Hi, It's so degrading to hear anyone refer to someone's effort as mere hobby engineering, whereas they will probably be using the device whatever to make real money, even modestly.........Wilbur and Orville were bike mechanics before they went to flying.
The subject of ball screws is a deep one, as most if not 99.9% of us would not be able to determine the difference between a rolled ball screw and a ground one apart from the appearance of the trackways.
Rolled screws ARE DEFINATELY HARDENED.....well mine are.....make no error on that part.....attempting to run a hardened ball on a soft surface will immediately cause the balls to dent the surface and so you have no ball screw.
The difference is it is cheaper to roll a track in a steel bar, and harden it than it is to roll one, harden it and finish grind it, one reason why it cost more.
I would hazard a guess (complete ignorance on the methodology for production here) that precision ground screws start off by being rolled then hardened and then ground to fine limits, whereas the rolled screws just rely on the accuracy of the rolling medium to produce the pitch over a specific length.
This is similar to screw cutting in the lathe.......just cutting a screw and hardening it leaves you with the accuracy of the cut thread and tolerances that are affected by the hardening process.........that is the first cost outlay.
If you went to a further stage and had the screw ground it would add at least the same amount again to the final product cost.
Provided the rolled screws have hardened tracks of sufficient hardness to last, then the fact that they have some backlash is irrelevant as backlash can be eliminated by fitting two ball nuts etc etc as I mentioned before, and so making a mongrel into a thoroughbred.
The big difference in quality would ultimately be the hardness of the ball track, irrespective if it is rolled or ground, as if the track is not all that hard it WILL wear out quicker or have backlash sooner.
What point would it be to have a cheap rolled screw with a softer track when a hard longer lasting track costs no more to produce.
Case hardening is case hardening and the quality of the case is purely a product of how long the bar is kept in the cyanide bathe or whatever it is case hardened with, but the big problem is controlling the tolerances for pitch and straightness of the bar after the hardening and quenching process........I hazard a guess that the hardening process is done with induction hardening as this can be done along the threaded bar progressively.
In that case, the bar is not case hardened by a case producing chemical additive process, but rather formed from a metal with high carbon material which only gets the top layer hardened by the induction and rapid quenching process, similar to the production of carbon steel band saw blades that are produced on a continuous roll and get the teeth hardened just at the tips.
I marvel at the ability of the manufacturers to produce the balls screws at such a low price.
Anyone who has lathe experience and can cut threads can cut a ball screw track on a piece of steel bar, but the crunch comes when it gets hardened and goes out of wack from the expansion and contraction making any pitch accuracy non existent......we can't even imagine that the bar will remain straight when it gets hardened and then quenched.
I have a rolled ball screw and nut with a second nut I bought to do the double ball nut experiment I mentioned, and when I have time I'll do the double ball nut exercise to prove that it is feasible and totally practical to have cheap ball screws that are backlash free.
Ian.