Friday, 26 August 2011
Tuesday, 23 August 2011
Thursday, 18 August 2011
Optical flats
Optical flats are optical-grade pieces of glass lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch (about 25 nanometres). They are used with a monochromatic light to determine the flatness of other optical surfaces by interference.[1] When an optical flat is placed on another surface and illuminated, the light waves reflect off both the bottom surface of the flat and the surface it is resting on. The reflected waves interfere, creating a pattern of interference fringes (Newton's rings), visible as light and dark bands. The spacing between the fringes is smaller where the gap is changing more rapidly, indicating a departure from flatness in one of the two surfaces, in a similar way to the contour lines on a map. A flat surface is indicated by a pattern of straight, parallel fringes with equal spacing, while other patterns indicate uneven surfaces. Two adjacent fringes indicate a difference in elevation of one-half wavelength of the light used, so by counting the fringes differences in elevation of the surface can be measured to millionths of an inch.
Usually only one of the two surfaces is made optically flat to the specified tolerance, and this surface is indicated by an arrow on the edge of the glass.
Optical flats are sometimes given an optical coating and used as precision mirrors for special purposes, such as in a Fabry–Pérot interferometer or laser cavity. Optical flats have uses in spectrophotometry as well.
Basics of interferometry and interferometers
Basics of interferometry and interferometers
(From: Sam Goldwasser (sam@stdavids.picker.com)).
The dictionary definition goes something like:
"INTERFEROMETER: An instrument designed to produce optical interference fringes for measuring wavelengths, testing flat surfaces, measuring small distances, etc."
"INTERFEROMETER: An instrument designed to produce optical interference fringes for measuring wavelengths, testing flat surfaces, measuring small distances, etc."
As an example of an interferometer for making precise physical measurements, split a beam of monochromatic coherent light from a laser into two parts, bounce the beams around a bit and then recombine them at a screen, optical viewer, or sensor array. The beams will constructively or destructively interfere with each-other on a point-by-point basis depending on the net path-length difference between them. This will result in a pattern of light and dark fringes. If one of the beams is reflected from a mirror or corner reflector mounted on something whose position you need to monitor extremely precisely (like a multi-axis machine tool), then as it moves, the pattern will change. Counting the passage of the fringes can provide measurements accurate to a few nanometers!
A simple version of a Michelson interferometer is shown below:
_____ Mirror 1 (Moving)
^
|
| Beam
| Splitter
+-------+ | / |
| Laser |=========>/<---------->| Mirror 2 (Fixed)
+-------+ / | |
|
|
|
v Screen (or optical viewer,
------- magnifier, sensor, etc.)
1. The laser produces a coherent monochromatic beam which is expanded and collimated by a pair of positive lenses (not shown).
2. Part of the laser beam is reflected up by the Beam Splitter (half silvered mirror), reflects off of Mirror 1 and back down. A portion of this passes through the Beam Splitter to the Screen.
3. The remainder of the laser beam passes through the Beam Splitter and is reflected from Mirror 2. Part of this is reflected down by the Beam Splitter to the Screen.
4. The two beams combine at the Screen resulting in an interference pattern of light and dark fringes. A magnifier, microscope, or other optical system imaging to a human observer or electronic sensor may be provided in place of the screen to view the fringe pattern in more detail or provide input to an electronic measurement system.
5. A microscopic shift in position or orientation of either mirror will result in a change to the fringe pattern. Presumably, the mirror designated as 'Moving' is mounted on some equipment such as a disk drive head positioner that is being tested or calibrated.
(Yes, about 50 percent of the light gets reflected back toward the laser and is wasted with this particular configuration. This light may also destabilize laser action if it enters the resonator. Both of these problems can be easily dealt with using slightly different optics than what are shown.)
A long coherence length laser producing a TEM00 beam is generally used for this application. HeNe lasers have excellent beam characteristics especially when frequency stabilized to operate in a single longitudinal mode. However, some types of diode lasers (which are normally not thought of as having respectable coherence lengths or stability) may also work. See the section: "Interferometers using inexpensive laser diodes". Even conventional light sources (e.g., gas discharge lamps producing distinct emission lines with narrow band optical filters) have acceptable performance for some types of interferometry.
