Week 1 - Eng. Metrology Flashcards
Metrology term meaning
(From greek ‘metron’ (Measure, and –logy) is a science of measurement
Metrology definition
Metrology include all theoretical and practical aspect of measurement
Metrology Importance
- Modern manufacturing development (concept of interchangeable parts)
- Ex: Ford rear axles same dimensions, easy to fit
- Ex: Royal Royce, parts hand fitted, takes 30min to fit
- Science of measurement
- Modern design, each part has certain dimension and tolerance to function together
- Metrology used by manufacturing engineer to measure parts within tolerance
Car manufacturing example
- Car assembly line worker only get 15 secs @station
- If part doesn’t fit, worker stops line
Dimensions and tolerances
Factors that determine performance of products, other than mechanical and physical properties:
- Dimensions: Linear or angular sizes of a component specified on the part drawing. (# value)
- Tolerances: Allowable variations from the specified part dimensions that are permitted
Dimensions (ANSI Y14.5M-1982)
- Represent nominal or basic sizes of the part and its features.
- Indicates the part size desired by the designer if it could be made w/ errors
Tolerances (ANSI Y14.5M-1982)
- Total amount by which a dimension is permitted to vary. Difference between max and min limits.
- Variations occur in any manufacturing process
- Tolerances used to define limits of allowed variation
Tolerance
- Tolerare, put up with, endure
- Impossible to make perfect parts
- Too small tolerance, high cost
- NIST tolerance shrink by a factor of 3 every 10 year . 001mium
Tolerance importance
- Parts from the same machine can be different:
- Speed of operation
- Temperature
- Lubrication
- Variation of incoming material
- other
- ISO system; definitions
Bilateral Tolerance
- Variation permitted in positive and negative directions from the nominal dimension
- Possible to be unbalanced. Ex: 2.500 +0.010, -.005
Unilateral Tolerance
-Variation from the specified dimension only permitted in one direction
-Either positive or negative
Ex: 2.500 +0.010 -0.000
Limit dimensions
-Permissible variation in a part feature size consist of the max and min dimensions allowed.
Ex: 2.505, 2.495
Ways to specify limits for dimensions
- Bilateral
- Unilateral
- Limit dimensions
Tolerance control (slide 17)
slide 17
Engineering drawing symbols
slide 20
Geometric tolerance
Tolerances that involve shape features of the part
Allowance
Specific difference in dimensions between mating parts
Basic size
Dimension from which limits of size are derived
Bilateral tolerance
Deviation from the basic size (+ or -)
Clearence
Space between mating parts
Clearance fit
Fit that allows rotation or sliding b/ mating parts
Datum
Theoretically exact axis, point, line or plane
Feature
Physically identifiable portion of a part, eg hole, slot pin, chamfer
Fit
Range of looseness or tightness
Interference
Negative clearence
Interference fit
The external dimension of one part slightly exceeds the internal dimension of the part into which it has to fit
Limit dimension
Max and min dimension of a part
MMC (Maximum material condition)
Condition where a feature of size contains the max amount of material within the stated limits of size
Positional tolerancing
A system of specifying the true position, size, and form of the feature of a part including allowable variation
Transition fit
Fit with small clearance or interference that allows for accurate location of mating parts
Surface technology
Concerns with:
- Defining the characteristic of a surface
- Surface texture
- Surface integrity
- Relationship b/ manufacturing processes and characteristics of resulting surface
Nominal surface
Designer’s intended surface contour of part, defined by lines in the eng. drawing
- Nominal surfaces appear as absolutely straight lines, ideal circles, round holes, and other edges and surfaces that are geometrically perfect
- Actual surfaces of a part are determined by the manufacturing processes used to make it
Surfaces
- Surfaces have properties and behaviour that is different from the bulk of the part
- Surfaces affect function, appearance of the manufactured parts
Surface appearance
- How the surface feels
- How it looks
- How it behaves for coating or sealing
Surface function
- How it behaves in contact with another surface
- How the surface will wear
- How well will it retain lubricant
- How well will it hold a load
Why surfaces are important?
