Dumb Question about Ball Screws

The mounted image in this post and the first image in the post after should help: M4: 1510SS Heavy Mk.I

 

The aluminum block mounts and centers the nut block, itself mounts to the C-Beam (since the fixed end nut block doesn't have 20mm C-Beam screw mounting hole spacing, it can't just be shimmed, it has to be bracketed), and mounts the NEMA 24 (yes, 24- nominal 60mm instead of 57mm) motor on the end. You may not be able to machine a block, but it shouldn't be hard to print it with high infill and be reasonably strong for a lighter machine.

All of it is designed to fit quite snugly inside the channel of a C-Beam section, with a carefully selected ballnut block- not all of them are less than 60mm wide, and machining down is tricky with the threaded hole spacings on each side.

 

So Rob you are using 16mm ballscrews? I read somewhere here in a post that the 12mm screws would not be good for 1000mm as they would be prone to whip, is this the general consensus or would 12mm suffice? Just thinking about space, unless of course the 16mm screws are a easy fit.

 

I wouldn't use 12mm in a rotating-screw configuration above about 500mm or so. 16mm are stable to around 1100rpm at 1500mm, so I'm happy to use them as rotating screws at 1000mm. I forget what the critical speed is at 1000mm- depends on the bearing configuration and tension- but it's plenty for stepper use and I'm pretty sure fine for most servos too. You can also control the screw pitch since what we want isn't a specific rpm, it's a travel speed.

 

Thanks for the reply Rob, what I'm having a hard time with is lead screw sizes I see in all of the machines are 8mm, and from my understanding are fine even at 1000mm+ , is there a particular reason why the 12mm ball screws are not a better choice over the 8mm lead screws?

And I don't want to be hijacking the thread too much but do you know if the 16mm ball screw assemblies fit within the cbeam extrusion? I am looking at a 1000mm spinx linear rail build and just trying to see what is going to be the best route.

 

They get away with what are kinda sub-standard screws because they're multi-start; while the pitch is 2mm, the lead is 8mm, so the screws aren't actually rotating all that fast. Maybe 200-300rpm at most. They're then tensioned and an anti-lash nut block installed, so they're always under tension/support from three points. There's almost certainly still sag and whip, but keeping the speeds down helps minimize the resulting vibrational forces. This is great for keeping prices down for new hobbyists, who would probably be put off by $500 of a $2000+ purchase being on four screws alone.

Ballscrews are explicitly high speed motion components, and are generally operated as such, using high voltage supplies for the motors to maximize acceleration and minimize torque drop (check the data charts for specifics) in an extended usable speed range, maybe also using step morphing capable drives like the gold standard for open-loop stepper operation, the Gecko G540. This makes knowing and understanding the critical speed of the screw really important.

As I implied above, SFU1204 screws are 4mm pitch- you have to run them twice as fast just to get the same speeds as the standard 8mm screws- 500rpm just to hit 2000mm/min "standard" rapids. That has a torque penalty. If you're running a 24V supply, a Blackbox/DM542/SilentStick/whatever driver that isn't optimized for really high-speed operation, etc, you're gonna struggle. Then, even if you can get high speed (800rpm+) operation out of it, you have to start paying attention to the critical speed. I don't know the root diameter of a 12mm ballscrew off the top of my head, I'd assume about 9.5mm, but you can try calculating that (there are many online calculators) to see if it's even within the realm of practicality.

Then the 12mm 4-channel ball nuts are 30x40mm. Good luck finding a traditional nut block that'll still fit inside a C-Beam. If you found or fabricated an "inverse" nut block that hangs off the end of the nut instead, it might be easier.

And after all of that, you have to oil ball screws and usually run them with a machine that has a controller capable of backlash compensation, because the balls aren't usually perfectly seated in the channel- otherwise they couldn't move. Often with the cheap ones you can have up to 20 thou/0.5mm of lash even though they claim they're "antibacklash" (ie. loaded with slightly larger balls) or otherwise quote around a 3 thou lash spec, which is basically made-up. If you can find a DFU1204 - a double-nut screw- then you're all set, because they've shimmed out the lash at the factory, but you'll pay for it. Otherwise, your only alternative to still be able to reliably use a ball screw with grbl is the traditional hobbyist hack; make your own anti-lash nut block, which would accept two ball nuts back-to-back, and then control the distance between them with screws to adjust the lash down to an acceptable number without causing excessive wear. Similar to the OB lead screw nut block.

Not a chance. I have them between two entire C-Beam extrusions in 60mm gaps as you can kinda see here- M4: 1510SS Heavy Mk.I - the Y axis screws are still in their green wrap here. Even finding a sub-60mm nut block for the X-axis was a bit of work on AliExpress.

So, the upshot is, control the rotational speed with different screw pitches, or go down the rabbit hole of overbuilding and learning new controls. Or both! Really depends on how familiar you are with machine design, and whether you have the capacity to fabricate your own new plates and blocks. If it's your first build, I don't know that it's the direction I'd go. Linear rail isn't exactly plug'n'play either. The higher the spec of the component, the more work it requires to use it effectively.

 

Fantastic reply Rob, thank you for taking the time to write it. I had my suspicions about the nut blocks fitting and you confirmed that. Sometimes a brief simple explanation makes the penny drop and what you have explained about ball screws has most certainly helped in the decision process. I was thinking about the whole thing earlier and really don't want to over complicate the build more than I have to and getting the ball screws to work just seems like to much work at the moment. Sticking with the lead screws is probably easiest for now I think.

To give you a better understanding of what I am doing, I had recently rebuilt my cbeam machine with a few alterations with the intention to mainly use it for small alloy work and to hopefully make better use of a 4th axis that I have sitting that does nothing. After finishing I'm not sure I happy with it, after seeing the linear rail sphinx I thought this seems like a better design and a more rigid setup. The v wheels aren't really working for cutting alloy, chips get embedded in the wheels and I have had some wheels crack, I think possibly from wd40 I was using when machining. I have a Stepcraft m.500 which I use as my main machine which is solely for wood as I don't like to mix wood and metal, hence the cbeam for everything else. Thanks again for the help, much appreciated, seems I will just stick with lead screws and go the linear rails for the build and see how I go from there. The cbeam is running through a g540 so I at least that is not needed to change.