Gauge Blocks – The Original and Best Calibration Tool
You’ve kitted out your workshop with the best hand-held measuring equipment money can buy (at a great price from the ace guys at DML, of course!). The accuracy and resolution of the digital micrometers, calipers and thickness gauges you’ve bought are first class! But how do you know for sure they are all reading correctly?
Hopefully, when purchasing, you considered and went for Traceable or UKAS calibration (ask us for details if not!), but even so, after time and use, how do you ensure every micron and mm is as it seems?
If that’s the question, gauge blocks are the answer.
What are Gauge Blocks?
Gauge Blocks, also known as Slip Gauges, or Jo Blocks, after their inventor, Swedish machinist Carl Edvard Johansson, are a simple tool used to calibrate metrology equipment.
A block of hard material, most commonly hardened steel in a rectangular shape, gauge blocks have extremely flat surface faces, ensuring they offer great accuracy as a calibration tool.
History of Gauge Blocks
Johansson invented Gauge Blocks in 1896 when he was working as an armoury inspector for Remington Rifle Manufacturing.
A large selection of limit gauges and reference gauges would be required during the inspection process. These gauges were manufactured specifically for each rifle part that needed inspecting and would need to be re-manufactured every time a part was re-designed.
A desire to simplify the inspecting and referencing process led Johansson to the idea of a set of blocks that could combine to make any needed length within a range.
The first set of gauges Johansson devised contained 102 blocks that provided about 20-thousand different dimensions from 1mm to 201mm in increments of 0.01mm
In 1898, Johannsson applied for a patent: ‘How to use a number of blocks of different thicknesses by stacking end-to-end to make up a given length’. However, confusion over what the patent was asking to cover led to Johannsson having to appeal to the Swedish Royal Family before the patent was finally granted in 1908.
The patent didn’t include any mention of wringing, an important feature of gauge blocks, which allows the blocks to be stuck, or stacked together. Johannsson first noted wringing in gauge blocks in 1900, when two blocks that had been placed together didn’t separate when dropped. However, steel pieces wringing together had already been reported and demonstrated as far back as 1875.
Gauge blocks are made to have an extremely flat and smooth surface, achieved by lapping and polishing. This flat surface means that blocks can be wrung together, a process where the blocks stick together due to the molecular attraction between the two surfaces and a thin film of molecules trapped between the surfaces.
Wringing allows several blocks to be stuck together to achieve the required length.
There are several techniques used to wring blocks together, all involve pushing and twisting the blocks together to achieve the bond.
A step by step example of a typical wringing method is shown below.
Care should be taken when using any technique to not damage the blocks.
This process can be repeated to add additional blocks on top of the two initial blocks to create a stack of blocks that make the desired length.
Note: Blocks shouldn’t be left wrung together for a long period of time as it may become difficult to separate them without damaging the blocks.
When blocks are wrung together to make a stack, wear blocks can be used at each end of the stack to take the wear of being handled and measured. This practice protects the gauge blocks in the set, ensuring they all wear at the same rate, keeping the integrity of the set.
Types of Gauge Blocks
Gauge blocks are available in several different materials. The properties of the material will determine how hard-wearing and accurate the gauge blocks will be.
The most commonly found materials are detailed below.
High-grade hardened steel is the most commonly used material for Gauge blocks and is suitable for a wide range of applications and environments.
Often used on shop floors for calibrating tools and machinery, their durability can be less than other materials if they are not cared for, being more susceptible to burrs and dents if exposed to abrasive substances or dropped.
Tungsten carbide is a 50/50 compound of carbon and tungsten, it’s hardness level is close to that of diamond, it has a high melting point, so remains strong at high temperatures and is also highly resistance to corrosion.
These properties make tungsten carbide blocks more hard-wearing than steel blocks, especially in environments where they may be exposed to abrasive substances. Of course, these appealing properties push the price of tungsten carbide blocks up higher than steel blocks.
Ceramic blocks offer an extremely hard, smooth surface that is also highly resistant to abrasive substances. Considering this, it’s no surprise that ceramic blocks are typically then the most expensive.
However, value for money is down to frequency and type of use. If steel blocks are going to suffer damage under the level of use you place them under and need replacing, then buying one set of ceramic blocks may well prove to be money better spent.
Which material is best for you?
The type of material most suited for your application and environment will depend on a number of factors, including:
- Environment – As discussed above, tungsten carbide and ceramic blocks have a higher level of resistance to abrasive substances and should be considered over steel blocks if the blocks will be exposed to them
- Temperature – The coefficient of thermal expansion is detailed below – all materials will expand and shrink in different temperatures, steel more so than tungsten carbide and ceramic
- Frequency of use – Blocks that will be used more frequently will naturally wear quicker than those kept in their box for 90% of the time – choosing a more wear resistant material for blocks used often makes sense!
