) coating: At the appropriate cutting temperature, TiCN-coated drills have higher hardness, higher toughness and better wear resistance than TiN-coated drills. They are also suitable for high-speed drilling of various materials (especially steel parts).
However, they should be used with caution when processing non-ferrous metal materials because TiCN coating has high chemical affinity for non-ferrous metals and is prone to wear.
③ Titanium aluminum nitride (TiAlN) coating: This coating can improve the service life of drill bits, especially in high temperature cutting environments. Similar to TiCN coating, TiAlN coating is not very suitable for processing non-ferrous metal materials.
Optimization of drilling parameters
In deep hole machining, to maximize the drill's cutting performance, the drilling speed and feed rate must be optimized based on the specific length to diameter ratio.
When the length/diameter ratio of the drilling process is 4:1, the cutting speed should be reduced by 20% and the feed rate should be reduced by 10%.
When the length/diameter ratio is 5:1, the cutting speed should be reduced by 30% and the feed rate should be reduced by 20%. When the length/diameter ratio reaches 6:1-8:1, the cutting speed should be reduced by 40%. Furthermore, when the length/diameter ratio is 5:1-8:1, the feed rate should be reduced by 20%.
Although the price of a parabolic drill is 2 to 3 times higher than that of a standard twist drill, its excellent performance in machining deep holes (length-to-diameter ratio greater than 4:1) significantly reduces the cost of each hole drilled, making -The preferred tool for mechanical technicians to process deep holes.
