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英文原文
Options for micro-holemaking
As in the macroscale-machining world, holemaking is one of the most— if not the most—frequently 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 carbide’s worst enemy—heat.
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 isn’t 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 5μm 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, it’s important to select one with an included angle on its point that’s equal to or larger than the included angle on the through-coolant drill that follows. The pilot drill’s 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 you’re catching the coolant-fed drill’s 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 you’ve got a lot of foam,” Davis noted, “the chips aren’t being pulled out the way they are supposed to be.”
He added that another way to enhance a tool’s 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 heat’s 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. That’s 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. “We’re 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 drill’s 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 Burton’s customers. “We recommend that our customers find the highest rpm at the lowest possible vibration—the 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 doesn’t 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 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 tool’s center while the tool tip is held steady, creating a stress point where the tool will eventually break.
Taking a Plunge
Although standard microdrills aren’t 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 can’t be very deep because the tools don’t 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 it’s 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 isn’t keen on increasing that line’s sales. “When people try to buy them, I very seriously try to talk them out of it because we don’t 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 what’s commercially available, but it also contained grains measuring 0.5 and 0.6 microns. “It just doesn’t help to have small grains if they’re 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 tube’s 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" (11μm) is Makino’s 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 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. “It’s 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. “It’s 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 10μm in diameter with 0.000020" roundness. Applying a 10μm-dia. electrode produces a hole about 10.5μm to 11μm in diameter, and blind-holes are possible with the company’s EDM. The workpiece thickness for the smallest holes is up to 0.002", and the thickness can be up to 0.04" for 50μm 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 company’s 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 doesn’t care too much about the cost,” he said. “We’ve done parts where there’s $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 company’s 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 applications is for producing 90μm to 110μm holes in diesel injector nozzles made of 1mm-thick H series steel. Gilmore noted that those holes will need to be in the 50μm to 70μm 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 technology’s 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 doesn’t require electrodes. “A laser system is using light to make holes,” he said, “so it doesn’t 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 Makino’s Kiszonas.
中文譯文
微孔的加工方法
正如宏觀加工一樣，在微觀加工中孔的加工也許也是最常用的加工之一?？椎募庸し椒ㄓ泻芏喾N，每一種都有其優(yōu)點和缺陷，這主要取決于孔的直徑、深度、工件材料和設備要求。這篇文章主要介紹了內冷卻鉆頭鉆孔、無冷卻鉆孔、插銑、電火花以及激光加工微孔的幾種方法。
易于孔加工的操作
無論孔有多大，在加工時將冷卻液導入到刀尖，這都有助于排屑并能降低刀具和工件表面產生的摩擦熱。尤其是在加工深細孔時，有無冷卻對加工的影響更大，因為深細孔加工的刀具比較脆弱，再加上刀具對切屑的二次切削和切屑的堆積會積累大量的熱，而熱量是碳化物刀具的主要“天敵”，它會加快刀具的失效速度。
當使用外冷卻液時，刀具本身會阻止切削液進入切削加工位置。“也就是到3-5倍的直徑深度后切削液就會很難流入到刀尖?！?哈維工具有限公司的副總工程師杰夫戴維斯說，“這時，就應該選用帶有內冷的鉆頭?！?
另外，在加工小孔時采用外冷卻液的冷卻方式產生的利要大于弊，“當鉆頭進入工件時，已經流入孔的冷卻液產生的壓力有時會繳壞鉆頭?！贝骶S斯說。
刀具生產商提供的標準鉆頭的直徑從0.039到0.125英寸，能加工深度小于12倍直徑的深孔，同時提供直徑從0.002到0.020英寸的不帶內冷的鉆頭。
盡管有內冷能力，但還是不夠的，冷卻液還需要一定的流動速度從而能夠將切屑清出孔外。戴維斯強調，冷卻液的最低壓力應為600-800磅/平方英寸，“加工狀況還會隨著所施壓力的增加而提高?！彼a充道。
為了防止這些冷卻液通口被雜物堵塞，戴維斯還推薦在鉆頭上加5μm孔徑或更加精密的冷卻液濾清器。
另外，他還推薦在加工孔時有必要在工件的上方先加工一個定心或導向孔，以防止刀具偏斜，并有助于保證所加工孔的垂直度。當選用定心鉆時，應使選擇的定心鉆刀尖上的坡口角小于等于其后內冷鉆的破口角。定心鉆的直徑還要稍微大一些。例如，如果定心鉆的坡口角為120°，內冷卻鉆頭的坡口角為140°，并且定心鉆的直徑小于內冷卻鉆的直徑，“在加工時內冷卻鉆的拐角處會與定心孔干涉而容易脫落”，戴維斯說，“這將導致鉆頭損壞。”
雖然沒加強調，但是加工細深孔時，啄式進給是一種很好的加工方式。戴維斯建議，根據工件的材料的不同，每次啄式進給的深度最好為孔徑的30%—50%。這種加工方式便于排出切屑，使切屑不在加工的孔中堆積。
潤滑及冷卻
為了更加有助于排屑，戴維斯推薦在金屬加工中用油基金屬切削液代替水基冷卻液，因為油具有較高的潤滑效果。但是如果車間更加青睞于使用水基冷卻液，液體中應該包括EP（極壓）添加劑，增加潤滑和減少發(fā)泡?！叭绻a生很多泡沫，”戴維斯說，“切屑就不會按著預定的方式排出?！?
