6. Drilling

Drilling is a process in which a rotating cutting tool is used to cut a round hole in metal.

The cutting tools commonly used for this purpose are known as twist drills, and are made from high carbon steel or high speed steel.

Drill Bits

There are two common types of material used for cutting tools which are High-speed steel (HSS) and Tungsten carbide. HSS is very popular due to its low cost, and ability to flex under impact without breaking or chipping. HSS can be combined with other alloying ingredients such as cobalt. Some cutting tools may have up to 8% cobalt added to them. These HSS tools are often labeled simply "cobalt". This variety of HSS offers the same advantages as standard HSS, but can operate at up to 10% higher speeds and feeds due to its slightly higher hardness, better toughness, wear resistance, and heat resistance.

Tungsten carbide offers superior tool life due to its extreme hardness, wear resistance, and ability to withstand heat. While carbide is good at many things, it is also very brittle, and easily chipped under impact. While some tooling is made of solid carbide, more economical tooling is available that uses small pieces of carbide as the tool's cutting edges. The carbide pieces can either be mounted to the steel tool body by brazing or by screws. Tooling using replaceble carbide held in place with screws is known as inserted or indexable tooling.

Twist drills

A twist drill can be divided into three main parts:

Drill point - cone-shaped area at the tip of the drill bit. The drill point is made up of sub-parts: the lips are angled cutting edges that shear the metal into chips as the drill rotates. The lips are the only part of the drill that actually cuts. The angle of the lips on a general-purpose twist drill is 118 degrees. The chisel edge is also sometimes called the dead centre and lies between the lips at the centre of the drill point. The rotating chisel edge is the pivot point of the drill and does not actual cutting.

Drill body - extends from the tip to the beginning of the shank and makes up majority of the drill bit. The drill body is made up of the following parts: flutes (pathway for chips to flow out), margin, body clearance, helix angle and the web (connects the flutes and makes up centre part of the bit).

Shank - provides an area for mounting the drill bit into some type of holder.Shanks can be tapered and are more common on larger drill bits. The more taper is a popular standardised style of taper used in drill press spindles.

Drill sharpening

Twist drill points will wear as they are used, and may become damaged, so it is important to know how to resharpen drills by hand to keep them sharp and reduce tool replacement cost. Twist drills can be resharpened many, many times before they need to be replaced. The following basic steps can be performed using a bench grinder:

Hold the drill with the cutting lip, at 59 degrees (approx. 60) to the wheel face, with the tip slightly higher than the shank.
Lightly touch the lip against the wheel and lower the shank end of the drill to move from the lip toward the heel. Apply more pressure when moving toward the heel to create the 8- to 12-degree lip clearance. Repeat the process three times.
Repeat with the other lip and as needed to remove any wear or chips at the cutting edge.
Check the drill point angle and lip length with a drill point gage and grind as needed to create the proper angle and make both lips the same
Check the angle of the dead centre and the lip clearance with a protractor.

Procedure for Drilling

Accurately mark the centre of the hole

Use a rule, scriber, try square or odd-leg caliper to mark centre lines which locate the centre of the hole.

Centre punch

Centre punch the metal sufficiently to guide the drill on-centre.

Select the correct size drill

Insert the drill shank in the drill chuck. Use a chuck key to firmly tighten the chuck. Check that the drill is central in the chuck.

Check the speed

Drills are damaged if used at the wrong speed of rotation. Small drills should be used at a high speed. Larger drills should be used at a lower speed.

Work must be held securely

When drilling small holes in thin metal, the work may be held in a hand vice and supported on a piece of wood to protect the drill table. Heavy drilling through thicker metals requires the work to be held firmly in a machine vice.

See that the drill hole is on-centre

After the point of the drill has cut a little distance into the metal, it should be withdrawn and the work examined to see if the hole is on-centre. If not, the hole should be drawn over with a centre punch or cold chisel.

Feed the drill slowly through the metal

A constant pressure is needed on a drill to force it through the metal. A small drill can stand less pressure than a large drill. To prevent the drill jamming, ease up on the rate of feed as the drill breaks through the underside of the metal.

Safety

Wear eye protection.

Only one person should operate the press drill.

Do not hold metal in your bare hand when drilling.

When drilling holes of a diameter greater than 10 mm, a pilot hole should first be drilled. The pilot hole should be approximately half the diameter of the final hole. A build-up of heat can damage the drill. This may be prevented by the use of a suitable lubricant.

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