大型葉片砂帶磨削加工方法及工藝研究*

張明德，王興龍重慶理工大學(xué)機(jī)械工程學(xué)院，重慶 400054

1．Introduction

Large-scale blades are the main parts of ship propeller and the typical parts of freeform surface．The precision of design and manufacture of surface directly determines the efficiency of the propulsive and noise of the ship．It will help to improve the efficiency and precision of machining large-scale blades by the research of processing method for the free-form surface parts．At present the machining processes of large-scale blades are that the first is casting，the second is coarse milling and leaving grinding allowance．Finally，we used the manual grinding and model testing to meet the technical requirements．The machining process results to big profile error，and the work environment is poor．At the same time，it is low efficiency，high cost，not suitable for the development of modern industry needs．

In this paper，it based on the theories of differential geometry and belt grinding of complex curved surface，applying independent platform based on ACIS to program and simulate，and using five axes linkage abrasive belt grinding machine to process blades．This paper also analyzes the error of machining，determines and optimizes the process parameters，and improves machining quality and efficiency of large-scale blades．

2．Design of grinding machine structure for large-scale blade

The large-scale blades which are machined are casting and the large grinding allowance in this paper．According to the machining requirements of the blade，which remove the tool marks of milling and that the material is most removed in the case of ensuring the quality of blade．Meanwhile，Blade surface is relatively flat，so 5 axis linkage abrasive belt grinding machine is designed．This machine can meet the requirements of parts processing，and can reduce the manufacturing cost of machine．

The axis distribution of grinding machine for large-scale blades as follows in Figure 1．Considering the large size workpiece，hoisting problems，etc，C axis is designed，and it can drive the work-piece to rotate．Because when the direction of principal curvature of the contact point and the direction of the axis of the abrasive belt wheel has been the same，cutting tool，and the workpiece contact states best，and processing efficiency is high．So B axis is designed，it can make grinding wheel rotate a certain angle to ensure that the contact state．In addition to this，machine has three linear axes，which are X，Y，Z axes respectively．X axis and Z axis make grinding head move around and move up and down．Y axis makes work-piece before and after exercise．

Figure 1．The axis distribution of grinding machine for large-scale blades

3．Planning processing path and calculating tool center point

Because work-piece has been through milling machining before abrasive belt grinding，so workpiece has obvious tool marks，and residual height is larger．In order to eliminate the tool marks of rough milling，the blade is carried on the coarse grinding processing．In order to eliminate tool marks and residual height，this paper plans the coarse grinding trajectory that the first step is the longitudinal coarse grinding and the second step for transverse coarse grinding．And then，the blade carries on the finish grinding aimed at polishing on the surface

The blade surface is S1，The equation of surface is r=r(u，v)．

The direction of principal curvature at cuttercontact of contact surface S1is rt=d u d v．

The unit normal vector of machining surface:

From Eq．(11)，it can getγ，as follows:

The grinding head rotatesβdegrees around the Y axis，it can make the maximum or minimum principal curvature directions of the contact point axis direction of abrasive belt contact wheel the same．The contact state of cutting tool and workpiece is the best and processing efficiency is high like this．The calculation formulaβis as follows:

The calculation formula of tool center point:

rcis the vector of tool center point;rpis the vector of processing contact point P on the surface;Rcis the tool radius．

As follows Figure 2:

Figure 2．Calculating tool center point

Equation can also be written as the form of components:

According to the above formula，it can generate NC programs，as follows:

……

This paper also applied Matlab to simulate and verify the processing trajectory to calculate．Figure 3 is the simulation of longitudinal grinding trajectory，F(xiàn)igure 4 is the simulation of horizontal grinding trajectory．

Hardness of contact wheel is closely related to the surface roughness and the real effective depth of grinding．Contact wheel harder，the greater the actual grinding metal removal amount，the greater the roughness．The contact wheel is softer，the actual grinding metal removal volume is smaller，and the roughness is smaller．Therefore when the blade is coarse grinding，it can use a hard contact wheel hardness to ensure that tool marks of the milling are removed the most．When the blade finished grinding，it can use soft contact wheel．It can remove less material，and at the same time realize the effect of polishing．

