You couldn't build a MFH model with a dremel even if you could slow it down. Its just to physically big and clunky . I have three arroxmax drills, a screwdriver and a high speed engraver so the full set

Battery life is not a problem, i doubt i have used the engraver (which i use as a grinder) for a year and i just checked and its still fully charged. They all charge of a USB on my bench so are charged in use anyway with auto stop when charged Because of the small dia you can get it at very shallow angles.
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