Re: [CAD_CAM_EDM_DRO] Ball screws

There is this affliction that tends to take over when many start in CNC that makes them feel the need to have everything they make needs to be within .001" or less deviation from the design. It leads to an affliction I have always called "Ballscrewitis". Ballscrewitis is best known for it's symptoms of making us think we need the absolute best possible component for everything we do regardless of the end use. It has a 2nd order effect of making us reach out to the cheapest approach that has the high end name.

So in the realm of CNC lead screws, ball screws have become the holy grail. Unfortunately as Jon alludes to, there is a very wide range of product that are properly called ball screws. Some are absolutely great and some are worse than a non-ball screw. From my recent studying on machine design I am actually finding that there is a different system that is considered even better than a ball screw and that is a hydrostatic lead screw and nut. Fortunately they are pretty obscure or we would see hundreds of ways to make them at a fraction of the cost of industry. 

Based on my experience first as a hobbyist learning and converting machines to CNC and then in becoming a commercial manufacturer with industrial equipment, I have learned that tolerances in parts are good things and much of what is machined really makes no difference if it is +-.001 or +-.010" and frequently even larger tolerances are fine. What I have also learned is that just because the machine is both accurate and precise to a .0001" move and makes perfectly round interpolated circles does not mean you will not make scrap that is off. Machining is a very complex subject and just worrying about one small aspect of the process does not make you competent in the process.

So my general advise to anyone that wants to get into CNC and actually wants to become knowledgeable, put together the cheapest CNC anything you can and actually start experiencing how it all works. With that experience and probably $200 or so invested you will now be exposed to the full range of challenges that CNC machining is. You can start playing and learning and refining your own ideas of what you need. Buy a couple of different types of screws on e-bay just to get them in your hands and see what the difference really is. If you actually have a top end ground screw and nut and a low end rolled unit the differences become very very obvious. Once you understand what the extra $$ buys you and how that individual piece contributes to the whole, you will be in a much better place to make the best decision for your needs.

Summary is, don't stress, keep reading everything you can, and make anything that is CNC controlled to learn all the skills involved. My first CNC step and one that just excited me to no end and made me feel I could do this was simply a 1/4-20 all thread rod taped with strapping tape to the shaft of a NEMA 23 stepper with the lead nut being 2 - 1/4-20 hex nuts a 1/2" or so apart on the all thread and with a pair of tongue depressor sticks rubber banded across the nuts. It all sat on the floor and when you commanded a move and held onto the motor by hand the tongue depressors would move up and down the rod the commanded distance. Total investment was maybe $50 to learn the basics of a complex subject.

One warning, please do avoid cheap rolled ball screws. Interesting story. We were making mills for another company that advertises their machine that looks like a monster mill has balls and plays up the ball screws. Well we had those specific ball screws in house and did the end machining on them and put them into the OEM version of the monster mill. They were terrible. I would say there was at least .008 - .010" of backlash, the rolled screws were not all that smooth, and the spec on the consistency of the thread is in the range of .008" per foot by memory. So while this pretty expensive vendor promotes they have ball screws but the reality is a quality acme screw would have better performance.

Sorry to ramble so much, but I hate to see some that has never worked with this stuff think they can design the perfect setup without building up some knowledge and more important practical skill and understanding first. Save huge sums of money in the long run and get you a much more robust system.

Tim G

A2Z Corp

Englwood CO USA

www.a2zcorp.us/store