Instead of relying on the grid, maybe try this free plugin by Rick Johnson/Graffix Adobe Illustrator plugins - rj-graffix It installs 2 new tools in the toolbar IsoTool Draws straight lines (paths) constrained to the isometric angles. EG: The smart guides in illustrator don't always work well for finishing the shape best to draw fast and overshoot, then select and drag those overshot points back with the Direct Selection Tool or quicker, select all and with the Shape Builder Tool Alt click to delete the extra lines, to leave just the cube. 2nd tool , lives underneath the first, Isometric mover tool is good too, lets you drag the whole face at correct angle shows a nice guide line too Though you have to Edit >Copy and Edit> Paste in front , first if you want to move and copy, as the Alt key is for free movement To set the construction guides Go to Preferences , Smart Guides , select Construction Guides, choose 30 degree angles, then highlight and delete the fields you don't want, leaving say , 30 ,. 90 , 150. Then change them back to original settings when you finished isometric work.
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