Re: Where to purchase 1605 double nut?
A 14mm ground screw is fine for the X and Y. The end work will have to fit.
As far as backlash in a axes you have three main sources. The ball nut, support bearing and the way/slide stiction. One thing most people dont realize is the sliding surfaces on the China mills are rarely perfectly flat and square. My Y on my IH mill has a ground screw with really nice matched pair of AC bearings and a really tight nut but I still get backlash. Why?
Because the slide rocks before it moves by about .001".
The Z on most the mills will do the same thing. They will rock before they move. The standard RF-45 are the worst, The 704 and PM-25 have a fairly long Z slide so they work better but still have the same effect.
From what I have seen a screw with zero lash at lets say 15 pounds of force will still have some lash as a system on the mill.
To get under .0008" is quite hard. Flex from the coupler, end plates, slide rock , stick slip and a little from the end bearings all add up.
I'm not saying a ground screw is bad but a mill with C7 screws that are tight and end bearings with good preload and slides that are flat and move well you wont notice any real difference compared to the .002" of flex you get from tool hop, chatter when milling.
On the double nuts.
They have two preload systems. The most used is just a shim to separate the two nuts. This makes the preload. Most have no preload and have lash. You can make a shim but its a real PIA. Sometimes you can go to a larger ball size on one or both nuts but as you might think that is no easier then re packing a single nut.
You can ever so slightly bend the shim to act like a spring but you have to be really careful because the are cast and crack.
I often replace the shim with a new spring loaded system but it takes a lot of work.
youtube videos of the G0704 under the name arizonavideo99