Engineering Drawing

Introduction

One of the best ways to communicate one's ideas is through some form of picture or drawing. This is especially true for the engineer. The purpose of this guide is to give you the basics of engineering sketching and drawing.
We will treat "sketching" and "drawing" as one. "Sketching" generally means freehand drawing. "Drawing" usually means using drawing instruments, from compasses to computers to bring precision to the drawings.
This is just an introduction. Don't worry about understanding every detail right now - just get a general feel for the language of graphics.
We hope you like the object in Figure 1, because you'll be seeing a lot of it. Before we get started on any technical drawings, let's get a good look at this strange block from several angles.

Figure 1 - A Machined Block

Isometric Drawing

The representation of the object in figure 2 is called an isometric drawing. This is one of a family of three-dimensional views called pictorial drawings. In an isometric drawing, the object's vertical lines are drawn vertically, and the horizontal lines in the width and depth planes are shown at 30 degrees to the horizontal. When drawn under these guidelines, the lines parallel to these three axes are at their true (scale) lengths. Lines that are not parallel to these axes will not be of their true length.

Figure 2 - An Isometric Drawing

Any engineering drawing should show everything: a complete understanding of the object should be possible from the drawing. If the isometric drawing can show all details and all dimensions on one drawing, it is ideal. One can pack a great deal of information into an isometric drawing. However, if the object in figure 2 had a hole on the back side, it would not be visible using a single isometric drawing. In order to get a more complete view of the object, an orthographic projection may be used.

Orthographic or Multiview Drawing

Imagine that you have an object suspended by transparent threads inside a glass box, as in figure 3.

Figure 3 - The block suspended in a glass box

Then draw the object on each of three faces as seen from that direction. Unfold the box (figure 4) and you have the three views. We call this an "orthographic" or "multiview" drawing.

Figure 4 - The creation of an orthographic multiview drawing

Figure 5 shows how the three views appear on a piece of paper after unfolding the box.

Figure 5 - A multiview drawing and its explanation

Which views should one choose for a multiview drawing? The views that reveal every detail about the object. Three views are not always necessary; we need only as many views as are required to describe the object fully. For example, some objects need only two views, while others need four. The circular object in figure 6 requires only two views.

Figure 6 - An object needing only two orthogonal views

Dimensioning

Figure 7 - An isometric view with dimensions

We have "dimensioned" the object in the isometric drawing in figure 7. As a general guideline to dimensioning, try to think that you would make an object and dimension it in the most useful way. Put in exactly as many dimensions as are necessary for the craftsperson to make it -no more, no less. Do not put in redundant dimensions. Not only will these clutter the drawing, but if "tolerances" or accuracy levels have been included, the redundant dimensions often lead to conflicts when the tolerance allowances can be added in different ways.
Repeatedly measuring from one point to another will lead to inaccuracies. It is often better to measure from one end to various points. This gives the dimensions a reference standard. It is helpful to choose the placement of the dimension in the order in which a machinist would create the part. This convention may take some experience.

Sectioning

There are many times when the interior details of an object cannot be seen from the outside (figure 8).

Figure 8 - An isometric drawing that does not show all details

We can get around this by pretending to cut the object on a plane and showing the "sectional view". The sectional view is applicable to objects like engine blocks, where the interior details are intricate and would be very difficult to understand through the use of "hidden" lines (hidden lines are, by convention, dotted) on an orthographic or isometric drawing.
Imagine slicing the object in the middle (figure 9):

Figure 9 - "Sectioning" an object

Figure 10 - Sectioning the object in figure 8

Take away the front half (figure 10) and what you have is a full section view (figure 11).

Figure 11 - Sectioned isometric and orthogonal views

The cross-section looks like figure 11 when it is viewed from straight ahead.

Drawing Tools

To prepare a drawing, one can use manual drafting instruments (figure 12) or computer-aided drafting or design, or CAD. The basic drawing standards and conventions are the same regardless of what design tool you use to make the drawings. In learning drafting, we will approach it from the perspective of manual drafting. If the drawing is made without either instruments or CAD, it is called a freehand sketch.

