At least once a week we get such an inquiry – someone looking for a ball nut and separate shaft long enough so they can cut it and machine the journals themselves, usually thinking this will significantly reduce lead time. And of course, compared to buying a completely custom ball screw, it may just do that. There are several reasons why we do not offer separate shafts and ball nuts, or complete ball screw assemblies with unfinished ends. The first is pretty simple and significant – once someone alters either the shaft or the nut, our warranty becomes void – and most customers prefer warranties to be in force and valid when they use ball screws – we do as well. Second reason is that a ball screw shaft is a pretty complicated element – it is not like a lead screw which is basically a threaded shaft with a plain nut. A ball screw has a very complex shape, in the thread groove – and careful machining is required to maintain the integrity of the groove. The shaft must be carefully cleaned after machining lest some fine powder or grit gets into the nut when it is installed. Not to mention the issue of possibly poor concentricity between the machined bearing points and the original shaft thread – which is often observed when customers attempt to alter it. The resulting eccentricity affects operation, especially for a preloaded ball nut – the result is usually binding with associated higher than normal torque. Operating in this manner shortens life, often considerably. So why do some other suppliers offer this? Simply, they have a different business model, or may not care what happens to their product once it’s out the door. Does Steinmeyer sometimes cut longer existing shafts to produce prototype units with a shorter lead time? Sure – see another blog for details – but we add several worksteps to ensure proper run-out between bearing points and shaft thread.