The nature of my work is such that i have all the materials and tools to make a mill so i am not intrested on buying one.I want to know if there is a standard gap between the rollers for all type of grains or i should make it adjustable and if so what is the higher and the lower gap.
Better barley roller mills have an adjustable gap because different lautering systems have different requirements, as mentioned by @Denny, but there is no standard.
This article says that a mill gap between 0.035 and 0.050 inches (0.89 and 1.3 mm) is thought to be a good, all-purpose setting for barley malt. That article also has some other info that may be useful to you. However, the Barley Crusher is one commercial model that is very popular with home brewers, and it has an adjustable gap that is adjustable between 0.015 to 0.070 inches. Its factory setting is 0.039 inches. A finer crush is especially useful for the BIAB and batch-sparge methods of mashing.
You can use a spark plug gap tool from the auto store to measure the gap.
No, there is no standard. There is a range, but the gap that works for you depends on your lautering system and the way you brew. Knowing what gap someone else uses is no assurance that gap will work for you. Make the mill adjustable.
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