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What is a Surface Gauge?

Mary Elizabeth
Updated May 17, 2024
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In the world of manufacturing and metal work, a surface gauge is a tool for scribing — which is basically a precise way of measuring — lines on certain fixed materials. These tools are also sometimes also known as machinists’ gauges or scribers blocks, and are particularly useful when it comes to doing things like determining whether planes are truly horizontal. They can also help locate the precise center of cylinders, which is really helpful in a number of mechanical settings and jobs that require precision in assembly. The term also has some relevance in the auto manufacturing world, though the meaning here tends to be really different. Vehicles often have surface gauges on their dashboards to indicate how the vehicle is performing in a variety of metrics, and users read these more or less as moveable dials. The name is the same, but very little about the functionality or usability carries over.

Physical Characteristics

The tool typically has a steel base on which an adjustable vertical arm is mounted. In most cases, either one or two scribers are mounted on the arm; in this context, a “scriber” is a sharp tool used to mark or otherwise scrape into a material in order to note the precise point of measurement. They can usually be adjusted with both a coarse and a fine adjustment, and choosing between them is most often a matter of circumstance and material.

In smaller size gauges, these adjustments may be 4 in and 7 in (about 10 cm and 18 cm) and in larger sizes, 9 in and 12 in (about 23 cm and 30). When used to mark material, scribers might scrape lines into a surface or remove a top cot of paint; both of these are more or less permanent, but they don’t change the functionality of the marked material and in most cases the markings are made in out-of-the way places that will not be noticed. In some models, the scriber can be replaced with a pencil to mark lines less permanently.

Leveling Uses

Perhaps the most common use of the tool is in determining whether the surface of an object is truly horizontal. The manufacturing of many different electronics and machinery requires this, as does the assembly of most high-tech equipment. When things aren’t level, pieces might appear to fit together, but they typically do so in such a way that the long-term functionality could be damaged or at least negatively impacted.

In order to get a precise reading, the machinist will usually manually move the gauge across the surface of the object. The gauge will precisely tell the machinist how far the surface of the object is from its base. If the number varies, the surface is not perfectly horizontal.

Cylindrical Measuring

Another frequent application is to find the center of a cylinder. This is more common than many people think, particularly in the world of mechanical repair and machinery assembly. The tool can be used to find the center of an object that has a round cross-section. The object, usually a cylindrical bar, is placed in a what’s known as a “vee holder,” which holds the bar horizontally. The surface gauge will measure how far the top of the circle is from the bottom, and the halfway point is determined and then marked. The cylinder is then turned about 90 degrees in the vee block and the process is repeated. The result is an "x" that marks the exact center of the cylindrical surface.

Drawing Comparisons

These tools can also be used to compare the measurements of different parts to make sure they have precisely the same measurements. This is particularly important in machines and other things that are built in tandem, with two parts that are supposed to be the same or mirror images of each other. It’s usually important to ensure that they’re exactly even before proceeding in order to pave the way for smooth operations, and this gauge can make that assessment fairly easily. It can also be used to draw parallel lines, which can help assembly workers know where to place various parts.

Unrelated Use in Automobiles

A surface gauge on a vehicle's dashboard is a plate that provides a driver with various performance indicators. This includes the MPH (Miles Per Hour) or KPH (Kilometers Per Hour) gauge that shows the vehicle’s speed, the RPM (Revolutions Per Minute) gauge that shows the engine rotations, and the gas gauge that shows how full or empty the gas tank is. Other common vehicle performance indicators include the engine heat gauge, the oil gauge, and the battery indicator. These tools don’t actually mark anything, and only measure in real time. Their main goal is usually to give drivers more control, which is quite unrelated to precision in manufacturing.

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Mary Elizabeth
By Mary Elizabeth
Passionate about reading, writing, and research, Mary Elizabeth is dedicated to correcting misinformation on the Internet. In addition to writing articles on art, literature, and music for About Mechanics, Mary works as a teacher, composer, and author who has written books, study guides, and teaching materials. Mary has also created music composition content for Sibelius Software. She earned her B.A. from University of Chicago's writing program and an M.A. from the University of Vermont.
Discussion Comments
By anon972628 — On Oct 05, 2014

Two lines won't find the center unless they are exactly 1/2 the diameter. you scribe 4 lines to make a square, then scribe diagonals to find the center of that square and that will be the center of the cylinder

By Logicfest — On Mar 08, 2014

How often is the term "surface gauge" used when it come to automotive gauges? It would seem the term "gauge" is much more common, but that could have to do a lot with what part of the world you happen to live in when discussing such things.

Mary Elizabeth
Mary Elizabeth
Passionate about reading, writing, and research, Mary Elizabeth is dedicated to correcting misinformation on the...
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