# Thread: Aluminium FEA optimalized CNC router

1. Originally Posted by PotatoMill
Thanks for the quick response Fenza and Remrendes

According to a quick estimate I did I dont think i'm going to need any more torque than what these closed loop steppers deliver; http://www.aliexpress.com/item/new-o...a-82b3fb4b4813

Estimates done with

-40kg weight of moving part
-130N force of cutter (excessive)
-20N friction
-85% efficiency
-10 m/s^2 acceleration (more than I think i'll need, but I haven built any CNC machines before so i'm not certain on this.)

Using T = (F*l)/(2*PI*n) where F is total force, l is lead per revolution, n efficiency
I get 0.44N well within

However I do know that theory and experience dont always match up. And in real life there could be some factor that make the estimate wrong.
Hi PotatoMill your math certainly is correct but it doesn't account for the full picture. I use Kollmorgen MOTIONEERING to calculate more precisely what kind of motors are required, it's a little tricky to understand in the beginning but sticking with it pays off massively! http://www.kollmorgen.com/en-us/serv...eering-online/ here's the link to their site. I use their older application at the bottom of that page, it looks like they have a new online tool available now.

Kollmorgen have a huge range of motors available and the MOTIONEERING app helps point you to which is suited for your machine, I just use the data from that motor to base my search for a cheap servo / stepper system off of.

Also always remember that steppers holding torque is not the same as a servo rated/max torque.

A good rule of thumb that I use (though I'm not entirely sure it's correct) is that a 3Nm stepper is roughly equal in power to a 100W Servo.

Hope this helps!

Fenza

2. ## The Following User Says Thank You to Fenza For This Useful Post:

3. Originally Posted by PotatoMill
Estimates done with

-40kg weight of moving part
-130N force of cutter (excessive)
-20N friction
-85% efficiency
-10 m/s^2 acceleration (more than I think i'll need, but I haven built any CNC machines before so i'm not certain on this.)
With 5mm lead is not the best ballscrew for you. You shoud use the motors up to 500rpm. Under this, you nearly can get a full holding torque.
Last edited by remrendes; 19-06-2016 at 05:01 PM.

4. It would probably be best to make a 3:1 gearing to utilize the holding torque best. Or is it possible to get cheap ballscrews with shorter than 5mm lead?
Last edited by PotatoMill; 19-06-2016 at 05:29 PM.

5. Originally Posted by PotatoMill
It would probably be best to make a 3:1 gearing to utilize the holding torque best. Or is it possible to get cheap ballscrews with shorter than 5mm lead?
I would go for 10mm pitch and direct drive.
On 500rpm it is 5000mm/min, more than enough.
Last edited by remrendes; 19-06-2016 at 06:42 PM.

6. Originally Posted by Fenza
Hi PotatoMill your math certainly is correct but it doesn't account for the full picture. I use Kollmorgen MOTIONEERING to calculate more precisely what kind of motors are required, it's a little tricky to understand in the beginning but sticking with it pays off massively! http://www.kollmorgen.com/en-us/serv...eering-online/ here's the link to their site. I use their older application at the bottom of that page, it looks like they have a new online tool available now.

Kollmorgen have a huge range of motors available and the MOTIONEERING app helps point you to which is suited for your machine, I just use the data from that motor to base my search for a cheap servo / stepper system off of.

Also always remember that steppers holding torque is not the same as a servo rated/max torque.

A good rule of thumb that I use (though I'm not entirely sure it's correct) is that a 3Nm stepper is roughly equal in power to a 100W Servo.

Hope this helps!

Fenza
Great tool, very helpful. I'm trying both the new and old now, the old one seems better actually.

7. Originally Posted by remrendes
I would go for 10mm pitch and direct drive.
On 500rpm it is 5000mm/min, more than enough.
Wouldn't I want shorter pitch? If not I would not get close to the corner speed at all. And waste a lot of usable torque.

8. Originally Posted by PotatoMill
Wouldn't I want shorter pitch? If not I would not get close to the corner speed at all. And waste a lot of usable torque.
With steppers? No, not really.
What do you mean with a corner speed?

Here you can see a torque curve for both. You can calculate with a 0-500rpm range.

Unfortuantely it is Hungarian, but here you can see my calculation for my X axis with servo. This is the optimal for a dynamics, but I chosen 2.5:1.
Last edited by remrendes; 19-06-2016 at 07:54 PM.

9. Here is some info on corner speed. https://www.google.no/url?sa=t&sourc...JAddXnAgnBfq5A it's more apparent on these speed torque curves http://www.sanyodenki.eu/IMG/pdf/Ste..._06.12.N_e.pdf

10. Originally Posted by PotatoMill
Here is some info on corner speed. https://www.google.no/url?sa=t&sourc...JAddXnAgnBfq5A it's more apparent on these speed torque curves http://www.sanyodenki.eu/IMG/pdf/Ste..._06.12.N_e.pdf
Yes, it is on the speed/torque curve, where the motor starts to loose his torque. Approx 4-600rpm on a cheap steppers.

11. Hi PotatoMill, I see you specced 10m/s^2 acceleration. Now I'm not too experienced with routers and steppers (Mills and Servos is more my thing) but 1g is kind of a lot! I did a quick calculation and a 20mm ball screw 1,200mm length 5mm pitch with 40kg load will require over 3Nm of torque at the shaft to accelerate all the moving parts at 10m/s^2

Cutting forces are only small but the force required to overcome inertia and accelerate a 40kg load is great!

I believe 2.5m/s^2 would be a good target to try and aim for, though It would be nice if someone experienced with steppers and routers could chime in and advise. I believe 50in/s^2 is considered not too shabby in Mach3 which would be 1.27m/s^2

Hope this helps! That Kollmorgen app really is a great tool once you get used to it

Fenza

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