Introduction: What are Digital Calipers?
Typically there are 3 types of engineering measurement calipers in the scope of this article. The three types are the standard mechanical vernier calipers, the Dial Calipers and the Digital Calipers. This article will focus on the digital version. Of course there are many other types of calipers, even calipers without any graduation or way of reading a measurement. For the purposes of this article we will focus on the digital type. All three types of calipers offer high levels or repeatability and accuracy. The mechanical way in which all these caliper types is also the same. The main difference is how the caliper transfers the measurement into a number of units that the operator, or user can read and interpret. A dial Caliper, displays the measurement on a dial, similar to that of an old fashioned speedometer. Vernier calipers take their measurements using a number of scaled graduations that the user reads, interprets and calculates in order the get the correct measurement. A set of Digital Calipers displays the measurement instantly on an electronic screen. In all of the options, assuming the same precision and resolution, the resultant measurement would be exactly the same. There is also another choice, which is called a Digital Vernier Caliper. We will talk about this in more detail later.
Calipers are often seen as a critically important and common tool to have to the majority of machining workshops. It would also be fair to say that the variety of caliper iterations could be limitless. Often, bespoke or unusual part design require custom made calipers, or more specifically, it is usual to have calipers with custom made jaws.
Calipers are extremely common in machine shops. In fact, it would probably be safe to say that every machine shop in the world has some sort of variation of the caliper. They are extremely useful for general dimension measurements as well as measuring the diameter of round objects or holes. A good example of the versatility of a caliper would be to imagine a hollow cylinder boss. A vernier caliper could be used to measure the outside diameter of the tub, the inside diameter of the hole and the depth of the boss.
What are Digital Calipers and how accurate are they?
Digital Calipers display the real time position, or distance measurement on a screen. This is most often a liquid crystal display (LCD). Some screens offer large displays to make them easier to read. Digital calipers are much quicker to use than Vernier Calipers and you don’t need to do any addition or subtraction to get the final measurement distance. Digital Calipers often have the ability to convert between metric and imperial measurements. It is also typical that digital calipers have the ability to zero the distance and any point, in order to make taking step measurements much easier.
It is generally considered that digital calipers, or calipers of any type, are less accurate that micrometers of the same league. It is typical for a pair of high quality vernier calipers to be correct to 10 microns, or micrometers (0.01 mm / 1/1000th inch.). A high quality micrometer on the other hand most often measures accurately and faithful to its name, down to single micrometers, or microns (0.001 mm).
Digital calipers use a simple linear input, to linear output while micrometers use rotary input to convert into rotary linear movement of the spindle. This enables micrometers to be generally more accurate. It also has the effect of making calipers easier to use. Micrometers can apply a large amount of force which if overtightened, can cause skewing of the measurement to smaller values that reality. The repeatability consistency of micrometer readings takes more experience and practice to become competent. Digital Calipers, or other types of caliper do not require this level of subtle skill to use.
Basic Principle: How do Digital Calipers work?
Without going into too much technical, mind boggling (well, mind boggling to me) detail about how these systems work, we will name the key components that make digital calipers work and hopefully cover enough of the basics to give you a good idea of how they function.
In the diagram above we have highlighted two of the main part areas in the digital measurement system. The Blue area is where the scale is marked on a pair of vernier scales. On a set of digital calipers there is also a scale on this area, but it is often hidden, and it comes in the form of a code that can be read and interpreted. The slider section, highlighted in yellow, is where the electronics are housed. In the slider housing there is a sensor that reads the signals taken from the hidden scale that is on the main steel body of the caliper (highlighted in blue). These signals are interpreted by a computer chip and converted into a digital readout that is displayed on the LCD screen.
What is the difference between an incremental and absolute digital caliper?
There are typically two types of technology for interpreting these signals. These types are commonly referred to as incremental and absolute. An incremental system relies on a reference point in order to calculate and display its position relevant to that reference. In other words, an incremental system can only read its position relevant to a set zero. An absolute system is able to read its actual position on the scale by reading its real time position, as opposed to incrementally adding or subtracting its position from a reference point.
What are the Advantages & Disadvantages of Digital Calipers?
What are the Advantages of Digital Calipers?
The headline features and benefits of Digital Calipers are as follows...
- Very quick and easy to take readings.
- Can quickly choose between Metric or Imperial.
- Able to re-set zero point.
- Incremental or Absolute Measurements are an option on some models.
- Longer lengths can be measured over micrometers.
What are the Disadvantages of using Digital Calipers?
The limitations and considerations of using vernier calipers are as follows...
- Batteries are required.
- Can be more sensitive to impact and shocks.
- Basic models can be sensitive to liquids.
What are the parts of a Digital Vernier Caliper?
What are the parts of a Digital Caliper?
1) Outside Large Jaws.
Typically used to measure the outside, or external diameter of an object, like a cylinder or rod. Also used for measuring the width of features like thickness or a boss width.
2) Inside Small Jaws.
Intended to measure the internal diameter or distance of an object like a hollow cylinder or a pocket feature width.
3) Depth Probe, or Depth Rod.
Used to measure the depth of objects like a hole of or cavity pocket feature.
4) LCD Screen
Displays to measurement in real time in either metric or imperial.
5) Vernier Scale (Imperial Graduations).
Scale marked imperial graduations, usually in inches and fractions to help measure length in imperial.
6) Caliper Controls.
On/Off Button. mm/inches conversion. Zero Button. Incremental or Absolute Mode.
7) Lock Screw or Retainer.
Used to lock measurement positions to make it easier to read results without worrying about false adjustment.
8) Vernier Scale (Metric Graduations).
Scale marked metric graduations, usually in millimetres and centimetres.
9) Main Body & Stator
The main spine of the caliper that contains the coded scale that the slider reads
10) Slider Body
The slider body contains all the electronics that read the position data from the stator and converts that into a display on the LCD Screen.
How to Interpret a Vernier Calipers Specified Resolution & Accuracy?
A set of vernier calipers should state at least two sets of accuracy specifications to let the operator know how accurate, or to what tolerance the calipers are designed to measure to. We will define what both these measurements mean below...
What is the Resolution on Digital Vernier Calipers?
A quality pair of Digital Calipers should state 2 measurements of accuracy. One of these is Resolution. This is basically the smallest graduation of markings on the smallest scale. In other words, it is the smallest distance that you can read from using the scales used on that model. In most cases, on a high quality pair of calipers, this is 0.01 mm.
What is the accuracy specificaiton of Digital Calipers?
The second quoted specification for how precise a set of digital vernier calipers are is the 'Accuracy'. This is the overall accuracy of the instrument over any measurement. For example, if a part is measured at 102.04 mm that part would be accurate to plus or minus whatever the accuracy specification is.
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