Rotating Ballnut - design ideas

[JAZZCNC]

hello im thinking to make a new cnc router for size 5x10 so my question is im going to use rotary ball nut,do i need 25mm or 32mm ballscrew for 10 feet axis and do i need high pitch screw such as 25 or 32 since im thinking rotary ball nut doesnt have critical speed maybe i can use 10mm pitch and just spin it 3000-5000 rpm with 1:1 ratio belt,
and im going to pair it with 750-1kw 3000-5000 rpm servo for it i already.

i already got quote for my supplier

rotary ballscrew 3232 10 feet=350 usd
rotary ballscrew 2525 10 feet=300 usd

my target rapid is around 1500-2000ipm

thanks

Hi,

I'm just finishing a 10x5 machine with rotating ball nut so it's good timing to ask this question.

The first thing to mention is how much the screws sag under there own weight at 10ft long. With NO tension on the screws, it's massive but even with tension on the screws which you must have, there is still more Sag than you would probably expect.
So when the machine is at Rapid feeds and when axes are near the ends it leaves a long length of screw unsupported, so the screw can still vibrate.

On the machine I've built it uses a 25mm diameter with a 25mm pitch with a 1.5:1 ratio. I'm using 220V closed loop steppers spinning which spin between 1500-2000rpm. The machine is tuned at 30mtr/min(1200in/min) and will reach 40mtr/min((1575in/min) however I wouldn't suggest that speed and here's why.

The ball nut was never designed to spin at those rpm's so while you havent got the critical whip factor to deal with you still do have to consider the nut speed and what it was designed to spin at. Most ball nuts expect to be fixed and are designed for low spinning screws. So with Servo's spinning at 3000rpm or higher then you will have trouble with premature wear. Also, the alignment will be much more critical with higher RPM's.

My advice is to use a higher pitch then apply a ratio to lower the rotation speed of the ball nut while still getting the rapid feeds you would like. You will also get the advantage of increasing torque so you can lower the size of servos.

Now, where it gets tricky is with the Ball screw size. The larger Diameter screw will probably Sag more under its own weight so more tension will be needed. So you will need to design strong brackets that allow for the screw to be adjusted for alignment.

My suggestion is to go with a 32mm pitch and ratio of 2:1 that will give the equivalent of 16mm pitch and 1500rpm at the nut with 3000rpm motor. Giving just short 1000in/min which could be pushed up 1575in/min with 5000rpm giving 2500rpm at the nut which I would suggest is pushing the limit.

Hope this helps. Oh and this machine is vertical but that doesn't make any difference.