1、哈尔滨理工大学 2008 届本科生毕业设计（论文）外文翻译 1 外文原文 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, depend
2、ing 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 w
3、hile 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 o
4、f 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 Har
5、vey 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 li
6、ne 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
7、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 recomm
8、endation 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
9、-coolant drill that follows. The pilot drills 哈尔滨理工大学 2008 届本科生毕业设计（论文）外文翻译 2 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 c
10、orners 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
11、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 E
12、P (extreme pressure) additives to increase lubricity and minimize foaming. “If youve got a lot of 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 tita
13、nium 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 s
14、maller 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. Ho
15、wever, 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 t
16、esting 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
17、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 diame
18、ter decreases. (Note: The equation for cutting speed is sfm = tool diameter 0.26 spindle 哈尔滨理工大学 2008 届本科生毕业设计（论文）外文翻译 3 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 vibrationt
19、he 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 wear
20、s 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
21、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 runo
22、ut 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 thi
23、nk 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
24、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
25、 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
26、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