Figure 12 - Drawing Tools

"Assembly" Drawings

An isometric view of an "assembled" pillow-block bearing system is shown in figure 13. It corresponds closely to what you actually see when viewing the object from a particular angle. We cannot tell what the inside of the part looks like from this view.
We can also show isometric views of the pillow-block being taken apart or "disassembled" (figure 14). This allows you to see the inner components of the bearing system. Isometric drawings can show overall arrangement clearly, but not the details and the dimensions.

Figure 13 - Pillow-block (Freehand sketch)

Figure 14 - Disassembled Pillow-block

Cross-Sectional Views

A cross-sectional view portrays a cut-away portion of the object and is another way to show hidden components in a device.
Imagine a plane that cuts vertically through the center of the pillow block as shown in figure 15. Then imagine removing the material from the front of this plane, as shown in figure 16.

Figure 15 - Pillow Block	Figure 16 - Pillow Block

This is how the remaining rear section would look. Diagonal lines (cross-hatches) show regions where materials have been cut by the cutting plane.

Figure 17 - Section "A-A"

This cross-sectional view (section A-A, figure 17), one that is orthogonal to the viewing direction, shows the relationships of lengths and diameters better. These drawings are easier to make than isometric drawings. Seasoned engineers can interpret orthogonal drawings without needing an isometric drawing, but this takes a bit of practice.
The top "outside" view of the bearing is shown in figure 18. It is an orthogonal (perpendicular) projection. Notice the direction of the arrows for the "A-A" cutting plane.

Figure 18 - The top "outside" view of the bearing

Half-Sections

A half-section is a view of an object showing one-half of the view in section, as in figure 19 and 20.

Figure 19 - Full and sectioned isometric views

Figure 20 - Front view and half section

The diagonal lines on the section drawing are used to indicate the area that has been theoretically cut. These lines are called section lining or cross-hatching. The lines are thin and are usually drawn at a 45-degree angle to the major outline of the object. The spacing between lines should be uniform.
A second, rarer, use of cross-hatching is to indicate the material of the object. One form of cross-hatching may be used for cast iron, another for bronze, and so forth. More usually, the type of material is indicated elsewhere on the drawing, making the use of different types of cross-hatching unnecessary.

Figure 21 - Half section without hidden lines

Usually hidden (dotted) lines are not used on the cross-section unless they are needed for dimensioning purposes. Also, some hidden lines on the non-sectioned part of the drawings are not needed (figure 12) since they become redundant information and may clutter the drawing.

Sectioning Objects with Holes, Ribs, Etc.

The cross-section on the right of figure 22 is technically correct. However, the convention in a drawing is to show the view on the left as the preferred method for sectioning this type of object.

Figure 22 - Cross section

Dimensioning

The purpose of dimensioning is to provide a clear and complete description of an object. A complete set of dimensions will permit only one interpretation needed to construct the part. Dimensioning should follow these guidelines.

1. Accuracy: correct values must be given.
2. Clearness: dimensions must be placed in appropriate positions.
3. Completeness: nothing must be left out, and nothing duplicated.
4. Readability: the appropriate line quality must be used for legibility.

The Basics: Definitions and Dimensions

The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end (figure 23).

Figure 23 - Dimensioned Drawing

An arrowhead is approximately 3 mm long and 1 mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line. The first dimension line should be approximately 12 mm (0.6 in) from the object. Extension lines begin 1.5 mm from the object and extend 3 mm from the last dimension line.
A leader is a thin line used to connect a dimension with a particular area (figure 24).

Figure 24 - Example drawing with a leader

A leader may also be used to indicate a note or comment about a specific area. When there is limited space, a heavy black dot may be substituted for the arrows, as in figure 23. Also in this drawing, two holes are identical, allowing the "2x" notation to be used and the dimension to point to only one of the circles.

Where To Put Dimensions

The dimensions should be placed on the face that describes the feature most clearly. Examples of appropriate and inappropriate placing of dimensions are shown in figure 25.

Figure 25 - Example of appropriate and inappropriate dimensioning

In order to get the feel of what dimensioning is all about, we can start with a simple rectangular block. With this simple object, only three dimensions are needed to describe it completely (figure 26). There is little choice on where to put its dimensions.

