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

[m_c]

You do realise you can get anti-backlash gears?
Essentailly two gears mounted together and spring loaded out of alignment. You could do something similar using two gears mounted apart with a spring loaded belt between them, however you have to remember high acceleration forces could overcome the anti-backlash spring and allow backlash to develop briefly until the forces reduce again.
Then you don't get prefect motion with a straight cut rack, as you get a slight notchyness as each tooth engages/disengages with a resultant increase/ (there is a proper term for this, but it's essentially what makes straight cut gearboxes so noisy), but for a plasma cutter it's not really an issue. If you're really concerned, you could use a helical rack, but I wouldn't worry about it.

[Davek0974]

Thanks, i have read of some innovative ideas re backlash removal.

Without sounding rude to anyone here, I have just had a reply from a very well respected heavy-duty expert in plasma cutting process and machinery via email, his examples of work are what i would like to see and are well known.

Due to my budget constraints, it is felt that I should be looking at V-Rails and rack and pinion, or, ground, flat plate and skates with rack and pinion for my build.

This makes a lot of sense and is in line with 90% of the commercial production built machines available - no point in re-inventing the wheel.

I will price up and sketch out a v-rail build.

There is another option for drive - open-ended toothed belt with an omega loop drive - this is simply a length of timing belt stretched tightly from front to back with the motor on the gantry and the belt wrapped in a loop over the sprocket. It is supposed to be a very good system as long as steel corded belt is used and it's pulled tight.

My existing build uses a belt but its looped which doubles the stretch possibility and its too small at only 10mm wide, also due to the poor design at the shafts, i cannot stretch it very tightly.

Its too easy to forget things like a budget at times :(

Need to get back on track now I think ;)

[Davek0974]

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.

[m_c]

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.

[Davek0974]

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

I bought an extra last time round.

[Davek0974]

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?

[m_c]

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.