2011年10月31日 星期一
以快速原型作為消失模型進行石膏模快速鑄造
以快速原型作為消失模型進行石膏模快速鑄造
陳源豐
南開科技大學 機械工程學系
南開科技大學 機械工程學系
摘要
在傳統的鑄造工業中，生產鑄件所需要的時程甚長且費用也很高，由設計完成到鑄件產出，可能需要數天到數周之久。在分秒必爭的現代工業中，如何利用先進技術的輔助來縮短鑄件的開發時程，將是有待解決的重要課題。本研究運用快速原型技術，直接以快速原型作為消失模型，配合快速包埋石膏的使用，可在一到二天之內完成鑄件之生產，大幅縮短鑄造流程時間。
在傳統的鑄造工業中，生產鑄件所需要的時程甚長且費用也很高，由設計完成到鑄件產出，可能需要數天到數周之久。在分秒必爭的現代工業中，如何利用先進技術的輔助來縮短鑄件的開發時程，將是有待解決的重要課題。本研究運用快速原型技術，直接以快速原型作為消失模型，配合快速包埋石膏的使用，可在一到二天之內完成鑄件之生產，大幅縮短鑄造流程時間。
研究分為兩大部分，第一部分為光硬化樹脂快速原型的製作與燒失性能的測試，透過適當地控制燒失溫度及時間，將鑄模內的樹脂模型徹底燒除乾淨，確保模穴的完整性。第二部分則為搭配快速包埋石膏，以石膏模快速鑄造製程進行造模、燒成、澆鑄等測試。經由實驗顯示，本製程可在鑄件設計完成後的8小時內產出首批鑄件，迅速提供設計確認或後續加工特性評估之用。
關鍵字：快速原型、消失模型、石膏模、快速鑄造、光硬化樹脂
1. 前言
傳統的鑄件生產流程是鑄造方案設計、模型（木模）設計製作、鑄模製作、金屬熔化、澆鑄與脫模…，由設計完成到鑄件樣件產出，往往需要一周甚至更長的時間，即使是採用脫蠟精密鑄造法，其製程也是相當耗時。這不僅拉長了鑄件的開發時程，亦是開發成本居高不下的主要原因。近來流行所謂的快速模具(rapid toolings)，也就是為了達成時間經濟性的要求，希望在最短的時限內，製造出可供實際生產使用的模具，如射蠟模、塑膠射出成型模、壓鑄模等。快速模具所使用的材質可以是矽膠、金屬樹脂(metallic resin)、易熔合金、鋁合金與其它金屬材料(如鋼材)，其選用端視產品性質、產量或模具成本要求而定。在要求工件精度與模具壽命的大量生產場合，金屬材質快速模具應是較為理想的選項，而精密鑄造正是生產金屬材質快速模具的有效方法之一。另外不論是鑄件原型(小量產鑄件)或快速模具，在講求時效性的前提下，對鑄件的尺寸精度與表面粗度的要求，也是開發過程的另一項重要訴求。
隨著科技的進步，人類亟思縮短產品開發時程以降低成本並提升產業競爭力，因而有逆向工程（RE）與快速原型（RP）的發展[1]，如何利用這二種快速技術來提升鑄件的開發效率，當是鑄造界所面臨的新課題。
就RP技術層面來說，如何利用它來提升鑄件的開發效率？此不外乎是經由模型（patterns）或由鑄模（casting molds）二方面來著手。Bassoli等人[1]以澱粉質素材作為快速原型系統的耗材，製作鑄造模型，再透過快速鑄造（rapid casting）方法，可在甚短的時間內得到輕合金（light-alloy）的鑄件原型（casting prototypes）。Dotcher等人[2]以粉末狀高分子聚合物製作模型，再浸滲蠟液以提高模型的性能，並針對影響模型精度的參數進行探討。在Maji等人[3]的研究中，提出了將逆向工程（RE）、快速原型（RP）與包模鑄造（investment casting）等三種技術作結合的概念，不僅可以大幅縮短開發時間，更能有效改善鑄件品質，達到近淨形（near net shape）製造的終極目標。Hsy等人[4]則利用粉末式RP作出工件的反向形狀，再將此RP件經過沾漿、淋砂等殼模製程，作成鑄造用鑄模，最終翻製出鋁合金的快速射出成型模具，用來生產塑膠材質螺旋槳。
在國內的研究方面，張仲卿[5]與李賢建[6]利用RP原型配合包模鑄造，製作出塑膠射出成型用的快速模具。蕭宇聲[7]則先將光聚合樹脂原型作成射蠟模，以射蠟模生產蠟模型後，再經脫蠟鑄造製程的組蠟樹、沾漿、淋砂、脫蠟、殼模燒成、澆鑄…等步驟順序，生產快速模具，並且在金屬熔液的澆鑄過程中，採用真空輔助失壓鑄造法，以提升鑄件品質。