Figure 26 - Simple Object

We have to make some choices when we dimension a block with a notch or cutout (figure 27). It is usually best to dimension from a common line or surface. This can be called the datum line of surface. This eliminates the addition of measurement or machining inaccuracies that would come from "chain" or "series" dimensioning. Notice how the dimensions originate on the datum surfaces. We chose one datum surface in figure 27, and another in figure 28. As long as we are consistent, it makes no difference. (We are just showing the top view).

Figure 27 - Surface datum example

Figure 28 - Surface datum example

In figure 29 we have shown a hole that we have chosen to dimension on the left side of the object. The Ø stands for "diameter".

Figure 29 - Exampled of a dimensioned hole

When the left side of the block is "radiuses" as in figure 30, we break our rule that we should not duplicate dimensions. The total length is known because the radius of the curve on the left side is given. Then, for clarity, we add the overall length of 60 and we note that it is a reference (REF) dimension. This means that it is not really required.

Figure 30 - Example of a directly dimensioned hole

Somewhere on the paper, usually the bottom, there should be placed information on what measuring system is being used (e.g. inches and millimeters) and also the scale of the drawing.

Figure 31 - Example of a directly dimensioned hole

This drawing is symmetric about the horizontal centerline. Centerlines (chain-dotted) are used for symmetric objects, and also for the center of circles and holes. We can dimension directly to the centerline, as in figure 31. In some cases this method can be clearer than just dimensioning between surfaces.

MEMS Technology

Knowles' Micro Electro-Mechanical Systems (MEMS) technology platform brings you smaller product design possibilities and related cost advantages, including silicon-based, surface mount manufacturing.

Traditional uses of silicon involve creating pathways for electricity within components such as integrated circuits. In contrast, MEMS transforms silicon into mechanically moving parts. During the past decade, this process has become useful in an increasing number of industries. For example, the automotive market uses MEMS accelerometers to sense crashes and deploy airbags.

Our research in MEMS led to the introduction of the world's first surface mount microphone in 2001 to 2002, the SiSonic™ series. We have since helped major brands design breakthrough cell phone and consumer electronics products, while eliminating the cost of traditional offline ECM microphone subassemblies. 

MicroElectroMechanical Systems (MEMS) and Microrobotics research spans design methodologies, physical investigations and manufacturing techniques involving various microsensors, microactuators and other microsystems. Selected applications include inertial sensor suites for control and guidance, miniature wall-climbing robots using micro/nano-fiber adhesives, arrayed MEMS probe manipulators for tip-based nanomanufacturing, gas chemical sensor arrays for early warning systems, and ultra-compliant neural probes for brain-computer interfaces.

MEMS and Microrobotics RI faculty have developed strong multidisciplinary research programs on integrated MEMS design and fabrication (Fedder), micro and nanorobotic systems (Sitti), and implantable medical microsystems (Fedder, Weiss). This research links the RI to many departments throughout the University through active collaborations with faculty from across CIT and MCS as well as with Pitt and UPMC. These RI faculty members also have joint appointments in other entities. Expanded research in micro-inertial navigation technologies (Kelly and Fedder) is also planned to target new DOD initiatives.

The future for research in MEMS and microrobotics remains bright, with trends toward use of the technology to access and manipulate nano-scale phenomena as well as to enable emerging biomedical applications. RI faculty are taking leads in these areas in collaboration with faculty throughout the University. One of the challenges faced is to merge systems across macro-, micro-, and nano-scales. CMU's planned Nano/Bio/Energy Building (2014 expected date of completion) for nano- and biotechnologies will facilitate efforts in this area by providing state-of-the-art shared infrastructure for nano/microfabrication. In addition, a budding collaboration in multi-scale factory automation (Hollis, Fedder) is leveraging renewed efforts in reconfigurable, high-precision minifactories (Hollis).

There are several different broad categories of MEMS technologies, outlined below:

Bulk Micromachining:

Bulk micromachining is a fabrication technique which builds mechanical elements by starting with a silicon wafer, and then etching away unwanted parts, and being left with useful mechanical devices.  Typically, the wafer is photo patterned, leaving a protective layer on the parts of the wafer that you want to keep. The wafer is then submersed into a liquid etchant, like potassium hydroxide, which eats away any exposed silicon.  This is a relatively simple and inexpensive fabrication technology, and is well suited for applications which do not require much complexity, and which are price sensitive.

