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What is the "actual" ring size based upon?

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When I received an order for some 1/2" bronze rings, I thought that they looked bigger than a 1/2" inch inner diameter, as conformed by my measurements.  I had apparently missed on the order pages that the "actual ID" differs from the listed size by a bit.  I'm wondering why this is the case?  It doesn't look like the sizes are an even fraction of inches or an exact number of millimeters, either, so I'm curious that the sizes are actually based upon, and why they don't correspond more closely to the listed sizes?


Note that this isn't a complaint, since the correct sizes are indeed listed in the stats for each ring, but I am wondering why it is done this way.


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Greetings, I'll give you the long answer. 


The actual ID sizes are as close as we can make the rings to the "nominal" ID. Nominal is in this case 1/2" - it can also be known as the mandrel size. 


Here is some history first. 

In general when you make a ring the wire is wrapped around a mandrel or arbor. This mandrel or arbor is the smallest that ring could be made.

The typical way to make rings is to use a 5/16" mandrel to make  5/16"  rings. The problem is the the wire will spring back to be larger than 5/16". If the wire is harder like stainless or titanium the ring will be much larger - it can even be close to 3/8".


So about 5 years ago we started listing the actual ID of our rings - instead of the mandrel size (nominal). 

At that time we also started a project to change all of our rings sizes to as close to nominal as possible.


So back to the present:

At this time all of our rings, across materials, will be the same actual ID for a given nominal ID. 

We no longer use the technique of using a 5/16" mandrel or arbor to make a 5/16" ring. We use the correct undersized mandrel to make a ring that has the actual ID closest to 5/16". 

We have tons of mandrels in tiny increments of size. So for small rings we can get really close to the nominal size for the actual ID. Our target is within 5 thousandth of an inch. 


For larger rings like 1/2" and larger though there are fewer mandrel sizes to work with - this means that a ring can end up being different from nominal by more than 5 thou. Fortunately on larger rings sizes the larger difference is typically not a problem. 

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