Ball screw size and pitch

[Slink76]

Im looking at building a CNC with a cutting area of X1500mm x Y900mm x Z250 - 300mm
This would make my ball screw lengths about 1800mm x 12000 x 600, I think I haven't modeled it yet.

Ive been researching whip in ball screws and i think X2510 would give me about 8000mm/min, Y2505 about 9000mm/min and Z1605 (not really a problem as the stepper won't be able to spin fast enough for it to be a problem) Do you think im on the right path with the size and pitch of these ball screws?

I will be running Hiwin rails on all axis, C7 rated ball screws with two on X and one each on Y and Z.

I will be cutting hardwoods on this machine around 2000 - 4000mm/min

I'm looking at getting hybrid steppers, what size's would you recommend for this machine? I'm a little unsure on how much torque and what sort of accuracy i can expect with the pitch of these ball screws. I'm tipping gantry weight will also be a factor.

Cheers

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[ger21]

Do not use 5mm pitch screws, except maybe the Z axis. To get 9m/min, you'd need 1800rpm, which you won't get out of steppers.
And you should be able to spin a 2510 screw much faster than 800rpm without whipping.

Accuracy won't be an issue.
Motor requirements depend on how much weight you are moving, what top speed you want, and how fast do you want to accelerate to that speed.
If you are going to spend over $300 per motor/drive for closed loop steppers, you could get at least 3-4x better performance from 400w AC servos for about $100 more per motor.

[Slink76]

Thanks for your response Gerry

I thought with my X axis ball screw being 1800mm long is reaching its limits at that RPM using nooks calculator Metric Critical Speed Calculator | Nook Industries
I went with 5mm with the Y axis as i thought it would bring it inline with the performance i can get from the X axis but i get your point about getting 1800rpm from a stepper
So you would recommend 2510 on both X and Y? Whats the best performance i could get out of a ball screw 2510 1800mm long? Would you look at getting a bigger ball screw all together for the X axis?

I initially was going servos but thought it might be overkill, price isn't the issue with motors as i would prefer performance any day. Will i need a gearbox with a servo motor?

[ger21]

I don't think that you need anything larger than 25mm. Keep in mind that the nut is supporting the screw, so the actual unsupported span is usually much less than the length.
If you go with servos, I'd get screws with 20-25mm lead, as the servos have much higher resolution. This gives you higher speeds, and takes whip out of the equation.

With servos and ballscrews, you'd usually want a 2:1 or 3:1 belt reduction. Not a gearbox.

[Slink76]

I'm probably missing something here but why do you need a belt reduction with a servo motor?
With those 400w servos you can go from 1 - 4000 rpm, looking at the speed torque curves there's virtually no loss of torque until after 3000rpm, higher resolution... Whats the problem with mounting servos direct?

[RCaffin]

Reduction between motor and leadscrew gives a higher torque at the ball nut, which gives better acceleration.
I cannot imagine wanting to spin a ball screw at 4000 RPM though! Shudder. It would whip like mad.

Cheers
Roger

[ger21]

A direct drive servo needs to be 3x bigger than one with a 3:1 reduction, to get the same performance. Or a screw with 1/3 the pitch.

[louieatienza]

I'm probably missing something here but why do you need a belt reduction with a servo motor?
With those 400w servos you can go from 1 - 4000 rpm, looking at the speed torque curves there's virtually no loss of torque until after 3000rpm, higher resolution... Whats the problem with mounting servos direct?

400W servos only have about 180oz-in torqur continuous, the power comes from the speed it runs at (3000rpm continuous typically.) Which is why you may need reduction to get that power at a speed you can use. Same as you would need gearing in a car. And as Roger mentions, better acceleration.

Many commercial machines direct drive the ballscrews with servos, though these are larger machines with larger servos and ground ballscrews. I run a Bridgeport mill part time that uses servos, belt-driven screws, and it runs smooth as silk, and with bone-crushing torque...

[Slink76]

Thanks all for your responses.
Don't worry RCaffin i wasn't going to try and spin a ballscrew at 4000RPM

In my first post i mentioned i would like to cut hardwoods at around 2000 - 4000mm/min, i forgot to mention the slowest i need to travel is 60mm/min.

Doesn't increasing the pitch of the ballscrew have the same effect as using gear reduction?
If i use a 20mm pitch ballscrew direct mount i only need 300rpm to hit 6000mm/min, i would only need 3rpm to hit my low speed of 60mm/min. That's awfully close to a servos min of 1 RPM, wont gear reduction make this even worse.

[RCaffin]

Doesn't increasing the pitch of the ballscrew have the same effect as using gear reduction?
DECREASING the pitch has the same effect as using gear reduction.
But gearing down a DC servo motor is always a good idea, to match good DC motor speeds to reasonable ball screw speeds.

That's awfully close to a servos min of 1 RPM,
A full DC servo motor system with a high-count optical encoder can go down to 0 (zero) RPM without any problem. Do a Y-axis move and the X-axis motor holds still - that's 0 RPM. That means using a proper driver for the motor of course. I use Gecko 320 drivers with 512 line encoders.

Cheers
Roger

[Slink76]

So servos need gear reduction basically because they work better at high RPM.

I would prefer to direct mount but I'm not opposed to doing a gear reduction setup, i just figure it's a little easier.

