微孔的加工方法外文文献翻译@中英文翻译@外文翻译

1 英 文原文 Options for micro-holemaking As in the macroscale-machining world, holemaking is one of the most if not the mostfrequently performed operations for micromachining. Many options exist for how those holes are created. Each has its advantages and limitations, depending on the required hole diameter and depth, workpiece material and equipment requirements. This article covers holemaking with through-coolant drills and those without coolant holes, plunge milling, microdrilling using sinker EDMs and laser drilling. Helpful Holes Getting coolant to the drill tip while the tool is cutting helps reduce the amount of heat at the tool/workpiece interface and evacuate chips regardless of hole diameter. But through-coolant capability is especially helpful when deep-hole microdrilling because the tools are delicate and prone to failure when experiencing recutting of chips, chip packing and too much exposure to carbides worst enemyheat. When applying flood coolant, the drill itself blocks access to the cutting action. “Somewhere about 3 to 5 diameters deep, the coolant has trouble getting down to the tip,” said Jeff Davis, vice president of engineering for Harvey Tool Co., Rowley, Mass. “It becomes wise to use a coolant-fed drill at that point.” In addition, flood coolant can cause more harm than good when microholemaking. “The pressure from the flood coolant can sometimes snap fragile drills as they enter the part,” Davis said. The toolmaker offers a line of through-coolant drills with diameters from 0.039 to 0.125 that are able to produce holes up to 12 diameters deep, as well as microdrills without coolant holes from 0.002 to 0.020. Having through-coolant capacity isnt enough, though. Coolant needs to flow at a rate that enables it to clear the chips out of the hole. Davis recommends, at a minimum, 600 to 800 psi of coolant pressure. “It works much better if you have higher pressure than that,” he added. To prevent those tiny coolant holes from becoming clogged with debris, Davis also recommends a 5m or finer coolant filter. Another recommendation is to machine a pilot, or guide, hole to prevent the tool from wandering on top of the workpiece and aid in producing a straight hole. When applying a pilot drill, its important to select one with an included angle on its point thats equal to or larger than the included angle on the through-coolant drill that follows. The pilot drills diameter should also be slightly larger. For example, if the pilot drill has a 120 included angle and a smaller diameter than a through-coolant drill with a 140 included angle, “then youre catching the coolant-fed drills corners and knocking those corners off,” Davis said, which damages the drill. Although not mandatory, pecking is a good practice when microdrilling deep holes. Davis suggests a pecking cycle that is 30 to 50 percent of the diameter per peck depth, depending on the workpiece material. This clears the chips, preventing them from packing in the flute valleys. Lubricious Chill To further aid chip evacuation, Davis recommends applying an oil-based metalworking fluid instead of a waterbased coolant because oil provides greater lubricity. But if a shop prefers using coolant, the fluid should include EP (extreme pressure) additives to increase lubricity and minimize foaming. “If youve got a lot of 2 foam,” Davis noted, “the chips arent being pulled out the way they are supposed to be.” He added that another way to enhance a tools slipperiness while extending its life is with a coating, such as titanium aluminum nitride. TiAlN has a high hardness and is an effective coating for reducing heats impact when drilling difficult-to-machine materials, like stainless steel. David Burton, general