I'm gearing up to upgrade a new CNC router based on the OpenBuilds system. I've been looking at the LEAD CNC as a model. But one thing just occurred to me while really digging in to understand how the whole system works. I realized that the Y rails are unsupported except at the ends. Since I'm planning on building a "bowling alley" machine with a long Y axis (48" working length) and a short X axis (at least 20" working length), this is a concern, so I thought I'd bring it up here. Why? Well, my present machine (Zenbot 2448) also has long unsupported rails, as well as a heavy gantry. As a result, the rails sag ever so slightly when the gantry is in the middle of the y rails. I didn't notice it at first, and the manufacturer sort of didn't believe me. The sag is very subtle, but it is there. Normally, something like this wouldn't matter, but I'm building bass necks with built in carbon D-tubes, so I want to be able to control the relief of the neck precisely. I actually had to write a post processor for Fusion 360 that would compensate for the sag so I could continue using my present machine. So when I realized just now that the LEAD CNC also has unsupported Y rails, naturally I became concerned. I'm wondering whether I need to consider coming up with a design where the rails are supported, probably using ball bearing linear rails. It would integrated into the router table, which I need to build anyway. Then I'd need to figure how to hook that into the Openbuilds system for the gantry and gantry side plates. Does anyone have any comments, and if this is potentially an issue, any suggestions as to how to approach it? Thanks in advance for any help!
See How to calculate V-Slot® deflection So yes, for a very long custom machine, you may want to design in additional supports. You could have a torsion box between the rails for example
But the way the LEAD CNC is designed, where the Y carriage completely encloses the rail, this seems impossible. I'd need to go to a different design completely for that part of the machine, correct? I do have some ideas about that, but would welcome experienced input
You might consider a full depth, thin steel plate on the back face of the rails. Just a 3mm plate would add a fair amount of stiffness. You would need to up the 6mm spacers on that side of the wheels to 9mm.
With a custom machine, you have to think about where to compromise, where to make changes, where to follow the examples of the parent its based from, and where to trailblaze your own path: for example. Changing from the stacked plates to a single plate - versus a very long unsupported rail - which would be worse? etc.
I'm also thinking about using supported linear rails with bearings for the long Y axis, and building the support for them into the router table, which I need to build anyway. I'd use the LEAD CNC setup for the "top end". It will take a while to develop this option, thanks for all the support so far!!
