We need more information, that said,kw's cut in a bore are normaly centred,but you will get the odd factory part that has it's kw off center, I speak of larger bore (over 3"in dia)parts as these kw are cut on a dedicated key seater machine. Is this an existing kw or are you trying to cut a kw? Is this kw on a shaft or in a bore? If you use a cnc mill, there is not much excuse for not doing keyways with the profiling method, since accomodating the backlash of the Y axis is usually much less of a guessing game. It might be worth roughing the keyway underwidth by 1/16" and then profile cut both sides with a nice sharp carbide to straighten it up. If the enduser is using oversize plated keystock, then the properly centered keyway is more important. That combined amount of clearance forgives a few thousandths of off-centeredness to a shaft keyway. Non-plated keystock might be one or two thousandths narrower than nominal width. It is possible to see a small amount of deflection from center, but with typical components, the keyways in most hubs will be a couple of thousandths wider than nominal. We cut most smaller keyways, up to 3/8" wide in one pass on a typical knee mill, using HSS tools. Centering the shaft perfectly beneath an endmill is no guarantee of obtaining a centered keyway, because of flex of the tool, chuck and quill (of a knee mill).
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