Machining Tolerances Research Articles

Design of a micro machine tool with double-toggle mechanisms

Micro machining plays an important role in manufacturing the miniature products which are made of various materials with complex 3D shapes and tight machining tolerance. In this paper, a new design of CNC micro machine tool with double-toggle mechanism is proposed. The machine has the characteristics of low cost, ultra-fine feed resolution, and high accuracy. With the double-toggle mechanism and regular driving components including AC motors, ballscrews, and a PC-based controller, the micro machine tool can perform good micro machining for miniature components. Because of the geometrical drive function, the errors of the machine are reduced by the double toggle structure without using high-end driving components and controller. In addition, the design of the structure avoids the drawback of the regular toggle mechanism which cannot have stable feed resolution for the full travel range. The kinematics model and characteristics of the machine were derived and analyzed. Modal analysis and dynamic compliance analysis were also conducted to evaluate stiffness and dynamic characteristics of the machine. A prototype of the machine was built, and the accuracy calibration results showed the machine can reach a positioning accuracy of 470nm.

A Novel Method for Measuring Polyethylene Hip Liner Wear Using a Coordinate Measuring Machine and a Dual Stylus Probe

Ex vivo high-resolution measurement of highly crosslinked (HXL) polyethylene hip liner wear is necessary to characterize the in vivo performance of these polymers that exhibit increased wear resistance. Current studies focus on using a coordinate measuring machine (CMM) to acquire data representing the bearing surface(s) of HXL hip liners and use this data to determine linear and volumetric wear. However, these current techniques are subject to error in both data acquisition and data analysis. The purpose of this study was to identify these sources of error and present a novel method for HXL wear measurement that minimizes these contributions to error: our novel methods use a CMM to measure both the articular and backside surfaces of HXL hip liners for subsequent data analysis in Geomagic Control and matlab. Our method involves a vertical orientation of the hip liner to enable one CMM scan of both sides of the hip liner. This method minimizes identified sources of error and proves to be an effective approach for data acquisition of HXL hip liner wear. We also find that our data analysis technique of calculating changes in wall thicknesses is effective in accounting for errors associated with data analysis. Validation of this technique occurred via measurement of two never-implanted HXL hip liners of different sizes (28 mm and 32 mm). In comparing the 32 mm hip liner to its corresponding computer-aided design (CAD) model, we found that our data acquisition technique led to a 0.0019 mm discrepancy between the scanned liner and its CAD model in measured thickness at the pole. We calculated 0.0588 mm and 0.0800 of linear wear for the 28 mm and 32 mm hip liners, respectively, based on our data analysis algorithm. We hypothesize that these reported linear wear values of the never-implanted hip liners are due to machining tolerances of the hip liners themselves.

STEP-NC machine tool data model and its applications

The machine tool data model of STEP-NC (ISO 14649) was conceived as a necessary extension to the original STEP-NC set of standards to make efficient control possible. The intention of this paper is to describe the background to the data model as well as related research work building on a higher level of information than can currently be found in the control information. The development of STEP-NC controllers promises improved manufacturing and resource use. However, even with legacy controllers there are advantages in using STEP-NC as an intermediate representation. This paper describes how the data model for describing machine capability was developed and what can be delivered by using this standard data model for machine tool. A machine tool selection algorithm is developed in order to validate the data model. Technical issues were derived from developing a system for process planning based on STEP-NC. Machine tools are selected automatically by the comparison with machine capability, work space and tolerance with the proposed data model. This function can contribute reconfigurable manufacturing systems and distributed and multi-controller-based manufacturing environment.

Design of a Passive Flow Regulator Using a Genetic Algorithm

Passive flow regulators are usually intended to deliver or drain a fluid at a constant rate independently from pressure variations. Microfluidic devices made of a stack of two plates are considered here: the first plate comprises a flexible silicon membrane having through holes while the second plate is a rigid substrate with a cavity, an outlet hole and pillars aligned with the through holes of the membrane. The liquid flows through the holes etched in the membrane and through the small gap between the bottom of the membrane and the pillars: each hole can therefore be considered as a valve which progressively closes as the pressure increases, thus leading to a non-linear fluidic behaviour. FEM simulations have been performed to ensure a constant flow rate in the specified range of pressure. To make the design reliable, the device characteristics have been optimized using an evolutionary algorithm. The fitness function notably takes into account machining and alignment tolerances. Typical designs dedicated to drug delivery and hydrocephalus treatment are discussed.

