Making in metric with inch-calibrated machine-tools (or vice-versa) is more a matter of inconvenience than of re-tooling your workshop!
All my machines are Imperial but I can handle metric work without too much difficulty, using appropriate conversion-charts.
I keep one of those poster-sized Tracy Tools charts hanging up the workshop, suspended on a old dress-hanger so easy to flip round between size conversion and tapping-drill sizes.
Some stock-bar sizes are close enough for fair direct swaps of metric for inch, though you will need to take due care to assess the effects on fits with other components, and on the finished form or function; and be prepared to machine the material to the appropriate size if and where necessary.
The common BA sizes (which are metric, by geometrical progression, but described in inches) have close metric equivalents. For example 0BA is almost M6, 2BA / M5, 5BA / M3, for their diameters – you can't of course mix them directly even if some seem to screw together. Watch out though for potential problems with the hexagon sizes on the fasteners, if converting a design from one range to the other
There is also a metric standard fine-pitch range which I've never used, but might be more suitable for some model-engineering purposes than the somewhat rough-looking "ordinary" M-series fastenings with their comparatively coarse threads.
You might find some of the older model-engineering drawings or handbooks specify letter/number or fractional-inch drills, usually for thread-tapping sizes; but in fact standard metric drills in 0.1mm increments will cover virtually all the threads and other detail work you and I are likely to ever need.
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The only areas where you would need additional or replacement tooling are conversion change-wheels if you intend cutting inch threads on metric lathes (or vice-versa), taps and dies, and measuring tools – though many electronic micrometers and calipers have a built-in inch/metric choice anyway, at the press of a button.
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So no, you do not need to "restock all your [inelegantly-named] tooling " to make inch-dimensioned model parts in a metric workshop; just buy the few extras genuinely necessary, such BA, BSP (aka G, RP or RC ) and ME-series taps and dies; and conversion-tables. Or convert the project to metric equivalent.
Edited By Nigel Graham 2 on 22/03/2021 23:15:24