A comparator is a device used to measure the dimensional difference by comparing the manufactured part with the standard part. The comparators are classified into multiple categories like mechanical comparators, pneumatic comparators, optical comparators, electrical comparators, fluid comparators, etc.
They are classified into the above categories depending on their construction and their working principle. Under each category, there are multiple comparator types available today. In this article, we will be discussing dial indicators in brief.
What is a dial gauge?
Dial gauges (otherwise called dial indicators) are small mechanical devices used to sense and indicate the dimensional variation in the workpiece or the manufactured part. From an appearance perspective, it mostly looks like a small clock-like device with a plunger protruding at the bottom.
It works on the basic principle of gear and pinion mechanism. Dial gauges are one of the simplest and most widely used mechanical comparators. Since it is a type of mechanical comparator, its construction and working mechanism are based on mechanical elements.
In most cases, dial gauges and V blocks are used together to check the roundness or cylindricity of the workpiece. Now let us see about the parts used in constructing this comparator.
Parts of dial Gauge (Dial indicator)
The dial gauge is constructed using various parts, they are
- Bezel Clamp
- Dust Cap
- Needle or pointer
- Scale / Dial
- Contact point
- Revolution counter
- Gears and pinions
- Rack guide
- Mounting lug
The bezel is an important part because it holds the entire dial setup within itself and that includes the internal arrangement of the parts. In other words, it is called a case. Bezels are mostly made up of metals.
A bezel clamp is placed over the bezel. It is also called a scale locking screw and locknut. The main function of the bezel clamp is to lock the dial indicator scale after it is calibrated. This acts as the main part to achieve precise measurement.
Dust cap can be found in most of the dial indicator variants. The main function of a dust cap is to prevent dial indicators from moisture and dust particles. In other words, the dust cap is named dust guard. Dust caps are made up of rubber.
The needle inside the dial is used to indicate the dimensional variations of the workpiece. The displacement of the needle is measured with respect to the graduated scale.
The graduated scale is otherwise called a dial. The dial has a set of graduation markings in an angular scale which is helpful in measuring the needle displacement and the least count.
The spindle with a stem or plunger holds the dial on the top and the contact point on the bottom. Also, it provides the necessary length for the instrument to measure.
The contact point is the most important part of the dial gauge. It is used to sense the dimensional variations in the workpiece. There are various shapes of contact points available for different applications. The different types of contact points based on their shapes are mentioned below.
- Standard contact point
- Tapered contact point
- Button contact point
- Regular flat contact point
- Button type flat contact point
- Wide face flat contact point
The contact points can be replaced and interchanged. They are made up of hardened and wear-resistance materials like heat-treated steel, boron carbide, diamond, etc. Each contact point is used for measuring different materials.
The Revolution counter in the dial indicator is a small dial with graduated markings that indicates the revolution for the main scale. The small scale tells the number of turned revolutions of the pointer in the main scale which is equal to the 1 mm distance of plunger displacement. This is the role of the revolution counter in the dial gauge.
Gears, Pinions, and racks
Since it is a mechanical comparator, the mechanical parts like gears, pinions, springs, and Racks play a major role. Their main purpose is to transform linear displacements into radial movement. These parts are made up of metals.
The rack guide is fixed along with the plunger to give necessary gauging pressure during displacement. This gauging pressure is created by the coil spring.
Mounting lug is a metal part used to hold dial gauge in steady position in its laboratory setup. This metal part plays a major in order to get precise measurements.
Construction of Dial gauge
Now let us see about the construction of dial indicator.
Since dial gauge is a mechanical comparator, it is fully constructed using mechanical parts. It has plunger with detachable contact points outside the dial. The contact points can be detached and replaced with any type based on the use cases. In internal side of the dial, Plunger has an inbuilt rack gear setup. There is a rack guide and a coil spring setup outside the dial to keep plunger always in contact with the workpiece. The rack gear in plunger always meshes with the gears and pinions inside the dial setup.
