All about drill bits

post-nov11-drillbits

By Grant Laidlaw

Gert asks: Grant, my staff have the idea that once a drill bit is no longer sharp the bit should be replaced. This becomes expensive. Although basic I think that this would be of considerable help. Thanks.

Hi Gert, yes this is basic but one of the skills that unless you have completed some form of apprenticeship you will not have picked up. As we have a large percentage of technical persons working without any qualification I would imagine that very few of our technical personnel could sharpen a drill bit.

Let us look at safety first.

Using Drills Safely

Drilling is a common operation undertaken throughout the industry, however, the dangers involved with the use of this equipment are often underestimated. When performing any drilling operation it is important that the safety precautions mentioned earlier in this resource are taken into consideration.

Choose the Right Tool and Bit

Choosing the correct tool and accessories for your application can help to reduce the risk of serious injury. When used according to the manufacturer’s instructions, the proper tool and accessory will do the job safer and faster. Some things to check are:

Does the drill bit shank fit properly into the chuck? Check the tool’s manual for the type fitted
Is the drill’s capacity adequate for the accessory? Make sure the size of the bit is equal to or less than the capacity on the tool’s nameplate.

Before Drilling

Before working with any drill whether portable or pedestal, make sure the tool and its accessories are in proper working order. Failure to do so may increase your risk of injury and may result in binding, stalling, and loss of control. These situations may cause an accessory to break, causing an injury. Check carefully for loose power cord connections and frays or damage to the cord and plug. Replace damaged tool or extension cords immediately. Find out what is behind a workpiece before you do the job. Do not drill or cut into existing walls or other blind areas where electrical wiring, water, or gas pipes may exist. If this situation is unavoidable, disconnect all fuses/circuit breakers, and shut off any water and gas lines feeding this work site. Warn any other workers on the other side of your presence and operations. Some other things to check before drilling are:

Where applicable be sure belt guards are installed and working properly.
Be sure the chuck is tightly secured to the spindle.
Tighten the bit securely in the chuck.
Remove all chuck keys or wrenches before starting the drill.
Carefully set the drill press speed for both the type of material and bit size you are using.
Remove material or debris from the area that might be ignited by hot swarf.

During Drilling

To prevent the workpiece and backup material from spinning, set them against the left side of the drill support column. Never hold the work piece by hand. Secure the work piece with a clamp or another appropriate fixture if it is not long enough to be braced against the table column. Some other things to watch for during drilling are:

NEVER overreach! Never reach around or under the working head, or grab the chuck to stop a drill press. This can result in hand injury or other serious injury should your clothing warp around the spinning drill.

Don’t force drilling. The tool will do the job better and safer at the rate for which it was intended.
As you get close to breaking through the bottom of the workpiece, reduce pressure and allow the bit to pass through the hole easily. Put a piece of scrap wood under your workpiece to reduce splintering and to protect the drill bit point and work table.
If the drill binds in the workpiece, release the switch immediately. Unplug the tool, then free the bit from the workpiece. Do not try to free a jammed bit by starting and stopping the tool.
with portable tools, firmly grasp the trigger handle and auxiliary handle (if provided) to maintain control.
Always hold or brace the tool securely. Brace against stationary things for maximum control.
In a binding situation, the tool will react in the opposite direction of the turning bit. When drilling into the workpiece, the tool will try to spin anti-clockwise. Watch out for this – larger hand held machines will throw you off a ladder. Check the safety clutch.
Always keep a firm footing when using portable drills. Be sure you have balance and control before you start the job.

After Drilling

Don’t touch the drill bit or chips/swarf. The drill bit and swarf are hot immediately after drilling.
Always shut off, unplug, and lock the drill press if a lock is available.
Store drill bits with care. Do not drop them or subject them to excessive heat, cold or humidity.
Unplug the tool immediately after use, and before removing or changing the bit and before performing any service or maintenance on the tool.
Always clean up around the pedestal drill or if using a portable drill, the area surrounding the job.

Drills need sharpening from time to time. This takes considerable skill, and cutting angles have to be reproduced very accurately. If not, the drill will cut badly, will often overheat and can be damaged beyond repair. A drill which has not been sharpened to provide equal cutting lips will wander when drilling, and will drill a rough and oversize hole.

