Ballscrews and linear rail Xaxis - Dual or Single drive?

[rabidhamster]

Greetings all. I'm building a ~ 4"x3" 8020 machine, and using ~32mm ballscrews and THK precision linear rails. I have 2 precision ballscrews outta a decommissioned mill that are matched in every way but length. My impulse is to just dual-drive the x axis to avoid any chance of racking, but my gantry will be, it seems, very rigid, so perhaps centerdriving would be the way to go. What would you do if it were your machine?

Heres a sort of mockup photo to help with your visualization. Linear bearings not pictured, the steel rail wont be used there

[crane550]

After seeing it both ways I have one honest recommendation.... DUAL! I cannot stress this more. At very minimum, run dual screws and drive them both with a belt, like the Solsyva's do. I switched mine to dual motors and the X and could not believe the difference. Dual screws over a single offer so much more rigidity and accuracy. Looking into the laws of force and torque the math certainly back this claim up.

That being said, it is possible to run 1 screw. People have done, and some with good success. However, (and this is big) your potential for racking and problems goes thru the roof. Any looseness in your system will show up there on the ends of your gantry.

All my designs use dual drive. Your free to do your own, but this is my recommendation since you asked.

[louieatienza]

It depends on how you're ultimately going to have your system set up. but normally, at even your shortest width 3', you'd probably have to use dual screws unless you space your LM blocks far apart, and then you'd lose travel.

I'vev asked before if there was an optimal ratio between bearing rail spread and distance between blocks (end to end) that would give good ridgidity without excessively reducing travel. Your system as it sits would have a mechanical disadvantage if the leadscrew is mounted under the table, since it would be at a maximim distance from the bearing blocks IIRC.

Barring commercial machines, it seems the most successful single leadscrew designs for DIY have the rails 24" or less, and larger widths with two leadscrews or even r&p.

[rabidhamster]

Thanks guys.

Dual drive seems like it'll still be the way to go then. And really, given how much I hate redoing things, it was foolish to consider not dual driving to begin with.

[mmomike]

I do not have a cnc router yet.

What happens if one of the motors, in a dual X configuration, stops working in the middle of a run?

Since only one side is working, will it tweak major parts of the machine and require serious repair?

[JerryBurks]

I do not have a cnc router yet.

What happens if one of the motors, in a dual X configuration, stops working in the middle of a run?

Since only one side is working, will it tweak major parts of the machine and require serious repair?

Probably depends, if the machine or the motors are stronger.

I had the same concern when I built my machine and I force synchronized both steppers with a belt. This is kind of unusual from what I have seen here but it works very well. In normal operation, the belt is just idling along without load and does not contribute to inaccuracies, but if a motor drops out or worse if it stalls hard, the belt prevents the other motor from twisting the gantry. Another convenience is I can move the x by hand, tugging on the belt or turning the pulleys without the gantry losing perpendicular adjustment.

[mmomike]

Has anyone here ever twisted a gantry or worse when a motor went out or stalled?

Someone must be able to speak from experience.

ps. JerryBurks, I love your Big Bamboo build. The most visually appealing DIY cnc I've seen.

[LazyMan]

Has anyone here ever twisted a gantry or worse when a motor went out or stalled?

I had an issues while setting up my machine for dual X drives. The motors were out of sync. One motor stopped moving, the other continued and it permanently damaged my SRB20 Bearing blocks. It has also caused premature wear on my dumpster CNC nuts.

[JerryBurks]

Has anyone here ever twisted a gantry or worse when a motor went out or stalled?

Someone must be able to speak from experience.

ps. JerryBurks, I love your Big Bamboo build. The most visually appealing DIY cnc I've seen.

Thanks

I was not sure about the likelihood of such a problem occurring but I see from the previous post that it does happen. My 1600 in-oz steppers on a 15mm pitch screw can deliver about 1000 pound thrust when going slow (a 381 in-oz stepper on a 10mm ball screw can push about 350 pounds). Although I built a pretty strong machine, I doubt it could take that without damage.

[mmomike]

I see the advantages to driving both sides of the x-axis, but LazyMan's experience is frightening.

I have seen builds with 2 ballscrews for the x-axis being driven with a single motor via 2 timing belts. Are there any disadvantages to driving both side with one motor?
Seems like the best of both worlds. Both sides are being driven and the chance of driving one side and damaging the machine is reduced.

I am just a newbie working on a design looking for as much advice as I can get.

[JerryBurks]

I have seen builds with 2 ballscrews for the x-axis being driven with a single motor via 2 timing belts. Are there any disadvantages to driving both side with one motor?
Seems like the best of both worlds. Both sides are being driven and the chance of driving one side and damaging the machine is reduced.

I believe this is the more common method. The disadvantage can be (depending on pulley size and belt type) that belt backlash, elasticity, long term stretch and thermal effects can affect the positioning accuracy of the x-axis. I did not like that.

I rather invested in two dual shaft steppers and 2 drives and coupled them with the single belt for safety. I do not "slave" the 2nd motor in the CNC controller but run both drives parallel on the same controller pulse output. Given same size motors, it gives me also more oomph to swing the heavy gantry around than a single motor.

[louieatienza]

I believe this is the more common method. The disadvantage can be (depending on pulley size and belt type) that belt backlash, elasticity, long term stretch and thermal effects can affect the positioning accuracy of the x-axis. I did not like that.

I rather invested in two dual shaft steppers and 2 drives and coupled them with the single belt for safety. I do not "slave" the 2nd motor in the CNC controller but run both drives parallel on the same controller pulse output. Given same size motors, it gives me also more oomph to swing the heavy gantry around than a single motor.

I run my two leadscrews with 1 timing belt and pulleys. While there may be stretch or backlash, I don't see it. For all intents and purposes, my system is as backlash free as it can be and I need it to be. Since there is very little friction on the linear bearings, not a lot of force is needed to push evevn a heavy gantry around, and any stretching of belts would be from trying to "push" a tool through the material, or using parameters or cutting material too hard for the machine to handle. Even some very large, heavy commercial machies use timing belts for gear reduction.

While I probably could use a more powerful motor for my X axis, I get plenty of power and speed. Keep in mind too that a belt can also serve as a damper, isolating vibrations to and from the motor, as well as a way to change gear ratios should the need arise.