Research Status of Wood Processing Technology

1 Introduction

Wood material is a kind of material that includes wood and wood (or its waste) as the main raw material, obtained through mechanical processing, physical and chemical treatment and has the basic characteristics of wood and major components. Wood, modified wood, wooden man-made materials, and wood composite materials are all wood materials. The tools used for processing wooden materials are collectively referred to as woodworking tools. They are characterized by the requirement to maintain the sharpness of the cutting edge of the tool for a long time under high-speed and impact-resistant cutting conditions, not only to have good hardness and wear resistance, but also to Has sufficient strength and toughness. Due to the special nature of wood materials, cutting must consider factors such as tree species, density, moisture content, wood grain, fiber direction, etc., which also makes the woodworking tool material selection and structural design have a strong pertinence. Although China is a major producer of man-made board production, furniture production and woodworking machinery, the woodworking tool manufacturing industry is relatively weak, high durability, high processing quality, and high-performance woodworking tools suitable for the current high-speed machining trends are still in short supply. Market share is occupied by foreign investment products. Therefore, understanding and mastering the current research status of the processing technology of wood materials will help us to clearly understand the characteristics of wood materials and their processing characteristics and the characteristics of woodworking tool structures, and increase the importance of woodworking tools in China and further research and development. It is of great significance to improve the technical level and processing efficiency of China's wood material cutting and shorten the gap between China and the international developed countries in this field.

2 Wood material and its processing characteristics

2.1 Classification of Wood Materials

Wood in general meaning refers to the trunk part of the tree, also known as logs or solid wood, and is a natural and organic polymer material composed of cellulose, hemicellulose, and lignin. It is naturally generated and has a natural texture, a lustrous texture, and a specific strength. Large, easy to process, with good heat insulation and sound insulation, is an indispensable material in furniture. Wood-based man-made materials include man-made boards, man-made boards, and molding materials. Wood composite materials include wood plastic, plastic particle board and wood composite structural board. Due to the shortage of wood resources, the increasing awareness of human environmental protection and the advancement of manufacturing technology, the world's wooden man-made materials and wood composite materials, especially the wood-based panel industry, have developed rapidly in the past decade. Common wood-based panels include: plywood, particle board, and fiberboard (low density, medium density, high density). Table 1 lists the types, uses, and processing properties of several typical wood-based panels.

Table 1 Typical wood-based panel materials

Panel Type - Material Composition - Surface Quality - Edge Processing Performance - Classification of Use - Typical Use - Special Purpose

Coniferous wood plywood - Coniferous veneer - Smooth surface after sanding - Poor - For structural - Roofing panels, flooring, wall panels - Veneer lumber (LVL), Concrete formwork, Ship plywood

Hardwood plywood - Hardwood veneer, Artificial board - Smooth surface after sanding - Poor - Mostly non-structural - Wall coverings, furniture, joinery - Decorative floor

Oriented strand board (OSB) - 12 x 150mm shavings (mostly poplar) - smooth surface after sanding - poor - for structural - roofing panels, floors, wall panels, floor liners - I-beams

Particleboard - shavings length <6mm; wheat straw - very smooth surface with fine shavings - good - mostly non-structural - furniture, joinery, floor liners, stair treads - none

Medium Density Fibreboard (MDF) - Wood fibres, fibre bundles, straw - very smooth surfaces - very good - non-structural - furniture, joinery, mouldings - without

2.2 Processing Characteristics of Wood

The research on wood processing performance at home and abroad has been relatively mature. Unlike wood, metal is a typical material with heterogeneity and anisotropy. Its cutting characteristics are:

(1) The nature and strength of wood materials vary from one direction to the other, and the angles that act on the direction of the wood fibers during cutting are different. The stress and breaking load of the wood are also different, and the resulting chip shape and workpiece surface quality are also different. For example, when cutting along a straight line, the material is chamfered in an obliquely upward direction at the edge of the cutting edge, which is easy to bend and break, and the cutting surface is good. In the case of a reverse-cutting cutting, the material advances the chamfering to occur obliquely below the cutting edge and is not easy to break and the cutting surface is poor. Because of the heterogeneity and anisotropy of the wood, different cutting directions require different structural tools.

(2) The hardness of the wood is not high, the mechanical strength limit is low, and the separation is good.

(3) The heat-resisting ability is poor, and the coking temperature (110-120°C) cannot be exceeded during processing.

(4) Suitable for high-speed cutting: Wood cutting speed is generally 40 ~ 70m / s, up to 120m / s, the general cutter shaft speed 3000 ~ 12000r / min, up to 20000r / min. High-speed cutting makes the chips less likely to crack in the fiber direction and is cut by the cutter, so that high geometric accuracy and surface roughness can be obtained.

(5) There are many factors that affect the cutting performance of wood. The physical and mechanical properties of different tree species are different. Tree species, density, moisture content, wood texture, fiber direction, annual rings, temperature, mechanical strength, etc., may produce different cutting forces. Consumption, or processing defects such as burrs. For example, the literature [3] studied the influence of the transition zone and material density on the cutting force by means of milling. The impact of the material density on the cutting force was studied in the literature [4] through the microstructure of different woods and the wood density. There is no obvious linear relationship between quartz content and wood density and tool wear. It is pointed out that the quartz content, size and density cannot be used alone to explain the cutting edge wear rate.