Drill Bits

The twist drill is the most common type used for drilling holes. The flutes of the drill are spiral shaped and as well as carrying the shavings away from the cutting edge and the lubricant to the cutting edge, they actually form the cutting edges of the drill.

As with any other cutting tool the drill has cutting angles clearance angles and rake angles, these angles may need to be changed for the drill to be used on some materials such as grey cast iron, alloy steels or plastic. Drill bits have a shank by which they can be held by the drilling machine and spun at high speeds and fed into the work.

Fasteing Bit

If the drill bit is below 13 mm in diameter, the shank is parallel allowing the drill bit to be held in a Jacob’s chuck like the one shown here. Drill bit over this size have a Morse tapered shank which allows the drill to be fitted directly into the spindle nose of the drilling machine. The Morse taper twist drill bits pictured below are typical of those used in the industry. The full range of Morse tapers is from 0 to 7, sleeves can be used to adapt smaller drills to fit the machine taper size. Drillbits

High speed twist drills are more suitable for harder materials and if used properly and looked after will last much longer than regular twist drills. Carbide (more correctly, tungsten carbide) tipped bits are used for masonry, stone and ceramic tiles. These generally need to operate at slower speeds and higher feed pressures. These bits are normally used with a hammer drill. The bit is both rotated and hammered into the workpiece. Carbide Drillbit

The hammering breaks up the masonry at the drill bit tip. The flutes of the drill bit carry away the dust rather than the swarf encountered when drilling metals. Rotating the bit brings the cutting edges onto a fresh portion of the hole bottom with every hammer blow.

When to Sharpen

Clearly, sharpening of drill bits is normally carried out when they have become dulled. The signs of a dulled drill bit include the need for higher than normal pressure, the generation of excessive heat, squealing, and a lack of cutting progress. However, drill bits might also need to be re-ground to suit different materials. As they are bought, drill bits have cutting surfaces that are configured for general-purpose work – drilling steel, for example. But for optimal results on materials like plastics, rubber or very hard materials, the drill geometry is best changed from the all-purpose shape. This can be easily achieved by grinding.

Terms

Hold the drill vertically and rotate it until it looks like this. Note the angles that the top edges of the drill bit form to the vertical – here, each face is at 59 degrees, giving an included angle of 118 degrees. This is a typical point angle for a general purpose drill bit. Scheme 3

Rotate the drill bit in your fingers until you can look at the ‘ramp’ behind one of the two cutting lips. If the drill bit is held vertically, the edge of the ramp forms an angle with the long axis of about 12 – 15 degrees. This is called the clearance angle.

Clearance angle.

Quenching

Most good quality drill bits are made from high speed steel. (You will see ‘HHS’ stamped on the shank). High speed steel should NOT be quenched in water. Do not allow the drill to overheat when sharpening. If the metal starts to change colour, and pick up blue hues, then it has been overheated. Take it easy. Don’t go this far.

Sharpening Tungsten Carbide drills

Concrete drills have tungsten carbide inserts. There are four distinct faces (Two small, on the leading edge, and two small, on the trailing edge) on the tungsten carbide tip. When you sharpen a masonry drill, it is necessary for you to reproduce these four faces. If you re-sharpen the drill before it has become too dull, the originals of these faces will still be apparent. If you work with care, you can line the originals up with care, and can grind these. There is no rotating motion to be done now. A masonry drill is easier to sharpen than a standard drill. A silicone carbide wheel is used for grinding tungsten carbide tips. This is a soft, green coloured wheel. The active sparking associated with conventional grinding doesn’t occur with a silicone carbide wheel. You will notice a gentle glow in the region of the tip being ground. Grind each of the four faces individually. Don’t grind any face for longer than 10 seconds, and then quench in water. As with other drills, be sure to keep the ‘dead centre’ at the true dead centre when grinding.

Gert in the next issue I will continue with sharpening of a bit to a 59 degree angle. Thanks to everybody for the overwhelming response. I receive on average, over sixty questions per month and cannot publish all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you.

Archive for November, 2011