manager of Performance Micro Tool, Janesville, Wis., disagrees with the idea of coating microtools on the smaller end of the spectrum. “Coatings on tools below 0.020 typically have a negative effect on every machining aspect, from the quality of the initial cut to tool life,” he said. Thats because coatings are not thin enough and negatively alter the rake and relief angles when applied to tiny tools. However, work continues on the development of thinner coatings, and Burton indicated that Performance Micro Tool, which produces microendmills and microrouters and resells microdrills, is working on a project with others to create a submicron-thickness coating. “Were probably 6 months to 1 year from testing it in the market,” Burton said. The microdrills Performance offers are basically circuit-board drills, which are also effective for cutting metal. All the tools are without through-coolant capability. “I had a customer drill a 0.004-dia. hole in stainless steel, and he was amazed he could do it with a circuit-board drill,” Burton noted, adding that pecking and running at a high spindle speed increase the drills effectiveness. The requirements for how fast microtools should rotate depend on the type of CNC machines a shop uses and the tool diameter, with higher speeds needed as the diameter decreases. (Note: The equation for cutting speed is sfm = tool diameter 0.26 spindle speed.) Although relatively low, 5,000 rpm has been used successfully by Burtons customers. “We recommend that our customers find the highest rpm at the lowest possible vibrationthe sweet spot,” he said. In addition to minimizing vibration, a constant and adequate chip load is required to penetrate the workpiece while exerting low cutting forces and to allow the rake to remove the appropriate amount of material. If the drill takes too light of a chip load, the rake face wears quickly, becoming negative, and tool life suffers. This approach is often tempting when drilling with delicate tools. “If the customer decides he wants to baby the tool, he takes a lighter chip load,” Burton said, “and, typically, the cutting edge wears much quicker and creates a radius where the land of that radius is wider than the chip being cut. He ends up using it as a grinding tool, trying to bump material away.” For tools larger than 0.001, Burton considers a chip load under 0.0001 to be “babying.” If the drill doesnt snap, premature wear can result in abysmal tool life. Too much runout can also be destructive, but how much is debatable. Burton pointed out that Performance purposely designed a machine to have 0.0003 TIR to conduct in-house, worst-case milling scenarios, adding that the company is still able to mill a 0.004-wide slot “day in and day out.” He added: “You would think with 0.0003 runout and a chip load a third that, say, 0.0001 to 0.00015, the tool would break immediately because one flute would be taking the entire load and then the back end of the flute would be rubbing. When drilling, he indicated that up to 0.0003 TIR should be acceptable because once the drill is inside 3 the hole, the cutting edges on the end of the drill continue cutting while the noncutting lands on the OD guide the tool in the same direction. Minimizing run out becomes more critical as the depth-to-diameter ratio increases. This is because the flutes are not able to absorb as much deflection as they become more engaged in the workpiece. Ultimately, too much runout causes the tool shank to orbit around the tools center while the tool tip is held steady, creating a stress point where the tool will eventually break. Taking a Plunge Although standard microdrills arent generally available below 0.002, microendmills that can be used to “plunge” a