2.3 Processing characteristics of wooden man-made materials

There are many kinds of wood-based man-made materials, and it involves many influencing factors, which makes it difficult to systematically study them. There is no systematic and in-depth study in this area. Since these materials are mostly made of wood as the main raw material, they are cut into different structural units of different shapes and sizes, and adhesives are added. Therefore, the heterogeneity and anisotropy of wood still have a great effect on the cutting performance of these materials. influences. The research at home and abroad mainly boils down to two points:

(1) Influence of environmental and other conditions on the material properties. Literature [5,6] studied the differences in static strength, hardness, fatigue life, and creep of MDF, OSB, and particleboard in different relative humidity (65% RH and 85% RH) conditions. It shows that the static strength of MDF is the largest and the particle board is the smallest, and the static strength of the three materials at 65% RH is greater than that at 85% RH. In addition, because the MDF fiber is smaller than the shavings, that is, its material uniformity is better, the higher relative humidity has an adverse effect on the elastic modulus and fatigue life of the OSB and the particle board than that of the MDF.

(2) Effect of material properties on cutting force, tool wear, and machining quality. For example, the literature [7] tested the MDF cutting force, studied the influencing factors of the MDF cutting force and put forward the empirical formula of the cutting force calculation; the moisture content and density of the artificial board have a great relationship with the cutting performance. A series of cutting tests to measure cutting forces and surface roughness to study tool wear.

Drilling experiments on the MDF using pin hole drilling show that the factors influencing the drilling mainly include feed rate, rotation speed and cutting speed. The cutting force in the axial direction is the largest, and as the cutting speed increases, the cutting force increases. When the cutting speed reaches 60 m/min, the cutting force begins to decrease. The MDF drilling chips are fine powders. The quality of the outlet and inlet quality deteriorates as the number of drilled holes increases, but the variation of the hole diameter is very small. After 17400 holes drilling, the hole diameter still does not exceed the machining deviation.

3 Structural characteristics of drills and milling cutters for processing wood materials

3.1 The basic structural features of wood processing tools

(1) As mentioned before, the cutting direction of wood materials does not require different structural tools. For example, wood drilling is divided into horizontal drilling and longitudinal drilling. Horizontal drilling shall adopt a sharp angle of 180° (the angle between the two cutting edges of the drill bit) and a drill with a sinker. The sinker cutter is used to cut off the wood fiber to ensure the wall of the hole before the main edge participates in cutting. quality. In the longitudinal drilling, a conical drill (ie, with a front angle of less than 180°) is used.

(2) The tool wedge angle (the angle between the rake face and the flank face) is small: the strength of the wood is much smaller than that of the metal. Therefore, during the cutting process, the wood first deforms due to the cutting force, and then it is separated and eliminated. The proportion of separation force of chip and cutting is large, and the sharpness of the tool has a great influence on the separation force. Therefore, the small wedge angle facilitates the separation of the wood.

3.2 Drill

The amount of drill bits in the wood processing industry is very large, and it is mainly used to process various blind holes, through holes, and to remove defects (knots). General use of 45 steel blade, carbide blade, copper and silver brazing. In accordance with the alloy blade welding can be divided into insert type and integral type, insert type only the cutter head is carbide, the overall cutter head and spiral groove is a solid carbide rod, welding and sharpening technology requirements are higher .

For drilling through holes for metal processing, such as solid carbide straight shank twist drills, the front angle is generally greater than 80° to ensure the strength of the drill bit. The sharpness of the woodworking drill bit is even more important. Insert-type hard alloy pin hole drill for horizontal drilling, the general blade diameter of 3 ~ 16mm, double-edged, three-pointed, the center of the drill bit is about 1mm higher than the two cutting edge, the cutting edge is 0.5 times higher than the main edge ~ 1.0mm, the standard length is 57mm. Integral hard alloy through-hole drills can be used for longitudinal drilling. The front angle is generally 60°~80°, and the 60° drilling effect is the best. The cutting force is small and the burrs are small. Woodworking drills use left-handed and right-handed drills on ordinary drills. With the shank as a reference for installation and sharpening, it is generally required that the diameter of the shank be 10mm, clamped on the drilling machine by means of elastic chucks, including drills, vertical drills, and bench drills, and the rotational speed of the drill is generally 3000 to 5000r/min.

3.3 Milling cutter

The milling cutter for the processing of wooden materials is mainly used in milling machines, routers and routers and various machining centers. It can be used to shape the surface, cut the groove, seal the edge, etc. The rotation speed is 1-20000 r/min. According to the structure is divided into the following three categories:

(1) Integral milling cutter: generally use 45 steel as the body, cemented carbide as the cutting edge, copper and silver brazing. The blade protrudes from the body 1 to 1.5mm, the relief angle 10° to 15°, and the rake angle 25° to 35°. This kind of milling cutter usually grinds its rake surface with a blunt back, and after repeated grinding many times, its cutting circle diameter will gradually become smaller, and the linear curvature will change accordingly, thus affecting the processing quality. Therefore, the integral cutter is suitable for machining line types that do not need to be fitted. The edge banding knife used in the edge banding machine belongs to this type.