Today, almost all pressure sensors are built with Bulk Micromachining.  Bulk Micromachined pressure sensors offer several advantages over traditional pressure sensors.  They cost less, are highly reliable, manufacturable, and there is very good repeatability between devices.

All new cars on the market today have several micromachined pressure sensors, typically used to measure manifold pressure in the engine.

The small size and high reliability of micromachined pressure sensors make them ideal for a variety of medical applications as well.

Surface Micromachining:

While Bulk micromachining creates devices by etching into a wafer, Surface Micromachining builds devices up from the wafer layer-by-layer.

A typical Surface Micromachining process is a repetitive sequence of depositing thin films on a wafer, photopatterning the films, and then etching the patterns into the films. In order to create moving, functioning machines, these layers are alternating thin films of a structural material (typically silicon) and a sacrificial material (typically silicon dioxide).  The structural material will form the mechanical elements, and the sacrificial material creates the gaps and spaces between the mechanical elements. At the end of the process, the sacrificial material is removed, and the  structural elements are left free to move and function.

For the case of the structural level being silicon, and the sacrificial material being silicon dioxide, the final "release" process is performed by placing the wafer in Hydrofluoric Acid. The Hydrofluoric Acid quickly etches away the silicon dioxide, while leaving the silicon undisturbed.

The wafers are typically then sawn into individual chips, and the chips packaged in an appropriate manner for the given application.

Surface Micromachining requires more fabrication steps than Bulk Micromachining, and hence is more expensive.  It is able to create much more complicated devices, capable of sophisticated functionality. Surface Micromachining is suitable for applications requiring more sophisticated mechanical elements.

LIGA:

LIGA is a technology which creates small, but relatively high aspect ratio devices using x-ray lithography. The process typically starts with a sheet of PMMA. The PMMA is covered with a photomask, and then exposed to high energy x-rays.  The mask allows parts of the PMMA to be exposed to the x-rays, while protecting other parts.  The PMMA is then placed in a suitable etchant to remove the exposed areas, resulting in extremely precise, microscopic mechanical elements.

LIGA is a relatively inexpensive fabrication technology, and suitable for applications requiring higher aspect ratio devices than what is achievable in Surface Micromachining.

Deep Reactive Ion Etching:

Deep reactive ion etching is a type of Bulk Micromachining which etches mechanical elements into a silicon wafer. Unlike traditional Bulk Micromachining, which uses a wet chemical etch, Deep Reactive Ion Etching micromachining uses  a plasma etch to create features.  This allows greater flexibility in the etch profiles, enabling a wider array of mechanical elements. The fabrication tools needed to perform Deep Reactive Ion etching are somewhat expensive, to this technology is typically more expensive than traditional Bulk Micromachining based on wet etching.

Integrated MEMS Technologies

Since MEMS devices are created with the same tools used to create integrated circuits, in some cases it is actually possible to fabricate Micromachines and Microelectronics on the same piece of silicon.  Fabricating machines and transistors side by side enables machines that can have intelligence.  A number of exciting products are already taking advantage of this capability.

The micro mechanical device embedded with electronics/electrical system fabricated through a mix of integrated circuit manufacturing and micro-machining process where material is shaped by etching away micro layers is called Micro Electro Mechanical System (MEMS). The intelligent electronic system part is integrated in the same way of IC device fabrication. The most popular material used for MEMS is Silicon for it's semiconductor , physical and commercial properties.

Micro-Electro-Mechanical Systems consists of mechanical elements, sensors, actuators, and electrical and electronics devices on a common silicon substrate.

The sensors in MEMS gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. The advantages of semiconductor IC manufacturing such as low cost mass production, reliability are also integral to MEMS devices.
The size of MEMS sub-components is in the range of 1 to 100 micrometers and the size of MEMS device itself measure in the range of 20 micrometers to a millimeter.

Some of the advantages of MEMS devices are,

1. Very small size, mass, volume
2. Very low power consumption
3. Low cost
4. Easy to integrate into systems or modify
5. Small thermal constant
6. Can be highly resistant to vibration, shock and radiation
7. Batch fabricated in large arrays
8. Improved thermal expansion tolerance
9. Parallelism

Typical Applications:

There are plenty of applications for MEMS. As a breakthrough technology, MEMS is building synergy between previously unrelated fields such as biology and microelectronics, many new MEMS and Nanotechnology applications will emerge, expanding beyond that which is currently identified or known.

