In the Shop: The Hole Story

November 12, 2011 07:52 AM
In the Shop: The Hole Story

Drill bits in farm shops lead a tough life. In many shops, dull drill bits outnumber sharp drill bits two to one. But it doesn’t have to be that way. Careful selection of the type and style of drill bit, along with correct drill speed and lubrication, can keep drill bits sharp through dozens of uses.
There are three basic types of drill bits: carbon steel, high-speed steel and cobalt.

Carbon steel drill bits are the lowest-cost drill bits and work well for drilling wood, plastic and soft metals. They are not recommended for drilling stainless steel or hardened steel alloys.

High-speed steel drill bits come in several grades and have "HSS" stamped on their shank. HSS drill bits are more durable, stay sharp longer and cost more than carbon steel drill bits. They work well in many steel alloys but aren’t recommended for use in stainless steel.

Cobalt drill bits resist dulling better than carbon steel and HSS drill bits but are more brittle and also more expensive. Cobalt drill bits are preferred for stainless steel and other tough drilling jobs.

Bit coatings reduce friction. To complicate matters, all three types of drill bits come with optional coatings: black oxide or titanium nitride. Coatings don’t improve cutting ability, but they do reduce friction, which improves feeding and reduces heating, which in turn might increase longevity.

Black oxide coatings give drill bits a dull black finish that reduces friction, not only against the sides of the hole but in removing chips and metal spirals created by the cutting edges. Tests show that black oxide coatings can increase drill bit life by 50% compared with uncoated HSS drill bits.

Titanium nitride drill bits have a golden finish that reduces friction even more than black oxide. In tests, titanium nitride drill bits drilled holes 75% faster with 25% less effort and lasted up to 600% longer than uncoated HSS drill bits.

Two more considerations when selecting drill bits are tip angles: While there are oddball tip angles available for specialty drilling, the most common drill tip angles available to consumers are 118° and 135°.

The 180° drill bits are designed for use in drill presses. The 135° drill bits are for portable hand-held drills and often come with additional "split-tip" machining to "self-start" holes.
The life span of a drill bit is directly related to how it is used. Drill speed, feed rate and cooling/lubrication are critical to drill bit performance and longevity.

In the drill press owner’s manual, there is a table that lists specific drilling speeds for different-sized drill bits in different metals. In general, the smaller the diameter of a drill bit, the faster the recommended speed.

For example, a ¼" drill bit in a drill press, drilling mild steel with appropriate lubrication, should spin at around 1,500 rpm. A ¼" bit in a portable hand drill, drilling mild steel without lubrication, should run at only 760 rpm.

Unfortunately, most holes drilled in metal in farm shops are created using portable hand drills. It’s up to the user to "guesstimate" the speed and feed rate to optimize performance. To do so, check the speed range on the drill’s label, then adjust the drill’s speed "by ear" to an approximate speed. The feed rate of a hand-held drill is … a guess at best.

However, an engineer with a drill bit manufacturer once told me it’s nearly impossible for an average-sized man to exert enough pressure to overfeed a drill bit larger than ¼". His rule of thumb was, "If you’re getting a steady stream of chips or a nice spiral of metal out of the hole, your feed rate and drill speed are probably OK."

He suggested WD-40 or JB-80 to lubricate and cool drill bits and discouraged locking a drill at full speed.

"Speed builds heat, and heat is the enemy of drill bits," he said. "A drill bit with a blued tip was spun too fast, probably without enough lubrication."

In the sop Cool toolCool Tool of the Month

Buyers either love or hate drill bit sharpeners such as the Drill Doctor. The secret is to precisely follow the instructions included with the machine to transform dull abused drill bits into sharp, well-centered, hole-drilling demons. Price: Less than $100 for a consumer-grade Drill Doctor, more than $150 for a professional-duty unit.

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