Common Problems in Deep Hole Machining

Deep hole machining of difficult-to-cut materials, deep hole axis deflection, and the appearance of spiral grooves on the machined surface are all difficult issues to resolve in deep hole machining, directly impacting deep hole machining quality and efficiency. Therefore, research into deep hole machining techniques for difficult-to-cut materials, controlling deep hole axis deflection, and suppressing spiral grooves on the machined surface have become key areas of focus in deep hole machining. Over the years, theoretical analysis and experimental research on these issues have yielded significant machining results.

Section 1: Deep Hole Machining of Difficult-to-Cut Materials

With the rapid development of the machinery industry, specialized materials with excellent physical and mechanical properties, corrosion resistance, anti-magnetic properties, and high-temperature oxidation resistance are increasingly being used in deep hole machining. While their machining characteristics vary, they all share a common characteristic: they are extremely difficult to cut. The following sections describe their respective cutting characteristics and the corresponding process measures. I. Deep Hole Machining of Stainless Steel
1. Machinability of Stainless Steel
Stainless steel is classified according to its microstructure into austenitic (such as 1Cr18Ni9Ti), austenitic-ferritic (such as 1Cr18Ni11Si4AITi), ferritic (such as 1Cr17), martensitic (such as 1Cr13), and precipitation-hardening (such as OCr17Ni4Nb, often referred to as PH17-4).
The machinability of ferritic stainless steel is similar to that of alloy steel. Martensitic stainless steel has high hardness and strength after quenching, making it difficult to cut. Furthermore, it is difficult to achieve a low surface roughness when finishing untempered stainless steel. The relative machinability grade (K) of austenitic stainless steel is 0.15-0.5, indicating that the material has a high plasticity and is prone to work hardening. Its thermal conductivity is only one-third that of 45# steel, making it the most difficult stainless steel to cut. Precipitation hardening stainless steel generally has a hardness of HRC32~HRC38 and a strength a≥1100MPa. In addition to its high mechanical properties, this material also has the corrosion resistance of austenitic stainless steel. It is currently widely used in logging instruments and chemical machinery. When processing this material, the tool wears quickly, the chip toughness is large, and the cutting difficulty is relatively large.
2. Processing measures for processing austenitic stainless steel
(1) Select the appropriate tool material: Generally, YW1, YW2 or YG8A cemented carbide materials are selected. YT cemented carbide should not be selected because there is an affinity between the workpiece and the Ti element in the tool, which will cause serious tool sticking and tool wear.
(2) Select the appropriate cutting rate: Generally, v≤20m/min and f is 0.01~0.07mm/r.
(3) Select the appropriate tool angle: Select a larger rake angle to make cutting light and labor-saving. (4) Choose the right chip removal method: Since austenitic stainless steel is not easy to break chips, if the low speed and deep chip breaker cannot break chips well, it is best to take the opposite measure of not breaking chips, that is, reduce the feed rate. Appropriately increase the speed to cut thin chips, so that the chips are long and continuous and discharged smoothly, and the cutting is very smooth.
2. Deep hole processing of titanium alloys
Titanium alloy materials are divided into three categories: TA1-TA8 are called. Type titanium alloys: TB1TB2 are called β-type titanium alloys: TC1~TC11 are called (a+β) two-phase titanium alloys.
1. Processing properties of titanium alloys (1) Low elastic modulus: The elastic modulus of titanium alloy is about 1/2 of that of steel (i.e. 108 GPa). Strong friction is generated between the back face and the machined surface. It is one of the main reasons for the high cutting temperature of titanium alloys.
(2) High unit cutting force Titanium alloy density r = 4.51kg/cm3 is a material with high "specific strength" (strength/density) and "specific stiffness" (stiffness/density). During cutting, the contact length between the blade and the chip is extremely short, so the unit cutting force is relatively large. (3) High chemical activity: At high temperatures, titanium reacts strongly with gas components such as O2N2, H2, CO, and CO₂ in the air. In particular, it forms interstitial solid solutions with oxygen and nitrogen, generating a hard layer with high hardness, which has a strong wear effect on the tool. (4) High affinity: During cutting, due to the strong friction between the chip and the blade surface, under the action of high cutting temperature and high cutting pressure, the titanium elements in the tool material and the workpiece material have mutual affinity, resulting in biting and sticking to the tool, causing the tool to produce adhesive wear. (5) Poor thermal conductivity: The thermal conductivity of titanium alloy is about 1/5 to 1/3 of that of 45# steel. When the cutting temperature is high, the cutting heat is concentrated on the cutting edge, and the tool wears faster. 2. Main process measures (1) Control cutting temperature: Practice has shown that reducing cutting temperature is an effective way to improve tool durability when processing titanium alloys. Therefore, it is advisable to use a lower cutting speed, a large rake angle, a large back angle, and a large secondary deflection angle in order to reduce friction, make cutting easier, and "generate less" cutting heat; use a coolant with a certain pressure and flow rate for sufficient cooling so that the cutting heat can be "dissipated quickly."
(2) Use tungsten-cobalt cemented carbide: This tool material has a low affinity with titanium and good thermal conductivity. The finer the cemented carbide grains, the better the cutting effect.
(3) Improve the rigidity of the process system: This can effectively reduce cutting vibration and improve the durability of the tool. For example, use a drill rod or center stand with good rigidity.
(4) Use an inner-slanted chip breaker: The bottom of the groove has a larger radius, which reduces the deformation of the chip. The curvature radius of the chip curl is larger, which seems to be "free curling" or "free chip breaking" rather than "forced chip breaking" or "impact chip breaking", which can effectively improve the durability of the tool.
III. Deep hole processing of high-temperature alloys
1. Categories of high-temperature alloys
High-temperature alloys are divided into three categories: iron-based, nickel-based, and cobalt-based. Of the three types of high-temperature alloys, iron-based alloys have the worst oxidation resistance, are also the cheapest, and are easier to machine. Iron-based alloys have an austenitic structure, but their relative machinability is only about half that of austenitic stainless steel, making them much more difficult to machine. Key grades include GH135 (Cr15Ni35W2Mo2Al2.5Ti2) and GH36 (4Cr12Ni8Mn8MoVNb).

