Machining is defined as the use of tools to remove material from a workpiece in the form of chips. This involves workpiece materials, machine tools, fixtures, tools and cutting fluids. Beryllium is a rare metal with properties different from most other materials. Beryllium is not difficult to machine when its properties are mastered and some basic principles are followed. Numerous articles and research literature have been published to aid in beryllium cnc machining, but current complete data collection is lacking. From time to time, Brash Wellman receives calls from new users asking questions, mainly about very basic information about beryllium copper cnc machining. The purpose of compiling this introduction is to familiarize potential users with beryllium products and to equip them with some common sense of machinery to produce qualified beryllium products. This guidance material is a collection of personal and beryllium processor experience, but this material alone will not make the user an expert. This introduction collects knowledge from various sources, hoping to provide a reference or entry point for new users.
Introduction Of Beryllium Copper Precision Machining
Beryllium metal is extracted from beryl or silicon beryllite, and beryllium beads are obtained through a long series of chemical treatments. Beads are then vacuum smelted into beryllium ingots or rolled into powders (as raw materials for vacuum hot-pressed beryllium or other special powder metallurgy materials). Vacuum hot-pressing sheaths are usually made into cylinders to isolate external pollution and are made into different sizes according to the needs of different users. At present, more than 90% of beryllium material preparation equipment adopts this process. Beryllium powder can also be consolidated without sintering or casting, such as cold isostatic pressing after vacuum hot pressing, cold isostatic pressing or hot isostatic pressing after vacuum hot pressing. Beryllium can also be made into hot-pressed beryllium foil and drawn beryllium wire through a special production line.
This introduction is mainly about the basic principles of machining most vacuum heat pressing materials. Special materials are not included. Brash Wellman provides 5 commonly used grades of beryllium. These grades are sorted according to machinability as follows: 1) S-200 general grade; 2) S-65 high elongation structural grade; 3) I-70 optical grade; 4) I-220 inertial instrument grade (micro yield strength 5KSI) ;5) I-400 inertial instrument grade (with excellent dimensional stability and micro yield strength 8KSI). (1KSI=6.89N/mm2)
The machinability statements in this brief apply to all grades of beryllium. Feed rate, cutting speed and depth of cut are for general grade S-200. When machining other grades of beryllium(as cnc machining beryllium copper 17500), the machining parameters should be more conservative, especially the I-400 grade. The high oxide content and low elongation of the I-400 grade limit the machinability of this material. If the processing of I-400 beryllium material does not adopt more conservative parameters, it will lead to material collapse.
In addition, the detailed machining parameters of S-65 grade beryllium material can be found in the Air Force Materials Laboratory report AFML-76-88 “S-65 hot-pressed beryllium material preparation, material characteristics and machining process evaluation”, the author is Bryce King of Brush Wellman Company.
Although machining beryllium is different from other materials, beryllium is not difficult to machine and it is important to understand these differences. The machining experience of beryllium is similar to that of hard cast iron. Tools used to machine cast iron require minor geometric adjustments to work with beryllium. Set the machinability of 1113 steel to 100%, and the machinability of beryllium to 55%. Despite the relatively low hardness of beryllium (RB90 or RC11), tool wear is rapid and discontinuous chips are produced. When choosing an unsuitable tool geometry or tool material, it will cause the cutting edge of the tool to wear quickly.
The second general-purpose carbide tool grade for machining cast iron and non-ferrous materials is suitable for beryllium machining. In this grade, you can choose a specific tool grade such as Valenite VC-2 or the corresponding grade. This grade of tool is suitable for precision machining of long workpieces, and the cutting edge can withstand miles of cutting paths.
Obtaining a damage-free surface is very important for precision machined parts of beryllium. Surface damage mainly occurs in the form of twins or microcracks, which will reduce the elongation of beryllium to a fraction of the undamaged state. Machining damage of beryllium materials is usually eliminated by appropriate technological means such as stress relief heat treatment or chemical milling. It is recommended to use the chemical milling ratio in Appendix A. The thickness removal rate of one side is 0.0127mm/min, and the thickness of one side should be removed by 0.1016mm. Using a disciplined machining process and using chemical milling before the component is used has proven to be very effective. In some cases, chemical milling will remove more, but the dimensional tolerance is not easy to guarantee. Experience has proved that when the removal amount of chemical milling is 0.1016mm, tighter dimensional tolerances can still be maintained.
