As we have known that the purpose of using gauge blocks is to check the accuracy of other precision instruments such as micrometers and calipers. As a result, we use them in inspection, verification, and calibration. Even, we use them in tool setups as well. But, how do we know that our gauge blocks are accurate too?
The answer is gauge block calibration. After several uses, the gauge blocks’ accuracy may slightly degrade. There are some reasons behind it such as corrosion, scratches, burrs, etc that unfortunately may be invisible by our eyes.
The calibration will tell you the deviation that each of your gauge blocks possesses and you could know whether it’s still within the tolerance or not.
Reasons to Calibrate Gauge Blocks
These are the factors that affect the accuracy of the flatness and length of gauge blocks.
1. Corrosion from handling and storing
Steel alloy and carbide gauge blocks are likely to rust since it is made up of metal. Holding metal gauge block without gloves will leave fingermarks since sweat on hands easily reacts to metal. Storing and using gauge blocks in an uncontrolled environment with high temperatures and humidity will contribute to the fast rusting of the metal gauge blocks.
Scratch is the most common problem encountered in gauge blocks during and after use. Improper wringing will cause scratches on gauge blocks and also the frequent use of gauge block, especially during workshop use contributes to scratches.
This problem can be minimized by proper handling and maintenance. Sometimes the wrong method of cleaning of gauge block also contributes to scratch.
Burrs can occur when the gauge blocks accidentally drop or hit a harder material. Proper lapping can correct this issue.
Dents and chipped gauge blocks must be replaced if cannot be repaired. It is usually caused by strong force or abusive use.
5. Thermal expansion
When the temperature of an environment where the gauge block is used is not controlled and set to a higher temperature than 20°C or 63°F, the gauge block expands and loses its accuracy.
6. Worn-out gauge block
Frequently used gauge blocks such as workshop gauge blocks wear in a shorter period compared to gauge blocks that are not frequently used. It cannot be avoided but proper use and care can prolong its life span.
How Often Do We Need to Calibrate Gauge Blocks?
The frequency of calibration depends on the owners’ needs. Its main purpose must be considered for the cycle of calibration. Frequently used gauge blocks should be calibrated more often.
Ideally, gauge block calibration should be performed once in 1 to 3 years according to Starrett. Again, it depends on your need.
Methods of Gauge Block Calibration
There are mainly two methods of gauge block calibration; is either by mechanical comparison or by interferometry.
Before sending gauge blocks to the gauge block calibration service and when deciding the method of calibration, the following things need to consider:
1. The length of gauge blocks that needs to be calibrated will determine the calibration laboratory that will conduct the calibration.
In case of a long gauge block, consult your calibration lab if they can calibrate a long gauge block. These gauge blocks starting from 4 inches or 100 mm above are classified as long gauge blocks.
2. The main function of gauge blocks will define the method of calibration to be done.
The calibration method of the gauge block is governed by standards defined in the traceability chain.
Higher accuracy gauge blocks that are used solely as a reference of comparison during calibration cannot be calibrated by the mechanical comparison method and are calibrated by interferometry. Calibration services of higher grades should be conducted by a higher-level standard defined in a metrology traceability chain.
Lower grade gauge blocks such as workshop and inspection grade gauge blocks are calibrated by mechanical comparison.
1. Gauge Block Calibration by Interferometry
This method is the primary method for calibration gauge blocks. High precision or higher grade gauge blocks (such as reference and master gauge blocks) are calibrated using this method.
The principle of measurement by the interferometer is by measuring the distance between the interfering light reflected from to optical flat or the auxiliary plate where the gauge block is wrung and reflected interfering light from the top of the measuring face of the gauge block.
Through optical interferometers, interference patterns are being detected which can tell the actual measurement of the length of the gauge block that undergoes calibration without any contact point, unlike calibration by comparators.
Interferometers solely rely on the light beam and how the gauge block is wrung to the auxiliary plate.
Nowadays, interferometers gather the necessary data that is processed by a computer program.
The following are the essential parts of an optical interferometer
1. A light source
2. A collimating lens system and projection optics
3. An optical view which can be a screen or an eyepiece for viewing the fringe patterns
4. Data recording and measuring devices which can be possible as needed
Here are some of the interferometers for gauge block calibration
Michelson Interferometer and Twyman Green Interferometer are interferometers that are double path interferometers wherein the reference beam and a sample beam travel along the same path.
A Michelson interferometer is also a kind of an amplitude-division interferometer where the light beam from a light source is divided into two when it passes the light beam splitter.
Mitutoyo Automatic Gauge Block Interferometer GBI and NPL Gauge Block Interferometer are examples of Twyman Green Interferometers.
Advantages of interferometers for gauge block calibration
- Very accurate
- Can calibrate higher grades of the gauge block
- Can calibrate the longer length of gauge blocks
- No contact point during measurement which will not damage the blocks during calibration
- Free from vibrations which will affect the accuracy of the result.
Disadvantages of interferometers for gauge block calibration
- Higher calibration service cost
- Longer time of calibration
- May scratch the gauge block during wringing to the auxiliary plate.