(2) Assembled milling cutter: The insert is generally a regular polygon. It is directly mounted on the cutter body with screws and can be used for indexing. After the blade becomes dull, turn the blade to the other edge (ie the other side of the blade) and continue to use it. The milling cutter can maintain the initial contour shape after adjusting, replacing or repairing, thus improving the processing quality. In addition, the same cutter body can be used to meet the processing requirements of different types of surfaces by using different linear blades.

(3) Combined milling cutter: Integral and assembled milling cutters can be combined into a combined cutter to process different profiles. For example, a single piece of finger knives uses a combination of machinable fingers. The fingertips are spliced ​​firmly and have good versatility, which is an important means to make full use of wood raw materials. The number of blades and the depth of cut can be adjusted according to the material width and splicing elasticity requirements. This blade is generally standardized mass production.

4 Wood material processing tool materials and wear

Carbide cutting tools are widely used for cutting various materials such as wood, plywood, particleboard, and high-middle-density boards. They can not only rough-process but also finish, and can also process veneers of various materials. The wear of PCD woodworking tools is about one-fifth that of tungsten carbide tools. By the end of the 1990s, the share of PCD woodworking tools has accounted for 40% of its total application. Each year, PCD tools used for wood processing around the world cost between US$40 million and US$50 million, and will grow at a rate of 15% to 20% in the future. China's PCD woodworking tools are currently used to strengthen the numerical control of composite flooring and high-hardness fiberboard.

The wear process of woodworking tools can be divided into three phases: initial wear, continuous wear and rapid wear, just like metal tools. According to research and analysis, the wear of woodworking drill bits mainly occurs on the flank face. When carbide drills are used to drill MDF, the wear area is still small after 15,000 holes are machined, and the wear amount tends to increase slowly with the number of drilled holes. After the drilling volume reaches 9000, the wear volume tends to be stable and has not yet entered the rapid wear stage.

Woodworking tool processing object is a multi-component, complex mixture, the special nature of the wood material determines the particularity of the three wear mechanisms common to woodworking tools.

(1) Abrasive wear: It is caused by mechanical abrasion of hard materials in wooden materials. These hard spots include: resin, quartz sand, knuckles, and gluing materials;

(2) Chemical corrosion and wear: The curing agent (ammonium chloride) added during the sizing process of particleboard and MDF and the high-temperature chlorination corrosion caused by the reaction of chlorine and oxygen in the tool; or the tannin and acetic acid present in the cutting environment of wood materials. Chemical reaction of polyphenol compounds with tools;

(3) Electrochemical corrosion: It occurs due to contact of various components of the tool material with aqueous solutions, organic weak acids, and polyhydric phenol compounds in the wood, constituting many tiny primary cells.

Therefore, to improve the wear resistance of woodworking tools, there are two main ways: First, to improve the wear resistance of the tool wear; Second, to improve the tool's ability to resist corrosion wear. The use of surface osmotic layer technology for tungsten carbide cemented carbide, boron, vanadium, chromium, nitrogen, to improve tool wear resistance. Plating tools also have varying degrees of improvement in wear resistance. Coating TiN, Ti (C, N), ZrN, (Ti, Zr) N, TiAlN and other hard alloy materials, tool life can be increased several times. Coating with CrN or Cr2N can significantly improve the frictional resistance between the tool and the wood material. Diamond has high hardness, good wear resistance and chemical inertness. Diamond-coated woodworking tools are an ideal anti-wear method. The NCD (Nanocrystalline Diamond) coating can also reduce the coefficient of friction. The amount of reduction is closely related to the thickness of the coating, the type of wood, and even its microstructure. The geometry of the cutting edge is also a key factor for reliable coatings.

5 Conclusion

Wooden materials are inextricably linked with our production and life. The processing technology has a significant position in our country's construction and improvement of people's living standards. With the development trend of high-speed and automation of woodworking machinery, there are good development trends and prospects for wood processing. Therefore, in order to further increase production efficiency and processing quality, reduce processing costs; improve the overall level of processing of wood materials in China, increase the competitiveness in the international market, and urgently need to conduct in-depth research and resolution of related issues in the field of wood material processing technology. The problems to be solved include at least:

(1) Research on the cutting mechanism of materials, especially the research on the processing mechanism of large-volume and difficult-to-machine materials (such as reinforced laminate flooring);

(2) In-depth study of tool wear mechanism for different materials processing, optimization of tool materials, optimization of tool structure;

(3) Improve CAD/CAM technology for tool design;

(4) to improve the blade welding and sharpening technology;

(5) Research on rationalization of tool use: Including different structure tools that should be used for different machining methods; and reasonable cutting parameters when using different tools.