經上述文獻分析得知，以RP原型搭配精密鑄造製程，可在相對較短的時程內生產出所需之鑄件或快速模具，充分發揮時間經濟效益。然而儘管上述的各項研究均已導入快速原型製程，但在後續的精密鑄造過程中，沾漿、淋砂、脫蠟、鑄模燒成等製程仍需耗費相當長的開發時間。爲期更加縮短製程時間，本研究引入快速包埋用石膏，並採用高精度的液態樹脂RP系統，以有效提升鑄件的整體精度。
傳統的鑄件生產流程是鑄造方案設計、模型（木模）設計製作、鑄模製作、金屬熔化、澆鑄與脫模…，由設計完成到鑄件樣件產出，往往需要一周甚至更長的時間，即使是採用脫蠟精密鑄造法，其製程也是相當耗時。這不僅拉長了鑄件的開發時程，亦是開發成本居高不下的主要原因。近來流行所謂的快速模具(rapid toolings)，也就是為了達成時間經濟性的要求，希望在最短的時限內，製造出可供實際生產使用的模具，如射蠟模、塑膠射出成型模、壓鑄模等。快速模具所使用的材質可以是矽膠、金屬樹脂(metallic resin)、易熔合金、鋁合金與其它金屬材料(如鋼材)，其選用端視產品性質、產量或模具成本要求而定。在要求工件精度與模具壽命的大量生產場合，金屬材質快速模具應是較為理想的選項，而精密鑄造正是生產金屬材質快速模具的有效方法之一。另外不論是鑄件原型(小量產鑄件)或快速模具，在講求時效性的前提下，對鑄件的尺寸精度與表面粗度的要求，也是開發過程的另一項重要訴求。
隨著科技的進步，人類亟思縮短產品開發時程以降低成本並提升產業競爭力，因而有逆向工程（RE）與快速原型（RP）的發展[1]，如何利用這二種快速技術來提升鑄件的開發效率，當是鑄造界所面臨的新課題。
就RP技術層面來說，如何利用它來提升鑄件的開發效率？此不外乎是經由模型（patterns）或由鑄模（casting molds）二方面來著手。Bassoli等人[1]以澱粉質素材作為快速原型系統的耗材，製作鑄造模型，再透過快速鑄造（rapid casting）方法，可在甚短的時間內得到輕合金（light-alloy）的鑄件原型（casting prototypes）。Dotcher等人[2]以粉末狀高分子聚合物製作模型，再浸滲蠟液以提高模型的性能，並針對影響模型精度的參數進行探討。在Maji等人[3]的研究中，提出了將逆向工程（RE）、快速原型（RP）與包模鑄造（investment casting）等三種技術作結合的概念，不僅可以大幅縮短開發時間，更能有效改善鑄件品質，達到近淨形（near net shape）製造的終極目標。Hsy等人[4]則利用粉末式RP作出工件的反向形狀，再將此RP件經過沾漿、淋砂等殼模製程，作成鑄造用鑄模，最終翻製出鋁合金的快速射出成型模具，用來生產塑膠材質螺旋槳。
在國內的研究方面，張仲卿[5]與李賢建[6]利用RP原型配合包模鑄造，製作出塑膠射出成型用的快速模具。蕭宇聲[7]則先將光聚合樹脂原型作成射蠟模，以射蠟模生產蠟模型後，再經脫蠟鑄造製程的組蠟樹、沾漿、淋砂、脫蠟、殼模燒成、澆鑄…等步驟順序，生產快速模具，並且在金屬熔液的澆鑄過程中，採用真空輔助失壓鑄造法，以提升鑄件品質。
經上述文獻分析得知，以RP原型搭配精密鑄造製程，可在相對較短的時程內生產出所需之鑄件或快速模具，充分發揮時間經濟效益。然而儘管上述的各項研究均已導入快速原型製程，但在後續的精密鑄造過程中，沾漿、淋砂、脫蠟、鑄模燒成等製程仍需耗費相當長的開發時間。爲期更加縮短製程時間，本研究引入快速包埋用石膏，並採用高精度的液態樹脂RP系統，以有效提升鑄件的整體精度。
2. 實驗設備、實驗程序與資料歸納
2.1 液態快速原型製程
採用Object EDEN330快速原型系統，以PolyJet技術，將液態光聚合材料經由噴嘴噴出，再以紫外光照射每一層斷面使樹脂固化，逐層堆積成型，其最小層厚可達0.016mm。實驗鑄件為自行車用零件，外型如圖一所示，快速原型成形所需時間為5小時。