Due to gears, wear can occur so light hair spring is placed inside the dial. Outside the dial there some important things like graduated measuring scale, revolution counter and needle to indicate the measurements. In order to prevent the internal arrangement of dial gauge from dust or moisture, a dust gap is attached with dial. And the dial gauge is mounted to laboratory setup using mounting lugs. This is how the dial gauge is constructed.
Working mechanism or principle of dial gauge
Now let us discuss about the working mechanism of dial indicator.
The plunger or spindle is always kept in a normal extended position with the help of rack guide and light coil spring setup. Both workpiece and plunger are always kept in surface contact. When there is a dimensional variation in workpiece, there is displacement in plunger. To prevent the rotation of the plunger, there is a rack gear present at its top end. The rack gear in spindle is meshed with gear A. Following gear, A, there a big gear B which is mounted on the same shaft where gear A is mounted.
The displacement of spindle gets transferred through gears A and B. It is also transferred to gear C and D, since gear B and C is in contact. Both gear C and D are arranged in a same way as gears A and B, where gear B and C are meshed together. Also gear D is meshed with gear E, which is mounted on the separate shaft or spindle.
The gear F which is mounted on same spindle where pointer is also mounted, also meshed with gear E. Finally, the displacement of spindle is converted to displacement of pointer with the help of gears, this helps to find the surface finish and dimensional variations present in the workpiece. Since the working mechanism consists of mechanical parts, this is a good example of a perfect mechanical comparator.
Types of Dial Gauge
There are many types of dial indicator available in the market today. The different types of dial indicator are listed below.
- Type A dial gauge
- Type B Dial Gauge
- Type C Dial Gauge
- Micrometer dial gauge
- Lever type dial gauge
- Digital Indicator
Type A Dial Gauge
The Type A dial gauge is with reverse measuring force with the help of rack guide setup. It is also shock resistance because there is sleeve which floats over the plunger. So, this is the reason of shockproof movement through sleeve in this dial gauge. Also, the coil spring with rack guide arrangement in this comparator provides a constant measuring force.
This comparator has a protective housing (called back house integrated in housing) which is coated with chrome metal. It also has built-in tolerance makers out of the box. These are the important features in Type A dial gauge.
Type B Dial Gauge
Type B dial gauge looks similar to type A. It also has shock proof movement via sleeve which floats over spindle, and it has constant measuring force due to the coil spring arrangement like type A dial gauge. It also has a chrome plated protective housing with built in adjustable tolerance makers. The readings in type will be 0, 1mm and 1 pointer movement on 10mm (measuring range is about only 10mm). This comparator has a plastic case.
Type C Dial Gauge
Type C Dial gauge has a vast difference compared to type A and type B. This comparator has a very long spindle compared to others, so it has extra-large measuring range. The measuring range of type C comparator is four times (40mm range) greater than type A. The strengthened spindle about 5 mm is there at the at top arrangement. The measuring spindle can also be raised via the lifting cap. It is shockproof movement while the spindle because of sleeve floating over the plunger. The Type C dial gauge is packed and delivered in folded protective case.
Micrometer Dial Comparator
Micrometer dial comparator looks similar to micrometer and the procedure to use this comparator is also same as micrometer. That’s why this type of dial indicator is named as micrometer dial gauge. The construction of this comparator consists of a dial integrated with the micrometer frame, sleeve, rachets stop, thimble and anvils. There are both fixed and movable anvils.
The micrometer frame is made up of stainless steel and chrome plated. It is heat resistance. Like frame, fixed anvil is also made up of stainless steel and movable anvil has carbide tipped faces at end for wear resistance.
The micrometre dial gauge is mostly used to measure the diameter of wire and cylindrical parts like bolts, small shafts etc. It is also used to measure thickness and some linear dimensions and to measure precise standard parts.
Lever Type Dial Indicator
Lever Type Dial indicator or test type dial indicator is similar to dial gauge, the plunger system is constructed in a typical way where it can achieve displacement in smaller range. In above mentioned dial gauges, the plunger moves in and out direction but in lever type dial indicator plunger moves up and down because of lever.