Such a setup is exceedingly sensitive to EVERYTHING since positional shifts of a small fraction of a wavelength of the laser light (10s of nm - that's nanometers!) will result in a noticeable change in the fringe pattern. This can be used to advantage in making extremely precise position or speed measurements. However, it also means that setting up such an instrument in a stable manner requires great care and isolated mountings. Walking across the room or a bus going by down the street will show up as a fringe shift!
Interferometry techniques can be used to measure vibrational modes of solid bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in ground position or tilt which may signal the precursor to an earthquake, long term continental drift, shift in position of large suspended masses in the search for gravitational waves, and much much more. Very long base-line interferometry can even be applied at cosmic distances (with radio telescopes a continent or even an earth orbit diameter apart, and using radio emitting stars or galaxies instead of lasers). And, holography is just a variation on this technique where the interference pattern (the hologram) stores complex 3-D information.
What Is A Tool Maker Microscope?
What Is A Tool Maker Microscope?
Understand what tool maker microscopes are and their specific uses. These microscopes are special type of microscopes that are used to create precision tools and measure small distances between two points of a specimen.
A tool maker microscope is a type of a multi functional device that is primarily used for measuring tools and apparatus. These microscopes are widely used and commonly seen inside machine and tools manufacturing industries and factories. These microscopes are also inside electronics production houses and in aeronautic parts factories. A tool maker microscope is an indispensable tool in the different measurement tasks performed throughout the engineering industry.
The main use of a tool maker microscope is to measure the shape, size, angle, and the position of the small components that falls under the microscope’s measuring range. More often than not, a tool maker microscope is outfitted with a CCD camera that has the ability to capture, collect, and store images into specialized computer software. Certain computer aided design software is commonly used for such applications. The image produced by the camera and processed by the software is normally a two dimensional image.
But what makes a tool maker microscope fully functional are its glass grading and optics system. Since what are being viewed under these microscopes are metals and precision instruments, it is important that the objectives and the eye piece lenses are made of fine quality glasses only. These essential parts are what makes the device very durable and gives it the ability to withstand the wear and tear associated with the everyday stress of factory usage.
And much because of this, it is also important that the body, structure, and mechanisms of a tool maker microscope are created with highly durable materials, most preferably good quality metals. Because the conditions inside an industrial laboratory are not as good as a home or office laboratory setup, the microscope’s body should be capable of low heat production. It should also be able to resist corrosion, oscillation, and pollution – because all of these elements are present inside an industrial laboratories and production plants.
There are tool maker microscopes that are equipped with a cross hair reticle on the eye piece, coupled with a protractor on the tube. These are good instruments used to accurately measure the distance or the diameter of the tool under observation. The microscope’s stage is also built with a millimeter measuring system that also allows for the measurement of the specimen. The stage when moved, produce the distance traveled with which the microscope effectively measures.
Right now, quality tool maker microscopes are using semiconductor laser devices as directors. Instead of the cross hairs, a red point is virtually marked on the microscope’s working surface in order to locate the parts that have to be measured by the microscope. The CCD imaging system can also be used as a measurement system as well. This is another advanced feature of the newer versions of a tool maker microscope models. A CCD camera that has the ability to measure diameters and distances is a lot more convenient to use, especially to beginners.
But aside from all of these, a tool maker microscope should also have a good illumination system. It is the lights that allows for the superior viewing of tools and specimens. The higher the luminance value of the light provided by the microscope, the better its performance is. If necessary, an incandescent lamp should not be used for these applications. The light that is ideal is the one that produces a nice level of brightness with less heat. Lamps have life spans too. And because most of a tool maker microscope uses a built-in lighting system, the light to be used should last for an extended period of time, if and when possible.
A tool maker microscope is primarily used for measuring the shape of different components like the template, formed cutter, milling cutter, punching die, and cam. The pitch, external, and internal diameters are specifically measured as well. The thread gauge, guide worm, and guide screw are conveniently handled as well. As far angles are concerned, the thread and pitch angle are of chief concern.
These are what make a good tool maker microscope. If you are not familiar with these devices, it pays to know more about their specifications. The magnification power of tool maker microscope is nothing better than a regular compound microscope. In fact, it has a total magnification power of only 80x. This is due to the fact that these microscopes require good working distances of around 100 millimeters.