2.2 快速原型燒失實驗
2.2.1燒失溫度設定
2.1 液態快速原型製程
採用Object EDEN330快速原型系統，以PolyJet技術，將液態光聚合材料經由噴嘴噴出，再以紫外光照射每一層斷面使樹脂固化，逐層堆積成型，其最小層厚可達0.016mm。實驗鑄件為自行車用零件，外型如圖一所示，快速原型成形所需時間為5小時。
2.2 快速原型燒失實驗
2.2.1燒失溫度設定
精密鑄造用的石膏模或陶瓷殼模在澆鑄作業前通常需要經過燒成（燒結）過程，正確的燒成可使鑄模完全乾燥並得到充分的強度與適當的澆鑄模溫。燒成溫度視鑄件特性與鑄造金屬種類而定，大約為750~900℃，持溫時間則為30分鐘~90分鐘，為配合實際鑄模的燒成作業，實驗中快速原型之燒失測試溫度依序訂為900℃、850℃、800℃與750℃。
2.2.2燒失實驗
將長X寬X高為30㎝X30㎝X30㎝之實驗電爐加熱至預定溫度，以310不鏽鋼托盤盛放快速原型件並將其放入爐室中，每隔兩分鐘取出測量其殘餘重量，直至完全燒失為止。圖二為在不同的爐溫下快速原型的燒失時間曲線，燒失率定義為（燒去重量）/（原始重量）X100％。接著將快速原型包埋於石膏漿中，靜置25分鐘使其充份硬化後，放入750℃的爐中進行燒失測試，持溫30分鐘後徐冷到室溫，取出並剖切石膏模，觀察模穴內模型的燒失情況。
2.3 快速包埋作業
採用德國ADENTA-Vest CB快速包埋石膏，石膏模灌注完成後經25分鐘的靜置硬化，即可進行燒成。其燒成過程可採用一般燒成與快速燒成，本實驗採用快速燒成方式。包埋石膏的操作參數如表一所示，燒成與澆鑄模溫曲線則如圖三所示。
2.4 金屬熔化
鑄件材料採用矽－鋁青銅(成份為Cu90％、Al7％與Si2％)，使用電熱式石墨坩堝熔解，以硼砂為除渣劑。矽－鋁青銅的熔化溫度為1000℃，實驗中的熔液澆注溫度為1050℃，鑄模溫度為600℃。
2.5 失壓鑄造
由於石膏鑄模的透氣性十分良好，故實驗採用真空吸引方式進行澆鑄，一來可使銅液快速而平穩的充滿模穴，另一方面可減少模內的殘留氣體，降低鑄件內部產生氣孔的機率。圖四為真空輔助失
2.2.2燒失實驗
將長X寬X高為30㎝X30㎝X30㎝之實驗電爐加熱至預定溫度，以310不鏽鋼托盤盛放快速原型件並將其放入爐室中，每隔兩分鐘取出測量其殘餘重量，直至完全燒失為止。圖二為在不同的爐溫下快速原型的燒失時間曲線，燒失率定義為（燒去重量）/（原始重量）X100％。接著將快速原型包埋於石膏漿中，靜置25分鐘使其充份硬化後，放入750℃的爐中進行燒失測試，持溫30分鐘後徐冷到室溫，取出並剖切石膏模，觀察模穴內模型的燒失情況。
2.3 快速包埋作業
採用德國ADENTA-Vest CB快速包埋石膏，石膏模灌注完成後經25分鐘的靜置硬化，即可進行燒成。其燒成過程可採用一般燒成與快速燒成，本實驗採用快速燒成方式。包埋石膏的操作參數如表一所示，燒成與澆鑄模溫曲線則如圖三所示。
2.4 金屬熔化
鑄件材料採用矽－鋁青銅(成份為Cu90％、Al7％與Si2％)，使用電熱式石墨坩堝熔解，以硼砂為除渣劑。矽－鋁青銅的熔化溫度為1000℃，實驗中的熔液澆注溫度為1050℃，鑄模溫度為600℃。
2.5 失壓鑄造
由於石膏鑄模的透氣性十分良好，故實驗採用真空吸引方式進行澆鑄，一來可使銅液快速而平穩的充滿模穴，另一方面可減少模內的殘留氣體，降低鑄件內部產生氣孔的機率。圖四為真空輔助失
3. 結果與討論
Object EDEN 330系統在原型的建立過程中，必須在工件有懸空或下切口的部位處，噴塗建構支撐材料(support material)，待原型完成後，再將此一支撐材料去除。實驗中發現該公司所使用的支撐材料極易去除，以常溫清水與軟性海綿刷便可迅速的加以洗淨，不會破壞工件與支撐的連結表面且不需要後加工處理，可謂是製作消失模型的理想系統。
