Gantry Sag: shimming Y-rails on the Gantry while Pre-loading for Z-assembly + Y-car
 

I have measured the sag with the wire method (and then shimmed my Y-rails accordingly), but I did this while I had placed the same amount of weight as my entire intended Z-axis + Y car assembly in the middle of my gantry by means of some lifting weights.

This action results in my y-rails are now bending upwards in a nice even curve with no load applied on them, but when the y-car is traveling over it, it should flatten out to 'perfect straight' instead of dipping. ( of course in theory it would only be perfect when the y-car is exactly in the middle again, but i am confident this practice will also account for all the other positions more or less ).

Reason I actually did this I have made an heavier completely custom Z-axis than the original, and even though I increased the wall thickness of my gantry tubes I was curious how stiff the relatively lightweight ( compared to professional CNC machines) gantry actually was. I noticed a considerable flex (- 0,3/0,4 mm !) when this weight was placed in the middle of the gantry.

I have not seen any documentation on the forum on the sag in the gantry, but I am pretty confident this sag must be roughly the same amount (or maybe even more) with the thinner original wall thickness and a standard Z-axis. In my opinion something good to account for (to improve accuracy on large 3d work for example). If you merely cut completely trough sheets it will be less/not noticeable nor a problem.

So....I am:

1) wondering if anybody else has measured this during building as I did ? Please note: of course one will flatten the spoilboard with the CNC itself, so you can not measure this deviation afterwards by saying I cut perfect onion skins, or I can slide the same feeler gauge exactly under the bit anywhere on the table. What you could do when your table is already operating and your spoilboard is nice and flat is lock the Gantry in place by a clamp (so it can't roll) move the y-car all the way to the left (or the right), place a height caliper under the middle of one of your gantry beams, measure distance to table, note, move your Y-car to the middle of your gantry, measure the exact same spot on your gantry beam, subtract. I am especially curious of the results with people with heavy spindles, dust extraction, water reservoirs on the Z, etc and the standard gantry. I suspect it could be sagging 0,5 mm or more ?.

2) leaving this topic here also for future builders to ponder on.

Does nobody have any experience with this ??

When I step on my Y-car (80kg weight, 4mm steel wall tube) positioned in the middle of gantry it falls down 0.5mm or more.
I had more of it with wheel axles first made with error (10mm so I put reducers in holes ), it improved with using M12 bolts as per plans.

interesting info Danilo, thnx. My measurements are with bolts M12 according to plans.
How heavy is your Z approx ?
I know you have done large 3d works in the past, did you ever noticed issues with V-carving etc in these works ( lines becoming wider in the middle ) ?

Usually the wood has more difference in thickness, after leveling the table there are no noticable unlevel with flat materials like mdf. 0.5mm depth difference shows pretty well in vcarving with 90 degree bit.

My Z is standard per plans for now, added weight is spindle 5.5kg and some weight from polyurethane 150mm dust extraction hose

That seems logical ... like I wrote, your material will follow the 0,5-1 mm 'dip' in your table easily, so you will not notice anything after surfacing your spoilboard. I think most of us will never notice this 'issue' when cutting trough 'soft' materials that will bend to the table, because of the surfacing... Which is kinda neat, cause this flex is like you wrote considerable and could be a major issue otherwise when aiming for the 0,1 mm accuracy on other fronts. When I first discovered it I was flabbergasted ... quickly realizing the built in 'fix'.

When you would mill large accurate 3d molds out of stiff Pu foam or something it could show depending on how you orient your work piece on the machine.