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硬車削加工------以車代磨
作者：克里斯 庫帊弗
當(dāng)在車床或車削加工中心加工硬度大于45ＨＲＣ的工件時，我們把這種車削定義為硬車削加工。由于這種車削能獲得１.６的表面粗擦度，硬車削加工通常被認(rèn)為是磨削的一種替代工藝，或是磨削加工的預(yù)加工。硬車削加工通常是在工件熱處理后獲得45-68HRC硬度后來進行的。
硬車削加工制定工序的原則與傳統(tǒng)軟車削有很多相似之處。和任何一種新工藝一樣，硬車削有它自己的特點，但最基本的原則仍然是我們在車間中進行普通車削時所遵循的原則。這種工藝自產(chǎn)生之初就有了比磨削更多的優(yōu)勢，在磨削加工中往往需要專業(yè)的理論知識和豐富的實踐經(jīng)驗，并且不是所有加工工人都能否掌握，盡管任何一種加工方法都夠為機加工人員學(xué)習(xí)，但更多工人和工藝員更容易掌握硬切削的方法。
正確應(yīng)用
盡管硬車削能達(dá)到令人滿意的效果，但它不能取代磨削加工，通過磨削完成的鏡面拋光，粗糙度可達(dá)0.3-0.8，而這在硬車削加工中是不可能達(dá)到的。和硬車削相比，磨削尤其特有的優(yōu)勢，比如可以獲得更高維度的表面粗擦度和圓柱度。另外由于在一種卡盤裝夾下便可完成加工，硬車削和其他工藝相比能使工件獲得較高的圓柱度和同軸度。
硬車削加工工藝在零件表面平面度要求為0.5-12um，粗糙度要求在0.8-7um時能收到良好的效果，硬車削的工藝范圍很廣，諸如齒輪，柱塞泵零件，液壓元器件，軸承座表面，硬盤驅(qū)動軸等。
硬車削工藝和磨削相比，生產(chǎn)成本低。最明顯的成本優(yōu)勢是支撐部件成本的降低，正像CNC車削中心一般來說要比磨床價格低。另外一般能在車削加工中心中心加工的零件往往需要數(shù)臺不同類型的磨床才能完成同樣的操作，其成本優(yōu)勢更加顯著。
如上所述，一臺車削加工中心使用一個卡盤能完成內(nèi)空車削，外圓車削，錐面車削，車槽等。除了有利于提高平面度，直線度和同軸度外，還很大程度上減少了裝夾和循環(huán)操作的次數(shù)。高精密螺紋車削同樣可以完成，與手工加工螺紋相比有很好的同軸度。
硬車削還可以作為圓弧面和自由曲面的精加工，而磨削加工需要定制的砂輪，這種加工方法很耗時，并且專用磨床很昂貴。
除了這種能夠合并各種加工的天然優(yōu)勢，硬車削加工的循環(huán)時間遠(yuǎn)比具有相同加工能力的普通車削短。在硬車削加工中，金屬能很快被車削，如果使用立方氮化硼和陶瓷刀具還能完成高速車削。另外，硬車削加工工件裝夾和拆卸時間短，車削加工中心還更容易完成自動化以提高生產(chǎn)效率。
除了成本低，硬車削加工還有很多減少對環(huán)境的影響的特點。車削加工中心相比磨削耗能較少。硬車削通常在干燥環(huán)境下加工，減少了潤滑成本和對潤滑系統(tǒng)的布置。硬車削加工更容易循環(huán)利用切屑，。而磨削中的切屑分離成本很高或直接作為工業(yè)廢料處理。
合理的車床
由于表面粗糙度和精度是硬車削加工的基本要求，不是所有的車床和車削加工中都能很好的完成硬車削加工，除了滿足刀具的切削速度要求外，用于硬車削的機床必須具備良好的抵抗熱變形的能力，剛度和硬度。
正確使用刀具
硬車削的一個關(guān)鍵效益是在任何硬車削加工中都可以使用的嵌入式硬質(zhì)刀具，和成本較低的砂輪相比，這些刀具價格昂貴，但是盡管刀具價格昂貴，我們還是會選擇硬切削加工，因為加工時間和轉(zhuǎn)換時間的大大減少會彌補刀具的成本。
CBN和PCBN是加工硬度大于55HRC的零件的理想材料，CBN是最硬的材料之一，其硬度僅次于天然鉆石。并且CBN這種材料的加工工藝較好，在和金剛石材料類似的加工條件下有很好的綜合性能。并且CBN刀具在高溫下仍有很好的穩(wěn)定性。CBN也可以制成PCBN，PCBN是CBN粒子燒結(jié)而成具有高硬度高熱穩(wěn)定性的表面粗糙的集成物。
陶瓷工具割削工具是非常地較不昂貴的比較 CBN 和 PCBN，以及提供一個優(yōu)良的表面光制由于他們的能力維持一個銳利刃口。他們典型地提供在 CBN 和 PCBN 上的可提高的軔性，讓他們變成更好在這 45-55 HRC 范圍中為硬旋轉(zhuǎn)材料適合了。然而，類似 CBN 和 PCBN，他們是易碎的而且不很好地為打斷的傷口適合，不光滑或半的意思鑄件清理。
由于這些材料及優(yōu)良的耐磨性，CBN和ＰＣＢＮ刀具比碳質(zhì)刀具具有很大的持久性。然而，這些材料很脆，因此除了切削量很小的精加工外，這些刀具不適于其他的操作。
相比ＣＢＮ和ＰＣＢＮ陶瓷刀具的價格要低很多，并且由于能保持切削刃的鋒利使陶瓷刀具很適合于做精加工操作。陶瓷刀具比ＣＢＮ和ＰＣＢＮ刀具有更強的韌性，使這種刀具更適合４５－５５ＨＲＣ的硬切削加工。但是和ＣＢＮ，ＰＣＢＮ一樣，陶瓷刀具很脆，不適合非連續(xù)型加工，粗加工或半精加工。
在個別加工操作中，ＣＢＮ＼ＰＣＢＮ刀具的形狀和表面涂料以及嵌入式陶瓷刀具的刀頭可以根據(jù)不同情況加以選擇，不管選擇什么樣的刀具，，都必須有高精度和剛度的刀架，使合模銷的長度最小以減小振動，提高局部精度，延長刀具壽命。即使這些刀具能提供１．６的表面粗糙度，車痕仍會留在工件表面，不利于加工有油封要求的表面，加工刀痕會導(dǎo)致油的泄露，或者由于工件表面的突起使做旋轉(zhuǎn)運動的工件卡死。為了更好地完成車削精加工，薄膜類型的磨削單元可以用來作為精加工以達(dá)到表面粗糙度為0.8的鏡面。薄膜型磨削單元用于塔倫驅(qū)動的車削加工中心。通過使用多種拋光薄膜來磨掉表面的突起以達(dá)到傳統(tǒng)磨削加工類似的表面精度。
