Tight tolerance is the extra room you have to account for in a model to allow for inconsistencies by the printer. A good way to visualize tight tolerance is to imagine laying hardwood flooring. If a room is 12 feet wide and each board is 6 inches in width, you will need 24 boards to cover the entire room. Now let's say that the boards are only 5.75 inches in width. While this may seem like a negligible change, across the entire room a gap of 6 whole inches will form. Accounting for this is the essence of tight tolerances. 

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Let us now apply this concept in the realm of 3D Printing and CAD. Despite being incredibly precise machines, 3D printers have at best a tight tolerance index of ±0.10 mm, which means that a CAD dimension of 4.00mm will print as either 4.10 mm or 3.90 mm. Unfortunately, most printers are not this precise. The average lower limit for the type of printers we have is ±0.50 mm, but even this is not a hard and fast rule.  

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Even beyond printer inaccuracies, PLA filament will shrink as it cools by around 2%. Luckily, trying to calculate the exact tolerance needed in most cases is unnecessary, as variations smaller than a millimeter are often unnoticed, even when needing to fit two parts together. A safe tolerance to set is to add about 1 millimeter to any dimension. 

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It is also important to decide what type of fit is wanted. Parts that are friction fit together have to be made with less tight tolerances to increase the strength of the joint, while pieces that are glued together need extra room for the adhesive to go into. 

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Finding the perfect tight tolerance may take some trial and error, but there is nothing more satisfying than two pieces that fit together perfectly, so keep at it!

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TL;DR: Add 1mm to dimensions to account for irregularities in the printing process. 

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**In machining, tight tolerances technically have a different meaning, but the same concept applies to additive manufacturing (3D Printing)**