MEMS technology finds applications in the below general domains

Automotive domain:
1. Airbag Systems
2. Vehicle Security Systems
3. Intertial Brake Lights
4. Headlight Leveling
5. Rollover Detection
6. Automatic Door Locks
7. Active Suspension

Consumer domain:
1. Appliances
2. Sports Training Devices
3. Computer Peripherals
4. Car and Personal Navigation Devices
5. Active Subwoofers

Industrial domain:
1. Earthquake Detection and Gas Shutoff
2. Machine Health
3. Shock and Tilt Sensing

Military:
1. Tanks
2. Planes
3. Equipment for Soldiers

Biotechnology:

1. Polymerase Chain Reaction (PCR) microsystems for DNA amplification and identification
2. Micromachined Scanning Tunneling Microscopes (STMs)
3. Biochips for detection of hazardous chemical and biological agents
4. Microsystems for high-throughput drug screening and selection
5. Bio-MEMS in medical and health related technologies from Lab-On-Chip to biosensor & chemosensor.

The commercial applications include:

1. Inkjet printers, which use piezo-electrics or thermal bubble ejection to deposit ink on paper.
2. Accelerometers in modern cars for a large number of purposes including airbag deployment in    collisions.
3. Accelerometers in consumer electronics devices such as game controllers, personal media players     / cell phones and a number of Digital Cameras.
4. In PCs to park the hard disk head when free-fall is detected, to prevent damage and data loss.
5. MEMS gyroscopes used in modern cars and other applications to detect yaw; e.g. to deploy a roll     over bar or trigger dynamic stability control.
6. Silicon pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors.
7. Displays e.g. the DMD chip in a projector based on DLP technology has on its surface several      hundred thousand micromirrors.
8. Optical switching technology, which is, used for switching technology and alignment for data      communications.
9. Interferometric modulator display (IMOD) applications in consumer electronics (primarily displays for     mobile devices).
10. Improved performance from inductors and capacitors due the advent of the RF-MEMS technology

MEMS devices:

Few examples of real MEMS products are,

1. Adaptive Optics for Ophthalmic Applications
2. Optical Cross Connects
3. Air Bag Accelerometers
4. Pressure Sensors
5. Mirror Arrays for Televisions and Displays
6. High Performance Steerable Micromirrors
7. RF MEMS Devices
8. Disposable Medical Devices
9. High Force, High Displacement Electrostatic Actuators
10. MEMS Devices for Secure Communications

MEMS devices used in Space exploration field include:

1. Accelerometers and gyroscopes for inertial navigation
2. Pressure sensors
3. RF switches and tunable filters for communication
4. Tunable mirror arrays for adaptive optics
5. Micro-power sources and turbines
6. Propulsion and attitude control
7. Bio-reactors and Bio-sensors, Microfluidics
8. Thermal control
9. Atomic clocks

Mechanical Measuring Tools

Different Types of Mechanical Measuring Tools:-
Measuring instruments and gauges are used to measure various parameters such as clearance, diameter, depth, ovality, trueness etc. These are important engineering parameters which describes the condition of the working machinery.







Popular mechanical gauges and tools used are:

1. Ruler and scales :
They are used to measure lengths and other geometrical parameters. They can besingle steel plate or flexible tape type tool.

2. Callipers :
They are normally of two types- inside and outside calliper. They are used to measure internal and external size (for e.g. diameter) of an object. It requires external scale to compare the measured value. Some callipers are provided with measuring scale. Other types are odd leg and divider calliper.

3. Venire calliper :
It is a precision toolused to measure a small distance with high accuracy. It has got two different jaws to measure outside and inside dimension of an object.It can be a scale, dial or digital type venire calliper.

4. Micrometer :
It is a fine precision tool which is used to measure small distances and is more accurate than the venire calliper. Another type is a large micrometer calliper which is used to measure large outside diameter or distance.

5. Feeler gauge :
Feelers gauges are a bunch of fine thickened steel strips with marked thickness which are used to measure gap width or clearance between surface and bearings.

