Just by way of interest for prospective builders there is always discussion about what rod should I use... 12mm or bigger...
Well here is the measured deflection of both 12 and 16mm rod (500mm long in SK type mounts) under 5kg load.
In this experiment the mounts were not clamped up nor were they bolted down so the deflection of the rails themselves will, in reality, be less than this although the MDF base will deflects some as does the MDF 'table'.
2 x 12mm rod = 0.25mm (theory .08mm for rigidly fixed ends to .3mm for unfixed ends)
2 x 16mm rod = 0.1mm (theory .025mm to .1mm as above)
(i.e. if the clamps aren't tight and the supports not bolted down the rod flexes more)
Last edited by irving2008; 01-09-2008 at 10:02 PM. Reason: was 16mm not 20mm the bigger one!
Whats on the dials as read outs mate?
errrmmm... 0.25mm and 0.1mm :)
Since I'm running 1m unsupported 20mm rails on my machine I may well do just that.Nothing is foolproof......to a sufficiently talented fool!
12mm probably good enough for a PCB miller on the X-axis supporting the table, with 16mm on the Y axis and 12mm on Z. I think 20mm is prob required for a lightweight ali miller on X and Y with 16mm on Z
Fully supported rail with open bearings is best but harder to work with...
there will always be deflection.... the question is at what point does the deflection become to small to be of interest.... clearly if you are aiming to mill Ali to .01mm tolerance you need something that wont deflect more than .005 under cutting load.... remembering that deflections combine... movements in gantry, table, etc all add up...
Just to answer the point made previously, now the rails are fitted and bolted up tight with the table attached, the deflection of the centre of the table (16mm rails) under 4.5kg of load is <0.05mm (so that is the combined deflection of the rails, the MDF table and the MDF base the rails are mounted on).
I want to build table 1.2x0.8 meter.I will use supported 16mm rails on 1.2m long side.On 0.8 what have to use to have no deflection - supported rails 16mm or 20mm unsupported.
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