- Aesthetic reason (smooth surface, favorable impression to customer)
- Surfaces affect safety
- Friction and wear depend on surface characteristics
- Surfaces affect mechanical and physical properties (surface flaws-stress concentration point)
- Assembly of part is affected by their surfaces (strength of adhesively bonded joint-increases when surface is slightly rough)
- Subsequent operation (painting, coating, welding, soldering)
- Smooth surfaces make better electrical contacts (good electrical and thermal conductivity)
Metallic part surface
OUT-IN
- Surface texture
- Altered layer
- Substrate
Surface characteristics
- Microscopic scale reveals irregularities
- Substrate: bulk of the part, under the surface
- Surface texture: roughness, waviness, flaws
- Altered layer: Work hardening (Mech E), Heat (thermal E), chemicals, electrical E.
- Surface integrity (definition, specification, and control of surface layer, includes surface texture and altered layer
Contamination
-Oxide film: Rust(Iron), Aluminum -Other Dirt, oil, cutting fluids, lubricants, absorbed gases. -Affects appearance -Inhibits joining Soldering, plating, adhesives
Surface structure of metals
- Contaminant
- Absorbed gas
- Oxide layer
- Amorphous (beilby) layer
- Work-hardened layer
- Metal substrate
Surface texture
Repetitive and/or random deviations from the nominal surface of an object
- Lay direction
- Crack
- Waviness spacing
- Waviness height
- Roughness height
- Roughness width
Surface texture
The topography and geometric features of the surface:
- When highly magnified, the surface is anything but straight and smooth
- Has patterns and/or direction resulting from the mech process that produced it
4 Elements of surface texture
- Roughness:
small, finely-spaced deviations from nominal surface
-Determined by material characteristics and processes that formed the surface - Waviness:
Deviations of much larger spacing
-Occurs due to work deflection, vibration, heat treatment, etc
-Roughness is superimposed on waviness - Lay:
Predominant direction or pattern of the surface texture - Flaws:
Irregularities that occur occasionally on the surface
Cracks, scratches, inclusions
Although some flaws relate to surface texture, they also affect surface integrity
Surface roughness
A measurable characteristic based on roughness deviations
Within surface texture
Surface finish
More subjective term denoting smoothness and general quality of a surface
Surface finish=surface roughness
Surface texture
Surface roughness
Average of vertical deviations from nominal surface over a specified
Surface roughness equation
slide 45
Surface roughness specification
slide49
Cutoff length
-Problem of Ra computation is that waviness may get included
-To deal with it a parameter called cutoff length is used as a filter to separate waviness from roughness deviations
-Cutoff length is a sampling distance along the surface
Shorter than the waviness eliminates waviness deviations and only includes roughness deviations
Surface integrity
- Metallurgical changes in the altered layer beneath the surface that can have a significant effect on a material’s mechanical properties
- Study and control of this subsurface layer and the changes in it that occur during processing which may influence the performance of the finished part
Energy forms in surface integrity
- Mechanical (forging, extrusion,machining)
- Thermal
- Chemical
- Electrical
Surface changes by Mechanical energy
- Residual stresses in subsurface layer
- Cracks micro and macro
- Voids or inclusions
- Hardness variations
Surface changes by thermal energy
-Metallurgical changes
(re-crystallization, grain size changes, phase changes)
-Redeposited
Metal is removed from the surface in molten state, reattached prior to solidification
-Resolidified material
Portion of the metal which was melted and then solidified without detaching from the surface
-Heat affected zone in welding
Region not melted but heated to undergo metallurgical changes
-Hardness changes
Surface changes by Chemical energy
- Intergranular attack (corrosion and oxidation)
- Chemical contamination
- Absorption of certain elements such as H and Cl in metal surface (lead to property change)
- Selective Etch: concentrate on certain components in the base metal
- Alloy depletion and resulting hardness changes
Surface changes by electrical energy
- Changes in conductivity and/or magnetism
- Craters, rough surface depression left in the surface, resulting from short circuits during certain electrical processing techniques such arc welding, electro dischange machining, electro chemical machining
Sawed surface
slide 57 pic
Turned surface
SLIDE 58 PIC
Dimensional tolerances as a function of part size
slide 60 graph
Dimensional tolerance range and surface roughness in various processes
slide 61 graph
Tolerance and manufacturing processes
- some processes more accurate than others
- Examples:
- Most machining accurate tolerances +/-0.05mm(0.002in) or better
- Sand casting inaccurate, tolerances of 10 to 20 times the machined parts
Surfaces and manufacturing processes
- Some processes produce better surfaces than others
- Processing cost higher with higher surface finish bc additional operations and more time are used
- Processes for superior finishes: honing, lapping, polishing and superfinishing
