MECH 311 Manufacturing Processes Section X

Transcription

1 MECH 311 Manufacturing Processes Section X Time: W _ F 13:15-14:30 Credits: 3.75 Session: Fall Introduction Lecture 1 Instructor: Sivakumar Narayanswamy Mech 311 Lecture 1 1

2 Objective of the course To provide the basic understanding of Measurements, Tolerancing and Design and understanding of different manufacturing methods ( Conventional and non-conventional). Feel for manufacturing: What machine at what situation? What is possible by existing manufacturing methods? Cost effective routes! Feel for numbers!

3 How to Ensure? what we produce is what we want. Inspection

4 Manufacturing Processes Measurement: Why measure? (QA) How to measure? Fundamental exercise of inspection; Act of measuring or being measured. Manufactured parts correspond to the specs of the product: Confirm functionality, performance, reliability, process capability, etc. Attributes ( Qualitative, go or not-go, Gaging, decisions) Variables (quantitative, dimensions, useful for analysis and decision) What to measure? Size/geometry of tools Benefits Size/geometry of a part from a machine tool Determines capability of a process. Indicates the need of maintenance. Feedback of manufacturing.

5 Attribute Vs Dimensional Qualitative Fast and economical Pass or fail Mostly for standard and less severe applications (Automobile) Useful after process development Large production volume Quantitative Slow and expensive Exact dimension is needed Useful for highly reliable applications (Aircrafts) Needed for development Small production volume

6 Standard Measurements:

7 Issues Units are different => Conversion scales Linear Standards Metric Systems ANSI System 1in ~ 41, wavelengths related to human body sizes Of orange-red light Kr86

8

9 Length Standards: Gage blocks or slip gages (Alloy steel with Rc65) Standard set of rectangular gage blocks with ± in. Wrung-together gage blocks in a special holder, used with a dial gage to form an accurate comparator. Screw gage blocks wrung together to build up a desired dimension.

10 Standard gages of meter exist in any workshop ( standard blocks); need to calibrate every specific period ( various precision). Calibrated at 20 C

11 Accuracy Vs. Precision Accuracy: ability to reach the aimed size Precision: repeatability of accuracy (Left):Accuracy versus precision. Dots in targets represent location of shots. Cross (X) represents the location of the average position of all shots.

12 Allowance and Tolerance: Allowance: intentional desired difference between the dimensions of 2 mating parts Tolerance: undesirable but permissible deviation from a desired dimension. Most manufacturing processes products with dimensions distributed normally ( clustering around the average) x n n x i i 1 n x i1 x i n 2

13 When mating parts are designed, each shaft must be smaller than each hole of a clearance fit. The manner in which the distributions of the two mating parts interact determines the fit. UNTL upper natural tolerance limit = 3 LNTL, lower natural tolerance limit = 3 Wear or lack of process control shifting

14 Clearance? Range of Fit? MMC? LMC? Bilateral? Unilateral? Limits

15 How to Specify Tolerances? ANSI 8 classes of fits Class 1. Loose fit: large allowance. Accuracy is not essential. Class 2. Free fit: Liberal allowance. For running fits where speeds are above 600 rpm and pressures are 600 psi ( 4.1 MPa) or above Class 3. Medium Fit: Medium allowance. For running fits below 600 rpm and pressure below 600 psi ( 4.1 MPa) and for sliding fits. Class 4. Snug Fit: Zero allowance. No movement under load is intended, and no shaking is wanted. This is the tightest fit that can be assembled by hand Class 5. Wringing fit: zero to negative allowance. Assemblies are selective and not interchangeable. Class 6. Tight fit: slight negative allowance. An interface fit for parts that must not come apart in service and are not to be disassembled or are to be disassembled only seldom. Light pressure is required for assembly. Not to be used to with stand other than very light loads. Class 7. Medium force fit: an interference fit requiring considerable pressure to assemble; ordinarily assembled by heating the external member or cooling the internal member to provide expansion or shrinkage. Used for fastening wheels, crank disks, and the like to shafting. The tightest fit that should be used on cast iron external members. Class8. Heavy force and Shrink fits: considerable negative allowance. Used for permanent shrink on steel members.

