CMMs and GD&T - HN Metrology IDW 2000 Presentation.pdf · CMMs and GD&T Dr. Henrik S. Nielsen HN...
Transcript of CMMs and GD&T - HN Metrology IDW 2000 Presentation.pdf · CMMs and GD&T Dr. Henrik S. Nielsen HN...
![Page 1: CMMs and GD&T - HN Metrology IDW 2000 Presentation.pdf · CMMs and GD&T Dr. Henrik S. Nielsen HN Metrology Consulting, Inc. 10219 Coral Reef Way, Indianapolis, IN 46256 Phone: (317)](https://reader031.fdocuments.us/reader031/viewer/2022030400/5a73c60e7f8b9ad22a8b8736/html5/thumbnails/1.jpg)
CMMs and GD&T
Dr. Henrik S. NielsenHN Metrology Consulting, Inc.
10219 Coral Reef Way, Indianapolis, IN 46256Phone: (317) 849 9577 Fax: (317) 849 9578
E-mail: [email protected]: http://www.HN-Metrology.com
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The History of Tolerancing (abbreviated)
• Initially there was linear +/- tolerancing– did not capture geometric relationships– did not assure assemblability
• Then came Geometrical Dimensioning and Tolerancing– Standardized in ANSI/ASME Y14.5
and ISO 1101– captures geometric relationships– ensures assemblability
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The Scope of GD&T
• Claims to cover part function– But the only function covered is assembly
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Philosophies of GD&T
• Only concernedwith Pass or Fail– An actual value is
only defined forsimple call-outs• such as form deviation
– For features referenced to datums all features must be considered simultaneously to determine pass or fail• Actual value is not defined and non-existent
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What GD&T Does Well
• Tolerancing for assembly– Properly used GD&T can ensure assemblability
• Interference avoidance– GD&T controls the extreme points in features
• Enable tolerance stackup calculations– By capturing geometrical relationships
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What GD&T does not do well
• Sealing interfaces• Interference fits• Adaptable (non-stiff) parts• Hydrodynamic interfaces• Roller bearings
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Sealing Interfaces
• Fuel Injectors– Ensure sealing– Avoid scuffing
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Interference Fits
• Controlling the location of a pin pressed in a hole
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Adaptable (non-stiff) Parts
• Piston rings in liners– Ring can adapt to long
undulations in liner• but not to short
undulations
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Hydrodynamic Interfaces
• Sliding bearings– Contact is avoided by:
• Oil film• Elastic deformation
– Some form errorscan be toleratedothers cannot
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Roller bearings
• Lobing relative to number of rollers– Vibration– Noise– Longevity
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So What About CMMs?
• CMMs (or other discrete point measuring machines) can characterize all these examples– By mapping the surface– By breaking the surface down into various
wavelength components– By finding average substitute features
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The Cost of Tolerances
• Cheap:– Stationary, non-
interference fit
• Expensive:– Bearing
• Roller• Sliding
– Seal– Piston/Liner
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CMMs and GD&T
• CMMs can only measure discrete points– usually the operator only takes very few points
• e.g. 5-10 points to describe a hole
• CMMs usually use least squares algorithms to find substitute features– these are used to represent the real features
• This is not in accordance with the definitions given in GD&T standards
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So Why Do We Use CMMs?
• GD&T is only aimed at the pass/fail decision• Measurement data is needed for several
other purposes– Calibration– Tolerance development– Process control– Process capability
• CMMs can provide variable data for these purposes
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What do CMMs do well?
• CMMs work best, when several points are averaged in a result, such as:– Position
• e.g. of the center of a hole
– Distance• e.g. between hole centers or from a hole center to a
plane
– Orientation• e.g. angle between two planes
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What are CMMs not good at?
• CMMs do not work very well for measurements, where the result is dependent on a single point, such as:– Form measurement
• e.g. roundness of a hole
– Finding datums based on extreme points
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Process Control(Feed Back Loop)
• What information is needed?– Position(s)– Distance(s)– Actual values– Individual features
• What is good control data?– Based on stable algorithms
• not sensitive to single point deviations
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Process Control Data
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Conclusions I
• CMMs are very useful tools– Even though they do not measure in accordance
with GD&T• Often the difference can be expressed as an uncertainty
– CMMs can provide useful information for• Calibration• Tolerance development• Process control
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Conclusions II(The Future of Tolerancing)
• GD&T needs further development– Handling other functions than fit
• There is work going on in ISO in this direction– Next generation GD&T will be a much richer
language enabling designers to express functional requirements more precisely
• CMMs will handle many of these functions better than they handle current GD&T– because these functions are not based on
extreme points