Tuesday, 16 August 2011
Slip gauges
Slip gauges (also known as gage blocks, Johansson gauges, or Gauge blocks) are precision ground and lapped measuring standards. They are used as references for the setting of measuring equipment such as micrometers, gap gauges, sine bars, dial indicators (when used in an inspection role). They are available in various grades depending on their intended use.
· Calibration (AA) - (tolerance +0.00010 mm to -0.00005 mm)
· Reference (AAA) -high tolerance (± 0.00005 mm or 0.000002 in) inspection (A) - (tolerance +0.00015 mm to -0.00005 mm)
· workshop (B) - low tolerance (tolerance +0.00025 mm to -0.00015mm
LINEAR AND ANGULAR MEASUREMENT
Definition of metrology-Linear measuring instruments: Vernier, micrometer, Slip gauges
and classification, - Tool Makers Microscope - interferometery, optical flats, -
Comparators: limit gauges Mechanical, pneumatic and electrical comparators,
applications. Angular measurements: -Sine bar, Sine center, bevel protractor and angle
Decker..
Monday, 15 August 2011
Gauge block
Gauge block
A gauge block (also known as a gage block, Johansson gauge, slip gauge, or Jo block) is a precision ground and lapped length measuring standard. Invented in 1896 by Swedish machinist Carl E. Johansson,[1] they are used as a reference for the calibration of measuring equipment used in machine shops, such as micrometers, sine bars, calipers, and dial indicators (when used in an inspection role). Gauge blocks are the main means of length standardization used by industry.[1]
Description
How gauge blocks are measured.
Each gauge block consists of a block of metal or ceramic with two opposing faces ground precisely flat and parallel, a precise distance apart. Standard grade blocks are made of a hardened steel alloy, while calibration grade blocks are often made of tungsten carbide or chromium carbide because it is harder and wears less.[2] Gauge blocks come in sets of blocks of various lengths, along with two wear blocks, to allow a wide variety of standard lengths to be made up by stacking them. The length of each block is actually slightly shorter than the nominal length stamped on it, because the stamped length includes the length of one wring film, a film of lubricant which separates adjacent block faces in normal use. This nominal length is known as the interferometric length.[3]
In use, the blocks are removed from the set, cleaned of their protective coating (petroleum jelly or oil) and wrung together to form a stack of the required dimension, with the minimum number of blocks. Gauge blocks are calibrated to be accurate at 68 °F (20 °C) and should be kept at this temperature when taking measurements. This mitigates the effects of thermal expansion. The wear blocks, made of a harder substance like tungsten carbide, are included at each end of the stack, whenever possible, to protect the gauge blocks from being damaged in use.
Wringing
36 Johansson gauge blocks wrung together easily support their own weight.
Wringing is the process of sliding two blocks together so that their faces lightly bond. Because of their ultraflat surfaces, when wrung, gauge blocks adhere to each other tightly. Properly wrung blocks may withstand a 75 lbf (330 N) pull.[4] While the exact mechanism that causes wringing is unknown,[5][4] it is believed to be a combination of:[3][4]
· Air pressure applies pressure between the blocks because the air is squeezed out of the joint.
· Surface tension from oil and water vapor that is present between the blocks.
· Molecular attraction occurs when two very flat surfaces are brought into contact. This force causes gauge blocks to adhere even without surface lubricants, and in a vacuum.
It is believed that the last two sources are the most significant.[3]
The process of wringing involves four steps:[3]
- Wiping a clean gauge block across an oiled pad (see the accessories section).
- Wiping any extra oil off the gauge block using a dry pad (see the accessories section).
- The block is then slid perpendicularly across the other block while applying moderate pressure until they form a cruciform.
- Finally, the block is rotated until it is inline with the other block.
