Ball screw size

[BriceO]

Hi,

I'm currently building a CNC router of 1.8x1.3 meter to cut wood. I need some advice to choose the ball screw size.
I was thinking about using a Hiwin RM2005 with KL23H2100-35-4B 381 oz In. motor.
I calculate critical speed that should be around 650rpm. Is it correct?
What do you think about it?

Thanks.

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[NIC 77]

Hi,

I'm currently building a CNC router of 1.8x1.3 meter to cut wood. I need some advice to choose the ball screw size.
I was thinking about using a Hiwin RM2005 with KL23H2100-35-4B 381 oz In. motor.
I calculate critical speed that should be around 650rpm. Is it correct?
What do you think about it?

Thanks.

5mm lead, 600 RPM, 600 x 5 = 3000 mm/min = 118 IPM

10mm lead, 236 IPM

20mm lead, 472 IPM

IMO, your choice is between 10mm and 20mm. It depends on the weight of the moving parts and the acceleration you want as to which one is better. Definitely do not use 5 mm on a machine with a 1.8m or 6' ball screw.

[ger21]

10mm should be the minimum pitch for a router.
Some people use 5mm for the Z, to get away with a smaller motor.

[BriceO]

Thanks a lot for your answers!
I think I will go for a 10mm.
Smaller pitch means more torque and more precision, is it correct?
20mm diameter screw is it large enough for 1.8m?

[joel0407]

I have 5mm screws as they can be found much cheaper. I have no trouble at all with speed at 1m long.

[NIC 77]

I have 5mm screws as they can be found much cheaper. I have no trouble at all with speed at 1m long.

I find it hilarious when people persistently recommend 5mm screws to others. His is 1.8m long, not 1m.

Please remind me what your top speed and acceleration settings are?

5 mm is a really bad choice for him. I have done the math for a few different machine parameters, and can say that confidently.

Thanks a lot for your answers!
I think I will go for a 10mm.
Smaller pitch means more torque and more precision, is it correct?
20mm diameter screw is it large enough for 1.8m?

More precision, yes, but keep in mind that my 2:1 gear reduction R&P router moves about 1.5" per rotation and is plenty accurate with 10x microstepping. That's about the equivalent of 38mm lead on a ballscrew for precision. So, it really doesn't matter to you as much as you think. A little runout from your spindle will cause more loss in precision than that.

More torque, it doesn't work like that. Because you have to spin the lower lead ballscrew faster to get to the same speed with the same linear acceleration as a higher lead screw. Acceleration is the factor that is important. To see the comparison, all the factors need to be known and you need to do the math. Or you can just trust me that 10 is better than 5.

If you had a motorized screw jack to lift a car, spinning and accelerating very slowly and with a small length, all compared to a ballscrew on a machine like yours, then the smaller lead would give you more force and be a better choice. But for a CNC machine it's more about acceleration than holding force.

20mm diameter, I think should be fine, I went with 25mm because I thought it would be better with a rolled ballscrew for end machining purposes for 15mm ID bearings. I think I made a mistake.

[joel0407]

I find it hilarious when people persistently recommend 5mm screws to others. His is 1.8m long, not 1m.

Please remind me what your top speed and acceleration settings are?

5 mm is a really bad choice for him. I have done the math for a few different machine parameters, and can say that confidently.

I'll have to check what I have them set at. I have them backed off as it was capable of far too fast for my liking.

I'm glade you've done the math because I have done the practice. LOL. Don't take offense, Just in jest.

I had all these crazy recommendations and went a little less than most suggested as I built a hobby machine and wasn't worries about it being a little slow. Turned out so crazy fast that I need to slow it down.

I agree that 5mm will give more accuracy than other parts of the machine but if you keep every part as accurate as possible then the total of all the inaccuracies will be less.

I'll update with my current speed.

Happy Days

[joel0407]

Yep. Currently set at 700mm/second squared for acceleration and 100mm/sec for velocity.

- - - Updated - - -

Just making a video of what that looks like.

