SFU1605 Ballscrew Standard vs Anti-backlash

Hi There

I am newbie to the site and should just point I am very much amateur in CNC although have been playing around with CNC for quite few years

I am slowly building a large CNC, I currently have a small A4 machine but want something I can use to make furniture out of.
The dimension will be roughly 1700x1100, it will be loosely based on the open builds rails but I want to use sfu1605 instead on acme 8mm screws.
So far I have bought the rails and a SFU1605(imported from china) for the Y axis and I trying to source the ballscrews for the X axis. One thing I regret not looking into before was whether to get a ballscrew with anti-backlash my current one doesn't have this built in.

My question is does anyone have any experience with the imported SFU1605 ballscrews and will the lack on anti-backlash be a massive problem this will very much be a hobby style machine rather than anything professional.

I've also seen you can buy anti-backlash nuts that i assume you can install on pre-existing rails has anyone tried this and again and does it give good results ?

Also if any one has recommendations for the X axis ballscrews about 1200mm in length that would be great i am afraid budget it tight so probably looking to import them again

Any advice would be greatly appreciated

Cheers

James

Ballnuts (assuming they're of reasonable quality) on a ballscrew inherently have very low backlash. Since they work on a rolling rather than sliding concept they can have close tolerances and low friction, something that's not really possible with a lead screw.

Hi James,

Well, my first advice is to tell you to stop buying stuff before you know what you need. Because already you have made a bad choice by buying a 5mm pitch screw for a machine this size and application.
With 5mm pitch, you will struggle to get the feeds you'll require to cut correctly also if the screw is for the long axis this will be compounded because the length will cause whip at higher rpm's unless it's 25 or 32mm ball screw, which I doubt it is, plus large screws will introduce other issues regards motors, etc.

The better pitch for a machine this length and application would be at least 10mm with a minimum diameter of 20mm. 16 or 20mm pitch would be a better choice as it allows for ways to deal with the whip and still have the correct speed for the feeds required to cut correctly.

Regards the Antibacklash nuts then don't worry about it because while they are not Zero backlash they are still very low and far better than is required for any DIY machine cutting woods, plastics, and light aluminum work.
Also, I'd forget fitting another ball nut at a later date to cheap Chinese screws as no two screws will ever be the same and you'll end up messing about trying to preload the backlash out with oversize balls, etc. You'd be better off buying the screws from the start with double nuts which are already preloaded.

Thanks @Voicecoil and @JAZZCNC for both replying to me it very much appreciated

Good news about the standard ballscrew having low backlash

@JAZZCNC I see your point about the pitch of the 1605 will result in slower speeds, I guess i can live with this for a bit but upgrade the Y axis once everything else is up and working, For the two X axis at 1200mm do you think 1610 will be be ok for the this length or should i push for 2010 or 2510 ?

Sorry if this is silly question but If I use a larger pitch does this require more force out of the stepper motor and if so is there a resource that i can get this info from

Many Thanks

James

Quote:

---

Originally Posted by JamesOAB

@JAZZCNC I see your point about the pitch of the 1605 will result in slower speeds, I guess i can live with this for a bit but upgrade the Y axis once everything else is up and working, For the two X axis at 1200mm do you think 1610 will be be ok for the this length or should i push for 2010 or 2510 ?

Sorry if this is silly question but If I use a larger pitch does this require more force out of the stepper motor and if so is there a resource that i can get this info from

---

Not a silly question at all, it's actually a good question because it shows you have either done a little research or have reasonable engineering knowledge.
Strictly speaking in theory, yes, because the torque will be lower, but in reality, going from 5 to 10mm won't make a jot of difference provided you have reasonable overhead in the size of your steppers because the difference isn't that great and you'll have more than enough torque with a 10mm pitch. In fact, It's a common mistake for new builders to underestimate the forces steppers connected to ball-screws provide. For instance, the Z-axis with just a 3nm motor will easily punch a nice clean hole thru 18mm MDF without the spindle turning so you can imagine what 2 x motors working together will do.

Regards the 1200mm screw then you'll just about get away with 1610 however I would advise you to have both ends machined to accept fixed BK bearings as it offers extra support which makes the difference.
You could go with 20mm to be sure but then you have to think about the extra inertia and the effect it has on the motors. If your motors are 4Nm or more then you won't have any troubles, even 3Nm will work but 4Nm gives you plenty of overhead.

If I was fitting 3Nm then I would buy 2020 screws and connect them to the screws with timing belts and use a 2:1 ratio, this will increase the torque and halfs the screw speed which lowers inertia and chances of whip.
To be honest, I would connect the motors to the screws anyway even with a 1:1 ratio because it lowers resonance which allows the drives to run the motors smoother and faster.

Thanks @JAZZCNC again that very helpful, lots to think about, I have a look around and see what i can that matches your recommendations