A novel approach to determination of wheel position and orientation for five-axis CNC flute grinding of end mills

In practice, the flutes of end mills are ground using CNC grinding machines via controlling the grinding wheel’s position and orientation to guarantee the designed flute parameters including rake angle, flute angle, helix angle, and core radius. However, for the previous researches, the designed flute profile was ground via building a specific grinding wheel with a free-form profile using two-axis CNC grinder. And, the free-form grinding wheel will greatly increase the manufacturing cost, which is too complicated to implement in practice. In this research, the flute-grinding processes were developed with a standard grinding wheel via five-axis CNC grinding operations. The mathematical representation of machined flute parameters was deduced in terms of the grinding wheel’s position and orientation. The geometrical constraints to avoid interference and abnormal flute profile for the five-axis CNC fluting were first developed in this work. Finally, the difference between the designed flute parameters and the machined flute parameters were formulated as a constrained optimization problem so as to determine the wheel’s position and orientation. The set of effective initial points for this optimization model was found mainly distributed the first quadrant of the contact area. The fminsearch function in Matlab toolbox was recommended to solve the optimization model due to its capability of handling discontinuity problem. The solution obtained in optimization model and the corresponding machined flute parameter were verified and compared with Boolean simulation in CATIA to confirm the validity and efficiency of the proposed approach. The results showed that the accuracy of machined flute parameters could achieve 1e-3 mm and 1e-2°, which satisfied the machining tolerance. This study provides a general solution for the CNC fluting operations and could be extended to grind complex surface of end mills in the future study.

트라이볼로지 손상을 억제하기 위한 구조물 모서리부 설계: 제2부 - 설계인자 분석 및 예

values obtained from the examples are very small, thedimensions of the corner radius normally designed in engineering structures are regarded acceptable. The vonMises stress evaluated for a typical example, which is far below the yield strength, confirms this interpretation.Nevertheless, the present work can provide a design criterion as well as a guideline for quality control in the man-ufacturing of, in particular, contact corners, which has not been attempted before to the best of the author’sknowledge. In addition, this paper considers the problem of a step that may be formed in the contact contour byusing a similar approach. The result shows that no size of the step is permissible.Keywords − structure corner(구조물 모서리부), corner radius(모서리 반지름), contact mechanics(접촉역학),contact length(접촉 길이), step(단차), machining tolerance(가공 공차)

Influence of artificial tip perturbation on asymmetric vortices flow over a chined fuselage

An experimental study was conducted with the aim of understanding behavior of asymmetric vortices flow over a chined fuselage. The tests were carried out in a wind tunnel at Reynolds number of 1.87×105 under the conditions of high angles of attack and zero angle of sideslip. The results show that leeward vortices flow becomes asymmetric vortices flow when angle of attack increases over 20°. The asymmetric vortices flow is asymmetry of two forebody vortices owing to the increase of angle of attack but not asymmetry of vortex breakdown which appears when angle of attack is above 35°. Asymmetric vortices flow is sensitive to tip perturbation and is non-deterministic due to randomly distributed natural minute geometrical irregularities on the nose tip within machining tolerance. Deterministic asymmetric vortices flow can be obtained by attaching artificial tip perturbation which can trigger asymmetric vortices flow and decide asymmetric vortices flow pattern. Triggered by artificial tip perturbation, the vortex on the same side with perturbation is in a higher position, and the other vortex on the opposite side is in a lower position. Vortex suction on the lower vortex side is larger, which corresponds to a side force pointing to the lower vortex side.

Analysis of Using Ferrofluid as an Interface Material in a Field Reversible Thermal Connector

