BILATERAL TOLERANCE
One of the most commonly asked questions we get at Gaging.com besides “Do you have it in stock?” is “What is the accuracy of the gage?”
Sounds like a simple question, but there is more to it than just answering “a tenth”. There are a few different ways manufacturers rate their gages for accuracy. If we are talking about an electronic caliper for instance, the accepted tolerance is: (± .001”/ 6”) or (± .001” for every 6” of scale travel). *Manufacturer’s specifications may differ.
Let’s take a closer look at the (± .001” for every 6” of scale) tolerance. This is an example of a BILATERAL TOLERANCE and a very common way for manufacturers to specify their gages. Since a bilateral tolerance allows for error above and below the target dimension, it is important to remember that you must consider the full tolerance when you are taking your measurements. In your head you may be thinking, “OK, this caliper is accurate to .001”, but remember it is really .002” overall. At the factory when the inspector checked the caliper on a standard for a 6” nominal dimension, the gage could have displayed 5.999”, 6.000”or 6.001” and it would have passed inspection. Rightfully so!
Being aware of your gage’s tolerance specifications can be more important when you are taking a step up to a higher accuracy gage. These types of gages are often used for Aerospace, Automotive or Medical applications where part tolerances are critical. Engineers who are in the conceptual stages of a new product design will have part tolerances in mind that they would like to achieve. They may be thinking they need a bore gage that can measure .00015”, find a gage with that specification listed and buy it not realizing they are actually looking at a .0003” total tolerance. That amount of difference could come back to haunt them later on!
Note: It is a common practice for manufacturers to omit the “±” before their accuracy specifications. A good rule of thumb is if you do not see the “±“ you should assume it is there, particularly for precision hand tools: mics, calipers, bore gages, indicators, etc. Fixed gages and higher-end gages can be a different story and we will get into that in a future blog entry. At Gaging.com we are always happy to discuss any tolerancing questions or concerns to end users. Use our knowledge to your advantage!
Bowers Metrology is one of many manufacturers who express their tolerances as “bi-lateral” for their 3 point Holematic XT Extreme bore gages and higher accuracy gages like the Bowers Ultima system.
Coming Soon: UNILATERAL TOLERANCE and BANDWIDTH TOLERANCE.
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