Quick question on drive screws

[minichopper6hp]

Hello,

I have a quick question on the resistance of the drive screws as in how hard it is to turn. I have the x and y axis done except the motors and was wondering if it is too tight? It is acme drive screws, using supported linear rails and it seems tight but it has no backlash either. I have messed with the adjustments a little and i have taken care to align the rails to the drive screw within +/- .010". You can barely turn it with your fingertips on the 1/4" side where the motor would mount. Is this too tight? I have taken a pic of the motors im using on all 3 axis. Any help is appreciated.

Thanks!!

[CarveOne]

If you have a spring loaded anti-backlash nut on the ACME screw that would cause that much resistance to turning by hand. If you disconnect the nut from whatever else it is mounted to then there should be very little resistance to turning the screw by hand, assuming that the motor is also not connected.

CarveOne

[minichopper6hp]

They are just delrin nuts, not spring loaded. At this point im wondering if the motors would be enough to turn it, im not familiar with the size of these motors and the power they have, it seems like if im able to turn with my fingers on the 1/4" part, that's not much resistance. I dont know though thats why im trying to get some feelers out there with someone who has experience building diy cnc tables.

Thanks!

[RomanLini]

You should check the sliding resistance of your tables without the leadscrews installed, by testing with a weight and find the basic amount of force needed to make them move. Ie if they slide easily with less than a kg of force that should be fine.

Then install the leadscrews without upsetting the rail alignment, and check the torque needed to turn the leadscrews. You can test the torque by gently clamping a g-clamp etc on the end of the leadscrew, then at a 3" or 4" radius apply some weight to it and easily calc the oz:inches of torque needed to turn the leadscrew.

Your Delrin leadnuts are probably a bit tight, which is good as you will get very little backlash. If you can turn the leadscrews with less than 50 oz:in of torque then you should be good to go. My machine has tight plastic leadscrew nuts and I use a similar system to set the machine up.

[minichopper6hp]

The resistance is definitely in the screw, like i said its probably ok im just worrying too much. If i can turn it by hand at the small end it should be good.

[acondit]

If your screw is out of alignment by 0.010" with respect to your rails that is not good.

Your rails should be aligned to each other very precisely. Check that without the screw attached. Your table should slide freely from one end of travel to the other just by lifting the end of the frame a few inches. If it needs adjustment, slide the table to one end loosen all of the rail attaching bolts on one rail. Then tighten one or two at the end where the table is, then slide the table to the other end of travel and tighten one or two at that end of travel. Then while checking alignment by sliding the table back and forth gradually tighten all of the remaining bolts.

Once that is accomplished, slide the table to one end, attach the screw at that end (other end loose). Turn the screw to move the table to the other end and then fasten the screw at that end. The effort required to turn the screw should be approximately the same as turning the screw in the nut while it is not attached to anything.

Alan

[minichopper6hp]

Its not quite .010", that was just saying that its pretty close. More like .005" when i measured. I understand i want perfection, but being a homemade first machine i dont expect perfection. The rails slide very easily with the lead screw disconnected. I believe that the resistance is just in the delrin nut being tight, it even has a little resistance when you just turn the delrin nut on the lead screw. I guess ill just have to see on the test run with the stepper hooked up if i need to adjust anything. Ill probably use this machine to build my next one out of alum.

[acondit]

If you use the method of alignment that I suggested, you almost don't need to measure. You let the adjustment of the components ensure the proper alignment.

Alan

[louieatienza]

Its not quite .010", that was just saying that its pretty close. More like .005" when i measured. I understand i want perfection, but being a homemade first machine i dont expect perfection. The rails slide very easily with the lead screw disconnected. I believe that the resistance is just in the delrin nut being tight, it even has a little resistance when you just turn the delrin nut on the lead screw. I guess ill just have to see on the test run with the stepper hooked up if i need to adjust anything. Ill probably use this machine to build my next one out of alum.

Alan's description of how to align the rails and screw is a good one. If your screw bearing mounts are off, you may find the machine moves easily in some spots (probably the center) and tight in others (the ends).

When I built my first machine, I used anti-backlash nuts, but they were very snug on the screw. I was able to "free" them up a little by chucking a scrap length on my cordless drill, and runnung it back and forth through the nutm and it seemed to help a lot.

You're going to need some power to cut through aluminum so you don't want to waste it with the screw fighting through the nut or any misalignment evevn if it's slight...

[RomanLini]

...
I believe that the resistance is just in the delrin nut being tight, it even has a little resistance when you just turn the delrin nut on the lead screw. I guess ill just have to see on the test run with the stepper hooked up if i need to adjust anything.
...

As I said above my machine has "tight" plastic nuts and I understand them pretty well. My post gave a very easy way to measure the actual torque needed to turn the screw in the nut. If you quantify that torque, and describe your motor and driver characteristics we can advise if the plastic nuts are too tight or will be ok.