Surface finishes Flashcards
What are the 2 methods of measuring surface finish
Contact and Non Contact
What is an example of contact methods
Skid & stylus Profilometers
What material is used for the stylus in Profilometers
Diamond
How do Profilometers work
The rider is dragged across the surface where the stylus measures the surface profile
What are some advantages of Profilometers
Acceptable standards
Accuracy using a 20nm tip provides high resolution
Independence- not sensitive to surface contaminants such as colour and reflection
List some methods of smoothing a surface in order of roughness
Turning
Rough grinding
Finish Grinding
Lapping
Name an non contact method for measuring surface finish
Optical profilomentry
eg. phase shift, interformetry and vertical scanning
What is another name for a phase shift interferometer
Laser interferometer
What type of light does a phase shift interferometer use
Laser/monochromatic
What are Phase shift interferometers used for
very smooth surfaces
Are phase shift interferometers accurate
Yes very, can be used to measure light up to a 100th of the wavelength used
What is another name for vertical scanning interferometers
Coherence scanning inferometers or scanning white light inferometers
What is vertical scanning suitable for
Very rough surfaces
What does vertical scanning use
White light
What values of n mean constructive interference and what mean destructive
Constructive when n is even
Destructive when n is odd
What is the equation for the difference in distance travelled by the light
diff=n*lamda/2
Why are phase shift interferometers so accurate
The combine information from multiple phase shifted images of same object by moving position of mirror
How accurate can phase shift interferometers measure
Up to lamda/100
Which is more accurate vertical scanning or phase shift interferometers
Phase shift
Why wont vertical scanning damage the surface
Because it is anon contact method of scanning
For dimensional measurement what are the techniques of measuring
Contact and Non-contact
What are the types of contact techniques for dimensional measurement
Portable and non-portable
For dimensional measurement what are the types of non-contact measurement
Active 3D scanning (uses radiation) and Passive 3D scanning (uses light)
What is are examples of contact dimensional measurement
Callipers and micrometres
How would a coordinate measuring system be used to measuring dimensions while making contact
for both portable and non portable
Portable- hand operated device with contact probe is placed on device and probe is tracked using a tracking system
Non-portable- Object is secured to a fixed bed and a computer controlled robotic arm is used to collect data
What are the 3 types of active 3D scanning
Time of flight- laser pulse and uses d=vt
Triangulation- how far away an object is effects the position of laser on camera FOV
Structures light pattern- light cast onto a surface and deformation of pattern is used to find position
Why is Passive 3D scanning cheaper than active 3D scanning
Because they do not emit light they use reflected light
Name 2 types of passive 3D scanning
Stereoscopic systems
Photometric systems
Which type of scanning can be used for reverse engineering
Passive 3D scanning
What is more precise portable or non-portable contact coordinate measuring systems
Non-portable
What is faster 3D scanning or Non-portable contact CM
3D scanning
What is the most accurate a non-contact CMM can be
A few nano meters
How expensive are portable and non portable contact measuring methods
Both can cost 10s of 1000s but non-contact is more expensive
To what precision can portable CM systems measure
Up to about 20 micro meters
How precise can non-contact triangulation methods be
Reasonably precise, 10s of microns
What is the price for a non-contact triangulation method which measures at a precision of 200 microns
About £450
What is the advantage of structured light over triangulation
Faster as it can scan multiple points at once
What is the typical price for a structured lights dimensional measuring system
About £2000 with an accuracy of about 60 microns
How easy to obtain are Photometric methods of dimensional measurement
Very can be purchased as an app on smart phones
Name a method used for measuring of a surface’s flatness
Optical flats
What is an optical flat
A clear extremely flat piece of quartz
How does an optical flat work
Under monochromatic light interference bands are created and the deformation allows the flatness to be calculated. Deformation is formed by a flaw in the surfaces flatness causing the optical flat to be displaced at an angle so the light will take a different path through the flat creating a different pattern.
Explain constructive and destructive interference
Constructive- 2 waves that are in phase combine to create a wave with greater amplitude
Destructive- 2 waves out of phase with each other combine to produce 0 or a wave of reduced amplitude
What causes the dark areas when using an optical flat
Destructive interference
Surface roughness increases as…..
Tolerance range increases
What is the importance of surface texture
It has an effect of friction, wear and lubrication
It effects the appearance- ability to be painted, coated, bonded or welded
Effects crack propagation
Effects thermal and electrical conductivity
What is roughness on a surface
Finley spaced surface irregularities resulting from machining operation.
What is surface roughness dependant on
Speed, material of tool, work piece material and depth of cut
What is surface waviness
A surface irregularity of greater spacing than roughness caused by tool deflection
What causes surface waviness
Cutting pressure, Work piece deflection, vibration of part, deposition of built up deflection and tool vibration
What is a lay
A term used to designate direction of pattern produced by machine process
All surfaces are taken……………… to lay direction
Normal
How do you measure a surfaces flatness
Using Optical flats
How do you measure a surface dimension
Using contact (callipers and micrometers) and non contact (active 3D scanning and passive 3D scanning) methods
How do you measure surface finish
Using Contact (skid and stylus Profilometers) and non contact (interferometry) including phase shift and vertical
What are the 2 methods for measuring surface roughness
Centre line average (CLA) Ra
Root Mean Square (RMS) Rq
What is centre line average
Average height from a mean line for all the coordinates of the surface regardless of sign
What is the root mean square value
Defined as square of the mean of the square of the coordinates of surface measured from the centre line
