[dazp1976]

Hey guys/girls.
In the process of building my clone. Done the column ish, haven't touched the table yet.
The ballscrew I have for lifting/lowering the spindle head is crap. If I tighten the gibs a little the backlash is huge and you get a bit of head drop.
If I leave it all pretty loose it moves better but it isn't right.

Thinking if I use some gas struts to hold the head up it will stay in position with the ballnut and should work better.
Question is what force to use?
Was thinking of weighing the head and getting some struts rated 30% heavier. (will be divided by 2 for 1 either side)

What do you think?
New ballscrew is out of the question atm.

[ninefinger]

Before you blame all the backlash on the screw and / or nut check the end support bearings for play - they can contribute way more backlash. Also - many people have improved their chinese ballscrews performance through repacking with slightly oversize balls. Might be a cheap repair option for the ballscrew if it is the culprit.
As far as the gas struts go its a reasonable approach - it will just preload the head with backlash in the other direction which is better / safer from a machining perspective (head won't drop into a cut due to backlash but may not move up if you command a small step up...)

[peteeng]

Hi Dazp - I have a gas strut on my Z axis as I use a belt drive for this on one of my machines. Just set up the Z axis as you want it and place a scale under the axis. This load will tell you what you need. When you go to buy one be careful. The rating on the strut is very variable. I bought one then had to take it back because it was nearly twice the rated load. So I took my bathroom scales in and worked through their box of struts until I found one that was right. They were a bit quizzed when I brought the scales in and they were quite surprised in the variation of the struts loads. Cheers Peter

[CL_MotoTech]

McMaster 4138T621 is what I used on my G0704. Four years and going strong.

[dazp1976]

Before you blame all the backlash on the screw and / or nut check the end support bearings for play - they can contribute way more backlash. Also - many people have improved their chinese ballscrews performance through repacking with slightly oversize balls. Might be a cheap repair option for the ballscrew if it is the culprit.
As far as the gas struts go its a reasonable approach - it will just preload the head with backlash in the other direction which is better / safer from a machining perspective (head won't drop into a cut due to backlash but may not move up if you command a small step up...)

Definitely the DFU nut at fault.
No matter where I set the pre-load on the AC bearings the results are always the same.
The DFU nut is so loose it wobbles like crazy and can roll down/off the ballscrew under it's own weight.

New plan is to make a twin nut mount system bolted together and put 2 SFU nuts on them that I have lying around set back to face. Then between the 2 nut mounts use thin foil sheets to pack them to set the pre-load between the 2 ballnuts.

Should work quite well once it's all set properly, just a pain of a design keep taking a ballnut off all the time to make fine adjustments.