快速原型材料的主要成份為碳、氫及氧，在適當的溫度與含氧濃度下，可被完全燒除而不致殘留灰燼。為配合石膏鑄模的燒成過程，將原型單獨置於電熱爐中以200℃/hr的速率升溫到750℃，當開啟爐門探視時，原型在數秒後即會起火燃燒。試驗溫度達到900℃時，原型在爐門開啟的瞬間便會立即起火燃燒。因此在燒失過程中，保持電爐通氣口之開啟或由外部供應適量的氧氣，均有助於模型的燒失。
將包埋原型的石膏鑄模以150℃/hr速率升溫到750℃並持溫30分鐘，冷卻後切開鑄模，發現原型已完全燒除乾淨，無任何灰燼殘留，故樹脂原型可搭配多種品牌的石膏進行快速鑄造製程。
Object EDEN 330系統在原型的建立過程中，必須在工件有懸空或下切口的部位處，噴塗建構支撐材料(support material)，待原型完成後，再將此一支撐材料去除。實驗中發現該公司所使用的支撐材料極易去除，以常溫清水與軟性海綿刷便可迅速的加以洗淨，不會破壞工件與支撐的連結表面且不需要後加工處理，可謂是製作消失模型的理想系統。
快速原型材料的主要成份為碳、氫及氧，在適當的溫度與含氧濃度下，可被完全燒除而不致殘留灰燼。為配合石膏鑄模的燒成過程，將原型單獨置於電熱爐中以200℃/hr的速率升溫到750℃，當開啟爐門探視時，原型在數秒後即會起火燃燒。試驗溫度達到900℃時，原型在爐門開啟的瞬間便會立即起火燃燒。因此在燒失過程中，保持電爐通氣口之開啟或由外部供應適量的氧氣，均有助於模型的燒失。
將包埋原型的石膏鑄模以150℃/hr速率升溫到750℃並持溫30分鐘，冷卻後切開鑄模，發現原型已完全燒除乾淨，無任何灰燼殘留，故樹脂原型可搭配多種品牌的石膏進行快速鑄造製程。
4. 結論
本研究借由液態樹脂快速原型系統與快速包埋石膏，可對鑄件樣件(小量產件)或金屬材質快速模具進行快速製作，大幅縮短鑄件開發時程並有效降低開發成本。
本法所生產之鑄件具有優異的尺寸精度與表面粗度，可適用於鋁合金、銅合金、不鏽鋼、鎳－鉻合金等多種鑄造金屬。相較於其它鑄造方法，本法除鑄件品質極為優異外，在生產的時效性上更是佔有絕對的優勢。表二列出以本法生產鑄件時的單項製程時間，並以鑄件精度相仿的脫蠟製造（表三）作相對比較，由圖五即可明確看出本研究的時效性優勢。
5. 參考文獻
1. Bassoli E, Gatto A, Iuliano L and Violante MG, ”3D printing technique applied to rapid casting”, Rapid Prototyping Journal, Vol.13, pp148-155,2007.
2. Dotchev, K.D., Dimov, S.S., Pharn, D.T. and Ivanov, A.I., “Accuracy issues in rapid manufacturing CastForm(TM) patterns”, Proceedings of the Institution of Mechanical Part B – Journal of Engineering Manufacture, Vol.221, pp53-67, 2007.
3. Maji PK, Banerjee PS and Sinha A, “Application of rapid prototyping and rapid tooling for development of patient-specific craniofacial implant: an investigative study”, International Journal of Advanced Manufacturing Technology, Vol.36, PP510-515, 2008.
4. Hsu CY, Huang CK and Tzou GJ, “Using metallic resin and aluminum alloy molds to manufacture propellers with RP/RT technique”, Rapid Prototyping Journal, Vol.14, pp102-107, 2008.
