Controlled Depth Milling Research Articles

INSIGHTS ON ABRASIVE WATER JET MILLING OF BLIND POCKETS

Abrasive water jet (AWJ) machining is one of the advanced machining techniques used in the industries for processing materials that are extremely difficult to machine using conventional machining techniques. Based on the flexibility of AWJ, this process is currently employed for milling blind pockets over different materials. The most frequent method for making blind pockets in AWJ is the controlled depth milling mode. This approach was carried out with the raster tool paths. The quality of the blind pocket surface is influenced by different AWJ parameters such as water jet pressure, traverse speed, step-over distance, abrasive flow rate, and abrasive types. Among these, the traverse rate was found to be an influencing factor in most of the AWJ milling operations as it determines the nozzle speed followed by the energy density of the abrasive particle drops while striking across the target material surface, which resulted in a controlled depth of cut. This review paper highlights the performance of the AWJ pocket milling operations with various materials. From these results, it is reported that most of the AWJ milled surfaces were found to be of rough quality even though they were using different milling tool path strategies and parameter conditions. In addition, the milled pocket defects, namely uneven flatness, grit embedment, and undercut were observed. Besides, future research and directions have been addressed in which some of the novel concepts/approaches have been introduced including the scale effect examination in AWJ with the use of different nozzle, orifice, and abrasive sizes. This study will be more helpful to produce blind pockets with tight tolerances and a significant reduction in the process defects. The outcomes of this study will bring new innovations to the AWJ milling technique in order to make a significant footprint in the manufacturing industries for machining quality blind pockets over the target materials.

Investigation and optimization for depth of cut and surface roughness for control depth milling in Titanium Ti6AL4V with abrasive water jet cutting

Abrasive water jet (AWJ) cutting has been widely used in industries because of its precise cutting technique. However, the effectiveness of AWJ cutting is dependent on machine operating parameters and the material properties. In this research AWJ cutting was applied on Titanium Ti6AL4V Grade 5 material to investigate the effects on depth of cut (DoC) and roughness. Taguchi L27 experiments were conducted at three levels with operating parameters such as water pressure, transverse speed, abrasive mass flow rate, abrasive orifice size; a novel approach of ratio 3:1 was adopted with nozzle and orifice diameter to accommodate more parameters for investigation and to inspect the variation of these parameters in real time. Minitab 2017 software was used to simulate the influencing parameters with ANOVA. (GRA) Grey relational analysis linked with Taguchi technique represents a novel approach to optimization. It is a normalization estimation technique extended to elucidate the complex multi-performance characteristics. GRA is used for optimizing the process parameter which helped in determining the optimal parameters for roughness and depth of cut, ANOVA was used for analyzing the effect of independent variables on dependent variables, as per ANOVA the P values and the S/N ration rank indicated that water pressure was the most influencing parameter for surface roughness and transverse speed was the most influencing parameter for the depth of cut. The prediction of cutting depth through GRA optimization proved a valuable tool for the controlled depth milling (CDM) in Ti6Al4V material.

Erosion mechanisms during abrasive waterjet machining: Model microstructures and single particle experiments

The erosion mechanisms during abrasive waterjet (AWJ) machining have been examined for a variety of materials. However, no systematic study has considered the effect of the microstructure–property relationship on the erosion mechanisms in metals. In this work, the influence of microstructure and mechanical properties on the erosion mechanisms is investigated using AWJ controlled-depth milling and single particle impact experiments performed on nanocrystalline, microcrystalline and single crystal nickel samples. The resulting footprints and subsurface microstructure evolution were analysed using advanced characterization techniques. The erosion rate of the target metal is found to correlate positively with grain size and negatively with hardness but this correlation is nonlinear. The subsurface microstructure of the single crystal and microcrystalline are altered, while only the texture of the nanocrystalline nickel is modified. The grain refinement mechanism observed in microcrystalline and single crystal microstructure is elucidated by electron backscatter diffraction. It proceeds by the generation of dislocations under severe plastic deformation, which transforms into subgrains before forming new grains under further strain. Therefore, severe plastic deformation induced by AWJ machining leads to surface nanocrystallization and induces substantial subsurface work-hardening, as revealed by nanoindentation tests and confirmed by single particle impacts, with the consequence that the erosion rate decreases with decreasing grain size. This work clarifies the erosion mechanisms in pure metals and highlights the dynamic nature of AWJ machining as a result of the complex interplay between microstructure, mechanical properties and material removal mechanisms, providing new insights into AWJ controlled-depth milling technique.

