Anyone know where I can buy 2000mm long 8mm lead screws? I'm planning to buy 2ea. 1500mm from OpenBuilds, but that seems to be the longest size offered.
Thanks! Looks like it. Although, have you used these before with OpenBuild parts (anti-backlash nut)? Open Builds says "Lead Screw Diameter: 7.8~(mm) Note - These lead screws have been customized to work directly with OpenBuilds system"
I don't know why OpenBuilds sell lead screws that are 7.8mm in diameter instead of lead screws that are 8mm. I had to make a tap out of a small piece of lead screw to enlarge the tapped holes in nut blocks that came from OpenBuilds.
That is way too long.. the screw will seriously deflect all over the place at that length.. if you want to go to 2m.. you really should be looking at rack and pinion drive.
These nut blocks work with ZYLtech leadscrews Befenybay 2PCS Anti-Backlash Nut 8mm Diameter Lead Screw Nut, Lead Screw Moves 8mm in One Turn for CNC and 3D Printer (2mm Pitch 8mm Lead)
Will the screw deflection actually be a problem though? Sure, it sags a little between the nut block and the end of the screw, but is that actually going to cause unwanted movement along the rail? I plan on using a tensioning system and thrust bearings. Brett, you have a lot of experience using that plastic nut block? I plan on using this a lot, worried about the plastic.
Yes, it will.. you have a slightly sagging screw.. add rotational motion.. it's going to cause vibrations and will affect accuracy.. you will basically have a steel jump rope in your c beams... (okay, probably not that bad.. but bad enough)
Yes, you will have to go aftermarket. Either rack and pinion (best option), or belt driven. See the builds here, there are a few 4x8 custom builds using both options.
You could also use two belts that will act as rack & pinion. If you do, use a big belt, like an G3 rather than a G2.
As @JustinTime mentioned, a double belted system will help avoid stretching. It is sort of like a rack and pinion. Many Ox CNC machines were built this way bag in olden times (4-5 years ago). If you search "double belt" on the forums here you will find many examples to include one that had a 3000mm Y axis.
Giarc, what is your thought on the 2000mm lead screw. You think the sag will cause problems? Even with the tensioning?
I think that unless you tension it really really well, which will require quite a beef-up at the point of tension, you'll get whipping from the long screw. As Giarc said, the double belt has no stretching and is much simpler to install and more cost effective and you can have high speeds with no whipping.
Remember every force has an opposing force. By the time you have enough tension on a 2m screw to avoid whipping, you are also pulling the C-beam into a Bow
A couple of resources you might look into include: Eliminating Lead Screw Whip on the Workbee CNC Router | OpenBuilds Won't be much help on solving the whip on a screw of the length you seek but will give you a better idea of what you're dealing with. And not the best example out there but I was unable to locate the previous discussion here on the forum. The basic concept is reducing the unbraced length of the screw by using a free floating shuttle system.
As mentioned by the others, an 8mm x 2000mm leadscrew is not ideal. I used a 1/2 inch 5-start precision lead screw for my 1500mm Y axis and have no whipping issues. If I were building a 2000mm long machine, I would look at rack and pinion.
My 1500mm lead screws have quite a whip, at high speed. They are 16mm thick but are not tensioned since they came without thrust bearings. I may upgrade in the future to thrust bearings.
