Runout in Ball Screw - is what I'm seeing acceptable?
Hi all,
First post here. I have a 3D printing background, and am building a 1500mm x 1500mm (usable 1200x1200) CNC. I've ordered some 1500mm x 16mm rolled ball screws. I'm testing just to ensure there's nothing out of the ordinary on them.
I've put the screws in the bearing blocks on my work bench and clamped them down so they don't walk. I'm noticing around 1-2mm deflection or run out I guess as the screw rotates at a very slow speed. As the speed increases, it finally gets to a point where it smooths out, but we're talking probably 300-400IPM. The nut is not attached to anything, it's just free floating, or I'm holding gently to keep it from just spinning and staying in place to watch it travel.
Is this normal on these until the ball nut has been attached to a carriage, or is this something I need to get warrantied out / returned?
Thanks!
Re: Runout in Ball Screw - is what I'm seeing acceptable?
Quote:
Originally Posted by
alexanderhendrickson
Hi all,
First post here. I have a 3D printing background, and am building a 1500mm x 1500mm (usable 1200x1200) CNC. I've ordered some 1500mm x 16mm rolled ball screws. I'm testing just to ensure there's nothing out of the ordinary on them.
I've put the screws in the bearing blocks on my work bench and clamped them down so they don't walk. I'm noticing around 1-2mm deflection or run out I guess as the screw rotates at a very slow speed. As the speed increases, it finally gets to a point where it smooths out, but we're talking probably 300-400IPM. The nut is not attached to anything, it's just free floating, or I'm holding gently to keep it from just spinning and staying in place to watch it travel.
Is this normal on these until the ball nut has been attached to a carriage, or is this something I need to get warrantied out / returned?
Thanks!
They are junk if they are that bent, for that length of travel they are too small a diameter as well, 600mm to 800mm is around the max length for a 16mm Ballscrew, you will need a 25mm x10mm minimum Ballscrew for that length, and may need more power to drive it
Re: Runout in Ball Screw - is what I'm seeing acceptable?
Quote:
Originally Posted by
mactec54
They are junk if they are that bent, for that length of travel they are too small a diameter as well, 600mm to 800mm is around the max length for a 16mm Ballscrew, you will need a 25mm x10mm minimum Ballscrew for that length, and may need more power to drive it
Thanks. While I'd like to believe they're junk and just ship it back, I've seen a lot of people that have straightened them successfully, and I'm not averse to doing so if needed. I measured deflection with a gauge, and at its worse in the center it appears to be about 15-18 thousandths of an inch. Which doesn't seem like a lot but when it gets spinning it swings like a jump rope. BUT, that is also without my ball nut actually attached to a carriage, so who knows if that could be cancelled out by a rigid carriage?
As far as the size, what does the diameter have to do with it? Is it the rigidity, or is it something else I'm missing? I'd been looking at using NEMA23, but now I'm looking more toward servo motors. OMC has the AC ones which are quite a bit more speedy, but it appears they also have some NEMA23 DC servo motors too. I'm guessing with enough torque those would work well.
Re: Runout in Ball Screw - is what I'm seeing acceptable?
Hi,
Quote:
As far as the size, what does the diameter have to do with it? Is it the rigidity, or is it something else I'm missing? I
Yes rigidity is important and is improved by a larger diameter but the real issue is 'screw whipping'. As the screw rotates it gets a vibration which pulls it out of a straight line.
At high speed the whipping becomes that bad that the machine vibrates and the ballscrew nut will perish within hours. In your case with the screws not straight to start with
the problem is going to be worse, much worse.
Clearly the bigger the diameter of the screw the less inclined it is to whip. Likewise having a coarser pitch, say 10mm verses the common 5mm, then you do not need
to spin the screws as fast and thereby avoid whipping. Note however that with a 10mm screw you will need twice the torque from your stepper/servo to get the same acceleration.
You can go too far with ballscrerw diameter though. The rotational inertia of a ballscrew varies as the fourth power of diameter, so while you may avoid the problem
of whipping by going to 25mm or 32mm the rotational inertia would mean you need huge servo to drive them.
As an example: when I was designing and building my new (about a year old now) I found some really superb C5 ground, double nut 32mm dimeter 5mm pitch ballscrews in near new
condition really cheap. I bought them and built my mill around them. What I did not know, or rather had not properly considered, was the rotational inertia of these screws. They are
700mm long so there is not even a hint of whipping even when I spin them at 5000rpm and at 32mm they are as stiff as old boot leather!. When I did the inertia calculation I found
the the ballscrews were 80%, the servo (750W Delta B2) was 12.5% and the axis mass only 7.5% The axis is 115kg of cast iron, and I allowed 150kg to accommodate the vice etc,
and yet its less than 1/10th of the inertia, while the ballscrew is 8/10ths!!
I could have used 25mm or even 20mm diameter screws and with the same servos achieved a much higher acceleration, and high acceleration is the best for toolpath following.
As it turns out my 32mm screw equipped mill still accelerates at 2.7m/s2 at rated servo torque and 7.5m/s2 at peak overload, so my machine works really well.
I got a bit lucky, I might have made an error in choosing too large a screws. Matching the screws to the machine and the drive is important, as is the quality of the screws. They are the heart
of your linear motion system after all. Once you build the machine you are pretty much stuck with them.
I would follow Matec's advise and return them. They may well be inclined to accept a return if you ordered 20mm screws from them.
Craig
Re: Runout in Ball Screw - is what I'm seeing acceptable?
Quote:
Originally Posted by
alexanderhendrickson
Thanks. While I'd like to believe they're junk and just ship it back, I've seen a lot of people that have straightened them successfully, and I'm not averse to doing so if needed. I measured deflection with a gauge, and at its worse in the center it appears to be about 15-18 thousandths of an inch. Which doesn't seem like a lot but when it gets spinning it swings like a jump rope. BUT, that is also without my ball nut actually attached to a carriage, so who knows if that could be cancelled out by a rigid carriage?
As far as the size, what does the diameter have to do with it? Is it the rigidity, or is it something else I'm missing? I'd been looking at using NEMA23, but now I'm looking more toward servo motors. OMC has the AC ones which are quite a bit more speedy, but it appears they also have some NEMA23 DC servo motors too. I'm guessing with enough torque those would work well.
Sure, they can be straightened, that's not the problem part, 1500mm of travel is the problem, the Ballscrew is 16mm but the core of the Ballscrew will be around 12mm and this is just too flexible over that length
For servos you should look at what you can get support for DMM is one of the better AC servo systems. https://dmm-tech.com/
Re: Runout in Ball Screw - is what I'm seeing acceptable?
Hi,
I use Delta servos, a Taiwanese brand made in China, that perform very well, have good support, technical documentation and most importantly free set-up and tuning software
at fair prices. They are very similar in terms of quality, performance and price to DMM.
There are even cheaper Chinese made servos than either DMM or Delta, and they seem to perform ok but the support is questionable, the documentation shocking and no set-up and tuning software.
If you are already familiar with servos then they are probably OK, but if you're not familiar with servos avoid these cheap things like the plague. They'll drive you up the wall trying to program them
and get them to work.
Craig