6. Telescopic feeler gauge :
It is also known as tongue gauge and it consists of long feeler gauge inside acover with tongue or curved edge. The long feeler strips protrude out ofthe cover so that it can be inserted into remote places where feeler gauge access is not possible.

7. Poker gauge :
This gauge is used to measure propeller stern shaft clearance, also known as propeller wear down.

8. Bridge gauge :
Bridge gauges are used to measure the amount of wearof Main engine bearing. Normally theupper bearing keep is removed and clearance is measured with respect to journal. Feeler gauge can be used to complete the process.

9. Liner measurement tool :
Liner measurement tool is a set of straightassembled rod with marked length in each set. It is used to measure the wear down or increase in the diameter of the engine liner.

10. American Wire Gauge :
American wire gauge or AWG is a standard tool which is circular in shape and has various slots of different diameter in its circumference. It is used to measure cross section of an electric cable or wire.

11. Bore Gauge:
A tool to accurately measure size of any hole is known asbore gauge, It can be a scale, dial or digital type instrument .

12. Depth gauge:
A depth gauge is used to measure the depth of a slot, hole or any other surface of an object. It can be of scale, dial or digital type.

13. Angle plate or tool :
It is a right angle plate or tool used to measure the true right angle of two objects joined together.

14. Flat plate :
Flat plate is a precised flat surface used to measure flatness of an object when it is kept over the flat plate.

15. Dial Gauge :
Dial gauge is utilised in different tools as stated above and can be separately used to measure the trueness of the circular object, jumping of an object etc.

16. Lead Wire :
It is a conventional method to used soft lead wire or lead balls to measure the wear downor clearance between two mating surfaces. The lead wire or balls of fixed dimension is kept between two surfaces and both are tightened against each just as in normal condition. The increase in the width of the lead wire or ball will shoe the clearance or wear down.