After use the blocks are re-oiled or greased to protect against corrosion. The ability for a given gauge block to wring is called wringability; it is officially defined as "the ability of two surfaces to adhere tightly to each other in the absence of external means." The minimum conditions for wringability are a surface finish of 1 microinch (0.025 µm) AA or better, and a flatness of at least 5 μin (0.13 µm).[3]
There is a formal test to measure wringability. First, the block is prepared for wringing using the standard process. The block is then slid across a 2 in (51 mm) reference grade (1 μin (0.025 µm) flatness) quartz optical flat while applying moderate pressure. Then, the bottom of the gauge block is observed (through the optical flat) for oil or color. For Federal Grades 0.5, 1, and 2 and ISO grades K, 00, and 0 no oil or color should be visible under the gauge block. For Federal Grade 3 and ISO grades 1 and 2, no more than 20% of the surface area should show oil or color. Note that this test is hard to perform on gauge blocks thinner than 0.1 in (2.5 mm) because they tend not to be flat in the relaxed state.[3]
Accessories
A gauge block accessory set
The pictured accessories provide a set of holders and tools to extend the usefulness of the gauge block set. They provide a means of securely clamping large stacks together along with reference points and scribers.
Slip gauges are made from a select grade of carbide with hardness of 1500 Vickers hardness. Long series slip gauges are made from high quality steel having cross section (35 x 9 mm) with holes for clamping two slips together.
A gauge block stone is used to remove nicks and burrs to maintain wringability.[3]
There are two wringing pads used to prepare a gauge block for wringing. The first is an oil pad, which applies a light layer of oil to the block. The second is a dry pad, which removes any excess oil from the block after the oil pad has been used.[3]
Grades
Gauge blocks (left in each picture, under optical flats) being used to measure the height of a ball bearing and a plug gage using interferometry.
They are available in various grades depending on their intended use.[6] Various grading standards include: JIS B 7506-1997 (Japan)/DIN 861-1980 (Germany), ASME (US), BS 4311: Part 1: 1993 (UK). Tolerances will vary within the same grade as the thickness of the material increases.
· reference (AAA): small tolerance (±0.05 μm or ±0.000002 in) used to establish standards
· calibration (AA): (tolerance +0.10 μm to −0.05 μm) used to calibrate inspection blocks and very high precision gauging
· inspection (A): (tolerance +0.15 μm to −0.05 μm) used as toolroom standards for setting other gauging tools
· workshop (B): large tolerance (tolerance +0.25 μm to −0.15 μm) used as shop standards for precision measurement
More recent grade designations include (U.S. Federal Specification GGG-G-15C):
· 0.5 — generally equivalent to grade AAA
· 1 — generally equivalent to grade AA
· 2 — generally equivalent to grade A+
· 3 — compromise grade between A and B
and ANSI/ASME B89.1.9M, which defines both absolute deviations from nominal dimensions and parallelism limits as criteria for grade determination. Generally, grades are equivalent to former U.S. Federal grades as follows:
· 00 — generally equivalent to grade 1 (most exacting flatness and accuracy requirements)
· 0 — generally equivalent to grade 2
· AS-1 — generally equivalent to grade 3 (reportedly stands for American Standard - 1)
· AS-2 — generally less accurate than grade 3
· K — generally equivalent to grade 00 flatness (parallelism) with grade AS-1 accuracy
The ANSI/ASME standard follows a similar philosophy as set forth in ISO 3650. See the NIST reference below for more detailed information on tolerances for each grade and block size. Also consult page 2 of: Commercial Gauge Block Tolerances (Length refers to the calibrated thickness)
History
The gauge block set, also known as "Jo Blocks", was developed by the Swedish inventor Carl Edvard Johansson.[7] Johansson was employed in 1888 as an armourer inspector by the state arsenal Carl Gustafs stads Gevärsfaktori [Carl Gustaf Stad's Rifle Factory] in the town of Eskilstuna, Sweden. He was concerned with the expensive tools for measuring parts for the Remington rifles then in production under license at Carl Gustaf. When Sweden adopted a tailored variant of the Mauser carbine in 1894, Johansson was very excited about the chance to study Mauser's methods of measuring, in preparation for production under license at Carl Gustaf (which began several years later). However, a visit to the Mauser factory in Oberndorf am Neckar, Germany, turned out to be a disappointment. On the train home, he thought about the problem, and he came up with the idea of a set of blocks that could be combined to make up any measure.
There had already been a long history of increasing use of gauges up to this time, such as gauges for filing and go/no go gauges, which were custom-made individually in a toolroom for use on the shop floor; but there had never been super-precision gauge blocks that could be wrung together to make up different lengths, as Johansson now envisioned.
