surface roughness and straightedge surfaceplate surface gauge visual inspection profilometer opticalflat

Surface roughness

• On any finished surface,imperfections are found and these take the form of a succession of hills and valleys
• Which vary both in height and in spacing and result in a kind of texture which in appearance or feel.

Straight edge

• Straightedges are commonly used for testing the straightness and flatness of plane surfaces
• Flatness is checked by a light test
• The narrow edge of the tool is applied to the surface to be tested and kept horizontally at the eye level

• Watching for light between the straightedge and the surface along the straightedge
• Light seen at one place or another indicates deviation from straightness

Surface plate

• Surface plate is used for testing the flatness of work itself and is also used for marking out work
• This is used for small pieces of work while the marking -out table is used for larger jobs
• It is used in conjunction with other gauges,the plate becomes a functional gauge in itself.

Surface gauge

• Surface gauge is used in laying out of parts that have to be machined
• It is used in conjunction with a dial indicator for checking surface
• The tool has a v-shaped groove at the bottom of the base for resting the block on a round bar
• So that dimensions may be set off from the bar to some other part of the work

Visual inspection

• The surface roughness can be judged by visual comparison of one surface with the other by nacked eye
• Sense of feel method is most often used as a measure of surface roughness
• In this method a finger nail is moved along the surface of the job and a sense of feel indicate the amount of resistance and depth of irregularities
• This can be compared with master roughness scale

Profilometer

• Profilometer is a tracer instrument used to measure surface roughness accurately
• The two main parts of profilometer are the tracer and an amplifier
• The tracer may be moved either manually or mechanically across the surface of the work
• Mechanical movements by motor drive give a more consistent and dependable roughness measurement

• As the tracer is moved across the surface the stylus follows the contour of the surface
• These up and down movements of stylus are converted in to electrical impulses
• These impulses are fed in to the amplifier where it is amplified to accurate roughness meter
• The meter shows roughness heights in micro inches
• The instrument is best suited for measuring surface roughness of deep grooves

Optical Flat

• Optical flat utilizes interference technique to measure surface roughness
• It is made of high quality optical quartz having extremely flat and parallel surfaces of about 25mm
• The flat is placed on the surface to be checked and the monochromatic light is directed at the flat
• The light is reflected from the back face of the flat and also the surface of the work
• The light reflected from the work surface will travel through varying distances causing to the surface roughness

• Some will be in phase and some will be out of phase with light reflected from the optical flat.
• The result is a series of light and dark bands,which appear to the necked eye
• The number of fringes that appear is related to the distance between the part and the bottom surface of the optical flat
• A truly flat work piece surface will not split the light beam and fringes will not appear
• When surfaces are not flat,fringes are curved.