Dial Indicator Types? ADVANCED Metrologist Guide 2021!
Welcome to the DML guide on dial indicator types. Before looking at the different types of dial indicators, let’s quickly recap what dial indicators are and discuss some of the applications dial indicators are used for across various industries.
What is a Dial Indicator?
A dial indicator (also known as a DTI, clock indicator, dial gauge, probe indicator or comparator) is a precision measuring instrument used for several applications across many industries where precision is necessary, such as manufacturing, packaging, engineering design.
What Are Dial Indicators Used For?
Dial indicators are well suited to quality inspections. They can be used to monitor machinery, ensuring the tolerances it’s operating to are true and that manufactured components are within desired tolerances .
How Does a Dial Indicator work?
Typically a dial indicator has a spindle and a clock-face. Any force or pressure applied to the contact point of the spindle will push the spindle up into the body of the indicator or rotate the spindle around an axis. Gears in the indicator body magnify the displacement as it moves. The clock face responds to this, showing a reading through a scale and needle on analogue models, and in digital format on digital models.
The Dial Indicator Types
There are two main dial indicator types:
- Lever Dial Test Indicator
- Plunger Dial Indicator
Both dial indicator types have similar features while functioning differently. Understanding the differences in function and the variations in the characteristics of each will help you determine which is the correct instrument for your application.
In this article, we go over each indicator type in detail. We discuss functions, features, and their typical applications.
Let’s start with Plunger Dial Indicators.
Plunger Dial Indicator
Plunger Dial Indicators (often referred to simply as Dial Indicators) use a plunger that moves in a straight line to magnify and convert the force applied to the contact point (at the bottom of the plunger) into a measurement.
Features and Uses
Generally larger and heavier than test lever dial indicators, plunger dial indicators are most suited to repetitive, comparative measurements. An example of this is that once set up on a stand or height gauge, plunger dial indicators are great at quickly checking the measurements of a batch of components to ensure they are within the required tolerance.
Plunger dial indicators are also often used in machine setups to monitor changes in the alignment of machine parts and components. The plunger dial indicator needs to be positioned at an angle of 90° to function accurately. Using an indicator stand is the best way to do this, with the plunger slotting into a mounting.
Plunger dial indicators are also better suited for measuring larger movements than lever dial test indicators, due to cosine error, explained below.
Lever Dial Test Indicator
Lever Dial Test indicators are also known as a lever-type indicator, a DTI or simply a dial test indicator. The bottom of the body houses a pivot with a probe (the lever), its distinguishing feature when visually comparing the two dial indicator types.
Features and Uses
Lever dial test indicators are more suited for measuring small movements than their larger cousins, for example checking linear and geometric features on a component. Lever indicators are commonly used to measure the amount of material removed during grinding or milling.
The lower spring force and angled movement of the lever responds dimensional changes. Generally speaking lever indicators are used to monitor small changes during machining or to measure geometric features such as ovality or runout.
Probe/Contact Point Length and Diameter
The length and diameter of the contact point needed will depend on the application. A long thin probe may be required to reach into a narrow recess for example. It is possible to purchase probes/contact points of different sizes for specific uses.
However, it is important to use the brands (or brand recommended) products and to calibrate the instrument ready for the new application. Using a length of probe, that the test indicator is not designed to accommodate can cause incorrect magnification of the force, resulting in incorrect measurements/readings.
Considering this, buying a lever dial test indicator with a specifically suited probe/contact point for each application is possibly the best bet. The shape, size, and material of the contact point is a consideration discussed later in the article, and is relevant for both dial indicator types.
A little basic trigonometry (if there’s such a thing!) is required to explain and fix the most common error when using a test lever dial indicator.
What is Cosine Error?
Before we get into any formulas, let’s explain what causes the error.
The test lever dial indicator provides the most accurate measurements when its probe is positioned parallel to the surface of the item being measured, as shown here.
If it isn’t possible to position the dial indicator parallel to the measuring object then the probe can be angled while the contact point achieves the required parallel position, as shown below.
When the contact point of the measuring probe is not parallel to the measuring surface, it will result in inaccurate measurements, an error known as a cosine error.
How to Avoid Cosine Error
To avoid cosine errors the test lever indicator must be set on a stand and positioned at a 0° angle, ensuring it is parallel or radial to the measuring surface. However, depending on the application, environmental factors such as material position and available space mean this is not always possible. Cosine errors will inevitably occur in some situations, and it is important to know how to adjust for them.
How to Fix Cosine Error
This is where the math comes in. The triangle formed between the contact point/probe and the measuring surface can calculate the amount of error present in the measurement.
We need to know the unknown angle of the right-angled triangle, which is calculated like this:
- We know the length of the hypotenuse – it is the length of the contact point.
- We (should) know the length of the adjacent side of the triangle – it is the length of (or part of) the measuring surface.
- We then calculate adjacent/hypotenuse.
- We then grab our calculator and hit the ‘cos-1’ key!
Now we know the angle we use our calculator again to establish the adjustment we need to make to the measurement.
- Put the angle into your calculator and push the ‘cos’ key.
- Multiply the resulting number by the measurement reading, which will give you the correct measurement – simple!
