You may actually want to construct some invention prototypes even before you get too fancy with computer drawings. You can burn up a lot of money on engineering drawings when maybe what you really need first is a chunk of
papier-mâché that shows what you're trying to do. Then, go back and do the computer-aided drawings. And in trying to assess the right time for invention prototype as it relates to your patent, don't fret. You don't need to possess a working prototype in order to file for or be issued a patent. You just have to be able to prove that you could create a product that works as you've described.
The biggest problem that is seen when developing a product is that people tend to show their invention prototypes to their mothers, cousins, aunts and uncles and get positive feedback from them, and then go off on a tangent with their product without knowing whether they'll have commercial success or not.
You need to have someone whose opinion matters look at your invention prototypes and say it's promising. And that is exactly why Build My Product was developed. They want to help inspired inventors like you achieve your goals. invention prototypes need to be aimed at the right audience in order to do its job. That means tailoring it to fit the requirements and withstand the critiques of any combination of potential investors, partners, customers, suppliers and investors, not just the casual appraisals of friends and family. The more operable and the slicker-looking your invention prototypes, the better, however, don't get too carried away. After all, invention prototypes aren't a production piece, and no one expects it to be. Keep this project inexpensive because there are plenty of other places for you to spend your startup capital!
If it isn't too difficult or expensive, be prepared to make multiple versions of the invention prototypes along the way, your design is probably going to evolve and improve as you develop it and you will want to have enough money to make invention prototypes that are the latest and greatest generation. Another form of this approach is to make one prototype for "show" and another for "go." In other words, you could make one of the invention prototypes that looks like a finished product and another that demonstrates the functionality.