When a machine is made as a one-of-a-kind, it may not make sense to include every fastener in the design. Any fastener that’s regularly stocked may only be used several dozen times and the use of inventory can easily be absorbed and replenished without delaying the build. But what if the project calls for ten machines, or a hundred? Hardware can easily reach 10-15% of a build cost, and as part count increases, so do the justification of labor to properly account for every nut and bolt so that they can be charged to the project, instead of the general shop account.
The same may hold true for any single fastener used commonly enough to fully exhaust the standard stock level. There’s no hard and fast rule about a specific quantity, that will depend on your inventory systems. A good general guideline is to use the unit of “about a bin-full”. Most likely, every bin on the shelf is the same size, so it follows that one may have several hundred 4-40X 0.25″ SHCS, but only several dozen 1/2-13 X 4″ SHCS. Especially in the automation industry when tooling may repeat on an indexer dial or conveyor, a single fastener pattern could be repeat enough to quickly deplete not just manufacturing stock, but the regularly held inventory of your normal vendors! Don’t delay a flagship project waiting on something as simple as a socket head cap screw, in these cases it’s best to add them to CAD so they can be reflected on the bill of materials.