In the world of drill bits, the term “spiral” can reference two meanings, one generic, one specific. Lets look at the generic term first.
Often lay people or home hobbyists refer to a drill bit as “spiral” because of the way the cutting edge spirals up the shank of the bit. However, in professional language, this type of bit is always termed a “TWIST” drill bit. The twist of the bit is designed so that the waste material created as the point of the bit eats into whatever it is you are boring a hole into can travel up the shank and be removed, This prevents the bit from becoming clogged, heating up and decreasing efficiency. It works on much the same way as a corkscrew or general purpose screw, the spirals acting as a means of transport or propulsion.
Lets twist and step
However a spiral bit is a specific tool and is what we will look at in more detail.
More often called a UNIBIT or STEP BIT these are cone shaped and blend a twist [as above] with a STEPPED design. This step design combined with its cone shape is one of the features that makes this bit popular in mass production situations. Using a conventional straight sided twist bit an operator would need to change the bit several times if a variety of hole diameters were required. The step bit allows the operate to do this with just one bit. Of course, even these spiral bits come in a variety of sizes so some limitations do apply. Smaller uni bits often begin with a point or pilot, which makes starting the hole considerably easier, [if you have ever encountered “bit wander” when trying to use a larger diameter bit you will know what I mean] whereas larger uni bits are often used for making an existing hole bigger and have blunt tips.
Just like their slimmer cousins, the straight shank twist or spiral bit, uni bits are designed for “soft” work and best suited to materials such as timber, plastic, light weight aluminium or laminated surfaces. They can handle other very thin metals but it is likely to decrease the life expectancy of the bit.
Not a messy bit
Another advantage of a uni bit is the end result. You may have experienced the “ragged” edge left after a straight shank bit has finished and often a quick tidy up with glass paper is needed if this hole is to be visible. A step bit however will leave a nice, clean hole, no burrs or frayed edges, again, a valuable time saving feature in commercial or manufacturing operations. Commercially, uni bits can be used in lieu of glass paper, ie, cleaning up the rough edges from conventional drill bits.
Now maybe you are thinking with all this talk of commercial activities that this particular spiral bit is designed only for those super powerful drills and not for the humble handy cordless drill? Wrong. Uni bits, just like their standard spiral sisters come in a variety of shanks, including a hex fitting for snapping into the do-it-yourself cordless drill. So, next time you need a series of smooth, clean holes, try a uni bit, you won't be disappointed.