5. 張仲卿，逆向工程技術及整合應用，第79-85及157-164頁，高立圖書有限公司，台北、台灣，1990
6. 李賢建，金屬樹脂、粉末冶金與精密鑄造於薄殼產品之快速模具製作，碩士論文，國立台灣科技大學機械工程系，台北、台灣，1998。
7. 蕭宇聲，開發快速原型系統與精密脫蠟快速模具製作之應用，碩士論文，崑山科技大學機械工程研究所，台南、台灣，2003。
8. 傅豪、陳武宏，精密鑄造技術，第301-303頁，文京圖書有限公司，台北、台灣，1990。
9. 顏永年、單忠德，快速成型與鑄造技術，第1-97頁，機械工業出版社，北京，2004。
10. 林英傑，產品開發的新利器－RP/RT技術，第1-10頁，模具技術資訊，高雄、台灣，1997。
11. 范光照等，逆向工程技術及應用，第167-195頁，高立圖書有限公司，台北、台灣，2003。
12. 鄒貴鉅，應用RP/RT於螺槳射出成型及精密鑄造之研究，碩士論文，龍華科技大學機械系碩士班，桃園、台灣，2004。
13. http：//www.adentatec.com
本研究借由液態樹脂快速原型系統與快速包埋石膏，可對鑄件樣件(小量產件)或金屬材質快速模具進行快速製作，大幅縮短鑄件開發時程並有效降低開發成本。
本法所生產之鑄件具有優異的尺寸精度與表面粗度，可適用於鋁合金、銅合金、不鏽鋼、鎳－鉻合金等多種鑄造金屬。相較於其它鑄造方法，本法除鑄件品質極為優異外，在生產的時效性上更是佔有絕對的優勢。表二列出以本法生產鑄件時的單項製程時間，並以鑄件精度相仿的脫蠟製造（表三）作相對比較，由圖五即可明確看出本研究的時效性優勢。
5. 參考文獻
1. Bassoli E, Gatto A, Iuliano L and Violante MG, ”3D printing technique applied to rapid casting”, Rapid Prototyping Journal, Vol.13, pp148-155,2007.
2. Dotchev, K.D., Dimov, S.S., Pharn, D.T. and Ivanov, A.I., “Accuracy issues in rapid manufacturing CastForm(TM) patterns”, Proceedings of the Institution of Mechanical Part B – Journal of Engineering Manufacture, Vol.221, pp53-67, 2007.
3. Maji PK, Banerjee PS and Sinha A, “Application of rapid prototyping and rapid tooling for development of patient-specific craniofacial implant: an investigative study”, International Journal of Advanced Manufacturing Technology, Vol.36, PP510-515, 2008.
4. Hsu CY, Huang CK and Tzou GJ, “Using metallic resin and aluminum alloy molds to manufacture propellers with RP/RT technique”, Rapid Prototyping Journal, Vol.14, pp102-107, 2008.
5. 張仲卿，逆向工程技術及整合應用，第79-85及157-164頁，高立圖書有限公司，台北、台灣，1990
6. 李賢建，金屬樹脂、粉末冶金與精密鑄造於薄殼產品之快速模具製作，碩士論文，國立台灣科技大學機械工程系，台北、台灣，1998。
7. 蕭宇聲，開發快速原型系統與精密脫蠟快速模具製作之應用，碩士論文，崑山科技大學機械工程研究所，台南、台灣，2003。
8. 傅豪、陳武宏，精密鑄造技術，第301-303頁，文京圖書有限公司，台北、台灣，1990。
9. 顏永年、單忠德，快速成型與鑄造技術，第1-97頁，機械工業出版社，北京，2004。
10. 林英傑，產品開發的新利器－RP/RT技術，第1-10頁，模具技術資訊，高雄、台灣，1997。
11. 范光照等，逆向工程技術及應用，第167-195頁，高立圖書有限公司，台北、台灣，2003。
12. 鄒貴鉅，應用RP/RT於螺槳射出成型及精密鑄造之研究，碩士論文，龍華科技大學機械系碩士班，桃園、台灣，2004。
13. http：//www.adentatec.com
20111025製造技術社群研討會
南開科技大學
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研討記錄
時間：100.10.25 10：00
地點：南開科技大學機械系精密加工實驗室
出席人員：如簽到表
一、陳源豐教授專題演講－以快速原型作為消失模型進行石膏模快
二、林憲茂教授專題演講－微量潤滑在模具鋼高速銑削應用之研究
三、交流座談：略
四、散會 (15：00)
五、活動照片
100.10.25 陳源豐教授專題演講照片一
100.10.25 陳源豐教授專題演講照片二
100.10.25 陳源豐教授專題演講照片三
100.10.25 林憲茂教授專題演講照片一
100.10.25 林憲茂教授專題演講照片二
100.10.25 林憲茂教授專題演講照片三
2011年10月26日 星期三
Prediction System of Magnetic Abrasive Finishing (MAF) On the Internal Surface of Cylindrical Tube
Prediction System of Magnetic Abrasive Finishing (MAF)
On the Internal Surface of Cylindrical Tube
Ching-Lien Hung1, Lieh-Dai Yang2+, Wei-Liang Ku3, Han-Ming Chow4
Submitted to
March, 2010
This paper has not been published elsewhere nor has it been submitted for publication elsewhere.
