Model engineers typically make models by machining working parts from stock metal and metal castings. Some models are intended as utilitarian working models, others as highly meticulous display models, or sometimes a combination of both. The most elaborate models involve hand manufacture of thousands of parts, taking thousands of hours to complete, often over a number of years or even decades. The model engineer is often guided by commercially available drawings, however some draw their own designs, or even work without drawings. Similarly, most model engineers will buy castings when required, but some buy or make foundry equipment to cast metal themselves.