Dalian Marine Diesel Engine Co., Ltd. Gao Bo
With the continuous advancement of manufacturing technology and the increasing market demand, the operating speed of low-speed diesel engines is constantly approaching the most efficient propulsion speed of the propeller, and its design index is closer to lower speed, lighter weight and more compact structure. In order to reduce the working speed, the stroke ratio of the low speed diesel engine is continuously increased. The newly developed G model from Mann and the X model from Wärtsilä have a stroke-to-bore ratio of 4.65 and a minimum speed of 66 r/min. This change poses a challenge to the processing of large structural components where the machining of the frame is most sensitive to changes in stroke.
1. Rack processing difficulties
(1) The rack is a large box-type part with a large size. The dimensions of the G90 and X92 racks are 15~22m long, 5.65m wide and 5m high. The holes and openings are large, and the workpiece is turned over. Large deformations will occur (see Figure 1).
Figure 1 Rack top view
(2) The whole frame is welded by multiple steel plates. The weld bead is many and criss-crossed, the internal stress is large, and internal stress release deformation is easy after processing.
(3) The size and shape accuracy of the frame guide slide are high, which is the biggest difficulty in frame processing. The long stroke of the main machine will increase the height of the frame and lengthen the guide plate. It is necessary to solve the problem of rigidity reduction and machining vibration caused by the overhang of the machine ram.
(4) The opening at the bottom of the two ends of the frame is large and the rigidity is poor. It is easy to generate chattering and chipping during the processing, resulting in poor surface roughness and serious blade loss.
2. Process solutions for rack processing difficulties
For the above processing difficulties, the following solutions can be adopted:
(1) Since the frame will be deformed during the turning process, the most important processing surface (such as the processing of the guide slide) must be 180° after the last station finished product, that is, the most important surface finished product. Flip.
(2) If conditions are met, after the rack welding is completed, it is required to enter the heat treatment furnace for stress relief annealing. If there is no heat treatment condition, the workpiece should be roughed and placed for a period of time to aging, the internal stress is released, and the finished product is processed.
(3) Rack guide processing plan. To machine the frame of the super-mainframe, first the machine hardware parameters must meet the following requirements: 1 The length and width dimensions of the CNC gantry milling machine table should be sufficient, and the width of the table should be 6m. 2 After the machine beam is raised, the height to the work surface should reach 7m (greater than the height of the pad + the height of the workpiece + the length of the milling head). 3 The travel of the machine ram should preferably reach 3.5m, so that the sum of the ram stroke and the length of the milling head is greater than the machining height of the frame guide by 5m (see Figure 2).
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For more information, please download the attachment or view Metalworking (Cold Processing), Issue 23, 2013:
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