1+, 2Department of Industrial Engineering and Management, Nan Kai Universtiy of Technology, Nan Tou, Taiwan, R.O.C.
3Department of Information Management, Nan Kai Universtiy of Technology, Nan Tou, Taiwan, R.O.C.
4Department of Mechanical Engineering, Nan Kai Universtiy of Technology, Nan Tou, Taiwan, R.O.C.
+Corresponding authorAbstract
This study mainly used the way of the magnetic abrasive finishing (MAF) to explore the cylindrical tube of stainless steel SUS304 related to the processing characteristic and the prediction system. The self-make adjustable electricity polishing mechanism was assembled on the magnetic abrasive machine. The magnetic abrasive which was consisted of the sintered iron and Aluminum Oxide powder filled in the cylindrical stainless steel tube. Magnetic abrasive in the electromagnetic field was absorbed on the cylindrical tube to become flexible magnetic brush. It could generate adjustable pressure on the work piece surface when the magnetic brush is grinding, it could make the workpiece face polished to the mirror surface degree.
This experiment used the non-magnetic stainless steel SUS304, following experimental design to conduct the experiments and to explore the effects of various parameters such as rotational speed, vibration frequency, current strength, abrasive, etc., to the surface finish characteristics. After statistical analysis, ANOVA was obtained, and then surface finish prediction system was constructed based on the significant parameters, and the system precision was about 97%. The system will be further to develop an adaptive control model for MAF in a real fashion.
Key words: Magnetic abrasive polishing; flexible magnetic brush, experimental design, prediction model, surface finish.
1. Introduction
The rapid development of the semiconductor, biotechnology, and optical electronic industries has increased the importance of geometrical precision and part surface quality. Finishing is regularly applied to parts to obtain precise surfaces. Hence, numerous finishing techniques have been applied for finishing parts to obtain parts with high quality. These techniques include chemical mechanical polishing (CMP), electrical polishing (EP), and many others. However, both CMP and EP suffer from the formation of pollutants during its operations, and also yield surfaces with limited quality. Consequently, researchers in the industry and academics have attempted to develop a better means of obtaining a high-precision surface, with low cost, high efficiency, easy operations and low environmental pollution.
Following recent technological developments, stainless steel materials with characteristics of anti-oxidizing, anti-corrosive, and shiny surface have been applied in electronic, biochemical and medical instrumentation equipments. The surface of stainless steel parts must be extremely smooth to prevent pollution. Optimally, the surface finish can reach a level in that it looks like a mirror. A smooth stainless steel surface not only improves the parts quality but it also prevents rusting and staining of the parts surface. Finished parts can prevent the occurrence of the following situations: powder particles remaining on the part surfaces, contact between parts and the stainless steel surface, rough surfaces residing with oil dusk or food particles, and stainless steel burr of processed parts falling off when two parts contact each other.
Stainless steel is a soft, tough, and a difficult finishing material. Thin plate stainless steel that uses traditional processes is not easy to achieve a good surface finish. Hence, manual finishing was usually applied to achieve a surface finish that looks like a mirror. However, it is very time consuming to achieve a good surface finish using manual finishing techniques for stainless container steel surfaces.
To resolve the above problems, magnetic abrasive finishing (MAF) was recently created. MAF involves using a permanent magnet or an electronic magnet to generate a magnetic field, and the magnetic abrasives are formed as a flexible magnetic brush for pressing the workpiece [1, 2]. Thus, the magnetic brush becomes a finishing tool, and the magnetic abrasives of the magnetic brush stick to the workpiece during the finishing. Moreover, the frictional force generated by the abrasive finishing can remove particles of free-form surface. The procedure is repeated until a desired surface finish is attained.
When a permanent magnet was installed on the topside of the workpiece, any uneven or concave areas on the part could be finished [3-5]. Moreover, when the magnetic pole was installed inside or outside of the part, the internal and external pipes could also be finished [6]. Therefore, MAF is a multi-function precise finishing method. Workpiece materials can be magnetic (such as steel) or non-magnetic (such as ceramic), and the material removal weight can also be adjusted based on the size of the magnetic abrasives. The finishing pressure is controlled via the magnetic field, so MAF is used for micro-pressure finishing [7, 8]. Thus, the MAF method achieved a highly efficient way of obtaining a good surface finish.
This study attempts to develop a surface finishing technique for stainless steel, with the aim of analyzing the effects of different parameters and constructing the prediction system for the development of a further adaptive control system. Secondly, this investigation seeks to enhance surface finish of parts in order to meet the customer requirements.
