4.2 Measuring Equipment and Decimals
D.M. Donner
What piece of measuring equipment should you use? The answer is based on a few details, such as:
- What type of dimension are you measuring? Length, depth, radius, diameter, inside dimension, outside dimension…
- What type of measuring instrument do you need? A gage or direct measurement instrument?
- Are there space limitations? Measuring an o-ring groove, for example.
- And finally, what is the resolution of the dimension? (.1, .01, .001, …)
Measuring equipment is designed to be used for specific purposes and, in some cases, has a singular purpose. One of these instances is when measuring threads to ensure they comply with an engineer drawing. For example, a ½-13 UNC 2B thread gage can only verify that specific class of fit thread. Many different gages are needed to verify different sizes and classes of fit. Threads and classifications will be discussed later.
On the other hand, the slide caliper can be used for four different measuring types and is good for three decimal places in an experienced hand. A slide caliper is a measuring tool used to measure inside, outside and depth dimensions. This tool uses a set of measuring surfaces attached to two different sliding mechanisms.
A gageis a specific type of measuring equipment that is primarily used to measure a single quantity or attribute. The figure above shows a radius gage. This radius gage is used to measure .260″ radius. A Go/No-Go gage is another type of gage used in quality control measuring. This type of gage measurement requires two pins: one that fits (go) a feature, and one that does not (no-go).
Due to their singular function, gages typically come in sets which can make this type of measuring equipment more expensive to purchase. As machinists, we are responsible for verifying that dimensions are within tolerance. The use of a properly set-up go/no-go gage, for instance, provides us with this information. We will discuss more about gages and how to use them later in this chapter.
The figure above shows a drawer full of assembled go/no-go pin gages designed to gage holes. Many go/no-go gages are locally assembled for specific dimensions, where the operator will ensure the go gage fits the feature and the no-go gage will not. Notice this practice does not provide the specific dimension of a feature but only provides IN tolerance or OUT of tolerance feedback. If you need to verify a tap drill hole for a ½-13 UNC 2B thread, you can use a go/no-go pin gage. If the tap drill is 27/64ths (.4219), and let’s assume there is a three decimal tolerance of ∓.005″ (.427-.417), we can set up a .417 go and .428 no-go gage pin set to verify this hole feature.
The above figure is an example of needing a specific piece of inspection equipment to measure a dimension. When measuring small grooves, a blade micrometer is the only tool that will fit, and it has a resolution of four decimal places.
Direct read measuring equipmentprovides numerical values or r