Surface and machining induced damage characterization of abrasive water jet milled carbon/epoxy composite specimens and their impact on tensile behavior

Controlled depth milling of composites structures by abrasive water jet (AWJ) is a new area of machining being explored and knowledge on this is bare minimum. Hence it is essential to investigate surface quality and damage induced to ascertain their mechanical reliability. Here, the mechanism of material removal is manifested by erosive wear. In this study, carbon fiber reinforced plastic (CFRP) laminates are milled using AWJ process and surfaces generated by varying process parameters are characterized using roughness systems, X-ray tomography and scanning electron microscopy (SEM). SEM images reveal presence of damages in form of craters, ridges, broken fibers and embedded abrasive particles. Crater formation due to erosion phenomenon is affected by jet pressure. It is seen that the crater volume increases by around 500% when pressure varies from 80MPa to 140MPa. In the literature reviewed correlation between roughness of the machined surface and the mechanical behavior is ambiguous and remains an open problem. Hence, novel attempt has been made to analyze the influence of damage (crater volume) on tensile strength. Mechanical tests on specimens with varying surface texture and crater sizes reveals that tensile strength of machined specimens is more influenced by crater volume rather than surface roughness.

Stochastic simplified modelling of abrasive waterjet footprints.

Abrasive micro-waterjet processing is a non-conventional machining method that can be used to manufacture complex shapes in difficult-to-cut materials. Predicting the effect of the jet on the surface for a given set of machine parameters is a key element of controlling the process. However, the noise of the process is significant, making it difficult to design reliable jet-path strategies that produce good quality parts via controlled-depth milling. The process is highly unstable and has a strong random component that can affect the quality of the workpiece, especially in the case of controlled-depth milling. This study describes a method to predict the variability of the jet footprint for different jet feed speeds. A stochastic partial differential equation is used to describe the etched surface as the jet is moved over it, assuming that the erosion process can be divided into two main components: a deterministic part that corresponds to the average erosion of the jet and a stochastic part that accounts for the noise generated at different stages of the process. The model predicts the variability of the trench profiles to within less than 8%. These advances could enable abrasive micro-waterjet technology to be a suitable technology for controlled-depth milling.

Process models for controlled-depth abrasive waterjet milling of amorphous glasses

An abrasive waterjet milling technique has been developed to machine brittle amorphous glass in a controlled-depth milling mode, without using sacrificed masking plates made of hard materials. The main mechanisms associated with the milled channel formation process, i.e. the particle impact erosion, jet flow characteristics and the dynamics of nozzle motion, are analysed to arrive at the models for predicting the material removal rate and the geometry of the channels milled by this technique. It is found that these milling performance quantities are dominated by nine dimensionless variables representing the processing parameters and material properties that govern the major mechanisms involved in the milled channel formation. An experimental verification has been carried out and shows that the model predictions are in good agreement with the corresponding experimental data.

A Study on Deviations of the Jet with Traverse Speeds on Different Materials in Pocket Milling Using Abrasive Water Jet Machining Process

The current trend in abrasive water jet machining process is getting focused on milling applications using this technique. Abrasive water jet machining (AWJM) process is a well defined process for cutting or part separation. The present paper reports on the geometry obtained in controlled depth milling process of different materials. The dimensions considered in this paper are the pocket depth and the change in the kerf profile. Experimental observations are made relating the kerf profile with traverse speed and the mechanical properties of the work piece material. Tool paths for obtaining the pocket of size 9 mm x 20 mm are generated in raster mode and machined using AWJM on materials of varying hardness and at different traverse speeds. It is observed that there is a significant change in the geometry of the kerf profile and also the depth of the pocket with speed in conjuction with the material hardness.

Identification of the role of machinability and milling depth on machining time in controlled depth milling using abrasive water jet

Abrasive water jet machining (AWJM) is mainly used to through cut materials that are difficult to cut by conventional machining processes. This process may also be used for controlled depth milling (CDM) of materials. This work focuses on making blind pockets of controlled depths for a set of materials with AWJM. The materials used in the present work are AL 6061 alloy, AL 2024, brass 353, titanium (Ti6Al4V), AISI 304 (SS), and tool steel (M2 Rc 20). The effects of the milling depth and material characteristics on milling time are investigated. It is observed that machinability index and mechanical properties of the materials milled play important role in establishing milling time and surface roughness. It is found that traverse speed of AWJ process is lower for the materials with low machinability index and vice versa. Besides, the milling time increases non-linearly as the depth of milling increases due to loss of energy of jet and increase in standoff distance (SOD).

A Method to Reduce Milling Time for Ti-6Al-4V Alloy for Controlled Depth Milling Using Abrasive Water Jet Machining

This Abrasive Water Jet Machining (AWJM) process is usually used to through cut materials which are difficult to cut by conventional machining processes. This process may also be used for controlled depth milling (CDM) of materials. This work primarily focuses on controlling the abrasive flow rate to reduce the time for machining the component. Here, an experimental setup is made with a modified attachment for abrasive feed system to machine for Ti-6Al-4V alloy. The work also investigates the surface morphology, tolerance on depth of machining and surface waviness for the modified setup. With change in mass flow rate of abrasive, the traverse speed is altered and its effects on the machining time are studied. It is observed that traverse speed is an important parameter in the case of CDM for AWJM. It is also shown that surface waviness can be reduced as traverse speed is increased by using modified abrasive feeding system.

