Read the book Patent It Yourself to get a good grasp on patent law fundamentals.
In your example, all the various homemade cheese graters would be "prior art" when evaluating whether the "professionally manufactured" cheese grater is patentable. It makes no difference if a product is created or described by an unwashed sociopathic alone in his hut or by staid IBM -- once disclosed the invention is (generally) prior art for everything that comes later.
The issue before the Patent Office in determining patentability would be whether the "professionally manufactured" cheese grater is sufficiently distinct from the prior art cheese graters. The questions are whether the prior art described all the elements of the "professionally manufactured" cheese grater and, if not, whether that invention is an "obvious" variation of the prior art.
Those inquiries are very fact specific and only a patent attorney who educates himself or herself about your particular invention and the relevant field of art can give you any meaningful advice. Good luck.
Also, just because the proposed product has never been professionally manufactured does not mean that it has never been disclosed in a patent application. It is possible that someone has a patent on your same concept but is not manufacturing it. It is also possible that someone attempted to receive a patent for the idea but was unsuccessful for some reason. If you are serious about getting patent protection you will want to have a patent attorney/agent perform a prior art search to see exactly what is out there and how any prior art differs from your concept.
Here, the fundamental question is what did YOU invent? In applying for a patent, an applicant represents that they are the true inventor of something novel and non-obvious. It may very well be that metal is not an obvious substitute for wood and that such an improvement is novel and patentable. Just because you might not be able to patent an entire device or process, there might be improvements that you could patent and help position yourself in the marketplace.