For provisional applications, just about any drawings will do. Like other aspects of provisionals, the better prepared the provisional, the better chance of being able to properly rely on the provisional filing date. Thus, even for provisionals, proper drawings are important.
For non-provisional patent applications, drawings are not an afterthought. There are a number of very specific PTO rules that must be followed in the drawings. The drawings must show every feature of the invention specified in the claims. This means that it is not enough to simply discuss the idea of the invention generally in the specification; the invention must be discussed and the drawings described in detail.
When preparing the specification of a patent application, it is important to provide different levels of specificity. The invention should be described broadly and generally, but also with increasing levels of detail. This provides support for claims of varying breadth.
The drawings include reference numerals that point to various parts or elements in the drawings. These numerals are referred to in the specification to describe the invention, including how it is made and used. In this way, the drawings are not cluttered with a lot of words that make them difficult to read and understand.
Inventors sometimes think they can save money by preparing their own formal drawings. There are also a number of specific requirements relating to margins, font size, and lead lines used in the drawings. The rules for PCT applications are even more specific. In a typical case, patent draftmen who specialize in preparing patent drawings only charge a couple hundred dollars to prepare the drawings for an application. When I work with inventors who want to do the drawings themselves, I often go through multiple iterations that require a great deal of my time to correct and explain as the inventor works to comply with the drawing rules. In these cases, the time involved costs a great deal more than a couple hundred dollars.
Like most aspects of patent preparation and prosecution, a patent attorney can easily provide advice and guidance on the preparation of drawings for a patent application.