Vocabulary Term	Definition
Accumulated Error	The collected inaccuracy in measurement that can occur when multiple elements are combined.
Accuracy	The difference between a measurement reading and the true value of that measurement.
Air Gage	A variable, non-contact pneumatic instrument that uses pressurized air to inspect the ID of holes.
Air Regulator	The part of a pneumatic inspection system by which pressurized air is controlled.
Backflow Pressure	Movement of pressurized air in the direction opposite to which it was pushed out of the device. In an air gage, backflow pressure is caused by resistance from the sides of the hole.
Bore Gage	A hole inspection gage that makes three points of contact within the hole. Bore gages are handheld, mechanical or electronic contact instruments with variable measuring systems.
Borescope	A non-contact optical inspection device consisting of a rigid or flexible tube with an eyepiece at one end and a magnifying lens at the other. Borescopes provide a view of hole interiors that are otherwise difficult or impossible to see.
Caliper	A handheld, variable contact instrument that functions as a precision slide ruler. The indicators on the top of the instrument expand to measure internal diameters.
Capable	A gage's predictable range of ability, even when under the influence of natural variation due to common causes.
Comparison Device	A measuring instrument that adjusts to assume the size of a feature.
Concavity	A curved surface condition like the inside of a ball.
Contact Instrument	A measuring device that actually touches the part in order to obtain its measured value.
Contact Probe	The main measuring member on a coordinate measuring machine that communicates its position on a workpiece to the CMMs control panel or computer. A probe often has a sphere of ruby at its tip.
Continuous Scanning	An inspection method used by a coordinate measuring machine in which the probe slides along the surface of the workpiece to collect seamless data in the form of a series of numerous points.
Coordinate Measuring Machine	A sophisticated electronic measuring instrument with a flat polished table that inspects parts in three-dimensional space using either a contact or a non-contact probe. All CMMs are variable devices.
Dial Indicator	A measuring instrument with a contact point attached to a spindle and gears that move a pointer on the dial. Dial indicators have graduations that allow you to read different measurement values.
Digital Readout	An indicator on a measuring device that presents data through a numerical display.
Electronic Instruments	Inspection devices that use electrical impulses to report either position changes or contact between the inspection device and the part.
Extension	A piece that is added to the head of an inside micrometer to expand it to the width of the hole.
Eyepiece	The part of a boroscope through which the inspector views the interior of the workpiece.
Flowmeter Tube	A cylindrical indicating device on a pneumatic measuring instrument that looks similar to a thermometer.
Gaging Instrument	An inspection device of a standard size that determines fit but does not determine actual measurement value.
Go-No Go Gage	An instrument that determines whether a part feature simply passes or fails inspection. No effort is made to determine the exact degree of error.
Indicator	A device, often numerical, that displays a measurement. An indicator may be a dial with a needle or a digital readout.
Inner Diameter	ID. The interior surface of a hole in a workpiece.
Inside Micrometer	A mechanical or electronic, variable, handheld contact instrument, usually cylindrically shaped, used to measure the inside diameter of larger holes.
Laser System	A non-contact, variable, optical inspection method that uses light to examine the inside diameter of holes. Laser systems send out a single light wave on a straight line that is detected by sensors and converted into an electrical signal.
Light Wave	A form of visible energy used by lasers.
Linear Scale	A series of parallel lines that represent a measurement standard. A ruler contains a linear scale.
Linearity	The amount of error change throughout an instrument's measurement range. Linearity is also the amount of deviation from an instrument's ideal straight-line performance.
Lobing	A condition in which the manufacturing process creates a rounded projection out from what would otherwise be a circular hole. A hole may have more than one lobe.
Master Gage	A measuring device of a standard size that is used to calibrate other measuring instruments.
Mechanical Device	A measuring instrument that must be physically manipulated by the inspector. Mechanical devices may be go-no go or variable.
Micrometer Head	The main component of an inside micrometer that includes the scale and indicating device. Extensions are added to the micrometer head.
Non-Contact Instrument	A measuring device that is able to obtain the measured value of the part without making physical contact. An air gage is an example of a non-contact instrument.
Optical Comparator	A sophisticated measuring instrument that projects an image of a part onto a screen to compare the shape, size, and location of its features. Optical comparators are non-contact, variable, optical inspection devices.
Optical Instruments	Inspection devices that use light and lenses to inspect parts. The part may be viewed directly through the lense or displayed on a screen.
Outside Micrometer	A handheld device consisting of an anvil, a shaft, and an indicator that is used to measure outside diameters.
Ovality	A condition in which a hole that should be round has two opposing lobes, resulting in an egg shape.
Pin Gage	A cylindrically shaped length of metal of a specific diameter used as a gaging inspection device. A pin gage is a handheld mechanical contact instrument.
Plug Gage	A hardened, cylindrical gage used as a handheld mechanical contact instrument to inspect the size of a hole. Plug gages are available in standard diameters and are often two-sided, with a "go" side and a "no go" side.
Pneumatic Instrument	A measuring device that uses a pressurized gas, such as air, to function.
Precision	The degree to which an instrument will repeat the same measurement over a period of time.
Probe Cable	The portion of a laser system that transmits information and power to and from the tip of the device and the computer.
Ring Gage	A circular measuring device of a standard size that is used to calibrate other instruments or inspect cylindrical parts.
Rotary Laser	An inspection device that projects a light wave on the surface of the part's internal diameter as it turns.
Scale	A standard of measurement that is often displayed as a series of lines.
Setting Gage	A measuring device of a standard size that is used to check or prepare a working gage for use. Bore gages often come with matching setting ring gages.
Split-Ball Gage	A cylindrical device with an expanding, flat-ended ball on one end and a locking device on the other. A split-ball gage is a handheld, mechanical contact instrument that is used for comparison measurement. It is also called a small-hole gage.
Taper	A gradual narrowing of an inside or outside surface.
Telescoping Gage	A T-shaped measuring device that has two spring-loaded measuring arms and a lock in the base. A telescoping gage is a handheld, mechanical contact instrument used for comparison measurement.
Thimble	A ring or cylinder that fits around the spindle of a micrometer. To advance the spindle, you turn the thimble.
Tolerance	The acceptable variation from a specified dimension.
Tolerance Range	The expected range of measurements produced by a given operation. It is also known as a tolerance zone.
Variable Instrument	An inspection device calibrated in standard measurement units. Variable inspection reveals the degree of variation from a given standard.
Video Borescope	A borescope that contains a video camera rather than an eyepiece.
Working Gage	A measuring device of a standard size that is used to inspect parts.