Rather like cameras, the best folding knife is the one you have with you when you need it. The Swiss Army Knives (made by Victorinox and Wenger [two separate companies]) are highly respected and come in a wide range of types. The biggest one has over 50 tools - and they all work, but it is big and heavy. There are also specialised penknifes; for computer use, small enough to easily carry, with built in memory sticks, watches or compasses.
smith&wesson stainless steel 420
The Buck Knife 110 is a folding hunter or lock back folding knife. This knife is foldable and features a stainless steel blade, wooden handle and a leather sheaf.
The cons of having a buck folding knife is you might cut yourself with it but even then, because it's a folding knife that is pretty rare, even if you don't handle knives often.
an automatic knife is a folding knife that opens with a spring by pushing a button
There are multiple advantages to using a folding knife. The one advantage that seems to really stand out is that with a folding knife verses a fixed, the folder is able to be easily concealed. This is beneficial if you were to be in an environment where a weapon is deemed inappropriate.
Japan
The Carraca Navaja folding knife was a type of large folding knife often carried by Corsican bandits. The "Carraca" comes from the sound made by the ratcheting lock mechanism on the knife,which produced a series of clicks as the knife was opened. When the bandits would stop someone on the road to rob them, they would open the knife quickly, and the ratcheting lock would make an audible "Zzzzzzzzzzzt" sound. The lock mechanism is released by lifting a catch which is similar to a clasp knife, that is located on the spine of the knife near the bolster where the blade pivots.
Folding knives do not auto open, a folding knife would refer to a knife such as a buck knife, or swiss army knife, both of which require to be manually opened. If you are asking how to convert a folding knife or "folder" into an automatic knife, well it is possible but it would have to be enabled for conversion. For instance, a boker magnum AK74 knife may be sold without a spring for legal reasons, then to make it the automatic switchblade it was meant to be, you would have to add a spring by taking it apart. If you are asking how to open an automatic knife (i.e., a switchbalde, or spring assisted knife), then the answer is it has to be engaged with the push of a button, a switch, or a lever; doing so will allow the spring to deploy automatically given you took the safety off if it has one.
no more than a 3inch blade
Websites such as eBay or Amazon will probably be the cheapest places to purchase folding tactical knives. There are also online shops specialising in knives, such as Knife Center.
A penknife, or pen knife, is a small folding knife originally used for cutting or sharpening a quill to make a pen.
$500-750