Joe's 1000x600x200 steel and aluminium router started

[devmonkey]

So do you think the difference with 40-50mm extra offset will make any notable difference to stiffness?

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Say your overhang to spindle centre from gantry centre (assuming a rectangular gantry profile) reduces from 150mm to 100mm as a result of mounting rails top/bottom then you have decreased the torque applied to twist the gantry to 2/3rds for a given force applied to the spindle, so it really depends on the torsional stiffness of your gantry and the spacing of X bearings. Ideally the front X bearing wants to be at or forwards of the spindle so usually face mounting the Y rails increases the X bearing spacing, this is no bad thing but it does mean the machine needs to be longer to accommodate the same X travel.

For reference the difference in gantry twist for a given spindle force between when the Z is fully up and fully down will be greater than the difference in twist saved by shifting the spindle 50mm backwards.

As everyone says on this forum it depends on what you want to do with the machine as to how stiff it needs to be, having built previous machines that weren't stiff enough I now make every design choice around increasing stiffness. My choice to top/bottom mount the rails was both to increase stiffness and produce as compact a machine as possible as it is going into my garage so I don't have to traipse down to the large workshop in the woods in mid winter.