The electrical functionality of an avionics chassis is limited due to heat dissipation limits. The limits arise due to the fact that components in an avionic computer boxes are packed very compactly, with the components mounted onto plug-in cards, and the harsh environment experienced by the chassis limits how heat can be dissipated from the cards. Convective and radiative heat transfer to the ambient are generally not possible. Therefore, it is necessary to have heat transferred from the components conducted to the edge of the plug-in cards. The heat then needs to conduct from the card edge to a cold block that not only holds the card in place but also removes the generated heat by some heat transfer fluid that is circulated through the cold block. The interface between the plug-in card and the cold block typically has a high thermal resistance since it is necessary for the card to have the capability to be reworkable, meaning that the card can be removed and then returned to the chassis. Reducing the thermal resistance of the interface is the objective of the current study and the topic of this thesis. The current design uses a pressure interface between the card and cold block. The contact pressure is increased through the addition of a wedgelock, which is a field-reversible mechanical connector. To use a wedgelock, the cold block has channels milled on the surface with widths that are larger than the thickness of the plug-in card and the unexpanded wedgelock. The card edge is placed in the channel and placed against one of the channel walls. A wedgelock is then placed between the card and the other channel wall. The wedgelock is then expanded by using either a screw or a lever. As the wedgelock expands, it fills in the remaining channel gap and bears against the other face of the plug-in card. The majority of heat generated by the components on the plug-in card is forced to conduct from the card into the wall of the cold block, effectively a single sided, dry conduction heat transfer path. Having started as a student design competition named RevCon Challenge, work was performed to evaluate the use of new field-reversible thermal connectors. The new design proposed by the University of Missouri utilized oil based iron nanoparticles, commonly known as a ferrofluid, as a thermal interface material. By using a liquid type of interface material, the channel gap can be reduced to a few micrometers, within machining tolerances, and heat can be dissipated off both sides of the card. The addition of nanoparticles improves the effective thermal conductivity of base fluid. The use of iron nanoparticles allows magnets to be used to hold the fluid in place, so the electronic cards may be easily inserted and removed while keeping the ferrofluid in the cold block channel. The ferrofluid-based design which was investigated has shown lower thermal resistance than the current wedgelock design. These results open the door for further development of electronic cards by using higher heat emitting components without compromising the simplicity of attaching/detaching cards from cooling plates.

The sensitivity of ArcCHECK-based gamma analysis to manufactured errors in helical tomotherapy radiation delivery.

Three‐dimensional measurement arrays are an efficient means of acquiring a distribution of data for patient plan delivery QA. However, the tie between plan integrity and traditional gamma‐based analysis of these data are not clear. This study explores the sensitivity of such analysis by creating errors in Helical Tomotherapy delivery and measuring the passing rates with an ArcCHECK cylindrical diode array. Errors were introduced in each of the couch speed, leaf open time, and gantry starting position in increasing magnitude while the resulting gamma passing rates were tabulated. The error size required to degrade the gamma passing rate to 90% or below was on average a 3% change in couch speed, 5° in gantry synchronization, or a 5 ms in leaf closing speed for a 3%/3 mm Van Dyk gamma analysis. This varied with plan type, with prostate plans exhibiting less sensitivity than head and neck plans and with gamma analysis criteria, but in all cases the error magnitudes were large compared to actual machine tolerances. These findings suggest that the sensitivity of ArcCHECK‐based gamma analysis to single‐mode errors in tomotherapy plans is dependent upon plan and analysis type and at traditional passing thresholds unable to detect small defects in the plan.PACS number: 87.55.Qr

Tolerance measurements on internal- and external-hexagon implants.

To measure the horizontal machining tolerances of the interface between internal- and external-hexagon implants and analogs with corresponding components after delivery from the manufacturer. These values may be a valuable tool for evaluating increasing misfit caused by fabrication, processing, and wear. Seven implants and seven analogs with external- and internal-hexagon connections (Biomet 3i) with corresponding prefabricated gold cylinders and gold screws, prefabricated cylindric plastic cylinders, and laboratory screws were studied. One set of components from the external and internal groups was measured manually and digitally. Measurements from the test subjects were compared with identical measurements from the virtual model to obtain threshold values. The virtual model was then used to obtain optimally oriented cuts. The horizontal machining tolerances for castable plastic abutments on external implants were 12 ± 89 μm, and for internal implants they were 86 ± 47 μm. Tolerance measurements on prefabricated gold abutments for external implants were 44 ± 9 μm, and for internal implants they were 58 ± 28 μm. The groups with metallic components showed the smallest tolerance at < 50 μm for the external group and < 90 μm for the internal group. The prefabricated plastic cylinder groups ranged from < 100 μm for external and < 130 μm for internal connection.