hole are. “When people want to drill smaller than that, they use our endmills and are pretty successful,” Burton said. However, the holes cant be very deep because the tools dont have long aspect, or depth-to-diameter, ratios. Therefore, a 0.001-dia. endmill might be able to only make a hole up to 0.020 deep whereas a drill of the same size can go deeper because its designed to place the load on its tip when drilling. This transfers the pressure into the shank, which absorbs it. Performance offers endmills as small as 5 microns (0.0002) but isnt keen on increasing that lines sales. “When people try to buy them, I very seriously try to talk them out of it because we dont like making them,” Burton said. Part of the problem with tools that small is the carbide grains not only need to be submicron in size but the size also needs to be consistent, in part because such a tool is comprised of fewer grains. “The 5-micron endmill probably has 10 grains holding the core together,” Burton noted. He added that he has seen carbide powder containing 0.2-micron grains, which is about half the size of whats commercially available, but it also contained grains measuring 0.5 and 0.6 microns. “It just doesnt help to have small grains if theyre not uniform.” Microvaporization Electrical discharge machining using a sinker EDM is another micro-holemaking option. Unlike , which create small holes for threading wire through the workpiece when wire EDMing, EDMs for producing microholes are considerably more sophisticated, accurate and, of course, expensive. For producing deep microholes, a tube is applied as the electrode. For EDMing smaller but shallower holes, a solid electrode wire, or rod, is needed. “We try to use tubes as much as possible,” said Jeff Kiszonas, EDM product manager for Makino Inc., Auburn Hills, Mich. “But at some point, nobody can make a tube below a certain diameter.” He added that some suppliers offer tubes down to 0.003 in diameter for making holes as small as 0.0038. The tubes flushing hole enables creating a hole with a high depth-to-diameter ratio and helps to evacuate debris from the bottom of the hole during machining. One such sinker EDM for producing holes as small as 0.00044 (11m) is Makinos Edge2 sinker EDM with fine-hole option. In Japan, the machine tool builder recently produced eight such holes in 2 minutes and 40 seconds through 0.0010-thick tungsten carbide at the hole locations. The electrode was a silver-tungsten rod 0.00020 smaller than the hole being produced, to account for spark activity in the gap. When producing holes of that size, the rod, while rotating, is dressed with a charged EDM wire. The fine-hole option includes a W-axis attachment, which holds a die that guides the electrode, as well as a middle guide that prevents the electrode from bending or wobbling as it spins. With the option, the machine is 4 appropriate for drilling hole diameters less than 0.005. Another sinker EDM for micro-holemaking is the Mitsubishi VA10 with a fine-hole jig attachment to chuck and guide the fine wire applied to erode the material. “Its a standard EDM, but with that attachment fixed to the machine, we can do microhole drilling,” said Dennis Powderly, sinker EDM product manager for MC Machinery Systems Inc., Wood Dale, Ill. He added that the EDM is also able to create holes down to 0.0004 using a wire that rotates at up to 2,000 rpm. Turn to Tungsten EDMing is typically a slow process, and that holds true when it is used for microdrilling. “Its very slow, and the finer the details, the slower it is,” said , president and owner of Optimation Inc. The Midvale, Utah, company builds Profile 24 Piezo EDMs for micromachining and also performs microEDMing on a contract-machining