Effect of Abrasive Flow Rate in Milling with Abrasive Water Jet

This Abrasive Water Jet Machining (AWJM) process is usually used to cut the materials which are difficult to cut by conventional machining processes. In this work, controlled depth milling (CDM) is done using AWJM. This work primarily focuses on controlling the abrasive flow rate to reduce the time for machining the component. Here, an experimental setup is made with a modified attachment for abrasive feed system to machine stainless steel. The work also investigates the surface morphology, tolerance on depth of machining and surface waviness for the modified setup. With change in mass flow rate of abrasive, the traverse speed may also be altered and its effects on the machining time are controlled. This work also employs Non-destructive Testing (NDT) method i.e. ultrasonic flaw detector to find out internal defects and cracks in the milled material.

Challenges in using waterjet machining of NiTi shape memory alloys: An analysis of controlled-depth milling

The nickel–titanium shape memory alloys (NiTi SMAs) have a very high potential for a wide variety of applications thanks to their unique mechanical properties: shape memory effect and pseudoelasticity. However, they have been proved to be more challenging to cut than other advanced engineering materials because of their high ductility, crystal-oriented and stress-oriented mechanical properties. In stark contrast to the extensive work on the metallurgical/microstructural properties of the SMA, there is limited research regarding non-conventional machining of this group of special alloys.Waterjet technology is well-known for cutting advanced difficult-to-cut materials owing to its benefits of reduced mechanical and thermal damages to workpiece surfaces. This paper reports for the first time the use of waterjet technology to mill the functional shape memory alloys and thus to open new avenues for the utilisation of these alloys for advanced engineering applications (e.g. aerospace, medical fields). However, when it comes to NiTi SMAs (characterised by low temperature phase martensite and parent phase austenite), the insignificant waterjet temperatures become critical to the material behaviour as their crystal structures are sensitive to the variations in both temperature and mechanical compression. This makes the processing (particularly waterjet controlled-depth milling) a real challenging task.By taking into consideration both of the waterjet temperatures at different material removal conditions (i.e. with and without abrasives in the focussing tube) and the transformation temperatures of NiTi, three different working zones (100% martensite; mix of austenite and martensite; 100% austenite) under waterjet process have been proposed. In addition, a combined phase and stress–strain diagram for shape memory effect in martensitic phase and pseudoelasticity in austenite phase of NiTi has been suggested. In this paper, Ni49.8Ti50.2 shape memory alloy was considered in which its transition temperature range is overlapped with the waterjet operating temperature; two approaches of waterjet processes (plain and abrasive waterjet milling) were proposed so as to investigate the mechanical and metallurgical effect provoked by the relationship between operating temperatures and transformation temperatures. It was found that abrasive waterjetting is more viable than plain waterjetting for controlled-depth milling of NiTi shape memory alloys.

AWJ Milling of Gamma Titanium Aluminide

A study was undertaken to determine the feasibility of the AWJ process for controlled depth milling of gamma Titanium Aluminide tiles. It was demonstrated that milling can be accomplished to 0.025-mm accuracy. To overcome undercutting near rib roots, the jet was clock-angled at about 15 deg to the vertical every set of passes. This allowed the milling to thin skins of about 0.5-mm. It was observed that as the material is milled, stresses were relieved, and either deformation or cracking may result. Accordingly, parts need to be annealed before milling. The milling to thin skins was successfully demonstrated on 150×300 mm2 parts without adverse effects. Also, the process of milling of the dual rib height was developed using the dual mask approach. Abrasive particle embedding on the milled surfaces was observed to be about 0.15% of the area, but cleaning with plain waterjets showed that all embedded particles can be removed. A detailed economic analysis confirmed that the AWJ milling process is relatively inexpensive and highly productive. The complete cost of milling including mask cutting, overhead, capital, and running cost is less than 300 USD/ft2.

Geometrical modelling of abrasive waterjet footprints: A study for 90° jet impact angle

Modelling of abrasive waterjet footprints is of critical importance when aiming to generate controlled freeform surfaces. The paper reports on a geometrical model of the jet footprint (kerf) in maskless controlled-depth milling applications. The model firstly needs to find the material specific erosion (etching) rate that is obtained from the jet footprint by taking the limiting conditions (high jet feed rates) of the model. Once this is found, the jet footprint can be predicted accurately for any jet feed speed. An example of model validation is presented for 90° jet impingement angle against a SiC ceramic as target material.