2. Magnetic abrasive finishing
2.1 Fundamental principle
Magnetic abrasive finishing (MAF) of free-form surfaces involves filling the gap between the circular magnetic pole and the workpiece with the magnetic abrasives. The magnetic abrasives consist of sintered pure iron powder (99.9% Fe) and Al2O3. The end face of the magnetic pole absorbs the magnetic abrasives and forms a closed-loop magnetic field with the workpiece holder. The magnetic abrasives are generated in a non-uniformly magnetic field; in which the abrasives will join each other and follow the direction of the magnetic force to form a flexible magnetic brush. Refer to Figure 2-1 to see how the magnetic brush acted on the free-form surface. The magnetic force lines generated power to apply pressure from the magnetic abrasives to the workpiece, and the magnetic brush became a tool for finishing the workpiece. Moreover, the magnetic abrasives in the magnetic brush stick to the workpiece. When the magnetic pole rotates and moves with the workpiece relatively, the frictional force generated from MAF cause the abrasives to finish the particles of uneven or free-form surfaces until it becomes smooth. Moreover, the magnetic brush continues to move on the x-y-z direction of the CNC machine, brushing the workpiece until it meets the customer’s requirements.
3. Experimental mechanism, designs and results
3.1 MAF mechanism
This investigation involved the MAF mechanism illustrated in Figure 3-1. Using a permanent magnet generated the magnetic force; the magnetic field formed a closed loop due to the interaction of the permanent magnet, magnetic abrasives, workpiece, and workpiece holder (S10C steel). The magnetic flux density was close to 1.2 Tesla in a 1.0 mm working gap (distance between magnetic pole and workpiece holder). The S pole of the magnet was established with a shank installed in the spindle of the CNC machine. Meanwhile, the N pole of the magnet was designed to absorb the magnetic abrasives. The magnetic pole had an external diameter of 20 mm and a length of 40 mm . Furthermore, the N pole with a 10 mm radius ball shape was processed into four grooves with sizes of 1.5 mm width and 10mm depth to reduce the ball area of the magnetic pole and boost the magnetic field strength for achieving an efficient finishing.
3.2 Experimental designs
Objectives of the experimental design, was first to determine which parameters are most influential on the part surface. Then, to determine where to set the influential parameters so that part surface is almost always near the desired target value. In this study, four possible parameters which are spindle speed (S), vibration frequency (F), discharge current(C), abrasive weight ratio (A) considered and used to conduct experiments. In the experiment, four factors with each three levels were selected respectively as Table 3-1. After the experiments, the collected data (Table 3-2) were analyzed statistically and the ANOVA (Table 3-3) showed the results. After the analyzing, the significant parameters would be applied to develop a surface finish prediction system for the MAF operations using the collected data.
4. The Proposed S-FN-IPSFP System
Figure 4-1 illustrates the proposed statistical-assisted fuzzy-nets in-process surface finish prediction (S-FN-IPSFP) system. The inputs of the system were spindle speed (S); vibration frequency (F); discharge current (C); and abrasive weight ratio (A). The predicted variable of the system was the predicted surface finish (Rmax).
In order to generate a system with fuzzy-nets theory, a five-step approach of constructing the rule bank was introduced as follows.
Step 1: Divide the input space into fuzzy regions.
The input vectors are: spindle speed (S); vibration frequency (F); discharge current (C); and abrasive weight ratio (A). The ranges for each input variable were: spindle speed (S) [S+, S-] rpm, vibration frequency (F) [F+, F-] times/sec, discharge current (C) [C+, C-] amp, and abrasive weight ratio (A) [A+, A-], where S+and S- were the maximum and minimum values of the spindle speed (S) in all experimental data, respectively. The range of the output variable surface finish (Rmax), was [Rmax +, Rmax -], where Rmax + and Rmax - were the maximum and minimum values of Ra in all experimental data, respectively. Thus, the input feature vector X and “domain intervals” were given as:
The domain interval indicated which variable would most likely lie within the interval based on experience. Each interval was divided into 2K+1 regions, which were denoted by SK, S(K-1), ….MD, ….L(K-1), and LK.
The shape of each membership function was triangular and the spread width (W) of each triangular function was the same.
Step 2: Generate fuzzy rules for the given data pairs.
The fuzzy-nets training procedure was based on the input and output signals collected from the experiment. The signal obtained from the control system generated the input feature vector X (Eq.3-1). The output surface finish (Rmax) indicated the output vector of the system. The following example of fuzzy degree of the input variable (Fi) was determined in different regions. The function is given as:
where c(Fi) and W(Fi) indicate the center point and the spread width of the input linguistic variable Fi (e.g. S2, S1, MD, L1 or L2); and i is the index of regions, in which i = 2K+1.