basis. Optimation produces tungsten electrodes using a reverse-polarity process and machines and ring-laps them to as small as 10m in diameter with 0.000020 roundness. Applying a 10m-dia. electrode produces a hole about 10.5m to 11m in diameter, and blind-holes are possible with the companys EDM. The workpiece thickness for the smallest holes is up to 0.002, and the thickness can be up to 0.04 for 50m holes. After working with lasers and then with a former EDM builder to find a better way to produce precise microholes, Jorgensen decided the best approach was DIY. “We literally started with a clean sheet of paper and did all the electronics, all the software and the whole machine from scratch,” he said. Including the software, the machine costs in the neighborhood of $180,000 to $200,000. Much of the companys contract work, which is provided at a shop rate of $100 per hour, involves microEDMing exotic metals, such as gold and platinum for X-ray apertures, stainless steel for optical applications and tantalum and tungsten for the electron-beam industry. Jorgensen said the process is also appropriate for EDMing partially electrically conductive materials, such as PCD. “The customer normally doesnt care too much about the cost,” he said. “Weve done parts where theres $20,000 in time and material involved, and you can put the whole job underneath a fingernail. We do everything under a microscope.” Light Cutting Besides carbide and tungsten, light is an appropriate “tool material” for micro-holemaking. Although most laser drilling is performed in the infrared spectrum, the SuperPulse technology from The Ex One Co., Irwin, Pa., uses a green laser beam, said Randy Gilmore, the companys director of laser technologies. Unlike the femtosecond variety, Super- Pulse is a nanosecond laser, and its green light operates at the 532-nanometer wavelength. The technology provides laser pulses of 4 to 5 nanoseconds in duration, and those pulses are sent in pairs with a delay of 50 to 100 nanoseconds between individual pulses. The benefits of this approach are twofold. “It greatly enhances material removal compared to other nanosecond lasers,” Gilmore said, “and greatly reduces the amount of thermal damage done to the workpiece material” because of the pulses short duration. The minimum diameter produced with the SuperPulse laser is 45 microns, but one of the most common 5 applications is for producing 90m to 110m holes in diesel injector nozzles made of 1mm-thick H series steel. Gilmore noted that those holes will need to be in the 50m to 70m range as emission standards tighten because smaller holes in injector nozzles atomize diesel fuel better for more efficient burning. In addition, the technology can produce negatively tapered holes, with a smaller entrance than exit diameter, to promote better fuel flow. Another common application is drilling holes in aircraft turbine blades for cooling. Although the turbine material might only be 1.5mm to 2mm thick, Gilmore explained that the holes are drilled at a 25 entry angle so the air, as it comes out of the holes, hugs the airfoil surface and drags the heat away. That means the hole traverses up to 5mm of material. “Temperature is everything in a turbine” he said, “because in an aircraft engine, the hotter you can run the turbine, the better the fuel economy and the more thrust you get.” To further enhance the technologys competitiveness, Ex One developed a patent-pending material that is injected into a hollow-body component to block the laser beam and prevent back-wall strikes after it creates the needed hole. After laser machining, the end user removes the material without leaving remnants. “One of the bugaboos in getting lasers accepted in the diesel