Using tolerance maps to validate machining tolerances for transfer of cylindrical datum in manufacturing process

Process planers typically utilize different datum features than designers use when specifying tolerances. Datum features used in process plans are chosen to simplify setups and achieve desired geometric accuracy. Meanwhile, proper machining tolerances are required to be reassigned to satisfy design requirements. However, existing methods to transfer geometric tolerances directly and accurately are still missing due to incompatible mathematical models of tolerances. Also, the affection of material conditions on datum and partial constraint situations have not been deeply considered yet. Since cylindrical features are often used as datum features, this paper describes the use of tolerance maps (T-Maps) (patent no. 6963824) and manufacturing maps (M-maps) to establish analytical relationship among all relevant design and machining tolerances for transfer of cylindrical datum. Firstly, a parametric model of datum transfer is proposed to describe factors involving the process. Next, based on spatial and geometric parameters, as well as tolerances information, variation analysis among features is implemented to formulate transformed T-Maps, sum of which constructs M-Map. Then, distinct bounding boxes of cross-sections in M-Map are extracted through computing vertex coordinates of their boundaries due to complete and partial constraint scenarios. Thereafter, by virtue of bounding boxes, relationship among design and machining tolerances are obtained through fitting M-Maps into T-Maps. Finally, an example is introduced to verify feasibility of the proposed model and method.

Design methodology for an air-lubricated seal

A methodology to design an air-lubricated piston seal for pneumatic actuators is described. Such a seal permits a small air leakage through a very narrow gap between the seal lip and the counterpart; this air film both avoids contact and ensures lubrication at the seal-counterpart interface thus permitting reduced friction and wear. Subsequently the use of grease could be limited or avoided. A seal geometry equipped with a self-pressurized balancing chamber was designed to make leakage as insensitive as possible to the supply pressure and to the barrel machining tolerances. A prototype was manufactured and tests were carried out.

Design and fast verification of pocket elliptical journal bearings

Purpose – The purpose of this paper was to conceive a fast method to verify design and performance of elliptical pocket journal bearings. Design/methodology/approach – The computer-aided verification of pocket journal bearings is performed by means of a suitable finite element analysis method. Findings – The results of sample analyses indicate that the machining tolerances are very influential on elliptical pocket bearing performances, and they must be included among the input data. Originality/value – Although elliptical pocket bearings are widely used in turbomachineries, the influence of their design on performance has not been specifically investigated. A lot of works about tribological models are available, but few of them focus on their application to bearing design at the industrial level.

Influence of surface errors on the performance of EFPI based on GRIN lenses

The graded index (GRIN) lens reduces the divergence angle of the beam exiting from the lead-in optical fiber, which results in the increasing of F–P cavity mirror areas. Surface errors of the increased mirrors cannot be ignored due to the machining tolerance. The influence of surface errors on the performance of the fiber extrinsic Fabry–Perot interferometer (EFPI) is researched. Theoretical analysis and numerical simulation results demonstrate that surface errors will diminish the interference contrast. The larger the surface error, the smaller the interference contrast will become. The results will contribute to the design of an EFPI with a certain interference contrast.

Development and Evaluation of a Pulse Driven-type Electromagnetic Control Expansion Valve applied to a Small Scale Refrigerant Cycle (Part I: Theoretical estimation of pressure drop)

The application of a new pulse driven-type electromagnetic control expansion valve to an R744 cycle is proposed and the characteristics of the valve are theoretically evaluated and experimentally inspected in this paper. In this theoretical estimation, the structure of the valve is simulated using elements such as ducts, a pin hole, a groove and an orifice. The pressure drops caused by each element that make up the valve are estimated individually. The V-shaped groove and its entrance and exit interfaces create the majority of the overall resultant pressure drop. Especially, the influence of the machining tolerance is also estimated because the elements that make up the valve are very fine. Then, the validity of this estimation is verified by an experiment. The experiment is conducted by constructing a refrigerator cycle. The theoretically estimated pressure drop agrees with the experimental results even through the two-phase refrigerant fluid flow field. As a result, it was shown that the electromagnetic control expansion valve proposed in this paper can control the pressure drop in a way suitable for satisfying cooling demand. Also, the theoretical estimation proposed in this paper may be able to be applied for the estimation of the pressure drop of the expansion valve. Keywords: Electric expansion valve, Electromagnetic control, Refrigerator, Pulse drive, CO2(R744)

Generalized kinematics of five-axis serial machines with non-singular tool path generation

Typical five-axis machine tools have three translational and two rotary drives. This paper presents a generalized kinematics model that allows automatic configurations of all five-axis machine tools using screw theory. First, each kinematic element is modelled as a revolute joint, prismatic joint, workpiece or cutting tool. The kinematic elements are mathematically assembled through screw theory by using the base coordinate system. The general inverse kinematics solutions for both rotary and translational motions are evaluated. The singularities in five axis contouring are avoided by deforming splined tool orientation vectors. The tool orientation vectors are represented by a fifth degree B-spline curve in a quaternion space, while the movement of tool tip is represented by a fifth degree B-spline curve in the Cartesian space. Both splines, which form the C3 continuous tool path, are fitted to curve length parameter of the tool tip positions. If the tool path traverses the singular area of the machine, it is deformed by modifying the control points of the tool orientation spline in the quaternion space while respecting the machining tolerance. The proposed kinematics module, tool path generation algorithm and method for avoiding kinematic singularities are experimentally verified on a five-axis machine tool controlled by an in-house developed CNC system.

