when a large dia. hole is to be drilled,why a smalldia. hole is often drilled first?

Question

when a large dia. hole is to be drilled,why a smalldia. hole is often drilled first?what functions are performed by the flutes on a drill?

Answer 1

1. Because it helps to guide the larger bit. Also, the small bit serves as a test drill that is unlikely to cause problems if it slips.

2. They help to carry swarf away from the drill.

Answer 2

The low relative speed at the center of the bit and the solid web means the chisel edge in the center sort of plows through the work instead of cutting a clean chip. Since the the thickness of the web is proportional to the bit size, drilling a smaller hole to remove material in front of the web significantly reduces the axial force needed. The small hole may also help guide the larger bit, depending on the method used to locate the hole, however the larger bit is stiffer so it depends on the application, if locating off a center punch mark a small bit is very helpful, but in, for example, a CNC operation the stiffer bit would probably be prefered.

Answer 3

To the above good answer I would add that a pilot hole greatly reduces the amount of time and effort required to drill the larger hole. Large bits do not generally drill near as well without a pilot hole.

Answer 4

It is easier to make sure your hole is in the proper position if you start with a small hole at the exact location.

Flutes carry away the chips and cuttings that the cutting edges on the tip of the drill have cut off. Otherwise the hole would fill with that debris and not make an accurate hole.

Answer 5

Drill bits tend to wander somewhat during the drilling process, and a hole can be more precisely positioned if a pilot hole is drilled first -- as well as saving wear on the expensive larger drill.

Answer 6

If you look at a large drill end-on you will see that there is a relatively large portion at the centre, giving it strength, that has only rudimentary cutting edges. A smaller guide hole is more easily drilled and allows the centre of the large drill to penetrate with little difficulty.