injector community is that light has a nasty habit of continuing to travel until it meets another object,” Gilmore said. “In a diesel injector nozzle, that damages the interior surface of the opposite wall.” Although the $650,000 to $800,000 price for a Super- Pulse laser is higher than a micro-holemaking EDM, Gilmore noted that laser drilling doesnt require electrodes. “A laser system is using light to make holes,” he said, “so it doesnt have a consumable.” Depending on the application, mechanical drilling and plunge milling, EDMing and laser machining all have their place in the expanding micromachining universe. “People want more packed into smaller spaces,” said Makinos Kiszonas. 6 中文 译文 微孔的加工方法 正如宏观加工一样，在微观加工中孔的加工也许也是最常用的加工之一。孔的加工方法有很多种，每一种都有其 优点和缺陷，这主要取决于孔的直径、深度、工件材料和设备要求。这篇文章主要介绍了内冷却钻头钻孔、无冷却钻孔、插铣、电火花以及激光加工微孔的几种方法。 易于孔加工的操作 无论孔有多大，在加工时将冷却液导入到刀尖，这都有助于排屑并能降低刀具和工件表面产生的摩擦热。尤其是在加工深细孔时，有无冷却对加工的影响更大，因为深细孔加工的刀具比较脆弱，再加上刀具对切屑的二次切削和切屑的堆积会积累大量的热，而热量是碳化物刀具的主要“天敌”，它会加快刀具的失效速度。 当使用外冷却液时，刀具本身会阻止切削液进入切削加工位置。“也就 是到 3-5 倍的直径深度后切削液就会很难流入到刀尖。” 哈维工具有限公 司 的副总工程师杰夫戴维斯说，“这时，就应该选用带有内冷的钻头。” 另外，在加工小孔时采用外冷却液的冷却方式产生的利要大于弊，“当钻头进入工件时，已经流入孔的冷却液产生的压力有时会缴坏钻头。”戴维斯说。 刀具生产商提供的标准钻头的直径从 0.039 到 0.125 英寸，能加工深度小于 12 倍直径的深孔，同时提供直径从 0.002 到 0.020 英寸的不带内冷的钻头。 尽管有内冷能力，但还是不够的，冷却液还需要一定的流动速度从而能够将切屑清出孔外。戴维斯强调 ，冷却液的最低压力应为 600-800 磅 /平方英寸，“加工状况还会随着所施压力的增加而提高。”他补充道。 为了防止这些冷却液通口被杂物堵塞，戴维斯还推荐在钻头上加 5m 孔径或更加精密的冷却液滤清器。 另外，他还推荐在加工孔时有必要在工件的上方先加工一个定心或导向孔，以防止刀具偏斜，并有助于保证所加工孔的垂直度。当选用定心钻时，应使选择的定心钻刀尖上的坡口角小于等于其后内冷钻的破口角。定心钻的直径还要稍微大一些。例如，如果定心钻的坡口角为 120，内冷却钻头的坡口角为 140，并且定心钻的直径小于内冷却钻的直径 ，“在加工时内冷却钻的拐角处会与定心孔干涉而容易脱落” ， 戴维斯说，“这将导致钻头损坏。” 虽然没加强调，但是加工细深孔时，啄式进给是一种很好的加工方式。戴维斯建议，根据工件的材料的不同，每次啄式进给的深度最好为孔径的 30% 50%。这种加工方式便于排出切屑，使切屑不在加工的孔中堆积。 润滑及冷却 为了更加有助于排屑，戴维斯推荐在金属加工中用油基金属切削液代替水基冷却液，因为油具有较高的润滑效果。但是如果车间更加青睐于使用水基冷却液，液体中应该包括 EP（极压）添加剂，增加润滑和减少发泡。“如果产生很多泡沫，” 戴维斯说，“切屑就不会按着预定的方式排出。” 他还补充到，另一种提高润滑并且提高刀具寿命方法是刀具涂层，例如氮铝化钛（ TiAlN）。 TiAlN具有很高的硬度，当钻削像不锈钢这样的难加工金属材料时，带有 TiAlN 涂层的刀具能有效地减少热 7 量冲击。 威斯康星州简斯维尔微型刀具公司的总经理大卫伯顿，对微加工刀具的小批量涂层有不同的看法，他说：“对直径小于 0.020 英寸的刀具涂层，会对从刀具的加工质量到刀具的寿命等每一加工方面都产生消极影响”。因为小刀具的涂层不能够做得足够薄，这样涂层就会改变刀具的前角和后角，从而不 利于加工。 不过，更薄涂层的开发正在继续，伯顿表示，现在微型刀具公司除了生产销售微型铣刀、刨刀和微型钻头外，还在和其他公司合作致力于开发一种亚细微涂层。伯顿说：“我们计划这种图层刀具会在六个月到一年的时间内上市。” 微型钻公司的产品主要是用于电路板加工的钻头，但也可用于有效的切削金属。所有的刀具都没带有内冷能力。“我有一个客户想要在不锈钢上面钻一个 0.004 英寸的孔，他当时非常惊讶这能用一把加工电路板的钻头完成。”伯顿还补充说，“采用啄式进给并选择高的主轴速度可以提高钻头的效率。” 微加工刀具要使用多高的转 速，这主要依赖于车间所使用的数控机床和刀具的直径，所需的转速随刀具直径的增加而加快（注：切削速度公式为 sfm=刀具直径 0.26主轴转速）。 虽然相对较低，但伯顿的客户也成功地应用过每分钟 5000 转的加工速度。伯顿说：“我们建议我们的用户找到一个震动最小的最高转速 最佳加工速度。” 为了减少震动，在用小的切削力通过刀具的前倾面去除适当的金属时，应使渗入到工件中的切削载荷连续而充足，如果钻头承受的切削载荷太轻，刀具前倾面的磨损速度就会加快，刀具变钝，从而影响刀具的使用寿命。这在加工细孔时应更加注意。 “用户们常常使用较轻的切削载荷来延长刀具的使用寿命， ”伯顿说， “这恰恰会加快切削刃的磨损，并在刀刃宽出切屑的位置形成圆弧，刀具会变得像磨削工具一样把材料强行除掉，只能成为报废刀。 ”伯顿认为，直径大于 0.001 英寸的刀具切削抗力小于 0.0001时，切削力抗力就已经太小了，即使刀具不会断裂，过早的摩擦也会导致刀具寿命缩短。 太多的跳动也可能是破坏性的，但是影响有多少还值得商榷。伯顿指出，公司打算设计一台具有0.0003 英寸偏差的机器，用以建立室内最坏情况下的铣削场景，还将能够加工 0.004 英寸宽的槽，“这迟 早会实现的”。 他还补充：“你还可以试想一下 0.0003 英寸的跳动和只有正常水平三分之一的切削载荷，也就是说0.0001到 0.00015，刀具将会立即破坏，因为刀具的一个排屑槽会承受所有的载荷，然后排屑槽的后面就会破坏。” 他还指出，在钻孔时，小于 0.0003 英寸的偏差是可接受的，因为当钻头深入孔内时，钻头末端的切削刃在外圆柱非加工表面的引导下会继续切削。偏差的最小值随着深度和直径比值的增加而迅速减少，这是因为当钻头越深入工件，排屑槽的吸震能力越差。最后强烈的跳动导致刀柄绕着刀具的轴线转动，而刀尖还仍然保 持稳定，从而产生使刀具最终断裂的集中应力。 插铣 虽然通常没有直径小于 0.002 英寸的标准微型钻头，但可以用微型端铣刀来“冲”孔。“每当人们想加工一个小于 0.002 英寸的孔时，他们可以选用端铣刀，效果也不错。”伯顿说到。但是这样加工的孔不能太深，因为刀具体不长，没有大的深度直径比率。因此一把直径为 0.001 英寸的端铣刀只能加工最深 0.020 英寸的孔，而同样直径的钻头可以加工得更深，因为钻头的设计使载荷全部作用在刀尖上， 8 进而传到刀柄上被吸收。 市面上能提供最小 5 微米（ 0.0002 英寸）的端铣刀，但是并没有大量 销售。“当人们想买这样的刀具时，我非常严肃的试着说服他们不要买，因为我们不喜欢制作这样的刀具。”伯顿说到。这种刀具的主要问题是，不但这种刀具的硬质合金齿处于亚细微尺寸，而且当一把刀有多个齿时，每个齿的尺寸还要保持一致。伯顿道：“一把直径 5 微米的端铣刀在其基体上就夹持大约 10 个刀齿。” 他还补充说，他曾经看到过带有 0.2 微米齿的粉末冶金硬质合金刀具，这是商业上能提供齿的尺寸的一半，但它还包括 0.5 和 0.6 微米的小齿。“如果齿的尺寸不统一，小齿是发挥不出作用的”。 坠 电火花加工 应用坠电火花的电火花加工是另一种 微孔加工方式。这不同于将放电导线穿过工件的电火花加工方式，应用坠电火花加工的微孔更加精密和精确，但同时花费也会很高。 坠电火花加工深细孔时，要用一个导电管作为电极。加工小而浅的孔时，需要用到一根导线或棒