What are your thoughts on the below options
Machine size
Ballscrews two on X1800mm and single on Y1200mm and Z600mm

If i go Direct Drive would just using steppers be okay, steppers high torque at low RPM.
Closed loop stepper with ballscrews X2520, Y2520, Z1610. With a 20mm pitch I'm only looking at 300 - 400 RPM on screws

If i go servo use 3:1 Gear reduction with ballscrews X2520, Y2520, Z1610

[ger21]

You really need to calculate how much weight your moving, and how fast you want to accelerate. And what speed you want to accelerate to.
That will tell you what size motors you need. You also need to factor in the inertia of those long 25mm screws.

[Slink76]

Thanks Gerry, just started modeling so will be able to work out the weight soon.

[RCaffin]

Steppers are easier to use up to a point.
Their limitations are peak power and high speed: they are weaker in both of those compared to servo systems.

A 3:1 reduction between servo motor and ball screw is very common, but absolutely not with a gear unit. That would introduce huge amounts of backlash. The standard reduction method is a toothed belt, but again, watch out for backlash. If you use one of the old toothed belt designs you will have backlash. If you use a modern backlash-free toothed belt design, you will be laughing. Try a GT2 belt 9 mm wide: plenty of power and torque. That is what I have on my VMC, and I machine steel and titanium as well as aluminium.

Cheers
Roger

[Slink76]

Does anyone know what type of steel they use for those C7 Chinese Ballscrews

[RCaffin]

It sticks in my mind that some of them are simply case-hardened steel.

Cheers
Roger

[NIC 77]

I have been reading this thread but didn't feel the need to reply earlier as I think Ger21already gave you good advice. I didn't have much to add beyond that.

But I was just thinking about a few things that I could add.

I have done the ballscrew math for my own build including inertia effects.

5mm pitch is not a good choice, as was already mentioned. Also, I believe that it is best to keep the same lead on x and y. I can't see a good reason for having 10mm on one and 5mm on the other. For my purposes 10mm is also better on Z.

If you are looking at C7 rolled ballscrews from China, same as I will be using, for 25mm OD your only good choices are 10mm or 25mm lead. 20mm OD screws should be fine for your application as well and those can come with a 20mm lead.

The problem I have with 20mm OD rolled ballscrews is that when they are machined, from the pictures I've seen, they don't seem suitable for 15mm ID bearings. I personally don't want to go below 15mm ID bearings on my build, and I have already bought the bearings and mounts. I am still considering 20mm OD screws with a 16mm lead from China but those are ground, so the end machining should be more suitable for the 15mm ID bearings.

If you compare with a rack and pinion system, my first build which was rack and pinion, I believe the movement was 1.5" per rotation with 2:1 gear reduction from CNCRP, so 20 or 25mm lead really isn't necessarily excessive, and if you were making a 4x8 machine, I'd say to use 20 or 25mm lead, or even 40mm lead with perhaps a 2:1 belt reduction.

As I believe was already mentioned, there is no point in going above 25mm OD. When you go to the next size up, the inertia of the ballscrew really starts to make things worse, and the 25mm OD are already stronger than you need.

I assume you are planning to use Nema 34's?

Some people have made 5mm work well on much smaller machines with low inductance stepper motors so that they can spin them faster and get good results. Your machine is too large for this.

One thing I want to mention is that when you get your end machining done, not every stepper motor has the same shaft length. I am using SYK MBA 15-F mounts for Nema 34, direct drive, so I will have the end machining done so I can switch out the stepper motors if I need to with some other manufacturers. There will be a little bit of a gap between the ballscrew shaft and the motors I have now, but if I replace those motors, I can use the replacements that have a little bit longer of a shaft length.

At this point, I will most likely be using the 2510 ballscrew, probably what you will do as well (although you could still larger leads). The 2510 Chinese rolled ballscrews can also be had in a double nut configuration, which is what I was planning on, but now am reconsidering. Anyone out there want to offer an opinion on single vs. double nut for the 2510 ballscrew?

[NIC 77]

Been thinking about it a bit more. 2525 might be your best option on x and y, direct drive with steppers. I was looking at the 1500 and 900 as opposed to the 1800 and 1200.

It's a shame that there isn't a 2516 in the C7 rolled Chinese variety.

You mentioned cutting hardwoods at 4000 mm / min or 157 IPM. 10mm lead would do this no problems and alot more, IDK, perhaps 400 IPM, and there is the option of getting a double nut.

But after you've made this, perhaps you'll want 600 or more IPM. Then the 25mm lead becomes the better option.

Without doing the math for your machine, I can't say for certain. The weights of the moving parts and the specific motors used will affect it. The acceleration you want will affect it. Probably you would be very happy with 0.15G acceleration. That shouldn't be any problem for either lead (10 or 25) with a direct drive Nema 34.

[Slink76]

Thanks nic for your input, I'm in the middle of drawing the Gantry so i can give feed back on weight etc..soon (as an estimate maybe about 40 - 50kg) Thinking either 400w leadshine AC servos or 1,133 oz·in hybrid steppers (i'll run something smaller on Z) If i can get something that cuts from 60mm/min - 6000mm/min, potentially rapid higher i would be a happy man, 400 - 600 IPM rapids would be more than enough.

[jahnj0584]

For what its worth, my 1500mm long X Axis (130+ lbs) can be rapided at 400ipm with the dual 2005 Y axis (425 ozin motors). My acceleration is somewhere around 30ips2