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          機械加工中哪些因素會造成工件變形?
          來源: | 作者:pro3c65e5 | 發布時間: 2017-09-01 | 3831 次瀏覽 | 分享到:

          機械加工中的工件變形問題,是比較難以解決的問題。首先必須分析產生變形的原因,然后才能采取應對的措施。

          機械加工中哪些因素會造成工件變形?

          一、工件的材質和結構會影響工件的變形

          變形量的大小與形狀復雜程度、長寬比和壁厚大小成正比,與材質的剛性和穩定性成正比。所以在設計零件時盡可能的減小這些因素對工件變形的影響。

          尤其在大型零件的結構上更應該做到結構合理。在加工前也要對毛坯硬度、疏松等缺陷進行嚴格控制,保證毛坯質量,減少其帶來的工件變形。

          機械加工中哪些因素會造成工件變形?

          二、工件裝夾時造成的變形

          工件裝夾時,首先要選擇正確的夾緊點,然后根據夾緊點的位置選擇適當的夾緊力。因此盡可能使夾緊點和支撐點一致,使夾緊力作用在支撐上,夾緊點應盡可能靠近加工面,且選擇受力不易引起夾緊變形的位置。

          當工件上有幾個方向的夾緊力作用時,要考慮夾緊力的先后順序,對于使工件與支撐接觸夾緊力應先作用,且不易太大,對于平衡切削力的主要夾緊力,應作用在最后。

          其次要增大工件與夾具的接觸面積或采用軸向夾緊力。增加零件的剛性,是解決發生夾緊變形的有效辦法,但由于薄壁類零件的形狀和結構的特點,導致其具有較低的剛性。這樣在裝夾施力的作用下,就會產生變形。

          增大工件與夾具的接觸面積,可有效降低工件件裝夾時的變形。如在銑削加工薄壁件時,大量使用彈性壓板,目的就是增加接觸零件的受力面積;在車削薄壁套的內徑及外圓時,無論是采用簡單的開口過渡環,還是使用彈性芯軸、整弧卡爪等,均采用的是增大工件裝夾時的接觸面積。這種方法有利于承載夾緊力,從而避免零件的變形。采用軸向夾緊力,在生產中也被廣泛使用,設計制作專用夾具可使夾緊力作用在端面上,可以解決由于工件壁薄,剛性較差,導致的工件彎曲變形。

          機械加工中哪些因素會造成工件變形?

          三、工件加工時造成的變形

          工件在切削過程中由于受到切削力的作用,產生向著受力方向的彈性形變,就是我們常說的讓刀現象。應對此類變形在刀具上要采取相應的措施,精加工時要求刀具鋒利,一方面可減少刀具與工件的摩擦所形成的阻力,另一方面可提高刀具切削工件時的散熱能力,從而減少工件上殘余的內應力。例如在銑削薄壁類零件的大平面時,使用單刃銑削法,刀具參數選取了較大的主偏角和較大的前角,目的就是為了減少切削阻力。由于這種刀具切削輕快,減少了薄壁類零件的變形,在生產中得到廣泛的應用。在薄壁零件的車削中,合理的刀具角度對車削時切削力的大小,車削中產生的熱變形、工件表面的微觀質量都是至關重要的。刀具前角大小,決定著切削變形與刀具前角的鋒利程度。前角大,切削變形和摩擦力減小,但前角太大,會使刀具的楔角減小,刀具強度減弱,刀具散熱情況差,磨損加快。所以,一般車削鋼件材料的薄壁零件時,用高速刀具,前角取6°~30°,用硬質合金刀具,前角取5°~20°。刀具的后角大,摩擦力小,切削力也相應減小,但后角過大也會使刀具強度減弱。在車削薄壁零件時,用高速鋼車刀,刀具后角取6°~12°,用硬質合金刀具,后角取4°~12°,精車時取較大的后角,粗車時取較小的后角。車薄壁零件的內外圓時,取大的主偏角。正確選擇刀具是應對工件變形的必要條件。

          加工中刀具和工件摩擦產生的熱量也會使工件變形,因此在很多時候選擇高速切削加工。在高速切削加工中,由于切屑在較短時間內被切除,絕大部分切削熱被切屑帶走,減少了工件的熱變形;其次,在高速加工中,由于切削層材料軟化部分的減少,也可減少零件加工的變形,有利于保證零件的尺寸、形狀精度。另外,切削液主要用來減少切削過程中的摩擦和降低切削溫度。合理使用切削液對提高刀具的耐用度和加工表面質量、加工精度具有重要作用。因此,在加工中為防止零件變形必須合理使用充分的切削液。

          加工中采用合理的切削用量是保證零件精度的關鍵因素。在加工精度要求較高的薄壁類零件時,一般采取對稱加工,使相對的兩面產生的應力均衡,達到一個穩定狀態,加工后工件平整。但當某一工序采取較大的吃刀量時,由于拉應力、壓應力失去平衡,工件便會產生變形。

          薄壁零件車削時變形是多方面的,裝夾工件時的夾緊力,切削工件時切削力,工件阻礙刀具切削時產生的彈性變形和塑性變形,使切削區溫度升高而產生熱變形。所以,我們要在粗加工時,背吃刀量和進給量可以取大些;精加工時,刀量一般在0.2~0.5mm,進給量一般在0.1~0.2mm/r,甚至更小,切削速度6~120m/min,精車時用盡量高的切削速度,但不易過為高。合理選擇好切削用量,從而到達減少零件變形的目的。

          機械加工中哪些因素會造成工件變形?

          四、加工后應力變形

          加工后,零件本身存在內應力,這些內應力分布是一種相對平衡的狀態,零件外形相對穩定,但是去除一些材料和熱處理后內應力發生變化,這時工件需要重新達到力的平衡所以外形就發生了變化。解決這類變形可以通過熱處理的方法,把需要校直的工件疊成一定高度,采用一定工裝壓緊成平直狀態,然后把工裝和工件一起放入加熱爐中,根據零件材料的不同,選擇不同的加熱溫度和加熱時間。熱校直后,工件內部組織穩定。此時,工件不僅得到了較高的直線度,而且加工硬化現象得到消除,更便于零件的進一步精加工。鑄件要做到時效處理,盡量消除內部的殘余應力,采用變形后再加工的方式,即粗加工-時效-再加工。

          對于大型零件要采用仿形加工,即預計工件裝配后的變形量,加工時在相反的方向預留出變形量,可有效的防止零件在裝配后的變形。

          綜上所述,對于易變形工件,在毛坯和加工工藝上都要采用相應的對策,需根據不同情況加以分析,都會找到一條合適的工藝路線的。當然,上述的方法只是進一步減小工件變形,如果想得到更高精的工件,還需要不斷的學習、探討和研究。

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