Optimization Of Auto-Body Tolerance Allocation Based On Welding Springback

The quality of auto-body assembly takes an important role in the car designing,and it is mainly affected by the tolerance of the body parts.In order to improve it,it's necessary to optimize the tolerance of body parts.However,many of the body parts are made of flexible sheets.When the welding process is affected by the error,the flexible deformation will occur.After the completion of welding,the deformation of the parts will cause the body assembly springback,by which auto-body's tolerance optimization will be affected.Therefore,in order to optimize the distribution of body tolerance,it is necessary to take the welding springback into account.First of all,on the basis of traditional tolerance analysis theory,the error analysis of the gap and flush between the front fender and bumper of a vehicle was taken as an example,and the relevant parts of the model were extracted.According to the welding sequence,three dimensional tolerance analysis software 3DCS was used to simulate the assembly process of the relevant parts.Then,the model of rigid tolerance was established.Secondly,considering the problem that the welding springback often occurs due to the flexible deformation of the parts,the main causes of the welding springback were analyzed in the paper,including parts' errors,fixture block numbers,positioning block arrangements,fixtures' clamping force,and welding guns' clamp force.Then,examples were given to illustrate the effect of each error.The relationship between the input error of the parts and the output error of the assembly was analyzed,and the calculation process was illustrated by the example of door sill beam outer plate assembly.Thirdly,the tolerance manufacturing cost model and the tolerance total quality cost model were established in the paper,so was the basis of the constraint function.The firefly algorithm,which is easy to implement with high computational efficiency,was introduced to the optimization of auto-body tolerance.In order to realize the optimization precision and global optimization,a properly improvement was made.Then,an example was given to demonstrate the applicability of the algorithm in the optimization of auto-body tolerance.Finally,based on the analysis of the gap and flush between the front fender and bumper,finite element model of related parts were made in HyperWorks.Then,the flexible deformation of welding springback was calculated and added to the rigid tolerance analysis model.The tolerance manufacturing cost model and tolerance total quality cost model were established for tolerance optimization,and the weight was assigned for related tolerance according to the tolerance sensitivity coefficient.Moreover,the constraint function was established for optimization.Each of the two optimization models were calculated using the improved firefly algorithm,and the results were added into the flexible tolerance analysis model to verify one by one.The results showed that both of the two models could meet the requirement of the assembly precision,which illustrated the effectiveness of weight assignment and tolerance optimization.Besides,the results of these two models were compared with each other from different angles.It was proved that the tolerance total quality cost model could better balance the interests between the factory and consumers.