September 7, 2015
Not sure about the 48" cabinet shelves, but I have a 36" wide cabinet and could not find a source for ready-made shelves. I needed one more shelf, so was compelled to make my own.
After looking at various options, I settled on a pine laminated project panel (3/4 inch thick) as the best choice. HD offers an “edge-glued panel” (sku 493562) but this is 6 feet long and I only needed 34.75 inches of this for my cabinet. From another source I found a “Craft Master's Lodgepole Pine Laminated Project Panel” that was available as 48 x 20 x 3/4 inches, readily cut down to fit my 36 inch cabinet with less scrap. This project panel may be closer to what you need for your 48" cabinet. In any case, it's important to cut to size accurately. I coated the entire surface of the shelf (including the edges) with clear polyurethane for protection.
Given that this material is basically a series of pine boards glued together (albeit well glued), I decided to reinforce the shelf by adding 3/4 inch aluminum angle stock to the short edges. I cut the long dimension of the pine project panel slightly short so that the shelf with the angle stock pieces added was the correct size. I secured the angle stock in place with two countersunk #6-1/2 inch flat-head screws along the edges, and two #6-1/2 inch pan head screws along the surface part, staggered so they would not interfere with each other.
Finally, I fashioned four new shelf support pieces from 1 1/2 inch lengths of rectangular molding, 1/4 x 1/2 inches. I had some pine molding on hand, but if I were doing this again I would try to find a denser wood. As it turns out, the shelf support holes provided in the cabinet walls by ClosetMaid had the correct tight clearance for metric M5 machine screws. #10 screws may also work, but are slightly smaller and the M5 screws fit more snugly. I drilled holes in each of the molding pieces to provide for a tight fit of 2 cm long M5 machine screws. Tricky part here is that the four shelf supports must be identical and the holes must be true (not tilted), so the holes for the M5 screws must be carefully drilled. Best to use a drill press and vice so these holes are identically placed and true.
The final shelf worked very well. It is probably stronger than the other shelves provided and less prone to warping over time given similar loading.
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