Flash Flatness Measurement System Using Digital Dial Gauges

Batch or mass production of workpiece has raised new challenges for high-efficiency measurement, especially the measurement of flatness, which is a key parameter of machining tolerance. This paper presents a novel flatness test system utilizing array of digital micrometer gauges. The software is developed based on .NET. Experiments on oil coolers are carried out with 20 sampling points, comparison with coordinate measuring machines and the steadiness of both the single gauge and the system is presented, and the results show that the system achieves high measuring speed and high consistency.

TH‐C‐137‐06: An Inter‐Fractional Morphing Aperture Based Reoptimization Tool for Adaptive Radiotherapy

Purpose: To dosimetrically evaluate a streamlined adaptive radiotherapy tool to account for inter‐fractional tumor shrinkage. A novel aperture‐morphed simultaneously optimized finite sized pencil beam algorithm (AMSO‐FSPB) was investigated over volumetric modulated arc therapy (VMAT) head and neck plans. Methods: AMSO‐FSPB plans were retrospectively designed for head and neck patients in the supine position. CT image sets between fractions were rigidly registered using normalized cross correlation. A demons based deformable image registration algorithm was used to obtain a deformation vector field (DVF) between fractions. The DVFs were used to map contours between fractions. An eclipse and pinnacle compatible contour mapping algorithm was developed to apply the DVFs to the structure dicom set. DRRs at each gantry angle were used to derive the ratio of tumor shrinkage at every MLC position. The DVF and the target contour shrinkage were used to obtain a new set of fluence maps. The machine tolerance guidelines for the Varian True beam were followed for this study. A graphics processor united (GPU) based FSPB dose calculation engine with an embedded weighting factor re‐optimization algorithm was developed to reassign beamlet weighting factors to every 4 degrees of arch length in the VMAT plan. Results: The dosimetric advantages of the AMSO‐FSPB VMAT plans were directly proportional to the amount of the tumor regression between fractions. AMSO‐FSPB VMAT plans were dosimetrically superior to the original plans for all cases. The ratio of the mean and maximum dose for the Paratoids, Spinal Cord, and Brain Stem were lowered for all cases. The mean for the PTV70Gy, PTV60Gy, and PTV54Gy was increased while the standard deviation was decreased for all cases. Conclusion: Implementation of the AMSO‐FSPB algorithm improved the dosimetric quality of the supine head and neck VMAT plans and was done so with minimal additional cost to clinical resources. This research is supported by CPRIT Individual Investigator Award RP110329.

Three dimensional micromachining on aluminum surface by electrochemical wet stamping technique

We proposed an up–down working mode of electrochemical wet stamping technique (EC-WETS) for three dimensional (3D) micromachining on aluminum (Al) surface. 3D microstructures on a Si mold were transferred on to an agarose hydrogel containing 15% NaNO3, 2% MgF2, 1% NaOH and 5% glycerin, which acted as the quasi-solid electrolyte for the electrochemical micromachining. The transferred 3D microstructures on agarose hydrogel were then duplicated onto Al surface through anodic dissolution. The micromachining quality was improved by pulse-potential method dramatically with a machining tolerance lower than 200nm and an average removal rate of 210nmmin−1 in the Z direction. This method was proved to be a highly efficient, low cost and green method for 3D micromachining on active metal surface, which would be valuable for the manufacture of microelectromechanical system (MEMS).

Note: A compact, rigid, and easy-to-build piezo motor: The intact-tube GeckoDrive

We report an extremely simple, rigid, low machine tolerance, yet high performance piezoelectric motor, in which two rings are coaxially glued at the ends of one intact piezotube, respectively, using the proper gluing method. A central shaft is pushed to press against the inner edges of the rings by a spring strip at a proper axial position and in the gap between the shaft and the inner wall of the piezotube. It is compared with other important forms of three-friction driven motors and shows advantageous structure and unexpectedly excellent performance, hence deserving a new name: the GeckoDrive.