The plunger in lever type dial indicator is an angular movement type but in normal dial indicator it is linear moving plunger. Actually, plunger is constructed in a different way in this comparator. It has a ball tipped lever arm pivoted with comparator body. The ball tipped lever arm is called stylus. This comparator is constructed using various parts such as stylus, pivot, lever, return spring, scroll and pointer. The stylus or contact points consists of a chromium plated ball.
The lever is supported with the help of small ball bearings in pivot. Simultaneously the overload protection is there which is provided by slip clutch in pivot. The lever setup is arranged inside a box type housing which is made up of metal with satin chrome finish. Whenever the stylus moves in either direction the lever rotates the scroll with the help of pivot.
The movement of scroll gets transferred to the pointer on the main scale. The range of magnification is very limited in lever type dial gauge. The measuring range is 0.030 which is usually smaller compared to dial gauges, this is because the range is limited by the scroll.
Lever type dial gauges are used in wide range of applications like checking cylindricity and run out for shafts, bores etc., checking parallelism and squareness of flat surfaces etc.
Digital dial indicator
Digital dial indicator is a modern type indicator which has more flexibility compared to other traditional comparators. It has the same operating mechanism, but the ready graduated dial scale and pointer is replaced by a digital screen. Sensors are placed inside the dial which helps to sense the displacement and shows the deviation measurement on the screen.
It has some extra features like, it can display multiple unit system like mm and inches at a same time which is not possible in ordinary dial gauges. Digital dial comparators are more precise and has higher measuring range. Although it has lot of advantages, there is some disadvantages. Since it is digitalized, it is mainly depended on external power source. A small battery powers this device.
Requirements of a good dial gauge
To define a good dial indicator, there are certain requirements. Let’s discuss the requirements of a good dial indicator.
- The dial gauge should be constructed and designed in robust way in order to give readings without any mistakes or errors.
- The needle or pointer should be properly damped with the pointer axis shaft. If it is loosely damped, it won’t give correct readings while readings being taken.
- The spindle or plunger displacement should not be larger, because that can damage the instrument. There should be some mechanism to control the displacement.
- While measuring, the plunger should able to move freely without any vibration or friction in order to get precise measurement.
- The pointer movement and the plunger movement should be relative while measuring.
- Since it is a precision device, it should give accurate readings.
- The instrument should be able to calibrate and the reverse measuring force should remain constant over whole range.
Advantages of dial gauge
- The design and construction of dial gauge is robust and compact.
- Dial gauges are portable, easy to carry since they are small in size.
- Even unskilled and semiskilled labour can use this, since it is easy to operate.
- The outputs are easy to read
- It is cheaper compared to other comparators
- The measurement output is accurate and precise
- It doesn’t require any external power source to operate
- Availabilities of different varieties for different applications
- It is the best suitable for mass production dimension control
- There is no vibration and less friction
- Compact in size and portable
Disadvantages of dial gauge
- Since it is a mechanical comparator, wear and tear occurs due to friction
- Wear in internal mechanical parts sometimes result in errors in measurements
- Due to gears and racks packed inside, backlash occurs and this leads to low accuracy compared to non-mechanical comparators
- Variation in contact pressure causes more errors
- It needs an external laboratory setup like base table, clamping stand etc.
- Digital type dial gauge which is non mechanical definitely needs an external power source to operate
- Due to internal mechanical parts, slowness may happen sometimes
- Looseness in gear arrangement may result in wrong measurement outputs
- Since reading scale is fixed, it has a limited range of measurement
Application & uses of dial gauge
- Dial gauges are mostly built in with other measuring system as a read-out device.
- It is widely used measure the linear dimensional variations of a workpiece.
- It is used to measure the workpiece alignment, squareness, roundness, parallelism, cylindricity etc.
- Even dial gauges can measure the thickness of wires cables etc, there is a specific type of dial gauge available for this kind of applications.
- It is used along with sine bar for angle measurement.
- It is also used along with Parkinson gear tester, pocket thickness gauge etc.
- It is also used along with lath machines to check the cylindricity and ovality of the workpiece.