[joel0407]

[joel0407]

I must say after all above. I'm not saying 5 is better than 10. Just it's not as big of a deal as many make out. And as others have said, you want 1.8m and my example is only 1m. Whip will become exponentially more of a problem the longer you go. Also the longer it is the more you'll want it to have some reasonable speed. Maybe consider driving a ball nut rather than the screw. It's worth considering and checking prices. I'd even say you'll do with a smaller diameter screw then. It may become more economical way to do it.

I'm commenting because I did get a little caught up in all the advise and I was a little worried my machine wouldn't be as good as it has turned out.

Happy Days

[Biggs427]

Yes 5mm is doable, but with it you're limiting yourself. Especially with 1.8m long axis.

With higher pitch you can get away with higher inductance steppers, lower voltage PS, etc.

It is also not true that 5mm is cheaper. Yes there are cheap 5mm screws all over internet but is the quality there?

From a good vendor (BST, automation overstock, etc) the difference between 5mm and 10mm pitch is minimal.

[ger21]

Yep. Currently set at 700mm/second squared for acceleration and 100mm/sec for velocity.

That's a velocity of 236ipm, which I would find to be painfully slow.

[joel0407]

That's a velocity of 236ipm, which I would find to be painfully slow.

Do you have a video of your router moving? I'd like to see what speed you are talking about.

[ger21]

My current machine is 10 years old, and IS painfully slow. My new one will be about 5x-6x faster than yours when it's finished.

[BriceO]

More precision, yes, but keep in mind that my 2:1 gear reduction R&P router moves about 1.5" per rotation and is plenty accurate with 10x microstepping. That's about the equivalent of 38mm lead on a ballscrew for precision. So, it really doesn't matter to you as much as you think. A little runout from your spindle will cause more loss in precision than that.

More torque, it doesn't work like that. Because you have to spin the lower lead ballscrew faster to get to the same speed with the same linear acceleration as a higher lead screw. Acceleration is the factor that is important. To see the comparison, all the factors need to be known and you need to do the math. Or you can just trust me that 10 is better than 5.

If you had a motorized screw jack to lift a car, spinning and accelerating very slowly and with a small length, all compared to a ballscrew on a machine like yours, then the smaller lead would give you more force and be a better choice. But for a CNC machine it's more about acceleration than holding force.

20mm diameter, I think should be fine, I went with 25mm because I thought it would be better with a rolled ballscrew for end machining purposes for 15mm ID bearings. I think I made a mistake.

Thanks all for your answers and your explanations. I'm learning a lot here.
A 10mm lead will give 236 IPM too.
Just because I'm interested, ger21 how would you achieve to have this speed?

[ger21]

He has is velocity set at 236ipm, which is the speed limit.
With a 10mm lead, you'll be moving twice as fast at the same rpm. While you may not be able to use the same maximum rpm, you should be able to get 50%-75% more usable speed with 10mm pitch screws.

[skrubol]

Also remember that the critical speed calculation, the length is squared, so all else being equal, a 1.8m screw will have about 30% the critical speed of a 1m screw, so only around 30mm/s.
My calcs put it at more like 75mm/s I guess, but that's just with some formula off the interweb.

[BriceO]

Sorry but I'm confused, how do you calculate this critical speed? I didn't get this result at all.
and why RM2010 ( KL23H2100-35-4B 381 oz In) will move twice as fast at the same rpm? I searched a lot on the internet to find a good way to calculate it.

600 x 10 = 236 IPM

you should be able to get 50%-75% more usable speed

75% of 236? = 177

[skrubol]

I calculated the critical speed of a 1.8m, screw to be around 900RPM using the formula at this site:
Lead Screw Critical Speed (N) Formula & Calculator
I used 1.47 for C and 16 or 18mm for dr. The length and root diameter needed to be converted to inches for that site.
I don't think all critical speed formulas are exactly the same because there are large variations on side loading, balancing, etc. as well as your tolerance for screw vibration that can lead to very different real world critical speeds for the same length and diameter screw.

[BriceO]

That's really slow! I wasn't expecting that. What do you recommend, should I use R&P?