In daily precise mechanical machining, it often happens that the part machined is not nonconforming requirements due to dimension problems. Here, we will discuss about accuracy problems that occur in precise mechanical machining and how to improve the precision machining tolerance in the part or components machining
1.1 Machining Accuracy and Machining Error
Machining accuracy is the degree of conformance of actual parameter (dimension, profile and position) of a part after machining to the ideal parameter. The actual parameter is not possibly exactly the same as the ideal parameter, deviation always exists. The degree of deviation of actual parameter from the ideal parameter is error. Error is not avoidable, but methods can be taken to make the actual parameter close to the ideal as possible.
1.2 Original error
The precise machining system (machining system) composed of CNC machine tools, clamping tools, cutting tools and workpiece have various errors which will be reflected as machining error in different ways (expended or reduced) under different working conditions. The original error of machining system mainly geometry error of the machining system, positioning error, machining error caused by forced deformation of the machining system, machining error caused by thermal deformation of the machining system, deformation of the workpiece caused by redistribution of internal stress, and theoretical error, calibration error and measurement error etc.
1.3 Methods to study machining accuracy
Methods to study machining accuracy includes analytical calculation method and statistical analysis method. By these studying methods, we can find solutions to minimize the errors.
- Collective Error of Machining system
2.1 Geometry Error of CNC Machine Tool
In machining, the movement of the cutting tools relative to the formation of the workpiece are completed with CNC machine tool, therefore the machining accuracy of workpiece largely depend on the accuracy of CNC machine tool. Wear and tear will lead to decreased working accuracy of machine tool and choosing machine tools with higher quality is also a way of reducing error. Generally, the domestically manufactured machine tools in China are not comparable with the imported one, since the machining error of domestically manufactured machine tools is bigger.
2.1.1 Spindle Rotation Error
Spindle of machine tool is the execution unit of clamping and transmitting movements and power to workpieces or cutting tools. The spindle rotation error will directly influence the machining accuracy of the workpiece.
2.1.2 Guide Rail Error
Guide rails are the reference in confirm the relative position of each component of a machine tool and are the foundation of machine movements. Accuracy requirements on guide rails of turning machine are the straightness in horizontal plane, straightness in vertical plane, and parallelism (twist) between the paired guide rails.
In addition to the fabrication errors, non-uniform wear and installation quality of the guide rails are also important factors that may arise guide rail errors. Guide rails wear is one the of the important causes of machine tool accuracy decrease.
2.1.3 Transmission Chain Error
Transmission chain error refers to the relative movement error of transmission elements between starting and ending of the chain. Usually, transmission chain error is measured by the ending element angular error.
2.2 Geometry Error of Cutting Tools
In the process of cutting, any cutting tools will be unavoidably worn which will cause machining changes in dimension and profile of the workpiece. Abrasion of cutting tool can be reduced by correct selection of cutting tool of new-type wear-resisting materials and reasonable selection of cutting tool geometry and amount of cutting parameter, sharpening the tools in a right way, suitable application of coolant and cutting fluid. If the condition allows, automatic compensation instruments can be applied to compensate cutting tool abrasion, and I think this instrument is necessary to machine parts of high accuracy.
- Positioning Error
3.1 Misalignment Error
The basis according to which the size and position of a face is defined in the part drawing is called design reference. The basis according to which the size and position of a machined face is defined in the process drawing is called process reference. In ordinary circumstance, design reference should align with process reference. When machining a workpiece on machine tool, several geometry elements shall be chosen as positioning reference (or measuring reference), if the positioning reference (or measuring reference) does align with design basis, misalignment error will occur. Misalignment error equals the displacement of positioning reference relative to the design reference in the dimension direction of working procedure.
3.2 Manufacturing Error of Positioning Pair
The position of workpiece in fixtures is determined by the positioning element. The positioning element in fixture is not possibly manufactured as precise as the reference size, the actual size (or position) can vary within in allowed tolerance. Meanwhile, there is also manufacturing error on the surface of positioning reference of workpiece. Workpiece’s surface of positioning reference and positioning element in fixture form a positioning pair. The maximum displacement of workpiece caused by manufacturing inaccuracy and fit clearance of the positioning pair is called manufacturing error of positioning pair.
- Error caused by deformation of machining system
4.1 Rigidity of Workpiece
In machining system, if the rigidity of workpiece is lower than that of machine tool, cutting tool, and fixtures, the workpiece will have deformation under cutting force due to insufficient rigidity, which will significantly influence the machining accuracy. The maximum amount of deformation can be calculated according to relative formula of mechanics of materials.
4.2 Rigidity of Cutting Tools
The rigidity of cylindrical turning tool in the normal (y) direction of machined surface is very large, and its deformation can be ignored. When boring a small diameter inner hole, the deformation of the boring bar can influence machining accuracy if the rigidity of boring bar is poor. Deformation of boring bar can be calculated according to relative formula of mechanics of materials.
4.3 Rigidity of Machine Tool Components
A machine tools is composed of various parts and besides experimental methods there is not a simple method to calculate component rigidity of a machine tool.
- Error Caused by Thermal Deformation of Machining System
Thermal deformation of machining system has a significant influence on machining accuracy, especially in precise machining and large component machining, machining error caused by thermal deformation of machining system account for 40%～70% of total workpiece error. Under the effect of various heating source, the temperature of machine tool, cutting tool and workpiece will increase gradually and remit heat to the surrounding substance and space. When the absorbed heat and remitted heat per unit time are equal, the machining system reaches heat balance status. The selection of a heat treatment technology is also very important, if the factory does not have its own heat treatment technology, it is necessary to outsource the machining service. When outsourcing machining service, it is important to use the factories with suitable heat treatment.
- Conclusion: ways to enhance machining accuracy
“reduce original error, transfer original error, balance original error, and compensate error” is the highly summarization of improving accuracy.