Get the biggest bite from your twist drill bits

“When it comes to drilling in steel, contractors often end up selecting the wrong bit for the job,” says Kim Carlisle at Magna/Primark. Users rely on old standby bits when another bit can make more holes, faster.

Consider these bit differences that affect productivity:

Conventional vs. split-point: The two most common twist drill types have different drill point geometries, says Jeff Major at Triumph Twist Drill. “The general-purpose twist drill point has a 118-degree geometry. It’s a good all-around bit for making holes in mild steel and aluminum.”

They are widely available and low in cost, but may “walk” when starting a hole in hard materials. A center-punch mark may be needed.

The split-point bit comes in 118- and 135-degree geometries. “They are self-centering, which eliminates walking when starting holes. They are excellent for hand-held drilling and on curved surfaces. They can last longer than conventional point bits and work well in alloy steels. They often take less effort because they have better penetration and break up chips as they drill,” says Major.

Metallurgy: You will find two types of metallurgy in most bits: molybdenum and cobalt.

Molybdenum high-speed steel grades work well in drilling carbon and alloy steel and other machinable metals, wood and plastic, says Carlisle. Cobalt bits are used to drill hardened stainless steel, titanium and cast iron,

Cobalt bits have a thicker web, increasing the drilling thrust needed to make a hole.

“When in doubt, try TiN (titanium nitride) bits first,” says Carlisle.

Coatings: A drill bit’s color results from heat-treating or coating processes. Most affect the bit’s drilling properties.

Low-cost bits with a bright, shiny finish have no additional treatment to help performance. They are a good choice for drilling in soft metals.

Low-temperature oxide surface treatments (350F to 600F), sometimes referred to as ferrous oxides, have a yellow-brown, bronze or blue look. This treatment has little or no effect on drilling performance.

Bits with a black oxide surface treatment are a dull, black color. They are good for drilling in iron and steel but not for drilling softer metals. “The coating helps reduce galling and chip-welding during drilling,” says Major.

A steam/nitride finish looks like a black oxide bit, but offers corrosion resistance and improved abrasive action.

A TiN coating makes the bit’s surface harder and gives it a slick surface for easy chip removal. It has a distinctive gold color. TiN twist-drill bits can drill at higher speeds and heavier feeds and work well in most metals, says Major.

Bit length: It’s best to use the shortest bit possible. “Most drilling jobs don’t require great depth. In fact, most bits are never used beyond four times their diameter. That means a 1/4" drill is rarely used to drill a 1"-deep hole,” says Lowell Frey of Champion Cutting Tool Corp.

Flute design: Flute design can be used to best match the bit to the job, says Dil Ahluwalia of Alfa Tools. A parabolic flute design is the most efficient for metal work. “It ejects chips fast,” he says.

If you are drilling very soft metals, Ahluwalia says to select a bit with a slow spiral and a larger groove. It has greater chip-carrying capacity.

Use tips
Watch your speed. “Running a bit too fast is the worst thing you can do,” says Carlisle, “Our mindset is that speed helps you get the job done faster, but it’s not true.” Generally, in tougher material, a slower speed and slower feed is needed. Use higher speeds and feeds in softer materials.

Watch your chips. The material removed from the hole should come out in small, manageable chips. A good, constant feed rate is the key to creating and removing chips from the hole. “If your chips break into small curls and evacuate the hole easily, you are drilling at the ideal parameters,” says Major, “Don’t overspeed and underfeed the bit. This can decrease the bit’s life.”

Never use a twist drill to enlarge a hole. “Don’t attempt to enlarge an existing hole with a twist drill,” says Frey. Instead, use a reamer which has side cutting edges.

Don’t drill pilot holes. Pilot holes are rarely needed, especially with a self-centering split-point bit, says Frey. “If you need a pilot hole, make it very small. A 1/8" pilot hole should work for most jobber drills up to 3/8". A too-large pilot hole will destroy the twist drill bit.”

Deep-drilling tips: If the hole depth is greater than three times the bit diameter, it is considered a deep hole. You must retract the bit periodically to evacuate the chips. In deeper holes, reduce the feed and speed rate. Do not back out of the hole until you are at least half-way though the material, otherwise the undrilled metal in the hole may temper and become very hard to drill.

Prevent bit lock-up. As you get close to breaking through, back off slightly on speed and feed to help prevent the bit from grabbing.

Use lubrication. Ahluwalia recommends you use a cutting oil when metal drilling. “It helps you get a cleaner hole and will greatly lengthen the life of the bit,” he says.

Published in the January/February 2002 issue of Contractor Tools and Supplies magazine.

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