[lagfish]

I've been thinking about the difference in backlash between the different ballnut options. For example, when asked what's the standard backlash you get with different ballnut options, people typically say about 0.004 with standard ballnut, 0.002 with preload, and 0.001 or less with double ballnut. Can there really be such large differences between the different nuts? And are those differences due to the ballnuts themselves? For example, for preloaded nuts, you insert slightly oversize (by less than 0.0005) balls than standard ballscrews. Wouldn't this mean that preloaded nuts would only yield a decrease of 0.0005 or less of backlash than standard nuts? I think most of the backlash in different ballnut setups come from other factors like bearing , pulley, and motor coupler play.
For me, it doesn't make sense to spend $1000 on double-preloaded ballnuts for a hobby machine, and then spending only $10 on cheap couplers with lots of play. The "weakest link" in backlash is usually not the ballscrew. As an example, Project5K who bought the standard CNCFusion ballscrew kit, only got 0.0005 backlash on all axis, which is as good as anyone could want for hobby applications. Where dneisler was getting 0.030 backlash from his CNCFusion DELUXE ballscrew kit because of a cheap coupler.
I think more attention should be paid to eliminating backlash by preloading your bearings, getting better motor couplers and timing belts, than spending hundreds of dollars opting for a better ballnut option that only marginally decreases your backlash.

What do you think?

[The Blight]

I bought some ballscrews with nuts from germany some time ago (I have seen them on sites in the US too, but I can't remember where), and I have less then 0.0005" backlash on my machine (it's even an X2). They cost a bit, but I think it's worth it. The special thing about these is that you can put a screw that pushes in on the casing of the nut and thefore compressing it a little. The nut and spindle cost about 100$ each. I use these both on my lathe and on my mill. I also use bearing on all the screws to elimit all backlash. So it has ended up costing quite a bit, but the machine is great.

The store in Germany:
http://www.metallstore.de/

[LongRat]

A double nut set up is not all that expensive. I'm running the standard 5TPI rolled screws that so many other people use. I run double nuts, cost $25 for each nut, and I machined the carrier for the double nut assembly myself (cost - $0). For the benefits it yields, it is cheap. A much greater constraint is the space a double nut assembly takes up. On an X2, this is an issue. I can't run 2 nuts on the Y axis because it would sacrifice too much of the already limited travel the X2 has.
The other benefit of double nuts is the removal of wear induced play over time. I ran hundreds of parts off my old single nut set up before I noticed an increase in backlash (went from <0.004" to 0.008"). This backlash was not acceptable, and the cause was wear in the ballnut/screw assembly (I suspect the balls). On with the double assembly and problem solved. Yes, it is true that you need a decent end-bearing set up, but the reality is that this is not difficult to eachieve, especially with the relatively light loads these machines operate under. I just use a pair of preloaded axial thrust bearings.