to slave or not to slave

[stef_d]

I am at the design stage of my second machine. This one will have a cutting table of 1x1.5m. mainly aluminium, and a 2KW spindle on top.
The main question i am facing now is to double up the x axis drive/screw, or not.
i can see the obvious bonus of more torque to move the gantry, but the trade off is cost and complexity. why not just use one screw and a beefier motor ?
the gantry will not be that heavy, i am gessing 40 pounds max. and i am not looking for lightning speed.


is there another trade off i am missing ?

[ger21]

With one screw, the gantry must be extremely rigid, and have no chance of racking, or you'll have problems. With 2 screws, you're gantry doesn't need to be nearly as strong, because the screws will hold each side in place.

[stef_d]

Originally Posted by ger21 With one screw, the gantry must be extremely rigid, and have no chance of racking, or you'll have problems. With 2 screws, you're gantry doesn't need to be nearly as strong, because the screws will hold each side in place.

yes, of course.. good point.
is there a scientific way to establish how much strength there is in a gantry, and establish, at the drawing stage, that 2 screws will be required ?
or is it a question of building it, trying it, and if one screw is not enough, then stick another one ?

[hemsworthlad]

Hi Stef,
If you look at my build http://www.cnczone.com/forums/showthread.php?t=92404 you will see that i,m using twin 5mm ballscrews turned with 2 htd belts and 1 motor.
I,m still in the process of refining the setup and to be honest with you i,m not completely happy with it.
I can't get much more than 3mtr min rapids, well i can get nearly 4mtr min but it's not reliable and at the cost of accelaration.
I was hoping for 5mtr min but with the way it's setup i think theres to much friction which is robbing me of speed.?

The twin screw setup i,m completely happy with because the gantry is super strong and because of 2 screws theres no racking, the belts are good for ensureing that they turn together which makes sure theres no racking but it does come at the price of less rapid speed and a lot more messing about setting up and design of machine.

I.m still considering slaveing but the thought of missed steps causing racking is making me unsure.?

Hope this helps.?

Also one other thing. . . for the size of machine your looking at which is just a tad narrower than mine you will be looking at a gantry weight more than 40Pounds.?
My gantry weighs over 80 pounds and its 100% made from Ali, dont think it could be made any lighter and keep the strength. . . You'll be surprised just how much rails, ballscrews, motors and spindles weigh.

[stef_d]

Thanks for the input.
your machine looks cool, but would you expect racking if you only had one screw ? i guess it pretty much depends on how your linear bearing are set up.
if the wheels are in the vertical plane, the machine is probably more suceptible of racking than if they where in the horinzontal plane.. but then again, i dont know.

i suppose i may have underestimated the weight of the gantry !