Tham khảo tài liệu 'process selection - from design to manufacture episode 1 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 104 Selecting candidate processes Supply of lubrication commonly phosphate based to the die surfaces is important in providing uniform material flow and reduce friction. Small quantities of sulfur lead phosphorus silicon etc. reduce the ability of ferrous metals to withstand cold working. Surface cracking tearing of the surface of the part especially with high temperature alloys aluminum zinc magnesium. Control of the billet temperature extrusion speed and friction are important. Pipe or fishtailing metal flow tends to draw surface oxides and impurities towards center of part. Governing factors are friction temperature gradients and amount of surface impurities in billets. Internal cracking or chevron cracking similar to the necked region in a tensile test specimen. Governing factors are the die angle and amount of impurities in the billet. Surface detail is excellent. Surface roughness ranging mm Ra. Process capability charts showing the achievable dimensional tolerances for impact extrusion and cold forming are provided see . Dimensional tolerances for non-circular components are at least 50 per cent greater than those shown on the charts. Cold forming 105 Cold forming process capability chart. 106 Selecting candidate processes Cold heading Process description Wire form stock material is gripped in a die with usually one end protruding. The material is subsequently formed effectively upset by successive blows into the desired shape by a punch or a number of progressive punches. Shaping of the shank can be achieved simultaneously see . Cold heading process. Materials Suitable for all ductile metals principally carbon steels aluminum copper and lead alloys. Alloy and stainless steels zinc magnesium nickel alloys and precious metals are also processed. Process variations Usually performed with stock material at ambient temperature cold but also with stock material warm or hot. Solid die single stroke double stroke three blow two die
