Application of a variable flow stress machining theory to helical end milling

In vitro study on high rotation (up to over 300,000 rpm) deep removal (up to 150 microm) of ceramic prostheses, made of a machinable ceramic, Vita Mark II, was performed in dental surgery using a high-speed dental handpiece. Dental clinical removal relevance, including tangential and normal grinding forces, specific grinding energy, and surface roughness, was investigated to establish the relationships among the surgery parameters, chip geometry, and fracture mechanism. The results show that both the tangential and normal grinding forces increased with increases in both depth of cut and maximum undeformed chip thickness, but decreased with an increase in grinding speed. Specific grinding energy decreased with increase in the depth of cut and the maximum undeformed chip thickness, but increased with an increase in grinding speed. Surface roughness and morphology appeared to be independent of the increases in depth of cut, grinding speed, and maximum chip thickness. The limitation for deep removal using the dental handpiece was found that the operation at the depth of cut of 150 microm or beyond resulted in a huge normal force exertion of 3 N with a great variation. The microfracture, the lateral fracture, and the ductile microcutting were found to occur simultaneously in dental surgery to remove the ceramic prostheses.

The thickness of the maximum undistorted chip (a gmax ) is a crucial component of the machining arguments and significantly affects the quality of the grind. For the purpose of investigating the impact of the a gmax on the grinding removal properties of SiO2f/SiO2 composites, grinding experiments of SiO2f/SiO2 composites were conducted. The findings indicate that crack propagation, fiber cracking, matrix breaking, and interface debonding are the primary removal modes of SiO2f/SiO2 composites. Grinding force, surface roughness, and chip size all have a positive correlation with the maximum undeformed chip thickness. Surface brittle fracture brought on by SiO2 fiber microcrack results in little surface roughness. The surface quality degrades when bending fracture and compression fracture take place. Controlling the a gmax can enhance the grinding surface quality of SiO2f/SiO2 composites.

This work proposed an improved mechanistic model for prediction of cutting forces in micro-milling process. The combined influences of tool run out, trochoidal trajectory of the tool center, overlapping of tooth, edge radius and minimum chip thickness are incorporated in this model to realize the exact cutting phenomenon. Moreover, an improved undeformed chip thickness algorithm has been presented by considering tool run out, minimum chip thickness and trajectory of all passing teeth for one complete revolution of the tool instead of only the current tooth trajectory. For estimation of tool run out, a model based on the geometry of the two fluted end mill cutter has been developed. Effects of trochoidal trajectory of the tool center and tool run out are found to be significant as each tooth has a different chip load. Further, the effect of minimum chip thickness is found to be significant at lower feed value. The proposed model has been validated by micro-milling experiments on Ti6Al4V-titanium alloys using uncoated flat end mill cutter. The predicted cutting forces were found to be in good agreement with the experimental cutting forces in both feed and cross feed directions.

In micromachining, when the undeformed chip thickness becomes comparable to the edge radius of the cutting tool, the effective rake angle becomes to be negative and has significant effect on the determination of the minimum uncut chip thickness. The determination of the minimum uncut chip thickness is essential in micro machining in order to achieve desired surface integrity and accuracy. In this paper, an Arbitrary Lagrangian Eulerian (ALE)-based numerical modeling is proposed to determine the minimum uncut chip thickness for Copper by changing the cutting tool’s nominal rake angle. According to the relationship between the minimum uncut chip thickness and the effective rake angle, a mathematical model that reflects the relationship between the effective rake angle and the nominal rake angle is established.

Machining of difficult-to-cut materials producing serrated chip formation has generated a significant interest among academic researchers and industry groups because of the need for understanding the fundamental mechanisms to model the process. Therefore, various analytical and numerical models have been developed in the recent past to investigate such machining operations. The slip-line theory is one of the established modeling methods, but slip-line modeling of serrated chip formation has not been investigated adequately. In this study, a new slip-line model for serrated chip formation in machining with rounded cutting edge tool is presented along with its associated hodograph. In addition, Oxley’s theory was integrated to the proposed slip-line model for stainless steel material, and the model was validated by experimental results. A good agreement between the experimental and analytically predicted results was obtained. The proposed slip-line model offers predictions of cutting and thrust forces, ploughing force, maximum and minimum chip thickness values, tool–chip contact length, chip up-curl radius, thickness of the primary shear zone, angular position of the stagnation point, shear strain, shear strain rate at the shear plane, and the prevalent flow stress.

Minimum uncut (tm) chip thickness is the promising factor of the microendmilling process, which is affected by many factors. The tool geometry is one among them. In this work, the effect of different tool geometries, two flutes and four flutes of microendmill (500 μm diameter), are investigated, during slot milling of copper alloy (BSS 249) by considering the surface roughness (Ra) and cutting forces. The results show that the feed rate has more impact on the Ra. A higher order of Ra value is observed with the two flutes microendmill than that of with four flutes microendmill, due to the closer tool path generation of the subsequent passes of the four flutes. It is also observed that the transition of chip formation from interrupted chips to continuous chips has occurred with two flutes and four flutes. From the cutting forces analysis, it is found that the feed force (Fy) is greater than that of the transverse forces (Fx) irrespective of the tool geometry. The behaviour of cutting forces also helps to identify the minimum uncut chip thickness (tm). The angular shifting of the cutting forces is also observed concerning the tool geometry. The material removal mechanism reveals that the ploughing effect is more dominant than that of the shearing effect with two flutes microendmill. The present investigations have shown that four flutes microendmill is more suitable for tool-based microendmilling in order to achieve better Ra with minimum cutting forces.

