top mounted ballscrew, would it work properly ?

[Rory13]

Hello, i am in the process of building my first cnc. I ve looked through many designs but cant really decide. The working area i m interested is 75x50cm. Mostly it would deal with wood but it could be nice if it could be design proof for metals with a spindle upgrade and some additional gantry supports in the future. The frame would be made of 3mm thick steel tube, 80x40mm in the long side(positioned vertically as pictured) and 40x40mm on the rest. The moving gantry would probably need to be from aluminium which is very expensive in comparison to steel, but i m hoping with aluminium and some steel angles where it matters the weight will stay under 30kg for the gantry including spindle, so i can get away with nema23 motor 270ozin. Where can i find some info on friction coefficients and weigh load capacity of sbr type rails ? Not even the manufacturers publish these.I suppose sbr16 supported would be fine for the long axis and sbr12 on the others. Is the ballscrew on the top of the gantry idea completely wrong ? Shall i redesign with the screw underneath ? I want to get away with a single ballscrew design to keep the cost reasonable. Initially i will just mount a dremel for tests and get something stronger like 1,5kw after my cad skills get better. Please share your opinions . Thanks.

[Jon.]

Each ball screw will give you a single direction of travel so unless you are only intending on cutting straight lines you are going to need at least 2 axis of linear actuation. 3 axis if you want to vary your depths of cut.

[m_c]

Rory, there is nothing preventing that design from being able to work, however there are benefits to mounting the screw underneath the table.
One benefit is improved access, as your design will restrict access to the table on 4 sides. With the screw under the table, both ends of the table should be clear (unless some other part of the design blocks/restricts access).
Another benefit is by using the table frame to mount the screw, you should get a far sturdier mounting for the screw for less materials, as you don't have to build an extra structure above the table.
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For design inspiration at that size of machine, I'd suggest looking at 3040/6040s on ebay. Upscale the general design to the size you're after, but redesign to use supported rails.
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As for friction details for SBR rails, it's not something I've ever personally seen, and might not even be available due to the number of variables that could affect the figures. Have you tried checking the websites for the manufacturers?

[m_c]

Each ball screw will give you a single direction of travel so unless you are only intending on cutting straight lines you are going to need at least 2 axis of linear actuation. 3 axis if you want to vary your depths of cut.

I suspect what Rory meant, was only having a single central ballscrew on the long axis, instead of twin ballscrews with one either side, not an entire machine that runs on a single ballscrew.

[Rory13]

I suspect what Rory meant, was only having a single central ballscrew on the long axis, instead of twin ballscrews with one either side, not an entire machine that runs on a single ballscrew.

Yeap thats what i meant. Reason i m looking for friction coefficients for linear rails is that i d like to compare to alternative methods like some rolling bearings on the steel frame. I suppose the more diy methods would require a bigger motor to overcome the additional friction. Waggon type rails are great but their price is out of the question at the moment. Another question, is configuring different motor for each axis a harder task ? I was thinking about a nema34 for the long axis on a single ballscrew and nema23 for the rest, perhaps movement by belt on Z axis.

[m_c]

The more DIY methods might not require a bigger motor, however they're not recommended unless you want to be constantly having to adjust things to try and get consistent and accurate performance.
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Different size motors on each axis are not a problem. Whatever controller you end up using, is only interested in the number of steps it takes to move the axis a set distance. The controller is only responsible for sending the correct number of steps to the motor drivers, and has no interest in what the motor driver is actually powering.

[Boyan Silyavski]

The more DIY methods might not require a bigger motor, however they're not recommended unless you want to be constantly having to adjust things to try and get consistent and accurate performance.
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Different size motors on each axis are not a problem. Whatever controller you end up using, is only interested in the number of steps it takes to move the axis a set distance. The controller is only responsible for sending the correct number of steps to the motor drivers, and has no interest in what the motor driver is actually powering.

In fact if a top notch machine is the final aim, there could be a problem with PSU choice. I was going to buy some stuff these days and almost for a moment missed that point.
Was looking to buy 2x 3Nm Nema 23 closed loop kits for x and z , plus 1x 8Nm nema 34 for the both long screws connected by belt. then i woke up and saw that the different sized drives need different voltage. This could be cured buying the intended motors but only with the bigger drives, which could drive the small motors. So feed all with same voltage.
For normal motors just consulted with Dean yesterday, and he confirmed me that to keep all same /speed, torque, acceleration/ and exchange 2 motors with 1, you need higher voltage for the nema 34motor. Basically in this case- 2 different power supplies. Which is not a big deal, just to have in mind