@Mark_Carew_OpenBuild Is there anyway to get longer c-beam than 1500 even if it’s a custom order? Something along the lines of 3000mm.
OR, is it possible to 100% securely attach two 1500mm pieces so they are dead-true with each other and the joints are as if it was 1 straight piece?
you might wanna look into raw 8020 extrusion for those lengths, you’ll need a flat table and a different way of linear motion then acme rod or belts.
@Julius_Jahn Why a different mode of linear motion? They make 3000mm acme rods no?
I recomend you go to the hardware store and feel a 8mm rod thats longer then 3’. It’ll whip like crazy over such a span and can’t stay straight. Belts will also stretch like crazy and be inconsistent. Look into R&P or ballscrews for such a size of machine.
3000mm I would go rack and pinion or chain drive
My original question was unrelated to CNC but as long as we’re talking about other options anyway - I’m looking to build an upright CNC with counterweight so there won’t be much load on the screw, just don’t want to sell a kidney trying to figure out how to span that distance and keep accuracy.
Assuming that the vertical axis could stay where it was without any holding torque, would belts be accurate enough?
It been month I think about a 2000x3500mm OX derivative and many comments I got about this idea have been quite discouraging… Nevertheless I’m hardly able to abandon the idea of building the machine with OpenBuilds V-Slots ! A couple of sellers in UK and DK sell profiles up to 4m and even 6m IIRC (still have to verify they at least honor the fair share program). I have several multi-profiles assemblies ideas for the table, X and Y axis in hope to reach the desired stiffness to get the most out of a 2,2KW spindle. Then for the motion, I still wonder if a double belt setup (eventually with double runs) could do the job as, if I understand it correctly, belt tension only happens between the wheels on each side of the pulleys (see @Martin_Barfoed 's video as an example here https://m.youtube.com/watch?v=qXjLuLqFb-U).
I run dual belts. You still can’t drive the machine as hard as a rod and while the accuracy is there I can only imagine some more power loss over a span
@Julius_Jahn thank you for sharing your experience ! Handling the power of the 4Nm steppers I have in sight is why I’m thinking to use two double belt runs on each actuator… But with the state of my knowledge, I may be misleading myself ? On the other hand, from what I’ve read so far, we’d need at least 20mm ballscrews for such length, which is huge and heavy 
I got 1605 screws for my 1m x 1m machine. 0 play or sag in those! I think it should be fine up to 1.5m, maybe 2 if you’re ok with a little bit of sag (definitely a lot less then 8mm acme!)
I paid about 200$ USD for all 3, shipped express to me. Cheaper then the belts from the OB store too! haha
@Julius_Jahn hmm, well seems like I have to think again… 
@Jace_Richardson The guys here have your question answered but I will throw in a word. If you need to go that long you would be better off with something like a chain drive or rack and pinion system, (chain drive would be interesting (: )that being said you could still use the belt and pinion system for a router and just do shallow step-downs for your cuts. To be honest I always use shallow step down cuts on all my machines because it’s easier on the machine over all and I can go a little faster then when trying to hog through material. We are working on some longer T nuts to allow connecting V-Slot end-to-end and we hope to have them available soon so that we can all run longer lengths.
Seen chain driven DIY CNC machines here and there and, while I have no doubt it can handle the torque, I’ve always wondered how it compares with other motion systems on precision and backlash ? But there must be better ones than those on my bike on these points ! 
Anyone with interesting pointers on chain systems, knowledge or product ?
Environmentaly, this must be quite a noisy system too, hmm ?
Nevertheless, following this discussion, I’ve skimed over ebay and AE, and the price for a set of 20mm or 25mm ballscrews is well above what I had planned for this part of the build. Not to mention that with such pillars to move, I’m questioning the shift to NEMA 34… Let the bill rocket !
+Mark Carew good news for the longer t-nuts ! This will be a valuable addition to the range 
When you say “step-downs”, you’re talking about how you enter the cut aren’t you ? This confirms what Bob Warfield recommends in it’s article about machining tough materials on small machines (http://blog.cnccookbook.com/2015/09/09/the-12-secrets-of-machining-tough-materials-on-small-cnc-machines/) on softening the plunge.
As he explains, pushing the limits of the machines requires advanced CAM features… +Peter van der Walt what’s the status and plans of LaserWeb on these ? Any LinuxCNC /MachineKit user around to tell us about it ?
Men, I’m getting chatty 