X-Axis Sag Tolerance Question

I've been reworking a Techzone Huxley I built 6-7 years ago, and I've decided to enlarge the frame to accommodate an 8" x 8" print bed.

I bought new 6mm 303 Stainless threaded and smooth rods to replace the old ones. However, it didn't occur to me until after I'd received them to check the rod diameters of larger printers like the Mendel, which are apparently 8mm.

My initial thought was to return the 6mm and order some in 8mm, but that would require drilling out all of the frame pieces, and I worry that that will weaken them too much.

So I decided to see how much of a difference that would actually make, and calculated the midpoint deflection for the x-axis smooth rods at the original length with 6mm, the new length with 6mm, and the new length with 8mm, and ended up with this:

7.5" - 6mm: 0.005mm
9.5" - 6mm: 0.010mm
9.5" - 8mm: 0.005mm

It's a Bowden setup, with an x-carriage mass of about 153g spread over 40mm distance.

Is that an acceptable amount of sag?

Fortunately I haven't cut up any of the new rod, so I'm still able to return it if it came down to it.

Edit: Really I'm just trying to figure out if I'm making a mountain out of a mole hill.

Edited 1 time(s). Last edit at 03/09/2017 11:55AM by ajsmcs.

No you're not, although I call it a hill rather than a mountain. 8mm smooth rods on an i3 sag around 0.15mm in the centre, resulting in a hill on a height map. Grid levelling can compensate for it.

I don't notice it so much with mgn12h rail.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

The other issue will be how easy the 6mm rods are to bend.
Can you make an adaptor that attaches to your existing bracket and provides new fixings for the 8mm shafts. The new shafts don't have to be in the same place as the current 6mm ones.
So if you have your new 8mm rods going into a block which positions them 9mm above (or below) the current ones you can use an M6 bolt through the existing brackets into the new block.
Obviously you have to check the operational X and Z range, bed height etc.
Or get a local machine shop to turn down the ends of the 8mm to 6mm.
Or use an adaptor with a 8mm hole one side and a 6mm spigot the other. This means you loose some x travel but may only be about 4mm at each end.
If only you could find some tube with 8mm OD and 1mm wall thickness.

On my build the 8mm rods are clearly the weakest link in the design. It's not just vertical sag you need to worry about, but also twisting. If the nozzle is offset from the rods (which it will be in most hot end designs) then the error from the carriage twisting will rapidly dominate the error from sag. I'm inclined to the view that unsupported 8mm rods are too flexible to design well with in a printer with a 200mm or greater span. Supported rods would be a different game, but don't seem to be readily available in the small diameters, which makes linear guides look like the best design choice for typical printer sizes.