Second CNC table build, here we go again....

In summary -

1 - Go for the belt option again this time, but get it right! I think belts, designed properly would outlast a rack, easy to replace too.
2 - Using Linear rails is a possibility, pretty much same cost as V-rail but easier.
3 - This time I will go for two motors on the Y-axis, this will remove having a long cross-shaft in the gantry and the resulting torque twist it suffers, at a risk of racking the gantry, but plenty of builds have two slaved motors and work ok.
4 - Keep the rails above the table, with a gap under for sliding in bigger sheets. Having the rails up high means a low gantry - good for stability etc
5 - Stick with the motors/drives I have, they are Nema23 size so easy to upgrade to Nema23 servos later on if needed.

Out of your list, personally I'd say go for two servos now for the twinned axis, rather than buying an extra stepper/drive.
Servos are what is going to make the biggest difference in achieving higher speeds while still maintaining accuracy.

Hmm, would be an idea but i already have the spare motor and drive :)

I bought an extra last time round.

Torque and RPM...


Is it best to use a small drive pinion on the rack with higher motor rpm or larger pinion with lower motor rpm??


A 20t pinion will move 62.83mm/rev and at 3:1 reduction I will need 429.73 motor rpm for 9000mm/min travel.


A 32t pinion will move 100.53mm/rev and at 3:1 reduction I will need 301.58 motor rpm for the same speed.


I am fairly certain a larger pinion meshes better than a smaller one, but what size would be best considering my 3.1Nm motors.


My existing table is belt drive but works out at 100mm/rev travel and 300 motor rpm for 9000mm/min speed. When i did my high speed test it was running at 450 motor rpm so I know they can reach at least that.


I have a heavier gantry on the bigger machine, but also twice the motor power as i'm using two slaved motors.


Any views guys?

Smaller will give you better acceleration, and bigger higher speed.
If you know what torque you'll have, and the weight of your gantry, then you can work out your (theoretical) max acceleration. As I'm sure you'll be aware, there's no use having a high top speed if you don't have the acceleration to make use of it.

I'm now looking at direct drive, a 17t pinion will give me 53.39mm of travel so I will need 168rpm to reach my top speed of 9000mm/min.

The gantry will weigh at a guesstimate 30kg and will have two 3.1Nm motors, direct driving it.

This might work better as it keeps the motor rpm way down in the high torque area.

Any chance you could illustrate the maths needed for the acceleration estimates???

Quote:

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Originally Posted by Davek0974

I'm now looking at direct drive, a 17t pinion will give me 53.39mm of travel so I will need 168rpm to reach my top speed of 9000mm/min.

The gantry will weigh at a guesstimate 30kg and will have two 3.1Nm motors, direct driving it.

This might work better as it keeps the motor rpm way down in the high torque area.

Any chance you could illustrate the maths needed for the acceleration estimates???

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2pi/.05339 gives you a 117.68 advantage over a 1m pulley

So 2 x 3.1 x 117.68 = 730N

So a 30kg gantry will accelerate at 730/30 = 24 m/s/s about 2.4G on a good day with a following wind.

Assuming I haven't totally cocked up, someone should check this :subdued:

Thanks, if that is indeed correct then I have slightly more acceleration than needed, even allowing lavishly for other losses:)

I have read that plasma cutting only needs around 0.2 to 0.5G for quality so it does sound encouraging.

So what is the bending force on your gantry from accelerating the Z axis components?
Suppose that is 3kg of motor, slide, torch, screws and stuff. Wild over estimate.
We know that 1kg force will accelerate 1kg mass at 1G because that is what happens when you drop it.
So to accelerate 3Kg mass at 0.5G requires a force of 1.5kgf
Why do you need 30kg of gantry to withstand 1.5kg in X and Y?
Perhaps it is the 3kg in Z you are worrying about?
Perhaps you should take a few gantry length bars, support them at either end, put a weight in the middle and see how far they actually bend.
Maybe 30kg is not necessary after all?
Maybe 30kg is only there to accelerate the 27kg it contributes all by itself?

Ideally, it needs a structure that resists torsion moments - this is where my current machine fails - the gantry is a length of 44x44 structural aluminium which is ok in the beam but poor in tension as the centre box is only 18mmx18mm.

The torch and axis are not balanced and when accelerating it twists one way and the opposite on decelerating - causing ripples in the cuts.

What is ideal is a simple, light structure that ca withstand torsion or twist as well as support 3-4kg in the centre.

What that is, I have no idea, most machines just use box section because it's there I guess.

It needs a thick enough wall section to take 5-6mm screws firmly so I guess 5-6mm thick.

Without getting a structural engineer on the job, what stock item would be best suited here?