To cut circular holes in our grill lets use the ‘Edge Slice’ tool. Activate ‘Edge Slice’, turn on Snapping (‘X’) and change its options (‘F11′). In the Snapping Options choose ‘Geometry’ snapping. Be sure to select the grill geometry and deselect the cylinders.
Next click on the polygon you want to cut. As long as you don’t click anywhere else, you can drag the vertex around. Be aware that if you click on the polygon and drag onto an edge, it won’t cut the edge you dragged to. To do that you need to click on the edge, not on the polygon.