Step 3: Avoid conflicting rules.
It was possible to have two or more conflicting rules from the experiments, i.e., rules that have the same IF part but a different THEN part. Top-down and bottom-up methodologies were proposed to resolve this conflict (Lou [11]). The top-down methodology assigned a degree to each rule. The degree of rule i {IF S is MD, F is L1, C is L1, and A is L1, THEN Rmax is MD} is defined as:
where mE is the data pair degree assigned by a human expert based on the data collection condition.
Step 4: Statistical assisted fuzzy-nets rule base.
From previous steps, the conflicting rules have been resolved, and it was very likely that the rule base had empty rules that needed to be filled. To fill the empty rules, a multiple linear regression (MLR) of the experimental data was used to assist filling the empty rules in order to build the fuzzy-nets rule base.
Step 5: Determine a mapping based on the fuzzy rule base.
After the fuzzy rules were developed, the following defuzzification strategy was used to determine the output Ra for a given input data set (S, F, C and A). The antecedents of the ith fuzzy rule used the minimum operation in order to determine the degree,
where
where c(Rmaxi) denotes the center value of region Rmaxi and m would be the number of fuzzy rules in the combined fuzzy rule base, t denotes fuzzy rule number t.
5. System verification and conclusions
Once the statistical-assisted fuzzy-nets in-process surface finish prediction (S-FN-IPSFP) system was constructed, the testing data shown on Table 5-1 was tested, and the results were summarized as followings:
1. Four possible parameters which are spindle speed (S), vibration frequency (F), discharge current(C), abrasive weight ratio (A) are all significant influences on surface finish in MAF processes.
2. This research uses 81 experimental data to establish the fuzzy-nets prediction model and 10 sets of experimental test data were conducted, the accuracy of the system was about 97%. Therefore, the proposed statistical-assisted fuzzy-nets in-process surface finish prediction (S-FN-IPSFP) system could be applied to the next step of adaptive control system development of MAF processes in a real time fashion.
Reference
1. Shinmura, T.; Takazawa, K.; Hatano, E. Study on Magnetic Abrasive Finishing (1st Report): Process Principle and a Few Finishing Characteristics. J. of JSPE 1986, 52, 851-857.
2. Tsuchiya, K.; Shimizu, Y.; Sakaki, K. Sato, M. Polishing Mechanism of Magnetic Abrasion. J. of Japan Institute of Metals 1993, 157(11), 1333-1338.
3. Shinmura, T.; Hatano, E.; Takazawa, K. Development of Plane Magnetic Abrasive Finishing Apparatus and its Finishing Performance. J. of JSPE 1986, 52, 1080-1086.
4. Kim, J.D.; Choi, M.S. Study on Magnetic of Polishing Free-Form Surface. Int. J. Mach. Tool. Manufact 1997, 37, 1179-1187.
5. Jain, V.K.; Kumar, Prashant; Behera, P.K.; Jayswal, S.C. Effect of Working Gap and Circumferential Speed on the Performance of Magnetic Abrasive Finishing Process. Wear 2001, 205, 384-390.
6. Shinmura, T.; Yamaguchi, H. Study on a New Internal Finishing Process by the Application of Magnetic Abrasive Machining (Internal Finishing Process of Stainless Steel Tube and Clean Gas Bomb). Transactions of the Japan Society of Mechanical Engineers 1993, 59(Part C), 293-299.
7. Shinmura, T.; Takazawa, K.; Hatano, E. Study on Magnetic Abrasive Finishing (3rd Report): Finishing Characteristics Non-ferromagnetic Substances. J. of JSPE 1987, 53, 1440-1446.
8. Shinmura, T.; Wang, F.H.; Aizawa, T. Study on a New Finishing process of Fine Ceramics by Magnetic Abrasive Machining: On Improving the Effect of Finishing Efficiency Obtained by Mixing Diamond Magnetic Abrasives with Ferromagnetic Particles. J. of JSPE 1993, 59, 1251-1256.
9. Baron, Y.M. Theory of Magnetic Abrasive Finishing and Development of this Method in Russia. Proceedings of 6th International Conference on Precision Surface Finishing and Deburring Technology 2000, 344-361.
10. Chang, G.W.; Yan, B.H.; Hsu, R.T. Study on Cylindrical Magnetic Abrasive Finishing Using Unbounded Magnetic Abrasives. Int. J. Mach. Tools. Manuf 2002, 42, 575-583.
11. Lou, M.S.; Chen, J.C. In-Process Surface Recognition in a CNC Milling Machine Using the Fuzzy-Nets Method. Int. J. of Comp. and Ind. Eng 1997, 33, 401-404.
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