他還補充到，另一種提高潤滑并且提高刀具壽命方法是刀具涂層，例如氮鋁化鈦（TiAlN）。TiAlN具有很高的硬度，當鉆削像不銹鋼這樣的難加工金屬材料時，帶有TiAlN涂層的刀具能有效地減少熱量沖擊。
威斯康星州簡斯維爾微型刀具公司的總經理大衛(wèi)伯頓，對微加工刀具的小批量涂層有不同的看法，他說：“對直徑小于0.020英寸的刀具涂層，會對從刀具的加工質量到刀具的壽命等每一加工方面都產生消極影響”。因為小刀具的涂層不能夠做得足夠薄，這樣涂層就會改變刀具的前角和后角，從而不利于加工。
不過，更薄涂層的開發(fā)正在繼續(xù)，伯頓表示，現在微型刀具公司除了生產銷售微型銑刀、刨刀和微型鉆頭外，還在和其他公司合作致力于開發(fā)一種亞細微涂層。伯頓說：“我們計劃這種圖層刀具會在六個月到一年的時間內上市?！?
微型鉆公司的產品主要是用于電路板加工的鉆頭，但也可用于有效的切削金屬。所有的刀具都沒帶有內冷能力?！拔矣幸粋€客戶想要在不銹鋼上面鉆一個0.004英寸的孔，他當時非常驚訝這能用一把加工電路板的鉆頭完成?！辈D還補充說，“采用啄式進給并選擇高的主軸速度可以提高鉆頭的效率。”
微加工刀具要使用多高的轉速，這主要依賴于車間所使用的數控機床和刀具的直徑，所需的轉速隨刀具直徑的增加而加快（注：切削速度公式為 sfm=刀具直徑×0.26×主軸轉速）。
雖然相對較低，但伯頓的客戶也成功地應用過每分鐘5000轉的加工速度。伯頓說：“我們建議我們的用戶找到一個震動最小的最高轉速——最佳加工速度?！?
為了減少震動，在用小的切削力通過刀具的前傾面去除適當的金屬時，應使?jié)B入到工件中的切削載荷連續(xù)而充足，如果鉆頭承受的切削載荷太輕，刀具前傾面的磨損速度就會加快，刀具變鈍，從而影響刀具的使用壽命。這在加工細孔時應更加注意。
“用戶們常常使用較輕的切削載荷來延長刀具的使用壽命，”伯頓說， “這恰恰會加快切削刃的磨損，并在刀刃寬出切屑的位置形成圓弧，刀具會變得像磨削工具一樣把材料強行除掉，只能成為報廢刀?！辈D認為，直徑大于0.001英寸的刀具切削抗力小于0.0001″時，切削力抗力就已經太小了，即使刀具不會斷裂，過早的摩擦也會導致刀具壽命縮短。
太多的跳動也可能是破壞性的，但是影響有多少還值得商榷。伯頓指出，公司打算設計一臺具有0.0003英寸偏差的機器，用以建立室內最壞情況下的銑削場景，還將能夠加工0.004英寸寬的槽，“這遲早會實現的”。
他還補充：“你還可以試想一下0.0003英寸的跳動和只有正常水平三分之一的切削載荷，也就是說0.0001″到0.00015，刀具將會立即破壞，因為刀具的一個排屑槽會承受所有的載荷，然后排屑槽的后面就會破壞。”
他還指出，在鉆孔時，小于0.0003英寸的偏差是可接受的，因為當鉆頭深入孔內時，鉆頭末端的切削刃在外圓柱非加工表面的引導下會繼續(xù)切削。偏差的最小值隨著深度和直徑比值的增加而迅速減少，這是因為當鉆頭越深入工件，排屑槽的吸震能力越差。最后強烈的跳動導致刀柄繞著刀具的軸線轉動，而刀尖還仍然保持穩(wěn)定，從而產生使刀具最終斷裂的集中應力。
插銑
雖然通常沒有直徑小于0.002英寸的標準微型鉆頭，但可以用微型端銑刀來“沖”孔。“每當人們想加工一個小于0.002英寸的孔時，他們可以選用端銑刀，效果也不錯。”伯頓說到。但是這樣加工的孔不能太深，因為刀具體不長，沒有大的深度直徑比率。因此一把直徑為0.001英寸的端銑刀只能加工最深0.020英寸的孔，而同樣直徑的鉆頭可以加工得更深，因為鉆頭的設計使載荷全部作用在刀尖上，進而傳到刀柄上被吸收。
市面上能提供最小5微米（0.0002英寸）的端銑刀，但是并沒有大量銷售?！爱斎藗兿胭I這樣的刀具時，我非常嚴肅的試著說服他們不要買，因為我們不喜歡制作這樣的刀具。”伯頓說到。這種刀具的主要問題是，不但這種刀具的硬質合金齒處于亞細微尺寸，而且當一把刀有多個齒時，每個齒的尺寸還要保持一致。伯頓道：“一把直徑5微米的端銑刀在其基體上就夾持大約10個刀齒?！?
