Stell frame cnc 1500X1000. Adjustable table your opinion please

[Jonathan]

Forget all the cutouts on the gantry which have been included to reduce its mass. The servos you have a plenty powerful enough to move achieve the required dynamic performance for a gantry several times heavier than what you have designed. If you continue with your plan to use 100mm diameter pulleys then the inertia of just one of those pulleys will be more than the equivalent inertia of all three axes! Decide on the rapid feedrate you want, work out what drive ratio is required, select pulleys, then check that the servo motors are capable of driving it with the required acceleration. It is very simple to calculate, so don't compromise the performance of the system by using very large pulleys when they're almost certainly not required. If you're not sure how to work it out then let me know and I'll show you.

In answer to your direct questions:

1. Depends what you want to do with it, but you seem to be in the right ball park.
2. The consequences of using a lower thickness and having to replace it are far worse than the small increase in cost to use 10mm, so unless you're prepared to calculate it just stick with the thickest you're happy to pay for (presumably 10mm).
3. It looks a bit risky to me - the tolerances on the parts would have to be very good for them to fit together properly.

[ba99297]

Jonathan thanks for your responce.
As for the pulleys i am planning to use the following setup

X axis: 2 X 2010 ballscrews 1500mm long with 2:1 gear
Yaxis: 1 x 2010 ballscrews 1000mm long with 2:1 gear
Z axis: 1 x 2005 ballscrews 350mm long with 2:1 gear

As for the speed calculations i have made this thread

http://www.mycncuk.com/forums/gantry...culations.html

So with 2010 ballscrew and 2:1 gear ratio

i will have

motor gear ratio 1
ballscrew gear ratio 2
ballscrew length 1500
ballscrew diameter 20
ballscrew pitch 10
ballscrew rpm 1500
ballscrew rpm threshold ~1200
rapids mm 15000


Vagelis

[Jonathan]

Looks like your motors are rated for 400W @ 3000rpm, so 1.27Nm. If you calculate for 15T pulleys on the motors and 30T on the ballscrews using the following specifications:

Mass of axis in kg: [x y z]=[25 25 20] (X-axis mass halved as two motors)
Cutting force in N: [x y z]=[50 50 250]
Acceleration in m/s^2: [x y z]=[2 2 2] (changing this makes a big difference)
Ballscrews: [x y z]= RM2010
Ballscrew length in m: [x y z]=[1.5 1 0.35]

Yields the following:

Feedrate in m/min: [x y z]=[15.0 15.0 7.50]
Inertia in g-m^2: [x y z]=[0.12 0.11 0.07]
Torque in Nm: [x y z]=[0.38 0.34 0.29] (X torque 0.80Nm if gantry mass=0)
Inertia ratios: [x y z]=[3.65 3.18 2.00]

If you instead used 20:60T pulleys on all axes:

Feedrate in m/min: [x y z]=[10.0 10.0 5.00]
Inertia in g-m^2: [x y z]=[0.24 0.23 0.22]
Torque in Nm: [x y z]=[1.03 1.00 0.97]
Inertia ratios: [x y z]=[7.24 7.03 6.50]

So as you can see, even though at a glance you may think changing the ratio would put less load on the motors, you'd actually more than double the torque requirement by using such large pulleys and at the same time get a lower feedrate. This is because most of the torque goes into accelerating the pulleys and ballscrews, so just to illustrate the point about the gantry mass, here's what happens if you use 15:30T and increase the mass of the gantry to something silly like 1000kg:

Mass of axis in kg: [x y z]=[500 250 250] (X-axis mass halved as two motors)

Feedrate in m/min: [x y z]=[15.0 15.0 7.50]
Inertia in g-m^2: [x y z]=[0.42 0.22 0.09]
Torque in Nm: [x y z]=[1.23 0.65 0.37]
Inertia ratios: [x y z]=[12.73 6.53 2.86]


etc...

In other words, your motors are a jot bigger than necessary!

[References:
http://www.nskamericas.com/cps/rde/x...ive_Torque.pdf
ttp://www.hiwin.com/pdf/bs/ballscrews.pdf]

[Boyan Silyavski]

Forget all the cutouts on the gantry which have been included to reduce its mass. The servos you have a plenty powerful enough to move achieve the required dynamic performance for a gantry several times heavier than what you have designed. If you continue with your plan to use 100mm diameter pulleys then the inertia of just one of those pulleys will be more than the equivalent inertia of all three axes! Decide on the rapid feedrate you want, work out what drive ratio is required, select pulleys, then check that the servo motors are capable of driving it with the required acceleration. It is very simple to calculate, so don't compromise the performance of the system by using very large pulleys when they're almost certainly not required. If you're not sure how to work it out then let me know and I'll show you.

In answer to your direct questions:

1. Depends what you want to do with it, but you seem to be in the right ball park.
2. The consequences of using a lower thickness and having to replace it are far worse than the small increase in cost to use 10mm, so unless you're prepared to calculate it just stick with the thickest you're happy to pay for (presumably 10mm).
3. It looks a bit risky to me - the tolerances on the parts would have to be very good for them to fit together properly.

I second that. The plates will distort not matter what you do. Use 10mm simple solid plates. Dont be afraid for the weight. When everything aligned and smooth, the gantry will slide with astonishing to you ease.

How you came up with such big pulleys? Another thing to point you is to make sure the pulleys are from Aluminum, cause last time i ordered pulleys from belting online assuming they were aluminum they send me steel pulleys. So ask and make sure before buying.

[EddyCurrent]

Dont be afraid for the weight. When everything aligned and smooth, the gantry will slide with astonishing to you ease.

This is correct, when I first assembled my gantry it was easier to slide it with me sitting on top of it.

How you came up with such big pulleys? Another thing to point you is to make sure the pulleys are from Aluminum, cause last time i ordered pulleys from belting online assuming they were aluminum they send me steel pulleys. So ask and make sure before buying.

Yes they sent me steel pulleys but that's what I wanted.

[ba99297]

As for the pulleys i have to say that my initial planning was to use 2:1 gearing but if i saw that 3:1 would be better, i want to have the room for the new pulley. So i use such big pulley in my plans just to be sure that future changes to the gear ratio will not need gantry rebuild. Other than the pulleys do you see something wrong or something that could be correted

Thank

Vagelis

[Boyan Silyavski]

Other than the pulleys do you see something wrong or something that could be corrected

Thank

Vagelis

Can you show pictures of the back of the gantry?

[EddyCurrent]

As for the pulleys i have to say that my initial planning was to use 2:1 gearing but if i saw that 3:1 would be better, i want to have the room for the new pulley. So i use such big pulley in my plans just to be sure that future changes to the gear ratio will not need gantry rebuild. Other than the pulleys do you see something wrong or something that could be correted

Maybe if you had enough adjustment on the motor bracket, it would allow you to use the same belt for all size pulleys. I tried to keep all belts the same size so no need to keep lots of spares, I ended up with two sizes.

[ba99297]

Silyavski at the previews page ia have some photos. Here i upload some new



Eddy as i am not an expert i have the opinion that it is better to keep the belt as short as possible.