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|کد مقاله||سال انتشار||تعداد صفحات مقاله انگلیسی||ترجمه فارسی|
|26986||2003||7 صفحه PDF||سفارش دهید|
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : International Journal of Machine Tools and Manufacture, Volume 43, Issue 7, May 2003, Pages 731–737
Machine tool deflections due to cutting forces can result in dimensional errors on workpieces. The problem is most severe when flexible tools such as end mills are used. When dimensioned features are specified with tolerances, process planning should examine the compromise between achieving high productivity rates and meeting dimensions within the specified tolerances. The use of geometric dimensioning and tolerancing permits interaction between size and position and makes bonus tolerances available. The errors occurring in end milling are first examined and modelled using regression methods. A procedure is proposed for selecting optimal feed rates that ensure that tolerances can be met. The process is demonstrated in machining a slot using the down milling mode. The use of a tolerance analysis chart clarifies the results of the test in relation to the tolerance standards. The need to consider the transient errors at the exit of the cut is demonstrated.
In modern machining practice, there are competing pressures for productivity and part accuracy. It has been pointed out  that a 50% increase in tool life would lead to a 1.5% reduction in production cost whereas a 20% increase in productivity would lead to a 15% reduction in production cost. Greater gains are clearly available by increasing metal removal rates but this in turn creates problems in holding accuracy on parts since an increase in metal removal rates, particularly if achieved through higher chip loads, leads to greater cutting forces. In a CIRP keynote paper  that deals with machining errors, it is reported that, “deflection of the machine due to cutting forces dominates the error budget”. End milling is a particular machining process that has received a lot of attention in the context of tool deflection , , ,  and . End mills are comparatively flexible tools that deflect easily, regardless of the rigidity of the machine in which they are used. Moreover, the magnitude, application point and direction of the resultant cutting force change with the rotation of the tool. There is thus an inherent and unavoidable periodic variation in the cutting force that is partly responsible for the dimensions that result on the cut surface. In addition, in selecting machining conditions, it is easy to stray into combinations of feed and speed that induce machining instabilities such as chatter that further affect surface finish and dimensions. A number of methods have been proposed to deal with tool deflection. A recommendation to use the shortest possible tool for the greatest rigidity is obvious. Feed rate regulation has also been proposed ,  and . However, a feed rate reduction may result in the tool operating at a level below its potential and frequent changes in feed rate may result in an inconsistent surface quality . Another proposed method is tool path compensation. Watanabe et al.  developed an adaptive control system on an NC machine that altered the tool path to compensate for surface errors. Suh et al.  investigated a tool path correction method based on an instantaneous deflection model whilst Yang et al.  proposed a tool deflection compensation method based on tool tilting. Law et al.  presented a method that predicts contour accuracy as a result of tool deflection and compensates for the error. Compensation methods such as these can reduce the errors occurring while maintaining the initial process conditions for maximum productivity but there is little work reported that considers machining errors in the context of process planning to meet specified part tolerances. The machining of complete part features often requires a combination of steady state cutting and transient cutting conditions where the cutting geometry changes. An example of the latter occurs at the entry and exit of cuts. Force variations in these instances pose problems in holding tolerances over the whole of a feature’s surfaces. Geometric dimensioning and tolerancing (GD&T) has been in use for many years as an alternative to traditional coordinate dimensioning. It is a method used to control variations of a part from its specified size and form to meet part functionality or interchangeability requirements. In particular, it introduces methods that link the size and position of features from datum surfaces. It offers the prospect of bonus tolerances on position when the size of features is targeted at one of the limits of size with the use of a material modifier. Instead of simply searching for a compensation methods to reduce the error of a cut surface, the wider problem is to consider actual part requirements at the process planning stage, to examine the tolerances that are required to be achieved, to evaluate the opportunities offered through the interaction between size and position and to recommend process conditions that can meet the tolerances specified. This paper considers the errors that occur in the end milling process and evaluates the process planning decisions for a simple part containing a feature specified with GD&T that is to be machined by end milling. The need to take a complete view of the machining requirements is demonstrated.
نتیجه گیری انگلیسی
A process planning method that selects machining conditions in order to meet tolerances, set out using GD&T, has been presented. The approach taken is to choose the highest feed rate that ensures that the part can meet the tolerances. A regression equation has been used to model the component of cutting force that is responsible for the surface errors. However mechanistic cutting force models could be employed to improve cutting force estimations and hence the reliability of the planning strategy. It is demonstrated that there is a need to consider exit errors, as compensation for steady state errors alone will not ensure that a toleranced feature will pass inspection over its full length. The use of GD&T allows some interaction between the size and position of features. Acceptable combinations can be represented on a TAC. This is useful as the errors associated with exit conditions result in a local increase in size and a shift in the estimate of the position of the feature. Under these circumstances the bonus tolerance available is particularly useful. It would be theoretically possible to alter the tool path as the tool exits the cut to ensure uniform dimensions compared with steady state cutting, though this would be an aspect for further work. A case study that employed limiting feed rates resulted in measured surfaces that only just passed inspection. Given that the predictions of cutting force are subject to errors of ± 20%, a more cautious plan might reduce feed rates to ensure that the points were contained nearer the centre of the polygon in the TAC. In any case, a more global selection of process variables would be required for optimization and avoidance of chatter inducing combinations.