Was about to get a toy- then I did some research...

[Juranovich]

Best would be if Spindle centre /the bit/ does not go further than than carriage, machine looked from side . But If not possible its not a problem . If square linear rail is used. Though for metal work i say better not, for a perfect finish i mean.

Red line shows the ideal, blue line the normal. I would say 250-260mm span for me is the minimum of the gantry legs/ bearings

I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.

[Boyan Silyavski]

I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.

It does not matter as far as you use Hiwin 20 size or comparable rails. You could offset al gantry rail to the back, to compensate a bit for this. As Dean says look at other build for inspiration and to figure the details.

I will not tire to say that machine is as strong as its weakest point, so do not overbuild / did i just say that / just take care not to have weak details

[JAZZCNC]

I will not tire to say that machine is as strong as its weakest point, so do not overbuild / did i just say that / just take care not to have weak details

I DO NOT BELIEVE WHAT I'M READING... . . . FINALLY.........We agree on something...

[JAZZCNC]

I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.

If it helps I use 300mm wide plates with this design.! Anywhere between 250 and 300 will work ok.
However, keep in mind how you are going to fasten the profile to the plates and fasten the plates to the bearings if going with gantry on rails setup.! It's easy to forget things like access to bolts or one blocking another.!