The key to controlling machining damage is to prevent excessive tool wear. Worn knives can cause deep damage. Tool life depends on the tool itself and the conditions of use. Properly trained and experienced machinists will be able to determine when a tool needs to be replaced, and finishers should be more concerned about this. A more critical lesson is controlling all aspects of the machining process and establishing safe tool change intervals. Failure to follow the above rules can lead to cracks or even scrapping of the workpiece.
Due to cutting stress, beryllium materials are prone to surface defects such as microcracks and twins. The machined surface that the cutting edge engages can cause damage to the workpiece. The depth of the damaged layer can be measured by metallographic detection method. The degree of defects such as twinning and microcracks is affected by a combination of material properties, tool geometry, feed rates, cutting speeds, machining methods, and wet and dry methods. The surface damage layer must be controlled within 0.1016mm. When finishing is completed, the above-mentioned damage is eliminated by chemical milling and stress relief heat treatment. The metallographic inspection of the specimen section by polarizing microscope (250X) can judge whether the machining process is appropriate or not.
General Process Description
Special attention needs to be paid to the discussion of beryllium machining in the following sections.
Beryllium is an expensive metal. The more economical method is to let the raw material supplier provide the blank as close as possible to the final size of the workpiece. In this way, the machining manufacturer can reduce the generation of chips in the subsequent machining process. Rigid clamping of the workpiece is very important, otherwise chipping or cracks will occur at the entry and exit points of the cut. The knife should cut from no material to material.
Proper beryllium dust extraction devices and installation locations must be used during beryllium machining. Beryllium dust monitoring should be carried out regularly. Airborne beryllium dust can cause a range of chemical symptoms (beryllium allergy) that impair bodily functions, especially lung damage. The United States Atomic Energy Commission promulgated the standard at that time decades ago. The concentration of beryllium in the indoor atmosphere should not exceed 2mg/m2 (average concentration of 8 hours a day). Even if it exceeds the above average level, no one is allowed to be exposed to more than 25mg/m2. m2 environment. The maximum allowable contact time for a short-term concentration reaching or exceeding 25mg/m2 cannot exceed one and a half hours. After years of practice, this standard has been proved to be effective.
Beryllium is usually dry-cut during rough machining, mainly because clean beryllium chips are easy to recycle, smelt and recycle. The beryllium swarf is pumped into a 55-gallon steel tank by using a high-efficiency suction system.
Cutting fluids should be used when drilling deep holes in beryllium, tapping threads, or when high tolerances, high tool life, and no surface damage are required. Emulsions containing sulfur and chlorine are not recommended because they can cause corrosion or surface blooming of beryllium.
When using cutting fluid, the workpiece must be strictly cleaned with standard or MEK solution and dried after machining. The advantage of using cutting fluid is that it helps reduce beryllium dust. The disadvantage of using cutting fluid is that it increases the cost of cleaning and recycling and reduces the recycling price of beryllium shavings (material supplier recycling).
Relatively speaking, beryllium is a soft material, so it is recommended to use soft jaws to clamp the workpiece and increase the contact area as much as possible. When using cutting fluid and brass fixtures at the same time, pay close attention to the occurrence of battery effect (causing corrosion).
Hot-pressed beryllium ingots can be rolled into beryllium flakes at high temperatures. Roll forming speeds are lower than aluminum, but rolling tool and process costs are similar.
Beryllium structural components can be assembled using most techniques such as mechanical fastening, riveting, bonding, brazing, and welding. However, the fusion welding process should be avoided as much as possible, because it will cause the grain transition to increase and reduce the mechanical properties.