16 ISO System of Limits and Fits Clearance fits Transition fits/ Location fits/ Assembly fits Interference fits What is Hole based or Shaft based system? WHY? Shaft-basis and hole-basis system for specifying fits in the ISO system

17 Geometric Tolerances: Permitted tolerance on shape/geometry/form/position MMC Parts are made with the largest amount of material possible LMC - Parts are made with the least amount of material possible RFS Regardless of feature size Datums- Concept/Feature common for design, manufacturing and inspection (Left) Geometric tolerancing symbols; (Right Up) Feature control symbols for part drawings; (Right Down) Example of use of geometric tolerancing (tolerancing for flatness)

18 Inspection methods for measurement Metrology: measurement laboratory selected according to certain criteria: Gage capability ( rule of 10) Measuring device has to be 10times more precise than the tolerance measured: Eg. +/ / / Linearity Linear working range (Input Vs Output) Repeat accuracy Repeatability of the measurement Stability Retaining calibration over time, no-drift Magnification Amplification of the output portion of the device, bigger dials. Resolution Sensitivity; smallest input value that can be detected or measured

19 Measurement instruments ( linear) * Ruler (0.5mm) * Combination set * Vernier Caliper(0.01mm) * Micrometer caliper * Optical Comparators (0.001mm) * Laser/ interferometers(0.0001mm) Combination set.

20 Three styles of calipers in common use today Internal and external Digital Micrometer for measurements from 0 to 1in., in in. graduations.

21 Vision Systems of measurement Coordinate Measuring Machines (CMM) Optical Comparator, measuring the contour on a workpiece. Digital indicators with in/.mm conversions add to the utility of optical comparators. Coordinate measuring machine with inset showing probe and a part being measured.

22 Interference bands can be used to measure the size of objects to great accuracy Resolution ~ 10nm a a)(bottom left) Calibrating the X-axis linear table displacement of a vertical spindle milling machine; b) ( top right) Schematic of optical setup ; c) ( bottom right) Schematic of components of a two frequency laser interferometer.

23 The principle of the optical flat

24 Angle Measurement Sine Bar: 1sec of arc Setup to measure an angle on a part using a sine bar. The dial indicator is used to determine when the part surface X is parallel to the surface plate

25 Gages for Attributes ( mass production): (Top) Plain plug gage having go member on one end and not-go member on other other; (Bottom) Ring gage with stepped go and not-go member Go and not-go ring gages for checking a shaft.

26 100+/- 0.1 Shaft 100+/- 0.1 Hole Ring Gage GO Plug gage GO Ring gage No GO Plug gage NO GO

27 Surface Roughness Measurement Instruments: Schematic of surface profile as produced by stylus device showing some typical y values with respect to the center line. Schematic of stylus profile device for measuring surface roughness and surface profile with two readout devices shown: a meter for AA or rms values and a strip-chart recorder for surface profile. Top) Microtopographer, a stylus profile device used to measure and depict surface roughness and character (surface profile); Bottom) Typical surface-roughness profiles.

28 Above left) Terminology used in specifying and measuring surface quality; Above Middle) Symbols used on drawing by part designers, with definitions of symbols; Above Bottom) lay symbols; Right) lay symbols applied on drawings

29 Comparison of surface roughness produced by common production processes

30 Tolerance, Roughness and process are interconnected. Selection of one determines the other. Eg: +/-0.5mm, Ra=0.1um no sense +/- 0.01mm, Ra=25um no sense

31 Nondestructive Inspection and Testing: Ex: Strength of a part Destructive Quantitative results Do not require interpretation Restricted for costly measurements High cost equipment Consistent results Non-Destructive Qualitative results Skilled interpretation of results Low material cost Low cost equipment More subjective results

32 Visual Inspection the first inspection method Liquid (Dye)-penetrant inspection (LPI or DPI) To be done prior to surface finishing or surface modifying operations, like, shot peeing, polishing, etc.

33 Only for Ferromagnetic Materials (Fe, Ni, Co Alloys) Surface and sub surface flaws Orientation dependent, Perpendicular to the lines Parallel defects will not be detected Magnetic particle inspection (MPI)

34 Ultra-Sonic Inspection: Deep defects Left) Ultrasonic inspection of a flat plate with a single transducer; Right) Plot of sound intensity or transducer voltage versus time showing the initial pulse and echoes from the bottom surface and intervening defect. Left) Dual transducer ultrasonic inspection in the pulse-echo mode; Right) dual transducers in through-transmission configuration.

35 Radiography: Xray, gamma ray, nuetron beam. Full-size radiograph of the Liberty Bell. The photo reveals the famous crack, as well as the iron spider installed in 1915 to support the clapper and the steel beam and supports which were set into the yoke in 1929.