Back home, Johansson converted his wife's Singer sewing machine to a grinding and lapping machine. He preferred to carry out this precision work at home, as the grinding machines at the rifle factory were not good enough. His wife, Margareta, helped him a lot with the grinding besides the household work. Once Johansson had demonstrated his set at Carl Gustaf, his employer provided time and resources for him to develop the idea. Johansson was granted his first Swedish patent on 2 May 1901, SE patent No. 17017, called "Gauge Block Sets for Precision Measurement". Johansson formed the Swedish company CE Johansson AB (also known as 'CEJ') on 16 March 1917.
Johansson spent many years in America; during his life he crossed the Atlantic 22 times. The first CEJ gauge block set in America was sold to Henry M. Leland at the Cadillac Automobile Company around 1908. The first manufacturing plant in America for his gauge block sets was established in Poughkeepsie, Dutchess County, New York, in 1919. The economic environment of the post–World War I recession and depression of 1920–21 did not turn out so well for the company, so in 1923 he wrote a letter to Henry Ford of the Ford Motor Company, where he proposed a cooperation in order to save his company. Henry Ford became interested, and on 18 November 1923 he began working for Henry Ford in Dearborn, Michigan. Hounshell (1984), citing Althin (1948) and various archive primary sources, says, "Henry Ford purchased the famous gaugemaking operation of the Swede C. E. Johansson in 1923 and soon moved it into the laboratory facility in Dearborn. Between 1923 and 1927, the Johansson division supplied 'Jo-blocks' to the Ford toolroom and any manufacturer who could afford them. It also made some of the Ford 'go' and 'no-go' gauges used in production as well as other precision production devices."[8]
In 1936, at the age of 72, Johansson felt it was time to retire and go back to Sweden. He was awarded the large gold medal of the Royal Swedish Academy of Engineering Sciences in 1943, shortly after his death.
Gauge pins
Similar to gauge blocks, these are precision ground cylindrical bars for use in Go-NoGo gauges or similar applications.
Gauge rollers and balls
These are supplied as sets of individual rollers or balls as used in roller or ball bearings
terms in emm
Calibration:
If a known input is given to the measurement system the output deviates from the given input, the corrections are made in the instrument and then the output is measured. This process is called “Calibration”.
Sensitivity:
Sensitivity is the ratio of change in the output signal to the change in the input signal.
Readability:
Refers to the ease with which the readings of a measuring instrument can be read.
True size:
Theoretical size of a dimension which is free from errors.
Actual size:
Size obtained through measurement with permissible error.
Hysteresis:
All the energy put into the stressed component when loaded is not recovered upon unloading. so the output of measurement partially depends on input called Hysteresis.
Range:
The physical variables that are measured between two values. One is the higher calibration value Hc and the other is Lower value Lc.
Span:
The algebraic difference between higher calibration values to lower calibration values.
Resolution:
The minimum value of the input signal is required to cause an appreciable change in the output known as resolution.
Dead Zone:
It is the largest change in the physical variable to which the measuring instrument does not respond.
Threshold:
The minimum value of input signal that is required to make a change or start from zero.
Backlash:
The maximum distance through which one part of the instrument is moved without disturbing the other part.
Response Time:
The time at which the instrument begins its response for a change in the measured quantity.
Repeatability:
The ability of the measuring instrument to repeat the same results during the act measurements for the same quantity is known as repeatability.
Bias:
It is a characteristic of a measure or measuring instruments to give indications of the value of a measured quantity for which the average value differs from true value.
Magnification:
It means the magnitude of output signal of measuring instrument many times increases to make it more readable.
Drift:
If an instrument does not reproduce the same reading at different times of measurement for the same input signal, it is said to be measurement drift.
Reproducibility:
It is the consistency of pattern of variation in measurement. When individual measurements are carried out the closeness of the agreement between the results of measurements of the same quantity.
Uncertainty:
The range about the measured value within the true value of the measured quantity is likely to lie at the stated level of confidence.
Traceability:
It is nothing establishing a calibration by step by step comparison with better standards.
Parallax:
An apparent change in the position of the index relative is to the scale marks.
Friday, 12 August 2011
Thursday, 11 August 2011
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