All conventional machining can be used on beryllium, including wire EDM and EDM. However, beryllium dust and beryllium salts can cause health damage and must therefore be carried out in plants with special conditions.
Common beryllium machining methods such as turning, grinding, drilling, milling, and lapping are discussed in the following sections. Regarding the unconventional process methods such as EDM, wire cutting, and chemical milling used in beryllium 17200 machining, it is recommended to refer to the MCIC-72-03 report “Non-traditional machining of Beryllium Materials” of the Battel Metal and Ceramic Information Center.
Turning, Boring, Milling
Beryllium is a relatively soft material, so it is recommended to use tools with positive rake angles for the machining of all grades of beryllium. It is easy to form a crescent on the rake face of the tool when machining soft materials, and the positive rake angle is beneficial to avoid the above situation. Single-point tools for turning, boring, and milling beryllium must have sharp edge angles (6° for primary relief and 10° for secondary relief.). When the tool adopts a positive rake angle of 1-5°, the processing effect is the best, and sometimes a 0° relief angle can also be used. To reduce cutting stress, the nose radius or chamfer should be as small as possible. Almost all beryllium processing uses cemented carbide tools. Due to the grinding effect of beryllium, it should be supported by a live center.
The use of cutting fluid can improve the cutting effect and control the flying of beryllium dust, which is very important to the operator. It is best to use cutting fluid when finishing beryllium. Prochem Triple C 5505 is a tool grade specially designed for beryllium machining. Dry cnc machining is only used under special conditions, for example, in some cases it is necessary to collect dry chips for subsequent chemical analysis. The use of cutting fluid can reduce the processing cost, because for some special processing technology, the beryllium dust extraction device is very expensive. The beryllium dust extraction device must be located on the right side of the cutter so that the chips can be extracted in time. Cutting fluid must be used when grooving and drilling so that chips can be washed away from the cutting area in time.
Cutting fluid can be used to flush chips into the chip flutes, chips should be kept moist and collected frequently. The chips should be submerged in the cutting fluid to prevent the chips from becoming beryllium dust that can fly in the air after drying.
The cutting speed of beryllium material during rough turning, rough milling, drilling and boring is 88.9 to 127mm/sec. The more reasonable cutting speed is 50.8 to 101.6 mm/s for the critical dimension of fine turning. The feed rate during rough machining is 0.15 to 3 mm/rev. In order to minimize surface roughness during finishing, the feed rate should be below 0.013 mm/rev. The depth of cut for roughing is typically 2.54 to 6.35mm. The maximum cutting depth during finishing is 0.762mm. Typical roughing speeds are 70 to 125 rpm. The speed usually used for finishing is 30 to 60 revolutions per second. Beryllium is not suitable for any plastic machining, (Beryllium does not lend itself to any dwell type operation, such as generating corner radii by plunging or changing direction when turning and facing). Continuous cutting processes, such as CNC contouring, can minimize cutting thermal cracks, which tend to appear on porcelain surfaces. Beryllium workpieces are prone to cracks at the cut edge. The above-mentioned cracks can be avoided when cutting with a small amount of cutting. Chamfering at both ends (finish turning one end for a short distance and then reversing ends, chamfering both ends prior to turning or using “run off” material.)
In order to avoid excessive cutting stress, the depth of cut for finishing should not exceed 0.762mm. In some cases, some machining personnel use the method of gradually reducing the depth of cut to gradually reach the final size. A series of cutting depths are 0.762, 0.508, 0.254, 0.127mm. This helps to reduce surface damage and improve surface quality. A depth of cut of 0.0254 mm or less and a cutting speed of 10 revolutions per second can be used for finishing to obtain high surface quality.
When the chuck is loaded, the three jaws should not use too much clamping force. In order to avoid cracks or chipping, custom-made soft jaws should be used for finishing. Mild steel, aluminum, or bakelite are often used as materials for soft claws. When using water-based cutting fluid while using soft claws, it should be noted that sometimes water-based cutting fluid reacts with soft claws and beryllium materials, resulting in corrosion of beryllium materials. Due to the low elongation of beryllium, care should be taken when machining unsupported slender workpieces with large aspect ratios.
