Continuing our series on measurement devices, today we are going to discuss Gage Blocks. Gage blocks are rectangular blocks usually made of hardened steel or zirconia ceramics. The surfaces are flattened and polished to a tolerance of only 2 to 8 millionths of an inch.
The purpose of gage blocks is to measure things to an astonishing accuracy. Gage blocks are used to determine the accuracy of fixed gages while checking to see if those gages have experienced excess wear or any other form of alteration, calibrate various adjustable gauges, to set up machine tool settings, and to measure the accuracy of finished parts.
There are three classes of gage blocks:
- Class AA – also known as the laboratory or master set. They are accurate to within +/- 0.000002 in and +/- 0.00005 mm. These gages are used in climate controlled settings to ensure the accuracy of lower class gage blocks.
- Class A – used for inspection purposes, they are accurate to within +/- 0.000004 in and +0.00015/-0.00005 mm.
- Class B- are commonly known as a “working set” and are used most often in shop settings for machine tool setups, layout work, and measurement. They are accurate to +/- 0.000008 in and +0.00025/-0.00015 mm.
Gage block sets can come with only three or four blocks to sets that number up to 115 blocks. The typical English gage block set consists of 83 blocks and two wear blocks. The wear blocks can be either 0.050 in or 0.100 in. The most common metric sets consist of 88 pieces with two 2 mm wear blocks.
Wear blocks are stacked on each end of the gage block set when measuring and are designed to take all the wear and erosion that occurs during the lifetime of measuring using the set therefore prolonging the set’s usefulness. NEVER place a non-wear block on a work surface that you are measuring. Work surfaces can contain minute amounts of abrasives that will degrade the accuracy of your blocks over time. Always use wear blocks on each end of the stack, with the same face of the wear blocks touching the surfaces all the time. Most wear blocks are marked so you not put the wrong face on the item you are measuring.
Gage blocks are designed to be used in environments that are climate controlled. Most blocks dimensions are set in a temperature of 68 degrees F (20 degrees C). For every increase of 1 degree F (0.5 C), a typical 4 in stack of gage blocks will expand approximately 0.000025 in. With the human body temperature being about 98.6 degrees, it is important that not only a climate controlled facility be used when measuring, but that the aerospace technician limit his contact with the blocks either by holding them by your finger tips as little as possible or using insulated tweezers. The work area that is being measured should also be the same temperature as the blocks in order to obtain the best accuracy. Some manufacturers suggest you go the extra step and use insulated gloves along with the insulated tweezers. If the part being measured and the blocks are not the same temperature, some books suggest you immerse both items in kerosene until they are equalized. That of course may not be practical due to fire hazards or the size of the part being measured.
To save time and reduce the chance of error when using gage blocks you should use as few blocks as possible. There is an actual procedure advocated by the authors of the book, Technology of Machine Tools, to use when calculating the exact blocks you will need to make a measurement.
Step One: Write the dimension required on a a piece of paper.
Step Two: Deduct the size of two wear blocks.
Step Three: use a block that will eliminate the right-hand digit.
Step Four: Use a block that will eliminate the right-digit and at the same time bring the digit to the left of it to a zero or a five.
Step Five: Continue to eliminate the digits from the right to the left until the dimension required is attained.
Now to see this in action, here is an example in the table below:
As you see in the left hand column, you are subtracting the blocks from the desired measurement while in the right hand column you are adding the block’s measurements together. You should achieve a “0” in the left hand column and the desired measurement in the right hand column if you have done your math correctly.
Gage blocks surfaces are flattened so accurately, that they can actually “stick” together and withstand a pull of up to 200 lbs! It is not known exactly why that is though some have suggested it is either a molecular bond or due to the slight film of oil left over due to cleaning. To stack or “wring” gage blocks together, you must first clean the blocks with a clean, lint free, and soft cloth. Wipe the contact surface area of the block on the palm of your hand or wrist. This has two functions; One, to wipe any remaining particulates from the block onto your hand using the oil from your skin to “grab” the particulates, and two, also using the oil from your skin to “lube” the blocks. Place the end of one block onto the end of the other block and while using pressure, slide the blocks together. They should stick together. If they don’t, then the blocks were not properly cleaned.
