What is the difference between a Feeler Gage and a Radius Gage?

Most people know that gages are devices used to provide precise measurements. These are necessary tools in fields like science and engineering, where there is no room for error. Precision is everything for science and engineering projects, so properly calibrated gages are a must.

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In order to provide measurements for every type of surface, space, or area, there are a number of different types of gages. It can be difficult to keep track of which does what, particularly when two gages might appear to have similar use.

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Such is the case with the feeler gage and the radius gage. Both of these tools can provide a tiny and precise measurement, but that’s not enough of a distinction on its own. A radius gage, as you would expect, measures the radius of an object, either by measuring its exterior or interior space. A feeler gage measures the width of a gap. These two gages must be treated uniquely to their purpose and to their calibration needs.


In order to provide a clear distinction between a feeler gage and a radius gage, we’ll tell you everything there is to learn about both of these tools.


Everything you need to know about a radius gage


A radius gage is a term used interchangeably with “fillet gauge,” although radius gage is the more common term. Its name says it all — a tool for measuring the radius of an object.


But what exactly are we referring to when we say radius? If you think back to geometry class, you might recall that a radius describes the length from the center of a circle to its edge. When we use a radius gage, however, we’re not measuring the distance from center to edge, and we’re not typically measuring a circle. Instead, a radius, in this context, refers to the rounded edge of an object. For instance, a hinge or a tool may have two edges that meet in a curve instead of a corner. The radius gage allows a person to measure the entire curved area.


A curved corner would be known as a convex radius, where the curve is outward. Radius gages can also be used to measure the concave radius of a curve, where an object has a curve that arches inwards.


Radius gage design - Radius gages are generally compact little objects that look similar to a pocket knife when they’re put away. The tool is made up of steel leaves that are attached by a hinge at either side of a small, slim case. On one side, the leaves each feature a convex radius cut out, gradually increasing in size. These are used to measure a convex radius. On the other, the leaves feature a complementary convex cut out, used to measure a concave radius.


Each thin piece of steel has a radius cut out, as well as a measurement on it. When not in use, the leaves tuck into the case for neat storage. When it’s time to measure, they fan out, so that it’s easy to see the inscribed measurement written on each leaf.


How to use a radius gage - A radius gage works by comparing the precisely measured, machine-made radius of the gage to the radius of an object. It’s quite straightforward to use, and you don’t need anything else except for a strong light source to help you read the measurements. You simply search for the gage that fits the object you’re measuring the best.


You’re looking for a leaf on the gage that will leave zero space between the gage and the object you’re measuring. This is where the light source comes in. In order to find the most precise measurement, you can place a light source behind the gage to help identify a potential tiny gap.


Everything you need to know about a feeler gage


Feeler gages are an essential tool in mechanical engineering and other fields where it’s necessary to get an exact measurement of the clearance between two parts. This simple, handheld tool is used to get the exact measurement of any small gap. Usually, feeler gages measure spaces smaller than 5.08 millimeters, or 0.200 inches, which are too small for other measurement tools. These tiny spaces may not seem like much, but when you’re checking something mechanical like the valve clearance in an engine, or setting a gap in your car’s spark plugs, a feeler gage is a must.


Feeler gage design - At first glance, a feeler gage looks similar to a radius gage in that both have a pocket knife-like design. Like the radius gage, a feeler gage is made up of many precisely measured steel blades, also known as leaves, that are designed to be put away into a slim carrying case. Since there is only one shape used in a feeler gage, as opposed to the concave and convex shapes of the radius tool, there is just one set of leaves attached to one end.


Each leaf has a measurement inscribed on it. This indicates the thickness of the leaf and will give you the measurement of a space. Most feeler gages also come with a lock device at the hinge of the leaves to hold them in place. If you need to remove a leaf in order to use it in a tight space, you can unlock the lock device and remove a single leaf.


How to use a feeler gage - Measuring with a feeler gage is as simple as selecting a leaf and checking it against the width of an area. You’ll want to choose the leaf that seems close in width to space, then fold the other leaves into the protective case so they don’t get damaged in the process. From there, you can see how the leaf compares in size to the gap and move up or down in size if necessary. If you find that the gap, you’re measuring is in between leaf sizes, you can stack different leaves together to get different measurements from the gage.


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Phil Wiseman

Phil Wiseman

Phil Wiseman is Chief Marketing Officer at Alliance Calibration. He earned a B.S. in Chemical Physics from Centre College. Phil is an ASQ Certified Quality Auditor and ASQ Certified Manager of Quality/Organizational Excellence.

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