Wide cutters should be avoided for finishing. When machining holes, boring rather than reaming should be used as much as possible. If reaming is required, a positive helix rake angle reamer must be prepared. Madison or Davis type chuck boring tools can efficiently process beryllium materials while maintaining dimensional and surface quality requirements. Tapping on beryllium is accessible to all operators through the use of flexible tapping units, advanced tooling and control specifications.
Interrupted cuts should be avoided as much as possible (especially during finishing operations.) Sometimes this is unavoidable when rough turning rough stock, such as turning square stock into round bars. Uncoated tools with large nose radii are excellent for rough interrupted roughing. In some cases the depth of cut exceeds 6.35mm. There is a special case of using SPU633 machine card blades (were circumferentially located in a bar turning device that literally chews up the stock at feed rates of ) at a feed rate of 0.2032 to 0.254mm/rev or 50.8 to 63.5mm/sec. After rough turning, a margin of 0.762 to 1.524mm is left on one side for subsequent fine turning. The feed speed of the fine car is 91.44mm/sec or 0.1778mm/rev.
CNC Milling Beryllium Copper
The successful experience that applies to turning beryllium also applies to milling beryllium. Equipment, cutting tools and processes must be selected reasonably.
The axial rake angle, radial rake angle and corner angle of the face milling cutter correspond to the longitudinal rake angle, transverse rake angle and cutting-edge angle of the turning tool one by one. Usually defined angles in terms of the relationship of the cutting edge of the tool to the workpiece. The most important angle in some machining operations is the actual rake angle. The actual rake angle directly affects the shear angle during chip formation, thereby affecting cutting force, cutting power and cutting heat. The larger the positive value of the actual rake angle, the smaller the cutting force.
Beryllium is a highly rigid material, which results in increased tool cutting forces. When the workpiece is machined, it is very easy to crack or break at the cutting-in and cutting-out points of the tool. The occurrence of such cracks can be controlled by using appropriate technological measures and special cutting methods. For example, in conventional milling, the workpiece is not prone to damage at the point of entry of the tool because the cutting forces are acting on the bulk of the stiff material. Cracks can occur at the point where the tool cuts out, if no protective measures are taken to avoid this. Another protective measure is to chamfer 30° on the cut edge, which can eliminate the depth of the crack. The tool will gradually cut away from the chamfered edge.
When machining beryllium with a face mill, select positive radial and axial rake angles. Carbide machine card inserts with 30° to 45° chamfer or 0.762 to 1.016mm chamfer will get good machining results. The generation of edge crumbling must be pre-attended.
Climb milling, when machining beryllium 17300, tool life is very important, and climb milling will obtain longer tool life. The cutting force of climb milling is directed to the workpiece and the table. The cutting edge cuts continuously into the workpiece without the shocks seen in conventional milling. The workpiece must be reliably clamped and positioned with a rigid fixture to prevent vibration between the tool and the workpiece.
Face milling or rough milling is usually done dry cutting. Under the action of the tool rotation, the chips mixed with the cutting fluid will quickly fly away from the tool without being sucked away, creating an unsafe area. The knives are completely surrounded by the extraction air flow so that all chips and beryllium dust are recovered into special containers. Rough milling of beryllium materials usually adopts a linear feed rate of 50.8 to 63.5mm/sec, a feed rate of 0.254mm per tooth, and a depth of cut of 6.35mm. For fine milling, the maximum cutting depth is 0.762mm, and the feed rate is 0.0762 to 0.2032mm per tooth.
