Making holes
Tips for accurate holes in metal

by Clair D. Urbain

For retrofits, repairs or improvements on nearly any job in a plant, you’ll need to pull out your drill and make some new holes.

But what seems to be a straightforward job can cause extra grief and work if you don’t sink the hole correctly.

“Setup is the key factor in making a clean, correct hole in a maintenance job.  Usually time takes precedence over materials and tools and there are so many variables with each repair job,” says Tom Pope, Milwaukee Electric Tool Corporation.  “The trouble is, making a hole is a one-shot deal.” 

Alignment is all-important
Pope says most hole-making troubles show up with steel.  

“The problem relates almost always to alignment," he says.  
"The hand pressure required to get a good bite often causes side forces that can get you off-center and your hole goes crooked.  Then as you try to compensate, it bends the bit and it breaks.” 

Center-punching the work piece first then drilling a pilot hole can help you cut any hole faster and more accurately, he adds.

Excess force dulls the drill and can cause the bit to suddenly emerge from the underside of the work, causing burrs that must be removed.  If the force is too great, it can even break the workpiece.

Use purpose-built tools
Selecting the right drill or cutter can make a huge difference in hole quality. 

“We often get requests from end-users who want us to put bubble levels on the side of our drills.  But we’ve found that the bubble level covers only one plane, so it is of limited help,” Pope says. 

Instead, consider using a portable drill press that can mount to the work piece to shoot a straight hole.  Several power tool manufacturers offer these models; they are especially helpful when sinking larger holes.

Drill alternatives
For applications in thinner metal, a hole puncher can greatly improve hole-making speed and accuracy.  Electro-hydraulic hole punchers can pop holes in up to 15/32" metal in less than 4 seconds with less operator fatigue, says Jeff Miller of Hougen Manufacturing Inc.

“Annular cutters, which actually machine a hole into metal instead of drilling it can be very helpful,” says Miller.  “Annular cutters do not need a pilot hole, cut three times faster than twist drills and hole saws yet make round, burr-free holes without drill wander.”

For larger holes in thick plate applications, a magnetic-based drill works best with annular cutters.  A vacuum base is available from some tool manufacturers for use on non-ferrous materials. 

“Sheet metal annular cutters can be used in materials up to one-half-inch thick using an electric handheld drill,” Miller says.

If a drill is the right tool for the job, then the next step entails correct bit selection, says Kim Carlisle of Primark Tools.  Carlisle suggests using the following drill bit types in these applications:

Stainless steel or other aerospace materials: Use cobalt steel drills.  They withstand the higher heat needed to effectively cut these materials.

Hardened steel: Tungsten carbide-tipped drill bits should be used, however, cobalt steel drills will outperform high-speed steel in hardened steel applications.

Carbon steel: These economical bits are ideal for drilling holes in wood, plastics or light non-ferrous metals.

Abrasive materials: Fiberglass, graphite epoxies, high silicon aluminum and various types of cast iron will quickly dull twist drill points.  Use abrasive-resistant high-speed steel or titanium nitride-coated twist drills for best results.

High-speed steel: The choice for boring in any machinable metal is nitride- or titanium nitride-coated drills.

Watch your speed
It’s difficult to measure on a maintenance job, but if you go too fast or too slow with a bit, you’ll get a poor hole and shorten bit life, says Pope.

“If the bit turns faster than the speed allowed by the bit’s geometry, the bit will heat and dull. If you go too slow, the bit will overload and bind,” he says.

When bits are dull, don’t try to resharpen them by hand, says Pope.  Instead, machine-sharpen them. 

“Hand-sharpening a 135-degree split-point bit is very difficult," he says.  " If you don’t have a bit sharpener, use a new bit.  The applications where you need bits in maintenance jobs are too varied to try to make do with dull bits.”

Need bigger metal holes?
Hole saws are another option to make metal holes up to 6 
inches in diameter.  Starrett, a manufacturer of hole saws, 
offers these tips:

Always wear safety glasses when cutting holes and keep 
loose hair, clothing and jewelry as well as hands away from 
the cutting area.

Use coolants and diamond hole saws when cutting metals.  Use diamond hole saws on hardened steels, fired ceramics or concrete.

Hold the saw perpendicular to the work area; sometimes a quarter-inch pilot hole helps guide the hole saw.  Run the saw at the recommended speed for the material and the hole diameter.  A variable-speed drill can help you control speed, especially at the beginning and near the end of drilling the hole.

Use enough pressure to assure cutting but do not allow it to 
rub the work, when little cutting action takes place.  Remove chips frequently.

This article appeared in the April/May 1998 issue of MRO Today magazine.  Copyright, 1998.

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