Tools For Gadgets: 2009

Tuesday, March 3, 2009

Fixture (tool)

A fixture is a tool of the manufacturing industry used in mass production. Fixtures are used to hold objects in place and clamp them to machines or operating surfaces, so that the object can be machined or assembled.

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Fixtures differ from jigs, in that the fixture holds the workpiece in one place while a tool or cutter is moved in relation to it. A jig guides the tool along a path defined by the shape of the jig. The jig may also hold the tool during this operation.

The purposes of jigs and fixtures are widespread however when used in mass production they have five key aspects.

Reduce the cost of production
Maintain consistent quality
Speed production
Prevent or reduce improper techniques
Improve the overall safety to the part, operator, and machine.

Gauge block

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. The wear pieces are included at each end of the stack whenever possible as they provide protection against damage to the lapped faces of the main pieces. After use the blocks are reoiled or greased to protect their faces from corrosion.

Adhesion
Wringing is the process of sliding two blocks together so that their faces lightly bond. When combined with a very light film of oil, this action excludes any air from the gap between the two blocks. The alignment of the ultra-smooth surfaces in this manner permits molecular attraction to occur between the blocks, and forms a very strong bond between the blocks along with no discernible alteration to the stack's overall dimensions. Gage blocks, when properly wrung, may withstand a 200lb (890 newton) pull. The detailed physics responsible for this phenomenon remains unclear.[1] Possible causes that have been suggested are: atmospheric pressure, molecular attraction, a minute film of oil, or a combination of these factors.

Accessory set

Gauge block accessory setThe pictured accessories provides 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.

Grades

They are available in various grades depending on their intended use.

reference (AAA): small tolerance (± 0.00005 mm or 0.000002 in) used to establish standards
calibration (AA): (tolerance +0.00010 mm to -0.00005 mm) used to calibrate inspection blocks and very high precision gauging
inspection (A): (tolerance +0.00015 mm to -0.00005 mm) used as toolroom standards for setting other gauging tools
workshop (B): large tolerance (tolerance +0.00025 mm to -0.00015 mm) 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.

Measurement of Discharge

Automated direct measurement of streamflow discharge is difficult at present. In place of the direct measurement of streamflow discharge, one or more surrogate measurements can be used to produce discharge values. In the majority of cases, a stage measurement is used as the surrogate. Low gradient (or shallow-sloped) streams are highly influenced by variable downstream channel conditions. For these streams, a second stream gauge would be installed, and the slope of the water surface would be calculated between the gauges. This value would be used along with the stage measurement to more accurately determine the streamflow discharge. Within the last ten years, the technological advance of velocity sensors has allowed the use of water velocity as a reliable surrogate for streamflow discharge. These sensors are permanently mounted in the stream and measure velocity at a particular location in the stream.

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December 12, 2001 photo of the USGS streamflow-gaging station at Huey Creek, McMurdo Dry Valleys, Antarctica.In those instances where only a stage measurement is used as the surrogate, a rating curve must be constructed. A rating curve is the functional relation between stage and discharge. It is determined by making repeated discrete measurements of streamflow discharge using a velocimeter and some means to measure the channel geometry to determine the cross-sectional area of the channel. The technicians and hydrologists responsible for determining the rating curve visit the site routinely, with special trips to measure the hydrologic extremes (floods and droughts), and make a discharge measurement by following an explicit set of instructions.

Once the rating curve is established, it can be used in conjunction with stage measurements to determine the volumetric streamflow discharge. This record then serves as an assessment of the volume of water that passes by the stream gauge and is useful for many tasks associated with hydrology.

In those instances where a velocity measurement is additionally used as a surrogate, an index velocity determination is conducted. This analysis uses a velocity sensor, often either magnetic or acoustic, to measure the velocity of the flow at a particular location in the stream cross section. Once again, discrete measurements of streamflow discharge are made by the technician or hydrologist at a variety of stages. For each discrete determination of streamflow discharge, the mean velocity of the cross section is determined by dividing streamflow discharge by the cross-sectional area. A rating curve, similar to that used for stage-discharge determinations, is constructed using the mean velocity and the index velocity from the permanently mounted meter. An additional rating curve is constructed that relates stage of the stream to cross-sectional area. Using these two ratings, the automatically collected stage produces an estimate of the cross-sectional area, and the automatically collected index velocity produces an estimate of the mean velocity of the cross section. The streamflow discharge is computed as the estimate of the cross section area and the estimate of the mean velocity of the streamflow.