硬車削加工的效益
盡管硬車削加工不能替代所有的磨削加工，這種加工方法由于能夠減少裝夾時間和循環(huán)操作時間以及設(shè)備的低成本具有很大的經(jīng)濟性。由于他和傳統(tǒng)車削加工遵循很多相同原則，硬車削加工由于能很容易應(yīng)用于加工車間而具有很多而外的效益。如果選擇了正確的加工方法，正確的機床，正確的刀具，硬車削加工能夠加強由于很寬的公差范圍而帶來的收益。
附錄二 外文文獻
Hard Turning as an Alternative to Grinding
Chris Koepfer
Hard turning is typically defined as the turning of a part or barstock of harder than 45HRC on a lathe or turning center. Since surface roughness of Rmax/Rz=1.6s can be achieved, hard turning is often considered a replacement for grinding operations or as a pre-grinding process. Hard turning is most often performed on post-heat treated parts with surface hardness ranging from 45HRC to 68HRC or even higher.
The process of hard turning shares many fundamentals with its “soft turning” sibling. As with any new application, there is a learning curve for hard turning, but the fundamental principles follow those of the same turning operations that are commonly performed in shops today. This gives it an inherent advantage over grinding, which requires specific knowledge and experience that not all machinists possess. While any new process can be learned, most machinists and programmers today will have an easier time absorbing the hard turning process compared with grinding.
The Right Application
While hard turning can achieve impressive results, it is not an alternative for all parts typically finished through grinding. Polished mirror surface finishes of Rz=0.3~0.8z that can be achieved through grinding are not possible by hard turning alone. Grinding has the additional advantage of being able to achieve higher dimensional roundness and cylindricity accuracies compared with hard turning. However, since parts can typically be finished in a single chucking, hard turned parts often show superior concentricity and perpendicularity characteristics to their ground counterparts.
The “sweet spot” for hard turning applications are for parts that have roundness accuracy requirements between 0.5 and 12 microns, and surface roughness requirements between Rz 0.8 micron and Rz 7.0 microns (see chart on page 26). This includes a variety of parts such as gears, injection pump components, hydraulic components, seat surfaces, and hard disk drive shafts.