Figure 3．Longitudinal grinding trajectory

Figure 4．Horizontal grinding trajectory

4．The error analysis of processing

4．1．The positioning error of clamping

It is the firstly that blade is installed on the workbench before blade for processing．Positioning is reached by a positioning pin and a pin hole which are on Workbench．But the weight of workpiece is more than 900 kg，when the clamping workpiece have a larger impact on the workbench，it will cause the positioning error．Because the horizontal width of the blade is 1977 mm and vertical height of the blade is 1510 mm，so it will make the error further enlarge and cause to machine the unqualified products［3］．

Therefore it needs to correct the fixtures benchmark after clamping，which will be reduced to the error．The blade clamping is shown in Figure 5．

4．2．The influence of abrasive belt abrasion and contact wheel hardness

Because the machining surface of workpiece is large，so abrasive belt wear is more severe during machining，and it causes uneven blade grinding quantity．

Figure 5．The blade clamping

4．3．Deformation of the blade

The thickness of the blade from the flange to the top is reduced gradually．Figure 6 is the distribution of blade thickness that is the 10 sections from the top to the flange in the central axis［4］．

Figure 6．The thickness distribution of the blade

From Figure 6，it can get that the thickness of the top of the blade is the smallest，only 16 mm or so．The thickness of the flange is the largest，there is more than 135 mm．According to mechanics of materials and theoretical mechanics，blade clamping structure is similar to a cantilever beam．So the rigidity of flange is better，its deformation is smaller．And the top of the blade is the small thickness，poor rigidity，so the deformation is more serious．In addition，combined with its material which is the copper alloy that is soft，and constant grinding force in grinding，it makes the blade torsional deformation．

Blade were measured 9 sections using coordinate measuring machine．These 9 sections as follows Figure 7．In order to study deformation in the process of machining problem，the following measurement is performed in this paper．Before grinding A surface of blade，B surface of blade was measured by coordinate measuring machine;when grinding A surface of blade was finished，but B surface of blade had not been machined，B surface of blade was measured by coordinate measuring machine again to observe machining deformation of the blade．The deformation as shown in Figure 8．To reduce such processing error，both sides of blade should be repeatedly machined．

Figure 7．Test sections of blade

Figure 8．The machining deformation of blade

5．Determination and optimization of process parameters

Abrasive belt grinding blade eliminates the above factors which affect the machining accuracy，it includes some process parameters such as abrasive belt，abrasive belt speed，feeding speed，grinding pressure and so on．

In order to eliminate tool marks and residual height，this paper plans the coarse grinding trajectory．And then the blade carries on the finish grinding aimed at polishing on the surface．Eventually it can meet the technical requirements［5-6］．The process parameters form the experiments are shown in Table 1．

Table 1．Process parameters

In the process of coarse grinding，because grinding allowance is the large，so it choses the coarser sand belt．After repeated experiments，it can determine the process parameters as follows:the accumulation of abrasive of Al2O3;abrasive grain size is 36#;belt speed is 23 m/s;feeding speed is 3 600 mm/min;grinding pressure S=300．In the process of finished grinding，grinding allowance is small，it demand to meet the surface quality．After repeated experiments，it can determine the process parameters as follows:the accumulation of abrasive of Al2O3;abrasive grain size is 60#;belt speed is 23 m/s;feeding speed is 4 500 mm/min;grinding pressure S=260．Here it is important to note that when abrasive belt is broken in the process，the worker should change the abrasive belt according to actual situation．After the worker changed abrasive belt to start the machine，it should ensure that all parameters of the machine are same with the parameters that before abrasive belt is broken．Otherwise there will be obvious phenomenon of groove shown in Figure 9．

Figure 9．Obvious phenomenon of groove

6．Conclusion

1)Traditional machining method of large-scale blade usually requires manual polishing．Using abra-sive belt grinding technique which is effect of the grinding and polishing can improve the machining accuracy，reduce greatly the labor intensity and processing cost．It optimized the processing scheme by analyzing machining error．

2)It is based on the theories of differential geometry and belt grinding of complex curved surface，and combined with structure characteristics of 5 axis NC grinding machine which is used to machine the large-scale blade．At last，it determined process parameters of the blade machining and the blades machined meet requirements．a(chǎn)s shown in Figure 10，it provided the gist for complex curved surface parts processing．

Figure 10．The finished grinding blade

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