他還補充說，他曾經看到過帶有0.2微米齒的粉末冶金硬質合金刀具，這是商業(yè)上能提供齒的尺寸的一半，但它還包括0.5和0.6微米的小齒?！叭绻X的尺寸不統(tǒng)一，小齒是發(fā)揮不出作用的”。
墜電火花加工
應用墜電火花的電火花加工是另一種微孔加工方式。這不同于將放電導線穿過工件的電火花加工方式，應用墜電火花加工的微孔更加精密和精確，但同時花費也會很高。
墜電火花加工深細孔時，要用一個導電管作為電極。加工小而淺的孔時，需要用到一根導線或棒，“我們盡量用導管做電極，”位于密歇根州的牧野公司總經理 Jeff Kiszonas說道，導管的排渣孔能使加工的孔有大的深度直徑比，并能夠在加工中將孔底的熔渣排除孔外。他又補充道“但是另一方面，沒人能制出小于一定直徑的導管。”一些供應商能提供直徑小于0.003英寸的導管可以加工出0.0038英寸的孔。。
現在Makino公司生產的雙邊墜電火花加工設備能夠加工出0.00044英寸（11微米）的微孔，這種設備主要用于孔的精加工。最近，在日本這種機床的開發(fā)人員用兩分鐘加工了八個這樣的孔，并用四十秒穿透了0.0010英寸厚的碳化鎢板。加工電極為一個銀鎢合金棒，由于電火花加工中在電極和工件間存在放電間隙，所以，所加工孔的直徑會比電極直徑大0.00020英寸。
當加工上述尺寸的孔時，旋轉的導棒上包裹著通電的放電導線。精加工時需要一個W軸附件，用來夾持電極導向的模具，另外還需要一個中間導向件，當電極旋轉時用來來防止其彎曲和擺動。應用這種加工方式的機床適合于加工直徑小于0.005英寸的孔。
另一種墜電電火花加工微型孔機床是三菱VA10機床，它用精加工孔的鉆模附件來裝卡和引導精制導線來腐蝕金屬。伊利諾伊州的MC機械系統(tǒng)公司產品加工經理丹尼斯德利說：“這是一種標準的電火花加工，但是借助于安裝在機器上的附件，我們同樣可以加工細孔。”他還補充說在電火花加工中用2000轉/分的轉速旋轉的導線可以加工小于0.0004英寸的孔。
鎢電極電火花加工
電火花加工是一種典型的慢加工，加工微孔時這表現得也很明顯。“電火花加工非常慢，并且隨著加工精度的增加而減慢” Midvale公司（ Midvale公司是一個位于猶他州，主要生產24伏低壓電火花加工設備和基于精密電火花加工的公司）的總裁迪恩約根森說。
鎢電極的生產是應用反極性接法，經機械加工、研磨加工使之直徑達到10微米、粗糙度為0.000020英寸。應用10微米的電極能加工10.5到11微米的孔，并能加工盲孔。用于加工最小孔的最大工件厚度為0.002英寸，加工50微米直徑的孔時工件的厚度能達到0.004英寸。
在激光加工之后用電火花加工是生產高精度孔的一種比較不錯的方法，約根森已經決定重新研發(fā)最好的加工設備?！拔覀冃枰匦卵邪l(fā)所有電子控件、程序軟件和機械。”約根森說重新研發(fā)這些軟件和機械需要花費180，000到200，000美元。
車間里的多數精細加工為100美元/時，包括特殊金屬的電火花加工，如：X射線加工金和鉑、
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