2. Gauge Block Calibration by Mechanical Comparison
This method is the secondary method of calibrating gauge blocks. The gauge block calibration of inspection grade gauge block and workshop grade gauge block that is used for manufacturing industries are usually done by mechanical comparison using a mechanical comparator. These gauge blocks have higher measurement uncertainty compared to the higher gauge block grades where the calibration method is conducted by interferometric calibration.
The method of mechanical comparison uses a gauge block comparator which has a lower and upper probe that determines the length difference of a standard gauge block or the reference gauge block to the measured gauge block or the customer gauge block. As mentioned earlier, long gauge blocks cannot be calibrated by mechanical comparison due to the unavailability of gauge block comparators that are designed for longer nominal size gauge blocks.
Methods of Comparison
For the process of gauge block calibration by mechanical comparison, there are two methods of comparison, the 1-point measurement cycle, and the 5-point measurement cycle.
A 5-point measurement cycle will assure that the uncertainty evaluation will be more precise than the 1-point measurement cycle. You could request the calibration to have a 1-point measurement if your gauge blocks were occasionally used and given proper care. However, for those gauge blocks that are used daily, it is best that the calibration process conducts the 5-point measurement. Do not be an owner who sends their gauge block into the calibration lab for the sake of calibration certificate only.
For the 1-point measurement cycle, the probe of the gauge block comparator only measures the central length of the reference gauge block and the client’s gauge block. However, for the 5-point measurement cycle, the points of measurement include the central length and the nominal length on the four corners of the measuring face of the gauge block.
Automatic Vs Manual Gauge Block Comparator
Gauge block comparators may be automatic or manual.
Automatic gauge block comparators automatically pick the gauge blocks that will be calibrated. It is more efficient to use since it will be in less contact with the person assigned to do the calibration that may contribute to a change in temperature measurement during the calibration process.
For manual gauge block comparators, padded tongs are used to hold the gauge blocks for the prevention of heat transfer from the human body to the gauge block and also to prevent finger marks.
Here is an example of an automatic gauge block comparator
Octagon Automatic Gauge Block Comparator
Here is an example of a manual gauge block comparator from Federal.
Advantages of Mechanical Comparison
- A cheaper method of calibration
- It is portable and easily transferrable from one location to another.
- Mechanical comparators are easy to handle and use.
- Fewer corrections for calculating the result of comparison for the length of gauge blocks.
- Time of calibration using this method is faster than calibration by an interferometer.
Disadvantages of Mechanical Comparison
- Limited only to lower grades gauge blocks and not used for calibrating higher grades
- Limited to certain lengths of gauge blocks
- The comparators have a lot of moving parts that develop friction which contributes to the accuracy
Equipments for Gauge Block Calibration
These are the equipment you may want to know in the gauge block calibration.
- A set of master gauge blocks
- Mechanical gauge block comparator
- Interferometric measurement instruments
- thermistor thermometers
- optical flats
- granite surface plates
- gauge block cleaning and preparation equipment
Process of Gauge Block Calibration
1. Clean the gauge block
Dirt, dust, fingermarks, and other foreign material must be removed in the gauge block. Cleaning agent, cotton towels, bristle brush, Arkansas stone, granite stone and, padded tongs are needed in the cleaning process of gauge blocks.
Wipe the surface using a clean towel and cleaning agent such as denatured alcohol.
Remove dust using an air puffer and hold it using a padded tong.
Use a padded tong in holding the cleaned gauge block.
2. Recondition the gauge block
Use a tool such as optical flats and platens to verify if the surface of the gauge block is still flat.
Lap the gauge block if there are burrs and projections. Arkansas stone can be used for tungsten carbide gauge blocks, and a granite stone can be used for steel alloy gauge blocks.
3. Demagnetize the gauge blocks
Demagnetize the blocks that may retain a magnetic field by using demagnetizers. Check the magnetic field by using a gauss meter.
4. Control the gauge block temperature
Store gauge block in an ambient chamber with 20°C or 63°F temperature at a standard soaking time depending on its length.
Placing it in a heat sink will prevent unwanted expansion. Store both the reference gauge blocks and the gauge blocks together.
5. Perform the calibration
Conduct calibration using comparators or interferometer instruments.
Note and doc all the measurement results.
7. Shield back the gauge blocks
Coat the gauge blocks with anti-rust oil after the test.
An attached calibration sticker will tell you that the gauge block set has already performed a calibration.
9. Calibration certificate
A calibration certificate must be provided.
Gauge Block Calibration Services
Individuals or gauge block owners should not calibrate gauge blocks on their own. The best practice is by relying the calibration on the calibration service that is accredited to ISO 17025, at least complied with that standard.
Since gauge blocks serve as the reference standard of nominal length in the calibration of instruments such as caliper, micrometer, height gauge, and other higher accuracy measuring instruments such as the CMM, it is better to understand the traceability chain in order to define the necessarily certified calibration laboratories for reference grade gauge blocks, inspection grades and workshop grade.