Note: We’ve tried to offer a quick and easy to understand calculation explanation. For a full recap on sine, cosine, tangent, and calculating angles check out this great article.
Getting into correction formulas as above may or may not be suitable or needed for your situation and application. Other factors and features of lever test dial indicators can help. A tear-drop shaped contact point on the probe can negate cosine error up to an angle of 35°.
Now we’ve discussed the two dial indicator types, let’s look at the common features of the two instruments that you’ll need to consider.
What to look for in a Dial Indicator
As discussed in the article, the two dial indicator types are better suited for different applications. Hopefully, after reading this far, you know exactly which one you need!
The range of the instrument on both dial indicator types will determine the size of difference it can measure. lever Dial Test Indicators generally have a much smaller measuring range then plunger type indicators.
The resolution of the instrument is the smallest change in the distance the dial indicator will read. The higher the resolution, the more accurate the instrument is. However, the higher resolution models will also be more expensive, so choosing a product with sufficient resolution for your application is important when considering both dial indicator types.
Is how close to the actual measurement the instrument will consistently measure. The measurement the instrument gives may be above or below the actual measurement within the stated accuracy. i.e. ±0.001mm
Is how accurately the instrument will repeat a measurement when used by the same person, in the same conditions to measure the same item.
Generally more of an issue on plunger dial indicators, hysteresis is the delay between a force applied to the contact point of the plunger/lever, being transferred through the gears and the dial responding to the force. Although an extremely small error on most models, it is a consideration for some precision applications.
Back Type – Flat or Lug
Both dial indicator types may be available with either a ‘flat back’ or a ‘lug back’. Flatback dial indicators are held on a stand by either fastening the stem to the stand, or a magnetic force holding the indicator in place. Lug backs have a lug to be held through a screw attachment.
Plunger/Lever length and Diameter
The length and diameter of the plunger/lever will determine where the dial indicator can be positioned in relation to the item that needs to be measured, and thus its suitability for your application.
Whether you need a dial indicator that measures using the imperial or metric measuring system will depend on your personal preference, your country of origin, and/or the industry you work in. However, most digital models will provide readings in both systems, convenient in several situations.
Types of Contact Points
The shape of contact points and the material they are made from makes them more suited for different applications. Here’s a quick breakdown of the main contact point types.
- Ball Point Contact Points – Standard contact points generally as supplied.
- Shell Type Contact Points – Larger points of contact.
- Flat Contact Points – Work well on convex surfaces
- Blade Contact Points – Great for convex surfaces with grooves
- Needle Contact Points – The pointed tip can reach into holes or deep grooves
- Knife Edge Contact Points – Can fit into thin grooves
- Conical Contact Points – Work well for positioning measurement points. However, the sharp tip can damage soft materials with
- Larger Spherical Contact Points – Bigger contact points which are used when the material needs to be moved from side to side
- Roller Contact Points – Similar to the spherical points, roller contact points are used on a moving test piece.
The material the contact point is constructed from also needs to be considered.
- Carbide – Strong and long-lasting, the most common points currently available
- Ruby – High levels of compression strengths, also resistant to corrosion
- Steel – Cost effective but will generally wear quicker than carbide or ruby
As described in this article, many applications require both types of dial indicators to be set on a stand to ensure accuracy. An indicator stand will hold your dial indicator in a fixed position to ensure it measures the item at the correct point in the best position. Using the most suitable type of indicator stand for your application is essential to ensure this happens.
Common types of stands:
- Comparator stand
- Magnetic stands
Comparator stands use a flat base with a smooth surface for the measuring application to be placed on and measured. Having a flat surface with rest points allows for precise measurements.
The indicator stand base plate can come in different forms. Cast iron, steel, and granite are common materials used.
Comparator Stand Base Types:
- Flat Base – perfectly flat and smooth base.
- Flat base with Grooves – some comparator stands have grooves in the base.
- Serrated Base – The item rests on the serrated anvil surface, maximising flatness.
The magnetic base allows the stand to be fixed to any magnetically attractive surface. An on/off switch enables and disables the magnet to allow for easy removal and positing of the stand, without damaging the mounting surface.
The arm of magnetic stands often incorporates joints and a fine adjustment mechasism. The joints allow for further flexibility when positing the instrument the stand is holding, and the fine adjsutment mechanism is for making precise adjustments to the instruments position.
Granite Based Comparator Stands
The rigid granite base remains free from burrs, pileups, rust, and deterioration over time.
- Indicator mount – this is where the indicator will attach. Granite stands, as most types of stand, will also mount lug back indicators.
- Coarse adjustment – this adjustment handle will adjust the height of the mounted indicator so that it can be positioned at the correct height to measure the chosen item.
- Fine-tuning – This adjustment fine-tunes the position of the indicator. These small changes in the height of the stand are useful when used in conjunction with the indicator stroke, fine-tuning the position of the indicator within its range.
Both plunger and lever dial indicators are commonly used and essential precision measuring instruments, offering accuracy and flexibility across a wide range of applications. Both dial indicator types are simple in operation but can be used in complex measuring setups. Lever Dial test indicators are often used incorrectly due to cosine error so be sure to take this into consideration next time your planning your test. If you are unsure about the suitability of a dial indicator for your application, we can provide expert advice.
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