Second ballnut for improve accuracy

[dazp1976]

Hi All

I'm new to the forum and the CNC'ing and am currently building my 2nd iteration machine - it's a steep learning curve!

I need to achieve 0.01mm accuracy and precision on my machine and are building based on Hiwin 15mm rails, two rails per axes, duel carriages per rail and 1605 ballscrews, C7 grade and aluminium extrusions. I've build a first version and am getting the following accuracy and backlash (measured using a dial indicator):

X: 0.15mm (800mm rails/ballscrew)
Y: 0.04mm (800mm rails/ballscrew)
Z: 0.04mm (125mm rails/ballscrew)

I'm investigating why the X is worse than the other two axes, checking the fittings, plates, etc. But I do want to improve the overall accuracy and want to know whether fitting a second ballnut will improve accuracy and reduce backlash? If so, does the second ballnut need to be tensioned relative to the first ballnut?

Also to add, I know that accuracy depends on a lot more that just the ballscrew and ballnut, but I want to get clarity on this.

Shared knowledge, experience and constructive critique are welcomed!

It is doable to get it close.
I've got my machine down to 0.025mm. It uses gibs/ways so is fairly tight unlike linear rails and my longest is X at 800mm.
I did 2 things.
1: Used decent angular contact bearings size 12x32x10mm in custom made mountings for my 1605 DFU screws.
2. I packed (shimmed) the double nuts with thin (0.01mm) aluminium foil (tin foil) between the back nut and center spacer until they were slightly bitey.

These 2 ways seemed to be key. You have to be carefull where you buy your screws as some don't bother with the spacer bit so it's useless.
My Y screw needed packing by 0.04mm and my X needed 0.07mm. Bit of a difference there. I reckon your X will take a fair bit.
It's well worth taking the time to do these.

You can still have issues with spindle run out too especially with ER collets. Generally I can get it to 0.01mm in the ER collets or 0.004mm direct collet.
I generally add 0.01mm oversize to my drawings and it comes out pretty bang on.

Otherwise you may have to get some proper expensive grounded C3-C5 type screws. And angular contacts to fit them.