You may be interested in my steel frame router Home Built GantryMill Improvements - YouTube the cutting area is 600mm x axis and 350mm y and 100mm z. It is built with materials I had to hand hence the mixture of steel and aluminum.

I would like to echo Robins comments about distortion caused by welding, it can be a nightmare so you will need to be very careful, also it is easier to weld thicker material but harder to remove the distortion.

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