Sheet metal forming is a manufacturing method used because of its versatility. Sheets are plastically deformed by a tool to create a product. A tool is expected to last for several 100,000 forming operations and efforts are made to optimize the tools. A common type of wear referred to as galling is the adhesion of sheet material to the tools. This problem has become more prevalent as new high strength sheet materials have been developed at the same time as lubricants have become heavily regulated. This has forced the development of new tool steels with improved resistance to galling. There are many parameters influencing the response to galling. In this work the influence of surface preparation, contact geometry, material selection and lubrication has been investigated. The surface of the forming tools has a large influence on the tools effective life. To refurbish a forming tool is expensive and often requires special shops and hand polishing. The influence on galling of different surface preparations suited for sheet metal forming was investigated using a strip-reduction equipment. The contact conditions of a tool sliding against metal sheets were investigated using FE models. The contact conditions were calculated for a U-bending test and for a sliding-on-flat-surface wear tester. The results were compared to those found in literature. One model incorporated the surface roughness of a sheet as measured by optical profilometry. The strength of the interface between the tool and the sheet material determine if material can be transferred onto the tool. The interface between the tool and adhered sheet material was closely studied using transmission electron microscopy of thin lamellas produced by focused ion beam milling. This showed sheet material adhering to the tool without the formation of an interlayer. Finally, several different combinations of tool steels and sheet materials were tested with regards to their ability to withstand galling. / Baksidestext: Sheet metal forming can be used to produce a wide range of products but the initial costs are high as the forming tools are expensive. Wear of the tools in the form of galling i.e. the adhesion of tiny pieces of sheet material to the tools has become more prevalent as high strength sheet materials have been developed and lubricants have become heavily regulated. In this work the influence on galling of surface preparation, contact geometry, material selection and lubrication has been investigated. It was found that tool surfaces should be polished as rougher surfaces quickly picked up material that adhered to the tools and subsequently scratched the sheets. The strength of the interface between the tool and the sheet material determine if material can be transferred onto the tool. The interface was studied using bright field transmission electron microscopy and the sheet material was found to adhere to the tool without the formation of an interlayer. The conditions under which galling occurs were studied using a slider on flat surface wear tester and several different material combinations were tested with regards to their galling resistance. The contact conditions of the test equipment were also modeled using FE models to better understand the strains of the materials involved.

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A strategy was proposed to suppress the wear effect of tool in a polishing process. So, machining rate of work pieces in polishing process would keep constant. With this strategy applied to HDP, stable machining rate will bring more advantaged for precision engineering. This study mainly discussed by two parts. First, the planning of the rock-and-roll motion and a wear analysis for tools is talked about. Second is discussing about experiment further. In first part, in order to increase tool¡¦s lifetime, expanding tool wear region by rock-and-roll motion is adopted. Meanwhile, simulate the wear behavior to develop a method of removing tool¡¦s materials in expectant form. In second part, there are two targets to compare with first part, one is the effect of planning of tool¡¦s materials removing and the other is the improvement of suppressive wear effect for polishing tool and stabilizing removal rate of work pieces in rock-and-roll motion. The result of experiments shown that planning of tool¡¦s materials removing is working, the radius of curvature and roughness of tool is nearly stable, and there are some effects in removal rate on work pieces. The result of experiment of rock-and-roll motion verified that the wear theorem for tools and prove improvement of wear is working. So, the goal of suppression of wear effect for polishing tool by expanding tool wear region by rock-and-roll motion is feasible.

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Scuffing is a severe form of surface damage which limits the performance of lubricated sliding machine components. Empirical work has shown that failure either occurs under relatively mild elastohydrodynamic conditions with barely modified surfaces or under severe conditions with the surfaces well run-in. Two hypotheses exist which may explain these experimental differences. This thesis examines their relevance. The first hypothesis is that, under elastohydrodynamic lubrication, the surface asperities either remain rigid or become elastically deformed-micro-elastohydrodynamic lubrication. A non-dimensional plot, developed by Baglin, predicts the occurrence of the regimes. An experimental study of running-in and scuffing for tests initially operating in the different regimes is described. Tests were run on a two disc machine with incremental loading. Running-in occurred both when tests started in the micro-ehl regime and when they apparently entered it during operation. High sliding prevented entry into micro-ehl; scuffing occurred with barely modified surfaces. This hypothesis discriminates between failure types but cannot alone predict scuffing. The second hypothesis, by Crook and Shotter, is that scuffing represents an inbalance between the rate of film thinning with increasing load and the rate of running-in. Increasing load increases the temperature which, due to its effect on viscosity, controls film thinning. Knowledge of the machine's thermal behaviour is required. A model is developed to predict temperature in a finite length cylinder subject to a discrete rotating heat source and convective cooling. Steps to apply the theory to a two disc machine are detailed and the results compared to previous experimental temperatures. Methods of changing thermal response are considered and preliminary tests with the discs insulated to increase the temperature rise are described. A marked reduction in scuffing load emphasises the importance of thermal design. Further experimentation is necessary to determine whether the Crook and Shotter hypothesis can quantify scuffing failure.

published_or_final_version / Industrial Engineering / Doctoral / Doctor of Philosophy

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Diamonds have many extreme physical properties that lead to a host of technological applications. Their main use as a tool or abrasive for the machining of hard materials involves diamonds rubbing against the material being worked and often against each other. The strength and frictional properties of diamond are thus of great practical, as well as academic interest. Studies of the reciprocating sliding of one diamond on another for extended periods have shown that the changes in friction and wear are linked with the formation of debris. Analysis of the debris shows it does not consist of diamond fragments. Investigations of the effect of low-pressure gases on diamond friction also indicate debris plays an important role, and that the friction and wear are markedly dependent on the state of adsorbants on the diamond surface. Details are also given of the friction and wear of steel, glass and sintered diamond in contact with diamond. An associated study of the solid particle erosion of diamond has shown that cracking can occur at impact velocities of 25 m s<SUP>-1</SUP>. To facilitate the friction studies two existing friction apparatus have been automated. Computer control now allows data to be collected and analysed more rapidly and thoroughly than was possible before. The equipment has also been adapted to extend the conditions under which friction studies can be made.

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No description available.