Optimum machining results depend primarily on the high rigidity of the workpiece and tool system. Due to the reduction of the rigidity of the tool or workpiece system, the vibration of the tool and the increase in noise will cause excessive wear or damage of the tool, damage to the workpiece, reduction or out-of-tolerance machining accuracy or surface roughness. CNC machine tools with automatic tool change system are the best equipment for cutting machining. These devices can reduce production costs and process consistency. More cutting tools are available, automatic control of cutting speed and feed,
For example, use a 6.35mm diameter three-edged carbide keyway milling cutter designed for machining numerous special-profile slots on aircraft brake pads and avoid milling cracks. During rough machining, the cutting depth is 6.35mm, the cutting speed is 53340mm/min, and the feed rate is 152.4mm/min. The cutting depth during finishing is 0.254mm, the cutting speed is 45720mm/min, and the feed rate is 152.4mm/min. Because of the many tight turns and motion cross over points this configuration required, a strong, tough tool was required to take the load.
When using end mills (end mills), more flutes, larger shear angles, and a maximum wear width of 0.254mm can effectively avoid cutting damage. In the initial cut, if possible use a 0.381-0.508mm end mill (compared to rough milling). Spring chucks smaller than 12.7mm diameter can be used. When using larger diameter end mills, provide set screw flats on the shanks. Spring clips are prone to loose sliding.
Cnc Drilling Beryllium Copper
Drilling holes in beryllium is not much different than drilling other materials. Problems arise due to: 1) tool wear due to beryllium grinding; 2) orifice chipping (relatively low elongation of beryllium); and 3) high torque and axial forces required. Other things that should be of concern are: drill material, drill construction, bluntness and chipping of drill bits, drilling vibration, centering holes, increased feed rates while drilling out and operator changes when using manual machines.
When all other factors are equal, carbide tools are better suited for beryllium drilling than HSS tools. Although the standard 118° tip angle machining is acceptable, there is an increased chance of chipping when the tip is blunted or the tip angle is increased. Thinned webs bits reduce axial forces. When drilling deep holes, it is recommended to use a drill bit with a taper of about 0.1016mm from the drill tip to the drill shank to reduce the cutting resistance due to wear and cutting heat.
The cutting speed and feed rate of drilling beryllium are not much different from drilling cast iron. Good machining results can be obtained with cutting speed ranging from 25 to 150SFM. In most cases, the speed used will choose the middle value of this range. The feed speed ranges from 0.01524 mm/ to 0.03048 mm/rev. In special occasions, the feed speed will be as high as 0.254mm/rev, which is of course not a routine situation.
The drill bit must be kept sharp and avoid chipped areas. Beryllium is more sensitive to these conditions than other materials. Using a center drill (bottom drill) to center the hole can avoid the deflection and vibration of the drill bit when drilling, otherwise it will cause the hole to break and cause the workpiece to break, especially when drilling through holes on thin-walled parts.
When the drilling requirements are high and free-hand drilling is not allowed, the torque monitoring drill chuck used in the aerospace industry has proven to be a very good method. Using this equipment, allowable values for rotational speed and feed rate can be determined experimentally. Since the torque is mainly determined by the feed speed, the feed speed will be automatically reduced when the monitoring computer on the drill bit generates a signal to reduce the cutting force during the drilling process or when it is about to drill through. Dull or damaged tools increase cutting forces and reduce feed rates.
Another good way to machine holes in beryllium is with a CNC machine. Advances in CNC technology and good drilling experience will yield the same good results, except on special installations. When the drilling depth is determined, the cutting speed and feed speed can be controlled steplessly, and a smooth hole can be obtained. Support material may also be employed at the borehole exit. When machining large diameter holes, it is recommended to use the boring process to reach the final size and remove the surface damage caused by drilling. The general rule of thumb for drilling is to finish drill to size, ie center drill cannot be used as it will cause excessive surface damage.
The above recommendations apply to routine drilling of vacuum-pressed beryllium. The special drilling technology designed to prevent the hot-rolled beryllium material from collapsing is recommended by NASA. Dyna Systems Ltd. recommends the use of a CNC machining program called “tornetic drilling unit”.
Deep hole machining with a depth-to-diameter ratio exceeding 5 is difficult and requires special techniques. The inability to dissipate heat is the main problem. In the special machining plant, the above problems can be solved by using a gun drill with central cooling of the spindle (coolant pressure 20670N/mm2). (1KSI=6.89N/mm2)