When beginning a design it really helps when you have a clear idea/concept of what you're attempting to create. I realize that designs are often an exploration en route to the final version, "what if" is what CAD modeling is great at. Yes the first two posters are quite correct in what they've stated but there can be additional factors involved. Does this design need to be manufactured/fabricated? If so a solid understanding of any process involved will determine how feasible it is to create said design and which process should be used. Also, a good understanding of dimensional tolerances and finishes required will determine cost/process, and influence the success of that design. Very often I've seen designs/drawings with unrealistic tolerances in an attempt to be "more precise" for non-critical features, thereby driving the cost into the stratosphere. Sometimes it's sheer laziness on the part of the designer/drafter, sometimes it's just a lack of practical knowledge. If the OP is about completing an assignment for academic purposes then what the first two posts states is sufficient. If this is a question for prep into the real world of design/engineering/manufacturing then take my suggestions to heart, you'll be better regarded if you come prepared in these subjects. JMO after 35 years in manufacturing. Hope this helped someone.