36 Eddy Current Testing Conductive specimen Relation of the magnetizing coil, magnetizing current, and induced eddy currents. This dynamic magnetic field induces the eddy currents and the changes in the eddy currents produce a secondary magnetic field which interacts with the sensor coil or probe Eddy currents are constrained to travel within the conductive material, but the magnitude and path of the currents will be affected by defects and changes in material properties.

37 Other non-destructive test methods Acoustic emission monitoring (Active Flaws) Leak testing (Bubble test, pressure drop test, advanced) Thermal testing (Composites, electronic devices) Strain sensing (brittle or photoelastic coating, strain gage) Advanced optical methods (hologram, hologramic interferometry) Resistivity methods Xray Computed Topography (CT scan) Acoustic holography (Ultrasonic Sound)

38 Process Capability and Quality Control

39 Example of process

40 Process Capability and Quality Control Any process exhibit a level of inherent variability inherent capability Process Capability : Ability of the process to yield consistently the aimed output. How to measure/quantify the Process Capability (PC)?

41 Process Capability and Quality Control A. Inspection * to find defects * to prevent defects B. Analysis * the nature of the process based on a sufficient amount of data

42 The basic principle of quality control quality improvement without cost increase

43 Steps to take: 1) Design an experiment ( machine, setting, speeds, cutting rates, feed, material etc.) 2) Define the inspection method 3) Define the sample size 4) Separate parameters from noise 5) Take measurements

44 Example: Step1: Select the sample size, n=5 Inspect the dimensions Estimate X, and (Sample mean, SD, range) Step2: R Do the step on k=14 samples. Estimate X, and R (grand average, SD, Range)

45 Process Capability Index, Cp Does the process meet the specifications? C p toleracnce spread USL LSL 6 ' 6 ' C p 1.33, acceptable Is the process centered (not shifted)? D estimated mean nom. 1 tolerance spread 2 X 1 2 x x nom ( USL LSL) 1 - Nominal 3 USL LSL For Acceptance 2 Measure of the capability? Capability index C pk min USL X, 3 ' LSL X

46 x, ', R Natural capability limit of the process x 3 ' x 3 ' What PC studies tell about the process.

47 ' c 2

48 Inspection and Quality Control (QC): How much one should inspect 100% selected items none Cost Ongoing manufacturing Destructive tests Too large production too expensive Statistical Process Control Histogram of a process to compare measured sizes with designed specifications Basic design of the X chart, R chart and chart used in SPC.

49 Six Sigma Model USL-LSL 12 x 6 ' 68.26% % % 7 out of 10, ppm

50 How a process can get out of control? 1. Wear 2. Inherent errors 3. Heating 4. Vibrations 5. Fault measurement/settings Solutions: Shift the job Relax the specs Sort the products Improve precision: switch the cutting tool switch the work holding device change the material overhaul the process find (eliminate cause of variability)

51 Taguchi Method: create an orthogonal array of experiments to determine the dominant inputs to variability of a process. The use of Taguchi methods can reduce the inherent process variability as shown in the upper figure; factors A, B, C, and D versus process variable V shown in lower figure

52 Quality Control Charts: Statistical quality control charts for mm ( in.)- diameter pins. Note the trend in the X curve before the tool change, indicating that the mean was shifting due to tool wear. UCL- upper control limit LCL-lower control limit x 3 x' A R x 2 x 3 x' A R x 2

53 Samples of Chart Patterns Examples of X control charts

54 Determining Causes for Problems Every one is an inspector making it right for the first time Cause-Effect Diagram Fishbone diagram used to determine the causes of quality problem

55 Assume that you are incharge of the process development to develop a process Aim: 16h11 dia / Nominal Dia= Average(15.89, 16)= Measured values Sample1 Sample2 Sample3 Sample4 Sample Sample Average X Sample Range R sigma,

56 X 4 X x A 2 R 3 d where n 5 d2 and A2 are given in Fig R 0.73* n R 4 R ' asc2 0.8 c2 0.8 for n4 UCLx X 3 x LCLx X 3 x UCL LCL R R D D 4 3 R R 2.28 * *

57 USL= LSL= ' 6 Cp LSL USL ) 2( ) 2( D LSL USL X Xnom ' 3, min X LSL X USL C pk

58 X Bar Variation UCLx LCLx Actual Mean Nominal LSL USL X Bar Sample # Is the process ACCEPTABLE?

59 R Variation LCLR Grand R Sample R LCLR R Sample #

60 Find Cp, D and Cpk