Micrometer

Some micrometers are provided with a vernier scale on the sleeve in addition to the regular graduations. These permit measurements within 0.001 millimetre to be made on metric micrometers, or 0.0001 inches on inch-system micrometers.

The additional digit of these micrometers is obtained by finding the line on the sleeve vernier scale which exactly coincides with one on the thimble. The number of this coinciding vernier line represents the additional digit.

Thus, the reading for metric micrometers of this type is the number of whole millimetres (if any) and the number of hundredths of a millimetre, as with an ordinary micrometer, and the number of thousandths of a millimetre given by the coinciding vernier line on the sleeve vernier scale.

For example, a measurement of 5.783 millimetres would be obtained by reading 5.5 millimetres on the sleeve, and then adding 0.28 millimetre as determined by the thimble. The vernier would then be used to read the 0.003 (as shown in the image).

Inch micrometers are read in a similar fashion.

Note: 0.01 millimetre = 0.000393 inch, and 0.002 millimetre = 0.000078 inch (78 millionths) or alternately, 0.0001 inch = 0.00254 millimetres. Therefore, metric micrometers provide smaller measuring increments than comparable inch unit micrometers—the smallest graduation of an ordinary inch reading micrometer is 0.001 inch; the vernier type has graduations down to 0.0001 inch (0.00254 mm). When using either a metric or inch micrometer, without a vernier, smaller readings than those graduated may of course be obtained by visual interpolation between graduations.

Torque repeatability via torque-limiting ratchets or sleeves
An additional feature of many micrometers is the inclusion of a torque-limiting device on the thimble—either a spring-loaded ratchet or a friction sleeve. Normally, one could use the mechanical advantage of the screw to force the micrometer to squeeze the material or tighten the screw threads, giving an inaccurate measurement. However, by attaching a thimble that will ratchet or friction slip at a certain torque, the micrometer will not continue to advance once sufficient resistance is encountered. This results in greater accuracy and repeatability of measurements—most especially for low-skilled or semi-skilled workers, who may not have developed the light, consistent touch of a skilled user.

Dial test indicator

A Dial test indicator, also known as a lever arm test indicator or finger indicator, has a smaller measuring range than a dial indicator and therefore has the ability to measure in smaller increments. A test indicator measures the deflection of the arm, the probe does not retract but swings in an arc around its hinge point. The lever may be interchanged for length or ball diameter and permits measurements to be taken in narrow grooves and small bores where the body of a probe type may not reach. The model shown is bidirectional, some types may have to be switched via a side lever to be able to measure in the opposite direction.

These indicators actually measure angular displacement and not linear displacement. If a force is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial. However, this error starts to become noticeable when the force is as much as 10 degrees off the ideal 90. These indicators are used to compare two surfaces and alert the user when they are at the same position relative to the body of the indicator. In this application the force can be applied at almost any angle since you are looking for the same reading on two different surfaces.

Digital indicator
With the advent of electronics and LCDs the clock face and analog display has been replaced with digital displays, these have the added advantage of sometimes being able to record and transmit the data electronically to a computer. This process is known as statistical process control (SPC) and involves a computer recording and interpreting the results, this also reduces the risk of the operator introducing recording errors. Digital indicators can also be switched between imperial and metric units with the press of a button, thereby increasing the DTI's versatility.

Probe indicator

Probe indicators typically consist of a graduated dial and needle (thus the clock terminology) to record the minor increments, with a smaller embedded clock face and needle to record the number of needle rotations on the main dial. The tool may be graduated to record measurements between 0.01 mm (.001", which is not a direct unit conversion) down to 0.001 mm (.00005") for more accurate usage. The probe (or plunger) moves perpendicular to the object being tested by either retracting or extending from the indicator's body.

The dial face can be rotated to any position, this is used to orient the face towards the user as well as set the zero point, there will also be some means of incorporating limit indicators (the two metallic tabs visible in the right image, at 90 and 10 respectively), these limit tabs may be rotated around the dial face to any required position. There may also be a lever arm available that will allow the indicator's probe to be retracted easily.