Comments sought on new build: A CNC Router for RC Gliders and Planes

[njhussey]

Not done anything on this since the last update but have been specifying a Nema34 and 240V driver for the router along with the pulleys etc needed. Just bought for work an Axminster horizontal band saw http://www.axminster.co.uk/axminster...utting-bandsaw which seems to work OK.



Gradually adding tools for making all sorts of stuff. I've got my small ARC welder and Pillar drill at work too so getting quite a nice little workshop together. only thing that would be better was if it was at my house instead of work but I do have access any time I like.

[EddyCurrent]

Gradually adding tools for making all sorts of stuff.

Tool collector or tool user ? that's the question.

[Jonathan]

That's the same saw as I have - they've just changed the colour scheme. Once I got the blade adjusted properly (i.e. not prone to fall off at awkward moments) it cuts well - nice and square.

Have you bought the motor & pulleys already? If not then did you calculate the required system or take an educated guess? If you post the parameters (mass of axis, length and diameter of ballscrews, desired feedrate etc) then I can verify...

[njhussey]

Tool collector or tool user ? that's the question.

Using them for work... making spacer rings for some of our products, making jigs and fixtures for pressing in and out bearings etc.



Soon hope to be using them on the router (and mill conversion) but it keeps getting put back as more and more "one offs" keep coming in that I need to modify/make....not complaining as I'm being paid to play with the tools which makes a change from sitting at a desk all day long!!

[njhussey]

That's the same saw as I have - they've just changed the colour scheme. Once I got the blade adjusted properly (i.e. not prone to fall off at awkward moments) it cuts well - nice and square.

Have you bought the motor & pulleys already? If not then did you calculate the required system or take an educated guess? If you post the parameters (mass of axis, length and diameter of ballscrews, desired feedrate etc) then I can verify...

Hi Janathan, I've only done a couple of small test cuts on some 25mm rod to test it worked...will be checking the blade and giving everything a once over before it's used in earnest.

I'll have to dig out the drawing of the gantry (estimated) to work out the mass, ballscrews are 1610's length is 1300mm, feedrate I've not calculated (or did but can't find the calculations on the spread sheet) but it will be cuttng balsa wood, plastic and aluminium.

[Jonathan]

I'll have to dig out the drawing of the gantry (estimated) to work out the mass, ballscrews are 1610's length is 1300mm, feedrate I've not calculated (or did but can't find the calculations on the spread sheet) but it will be cuttng balsa wood, plastic and aluminium.

Just to be clear that's two ballscrews at 1300mm long, not one? By feedrate I meant what feedrate do you want, not expect, so for cutting those materials I'd say around 8-10m/min would be suitable. The mass doesn't have to be that accurate - within 10% is plenty good enough.

[njhussey]

Just to be clear that's two ballscrews at 1300mm long, not one? By feedrate I meant what feedrate do you want, not expect, so for cutting those materials I'd say around 8-10m/min would be suitable. The mass doesn't have to be that accurate - within 10% is plenty good enough.

Yes twin ballscrews 1300mm long! Going to drive it with one Nema 34 8.7Nm stepper (or 4.5 if I can get away with it) and a 2M2280N High voltage digital microstepping driver and a long belt 55kg weight springs to mind but I'm at work and the details are on my laptop at home.

[Jonathan]

Based on:

• 2 of RM1610-1300mm
• 55kg actuated mass

Assume:

• 10m/min feedrate
• 2m/s^2 acceleration
• 20T pulleys for 1:1 ratio, 30:N pulleys for 30:N ratio.
• Cutting force of 25N
• Not pre-loading ballscrews

Variables to select:

• Motor size
• Drive ratio

For the common 3Nm Nema 24 motors the corner speed is about 1000rpm, so lets start with 1:1 ratio and see if the torque calculated is low enough to use two of those motors. Using the script (posted here, and attached), the required torque from each motor is 0.47Nm. Up to their corner speed stepper motors output about 2/3rds of their holding torque, so 2Nm up to 1000rpm for the 3Nm motor, so since you only need 0.47Nm this system will definitely work up to 10m/min (1000rpm*10mm) plus a bit more since the torque will only drop below 0.47Nm at a fair bit more than 1000rpm, so you have a good factor of safety. You could even use 30:20T ratio and get 15m/min feedrate since the torque for these parameters is 0.78Nm, though you would need to used fixed-fixed bearings on the ballscrews to get the required critical speed.

One interesting thing to note is that if you use 30T pulleys instead of 20T, the torque required increases from 0.47Nm to 0.70Nm, so don't use bigger pulleys than necessary.

Lets try one motor driving both ballscrews:

Assume:

• 10m/min feedrate
• 2m/s^2 acceleration
• 20T pulleys for 1:1 ratio, 30:N pulleys for 30:N ratio.
  
  • One 20T pulley on each ballscrew
  • Two belts (length roughly 1/2 distance between ballscrews) with two 20T pulleys attached to motor.
  
  
• Cutting force of 25N
• Not pre-loading ballscrews

Since the system is symmetrical, we can model it as a single ballscrew with the length doubled and twice the pulley inertia. The calculated torque is 0.75Nm (or 1.09Nm with 30T pulleys). That's still within the ratings of the 3Nm motor... so no need for a Nema 34 motor (though it would clearly work, just be more expensive). If you want higher resolution, you could use 16:20 ratio (16T on motor) and the torque required is 0.55Nm.

My script also now calculates the ballscrew (nut, shaft & bearing) stiffness and the torsional stiffness of the belts. For example if we assume 100mm center distance, then for two motors the linear error due to belt compliance is 5.6um, compared to the error due to ballscrew stiffness of 0.7um. That's assuming worst case scenario - so machine operating with maximum acceleration and cutting force. If you use one motor, say the belts have about 700mm center distance then the belt stiffness goes down by a factor of 7, so it's now about the same as the torsional stiffness of the ballscrew. That sounds like a big difference, but you have to consider the stiffness of the whole system as clearly if the gantry is not terribly strong that will make a bigger difference than the belt stiffness (or lack of).

I've attached the scripts with your parameters entered for X - change the file extension from .txt to .m. The script runs in MATLAB, or GNU-octave which is free/open source and can be downloaded here. I encourage you to experiment with the numbers to get a feel for what parameters affect the torque requirements the most. You could put your numbers in for X and Z...

servo_motor_sizing_hussey_1.txtservo_motor_sizing_hussey_single_motor.txt

[JAZZCNC]

Since the system is symmetrical, we can model it as a single ballscrew with the length doubled and twice the pulley inertia. The calculated torque is 0.75Nm (or 1.09Nm with 30T pulleys). That's still within the ratings of the 3Nm motor... so no need for a Nema 34 motor (though it would clearly work, just be more expensive). If you want higher resolution, you could use 16:20 ratio (16T on motor) and the torque required is 0.55Nm.

No sorry lovely calculations and all that but I can tell you from experience that 1 x 3Nm Nema 23 motor connected to 2 x screws with timing belts won't handle what will become 60Kg+ gantry and be reliable.!
Yes it will move it no problem and work but when the cutting gets hard or the speed and direction changes are fast it will struggle and the chances of lost steps makes it unreliable IME.
Either twin 23's or Single 34 is the way to go for this size/length and weight IMO.

[njhussey]

I calculated that I only needed one Nema 24 3Nm stepper for the twin X axis ballscrews but on thinking about it and listening to Dean I'll be going with a single Nema 34 on the X axis.....just to err on the safe side...after all it's not my money now so I'll be getting a 240V driver to go with the Nema 34 as well...