- Budget – As with everything, top end blocks will cost less than bottom end ones, but it may be the bottom end ones are more than suitable for your application!
Thermal Conductivity & the coefficient of thermal expansion
All materials expand and contract to some degree due to increases and decreases in temperature. How much and how quickly the material expands varies due to the materials properties. The degree of expansion of a material can be calculated using the equation of thermal expansion:
|=||particular length measurement|
|=||change in length|
|=||change in temperature|
Taking the three materials we’re considering in gauge blocks, steel, tungsten carbide, and ceramic. Steel has the highest level of thermal expansion and ceramic the least.
Because of this, it is often stated that an advantage of steel blocks is that they will expand at the same degree as the work pieces and metrology equipment being used on a workshop floor and so steel blocks are more suited to environments where temperatures are likely to fluctuate.
In reality, the coefficient of expansion between the different types of material used in gauge blocks is too small for this to be an issue in most situations.
However, when any type of gauge blocks are being used in an environment where the temperature isn’t controlled, it is important to consider that the blocks will take longer to adjust to temperature changes than workpieces and metrology equipment due to their material properties, size and density.
Gauge blocks are available in different grades, the grades relate to manufacturing standards and state the accuracy of the blocks. The higher accuracy blocks are more expensive and may not be needed for your application, so it is worthwhile knowing what accuracy you need for the gauge blocks.
Here is a rundown of the different grades and their suitability for use.
Typically used on the shop floor for setting, checking and calibrating machine parts and instruments
Often found in inspection areas to verify the accuracy of other gauges as well as calibrating and setting measuring instruments
High accuracy gauge blocks, used in temperature-controlled laboratories to set and calibrate precision measuring equipment.
Grade 00 or Grade K
The most accurate grade available, used as master gauges in calibration laboratories
The grade of gauge block required for your use will of course depend on your environment and application. However, grade 2 blocks offer sufficient accuracy for the majority of workshop floor uses and
How to Use Gauge Blocks
The ‘selling’ feature of Gauge blocks is they can be used as individual lengths or in stacks of blocks to create multiple specific lengths. The flatness of the Gauge Blocks surfaces allows them to be wrung together in the bonded stacks.
When measuring gauge blocks, to ensure an accurate measurement, it is important to ensure the gauge block is parallel to the jaws or contact points of the measuring instrument you are using. By design, measuring faces tend to be extremely flat and parallel. This means you can often wring the gauge block to a measuring face making it parallel with the opposite measuring face and ensuring you take an accurate measurement.
A method of doing this is to take the measurement and then gently slide the block back and forth between the two measuring faces while applying a gentle force to the jaws, either manually or through a ratchet. This will wring the block to the measuring face.
If your measuring device has spherical or pointed contact points, then parallelism isn’t as crucial, but can still affect the reading if you are not careful. In this situation, try to take the measurement as close to the middle of the Gauge Block as possible. This will prevent the block from tilting in the jaws.
Care for Gauge Blocks
It is important to care for your gauge blocks to ensure they provide a long accurate life as your calibration tool. The level of care will vary depending on the environment and the type of blocks (grade & material), however, some general tips are:
- Wipe blocks clean before and after use
- Keep the blocks in provided case and a clean workspace
- Apply a rust inhibitor to steel blocks after wringing
- Do not leave blocks wrung together for long periods – leave them overnight and you may not get them separated again without damaging them!
- As with all metrology equipment, gauge blocks will at times need to be calibrated to check their accuracy. How frequently calibration is needed on gauge blocks depends on various factors, including the type of use and the environment they are stored in.
Gauge Blocks care kits are a good investment, such as this one from Mitutoyo.
As with all metrology equipment, knowing your applications requirements before you purchase the equipment will ensure you are getting value for money, with tools that are suitable for your application and hard wearing enough to withstand your intended frequency and level of use.
Steel gauge blocks are the cheapest option and are suitable for a wide range of uses in most working environments. However, they are more susceptible to burrs and damage than Tungsten Carbide and Ceramic Gauge blocks which offer superior hard-wearing properties but are more expensive.
Grade 2 Gauge Blocks will provide enough accuracy for the majority of uses, higher grade blocks are required for calibration uses.
If you are considering Gauge Blocks as a calibration tool, please contact us if you need any advice.
A great starter set of gauge blocks is the Moore & Wright MW700-12 32 Piece Steel Gauge Block Set available for a limited time with over 20% discount!
If you are looking to recalibrate your equipment with certification, please get in touch for details of our Traceable and UKAS calibration service.
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