The effect of particle hardness and shape when abrasive water jet milling titanium alloy Ti6Al4V

Legislative restriction on effluent disposal has resulted in an increase in the environmental costs of chemical milling and replacement methods are being sought. Abrasive water jet cutting (AWJ) is a mature process that is employed to through cut materials that are difficult to process by more conventional methods and the process is also being developed for controlled depth milling (CDM) to produce three-dimensional features which in the past might have been produced by chemical or etching processes. A major problem to be solved when using AWJ as a CDM technique is that of tolerance on depth, surface waviness and surface roughness of the milled area. In the current work, the effects of milling parameters on the surface characteristics are investigated when milling a titanium alloy (Ti6Al4V) with different abrasives, namely white and brown aluminium oxide, garnet, glass beads and steel shot. It has been demonstrated that the ratio between the hardness of the workpiece and the abrasive is more important than particle shape. Material removal rate and surface roughness increased when particle hardness is increased. Shape factor and particle hardness have no significant effect on surface waviness. For the abrasives investigated; traverse speed is shown to govern the operative mechanism of material removal and thus the material removal rate. It is also shown that the surface waviness can be reduced as the traverse speed is increased whilst, the surface roughness is not strongly dependent on traverse speed.

Abrasive water-jet controlled depth milling of Ti6Al4V alloy – an investigation of the role of jet–workpiece traverse speed and abrasive grit size on the characteristics of the milled material

Due to difficulties in the use of traditional mechanical methods to mill of difficult-to-machine materials (especially in thin section), methods such as chemical milling or etching have commonly been employed. However, increasing legislative restriction on disposal of effluent has resulted in an increase in the environmental costs of such processes, and has prompted examination of alternative processes for milling to a controlled depth. Abrasive water-jet (AWJ) technology is used in a routine manner in manufacturing industry to cut materials that are difficult to cut by other methods. A similar process, where AWJ is used for controlled depth milling (CDM) is thus considered here. The main problem to be solved in the use of AWJ as a CDM technique is that of tolerance on depth, surface waviness and surface roughness of the milled area. In the current work, the effects of jet–workpiece traverse speed, number of passes of the jet and abrasive grit size on the material removal rate, surface waviness and surface roughness are investigated. Traverse speed is shown to govern the operative mechanism of material removal and thus the material removal rate. It is also shown that the surface waviness can be reduced as the traverse speed is increased, but it should be noted that waviness increases with number of passes of the jet over the workpiece. The surface roughness is not strongly dependent on traverse speed. Smaller sized grit leads to a reduction in material removal rate but also to a decrease in both waviness and roughness.

A technical note on grit embedment following abrasive water-jet milling of a titanium alloy

Abrasive water-jet (AWJ) technology is routinely used to cut materials that are difficult to cut by other methods. However, whilst the technology for through-cutting of materials is mature, the process is also being developed for controlled depth milling (CDM) of materials since other processes such as chemi-milling are under increased pressure due to legislative restrictions and costs associated with effluent disposal. Grit embedment and surface morphology is considered to be an issue for the service life of components manufactured by AWJ-CDM, especially in environments where fatigue failure is a concern. In the current work, the effects of milling parameters (namely jet-workpiece traverse speed, jet impingement angle, milling direction and grit size) on the surface morphology and degree of grit embedment are investigated for the milling of a titanium alloy (Ti6Al4V) with garnet grit. Up to 40% of the area of a milled surface has been shown to consist of embedded grit. It has been demonstrated that grit embedment can be minimized either by milling with a high jet traverse speed at low impingement angles or by low speed milling at jet impingement angles up to 45° in the backward direction only. However, even in the best cases, 5% of the area of a milled surface was composed of embedded grit. The levels of grit embedment and development of surface morphology have been observed to depend upon complex interactions of the various processing parameters, and a rationale for the observed behaviour has been proposed.

Controlled-Depth Milling of Isogrid Structures With AWJs

An experimental investigation was conducted to determine the feasibility of using abrasive-waterjets (AWJs) for controlled-depth milling of aluminum and titanium isogrid parts used in aerospace and aircraft structures. The effects of a wide range of milling parameters were studied. It was found that the traverse rate is the most critical parameter that affects the uniformity of the milled surfaces. Traverse rates on the order of several meters per second were found necessary to control the milling depth accuracy to within 0.025 mm. The problem of dynamically controlling the AWJ process parameters was negated by using a maskant material over the workpiece. The mask was AWJ-cut with the required pattern to be milled. The general topography of the milled surface was found to be more dependent on process kinematic parameters than AWJ process parameters, while surface morphology was found to be highly dependent on abrasive parameters. Milling to thin skins of 0.5 mm was demonstrated.