The cost advantages of hard turning compared with grinding are numerous. The immediately apparent cost advantage is the reduced cost in capital equipment, as CNC turning centers are generally less expensive than grinding machines. Additionally, several types of grinding machines may be needed to perform the operations able to be performed on a single turning center, further opening the possibilities for equipment cost savings.
As mentioned above, a turning center can complete ID turning, OD turning, taper turning, and grooving in a single chucking. In addition to improving the accuracy of squareness, concentricity and straightness, this drastically reduces cycle and setup times as well. High precision threading operations can also be performed, guaranteeing concentricity with other part features compared with offline threading operations.
Hard turning also allows for the finishing of radius and free-curved surfaces. Grinding processes require a custom-dressed wheel, which is time consuming to produce, or highly customized grinding machines that can be expensive.
In addition to the inherent cost advantages of combining multiple operations into one, hard turning cycle times are drastically shorter than comparable turning operations. Metal can be removed much faster in hard turning operations, and high speed turning is possible with both CBN and ceramic cutting tools. Changing grinding wheels is also time consuming, whereas switching out inserts on turning centers can be quick. Part loading and unloading times are also shorter for turning centers, and turning centers are typically more easily automated for additional productivity.
A number of features of the hard turning process reduce environmental impact as well as cost. Turning centers consume less electricity than grinding machines, reducing both electrical consumption and the monthly electrical bill. Hard turning is often performed dry, eliminating both coolant costs and the need for coolant disposal. Hard turning produces easily recycled chips, whereas grinding produces sludge that must go through a costly separation process or be disposed of as industrial waste.
The Right Machine
As accuracy and surface finish are fundamental requirements for hard turned parts, not all lathes and turning centers are ideally suited for hard turning applications. In addition to being able to meet the speed requirements of the cutting tools, machines used for hard turning must maintain thermal stability, rigidity and precision over time.
The Right Tooling
One of the key benefits of hard turning is that off the shelf inserts can be used for virtually any hard turning operation. However, the CBN, PCBN and ceramic inserts can be expensive, especially when compared with the relatively low costs of grinding wheels. But don’t let the high cost of inserts scare you off from the hard turning process, as the high tooling costs are quickly offset by reductions in processing time and change-over time.
CBN and PCBN are ideal cutting tool materials for the hard finishing of parts above 55HRC. CBN is one of the hardest materials, second only to diamond. It is a manufactured material, synthesized under conditions similar to those used for synthetic diamond. However, unlike diamond cutting tools, CBN demonstrates increased stability at higher temperatures. CBN is also used to manufacture PCBN, a composite formed by sintering CBN particles into a hard, tough and thermally stable material.
Thanks to their incredible wear resistance, CBN and PCBN cutting tools outlast carbide by a large factor in hard turning applications. However, they are extremely brittle, and therefore, not well suited for anything other than finishing operations with smaller depths of cut.
Ceramic cutting tools are considerably less expensive than CBN and PCBN, and also provide an excellent surface finish thanks to their ability to maintain a sharp cutting edge. They typically offer enhanced toughness over CBN and PCBN, making them better suited for hard turning materials in the 45-55 HRC range. However, similar to CBN and PCBN, they are brittle and not well suited for interrupted cuts, roughing or semi-finishing.
Geometries and coatings on CBN, PCBN and ceramic inserts can be selected for optimal performance in individual applications. No matter what cutting tool is chosen, an accurate and rigid toolholder is also essential to the process. Maintain a minimal projection length to minimize vibration, enhance part finish and maximize cutting tool life.
Even though these cutting tools can provide surface finishes of Rmax/Rz=1.6s, feed turning marks (similar to the peaks and valleys of threading) remain on the surface. This may be an issue for parts such as oil seal contact surfaces, where the valleys may cause oil to leak, or bearing rolling surfaces where the peaks may cause the bearing material to detach. For better surface finish on a turning center, a film-type grinding unit can be used to achieve surface finishes of less than Rz0.8 micron. Film-type grinding units are for turning centers with driven turrets, and attach to the tooling turret as a driven tool would. They use a variety of polishing films to grind off the peaks left by hard turning, providing surface finishes similar to traditional grinding methods.
Benefits of Hard Turning
Although hard turning it is not an alternative for all grinding operations, the potential cost savings from reduced setup times, faster cycle times and lower equipment costs are too big to ignore. Since it shares many fundamentals with standard turning processes, hard turning has the additional benefit of being able to be easily assimilated into most shops. With a little help choosing the right application, right machine and right tooling, hard turning can quickly enhance the profitability of a variety of tight tolerance applications.