To take care of your gage blocks and ensure that they have a long life, you should:
- Keep the case closed at all times except when you are getting a block or placing back a cleaned block.
- Do not play dominoes with them.
- Do not unnecessarily finger the surfaces of the block to avoid rusting and tarnishing due to your skin oils and moisture.
- Do not drop the blocks or scratch the surfaces of the blocks.
- Do not use them in your juggling act at the comedy club.
- Immediately after use, each block should be cleaned, oiled, and placed back into the box. (Don’t forget to close the box!)
- Never leave your gage blocks wrung together! Leaving them this way will encourage rusting from the oils and moisture from your skin.
The Quality Technician’s Handbook (Griffith, 2003)
Technology of Machine Tools (Krar & Check, 1997)
We are going to start a series talking about the use and type of measuring tools commonly used by the aerospace technician. In this part, we will cover the basics that are needed to keep in mind when selecting and using measuring tools.
There is an old phrase that says, “When the only tool you have in your tool box is a hammer, then every problem looks like a nail.” The same goes with measuring tools. If all you have is a ruler, then you have the tendency to measure everything with it. As we all know, various things such as surfaces, threads, thickness, etc. need to be measured which require a variety of measuring tools, from rulers, dial calipers, to gages.
Though accuracy is needed in the aerospace field, accuracy can actually be too much. When you have need of knowing the measurement of a part within 0.01 of accuracy, you do not need to use a tool that measures within 0.000001 accuracy. That much detail is wasteful and not needed for the task. “The rule of thumb is to select a measuring tool that is ten times more accurate than the total tolerance to be measured, or the tool can discriminate to one-tenth of the total part tolerance.” (Griffith, 2003) This is called the 10 % or 10-1 rule. So if you wish to measure something within 0.01 accuracy as called for by the tolerence listed on the process, you only need to measure ten times that accurate or 0.001 in order to follow the 10-1 rule.
When I took a chemistry lab in college, I had the instructor once comment that I was being too accurate in my measurements. I had taken that as a compliment at the time not understanding that his “tolerance” did not require as much accuracy as I was using in measuring my chemical compounds for the experiment. Though my outcomes were more inline with what the lab book had said we would come up with in the results of the experiment, I was always last to leave the class, and I missed the point that he wanted us to grasp the concept of the experiment, not the technique.
Know your measuring tool! What does it measure? What is the fixed end (reference) and what is the measured surface (movable)? Do you know how to read the measurement displayed? What are the divisons on it and what do they represent? You must be proficient with your tool in order to use and read the correct measurment.
Strive for accuracy and precision. “Accuracy is the difference between the average of several measurements made on a part and the true value of that part. Precision means getting the consistent results repeatedly.” (Griffith, 2003) If you are using the wrong tool during a measurement or reading it wrong, but getting consistent results, you have precision but not accuracy. That is because you are getting the same results each time, but the measurment process is inaccurate. But if you are using the right tool and reading it correctly, then you will get accuracy and precision.
Pay attention to the pressure used when measuring. If you use too much pressure or too little, you can get an less precise reading (especially if the pressure changes each time you measure) and therefore an inaccurate reading. Don’t use a micrometer like a C-clamp to hold a part in place. That is too much pressure and will affect the reading. Same goes for using too little pressure. If contact with the part is too light, then the reading is skewed. Always use consistent pressure.
Take care of your measuring tools. First of all, inspect the tool. Is there a calibration sticker on it stating when it was calibrated and when the calibration expires? Is the tool in good condition? Does all the parts of it work as expected? Is the tool clean of debris and dirt that could affect its use or the measurement? Has the tool been dropped? Have you tested the tool on a part that already has had its measurment determined? Has the tool been stored properly to prevent damage? Measuring tools should not be piled on each other and usually should be kept in individual cases. Is the tool showing any sign of wear, especially on the measuring and fixed surfaces?
Learning about and becoming proficient with your measuring tools will ensure that your work will be more consistant and error free. And, don’t forget the old adage when doing your work; “Measure twice and cut once!”
Sources: The Quality Technician’s Handbook by Griffith
Technology of Machine Tools by Krar and Check