The cutting edge radius Re leads to the minimum uncut chip thickness in micro machining. The single-edge single-grit model was developed for micro mill-grinding in this paper. Simulations for Ti6Al4V were performed in various cutting depths, and the minimum uncut chip thickness of micro mill-grinding was confirmed to be between 0.25Re and 0.3Re (approximate 0.275Re). Various grain heights of protrusion were investigated to discuss the influence of grain depths of cut on the minimum uncut chip thickness. The minimum uncut chip thickness will increase if the grinding debris forms, while it will get smaller if the grinding debris cant be generated.

The characteristics of undeformed chip geometries generated by a dedicated cutting tool are complex in orbital drilling of Ti6Al4V and they have an important influence on the quality of hole-making. This study investigates the orbital drilling kinematics and the undeformed chip geometry based on the definition of three machining stages. A model of undeformed chip geometry is developed through simulation. The thickness of the undeformed chips and their resultant cutting volume at different stages are further analyzed and calculated. Experiments are conducted by orbital drilling of TC4 titanium alloy to verify the characteristics of the undeformed chip and cap geometries, and to ascertain their relevance to the cutting forces as well as the hole quality. It is found that the main type of chip with “a tadpole shape,” considerable chipping, and two types of caps geometries appears at three machining stages of orbital drilling process. The corresponding cutting forces reflect an approximately linear increase, stable fluctuation, and a gradual decrease, which are nearly consistent with the characteristics of the simulated unformed chip geometries. Two types of cap geometries can be used to monitor tool wear and improve the burr situation at the hole exit.

This article introduces a methodology for extracting specific cutting force coefficients by performing micro drilling experiments with tool edge radius effect.Tool edge radius mainly affects the effective rake angle that varies according to undeformed chip thickness. Ploughing effect is also considered for undeformed chip thickness lower than the minimum chip thickness. In this work specific normal and frictional cutting coefficients for both ploughing and shearing are determined from mechanistic approach of fitting experimental specific thrust forces of the micro drilling process. The variations of these cutting coefficients with respect to cutting speedand feed are presented. Finally these coefficients have been applied to the mechanistic model to predict thrust force in micro drilling. The predicted thrust force values at different feed show good agreement with the experimental results.

Distribution of the maximum undeformed chip thickness can be approximated as the reproduction of grooves and protrusions of the grinding surface. It plays a very important role in grinding process as it has a close influence on the prediction and modeling of grinding forces, tool life, and surface quality, as well as process stability. In this study, it is investigated from the perspective of the real interference depth of the active abrasive grain to cater to the performance evaluation of the grinding surface. Firstly, image processing techniques combined with three dimensional topography tests are utilized to extract grains' essential characteristics such as the protrusion height, shape, distribution and density. Then, grains' wear is quantified by the probability assignment function and Dempster-Shafer evidence theory. Based on this, the actual interference depth of a single grain is determined. Through grain's kinematics analysis and considering the effect of contact deformation on the actual contact arc length and affective cutting edge density, the distribution of chip thickness in the current grinding area is defined and its distribution model is established. Model's correctness and rationality are verified by grinding experiments of the slider raceway. Results demonstrate the grain's interference depth highly depends on its protrusion height, wear amount and grinding depth which is a primary contributor to the size of undeformed chip thickness, and grains' density, contact deformations mainly affect its distribution. The quantified distribution model of the maximum undeformed chip thickness lays a foundation for the topography modeling and integrity research of grinding surfaces.

Hard gear finishing viewed as a process of abrasive wear

A numerical simulation technique has been developed to generate the grinding wheel topography using square pyramidal grits. The ground workpiece surface has also been generated simulating the trajectory of all the abrasive grits and removing the interfering material. The average workpiece surface roughness is calculated and the effects of different grinding parameters on the average surface roughness of the generated workpiece have been studied. Finally, the variation of surface roughness with the maximum uncut chip thickness has been studied.

The formation of surface texture in milling is a complex process affected by numerous factors. This paper focuses on the surface roughness of X37CrMoV51 steel machined by shoulder milling. The aim of the study was to develop a mathematical model to predict the surface roughness parameter Ra. The proposed model for predicting the surface roughness parameter Ra in shoulder milling takes into account the feed per tooth, fz, the corner radius, rε, and the actual number of inserts involved in the material removal process as well as hmin and D(ξ). The correlation coefficient between the theoretical and experimental data was high (0.96). The milling tests were carried out on a three-axis vertical milling machine using a square shoulder face mill. The geometric analysis of the face mill shows that at a feed rate of 0.04 mm/tooth, cutting was performed by three out of five inserts, and when the feed rate exceeded 0.12 mm/tooth, material was removed by all inserts. The minimum chip thickness parameter and the standard deviation of the relative displacement increased as the feed increased. Over the whole range of feeds per tooth, the displacement increased by 0.63 µm. Higher cutting speeds resulted in lower minimum chip thicknesses and the average standard deviation of the relative displacements for the whole range of cutting speeds was 2 μm.

Prediction of cutting forces during micro end milling considering chip thickness accumulation

This paper discusses the development of an enhanced, static model of chip formation in micromilling processes that is able to describe the intermittency of the chip formation observed at low feeds per tooth due to the dominance of the minimum chip thickness effect. Experimental analyses demonstrate the validity of the proposed model by verifying the level of periodicity in the cutting forces present at various feeds per tooth. A key finding of this study is the identification of a local maximum in the radial thrust forces in the micromilling process during the noncutting regime, at feeds per tooth that are of the order of the minimum chip thickness. To overcome the challenges in the direct measurement of the minimum chip thickness, this paper presents a method for estimating the minimum chip thickness of various combinations of tools and workpiece materials based on easily attainable cutting-force data. A discussion on the selection of process parameters to avoid intermittent chip formation is also presented.