Nickel-based high-temperature alloys have better oxidation resistance than iron-based alloys but worse than cobalt-based alloys. They are more expensive than iron-based alloys but cheaper than cobalt-based alloys. They are more difficult to machine than iron-based alloys but less difficult to machine than cobalt-based alloys. Key grades include GH33 (Cr20Ni77AlTi2.5) and K3 (17Cr12Ni68W5M04C05Al5Ti3). GH33 is a wrought alloy, while K3 is a casting alloy.

2. Cutting characteristics of nickel-based high-temperature alloys

(1) Severe work hardening, the hardening degree of the processed surface can reach 200%~500%. (2) Large cutting force, which is 2~3 times that of 45# steel. (3) High cutting temperature, which can reach 1000℃. (4) Fast tool wear.

3. Main process measures for nickel-based high-temperature alloys

(1) Select YD15 hard alloy material, which is resistant to high temperature and has high resistance to oxidation and diffusion wear.

(2) Perform "quenching" treatment on nickel-based high-temperature alloys to convert the internal metal compounds into solid solutions, thereby reducing cutting force.

(3) Improve the rigidity of the process system and use a center stand and drill rod with good rigidity as much as possible; in addition, some measures can be taken to improve the rigidity of the tool during cutting when designing the tool, such as appropriately reducing the eccentricity, increasing the radial force pressing on the guide block, using vibration damping blocks and vibration damping strips, and using four-edge drills instead of three-edge drills.

(4) Reduce the cutting speed, but do not reduce the feed rate too much to avoid cutting the blade on the hardened layer. (5) Ensure sufficient cooling and smooth chip removal.