Straight, Smooth, and Round

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by Kip Hanson

Ensuring drill hole integrity


Achieving good tool life while cranking out lots of drilled holes is a sure way to give any machinist a warm fuzzy feeling. But if the holes don’t meet print, what’s the point? Taper and straightness problems, poor surface finish, ovality and triangulation—these are just a few of the quality concerns that often make holemaking a real pain in the neck.

Holes can be EDMed, EBMed, ECMed, stamped, lasered, and burned, but in machine shops, they are nearly always drilled, then reamed or bored if needed. Whatever the process, hole integrity (i.e. accuracy) is usually defined by one or more geometric dimensioning and tolerancing (GD&T) values:

Straightness—tie a pair of tin cans together and pull the string tight: that’s a straight line, something your drilled or reamed hole should follow.

Roundness—of course holes should be round, but instead are often afflicted with ovality and lobing, two conditions measured by roundness.

True Position—if you just drilled a hole in the wall and missed the stud, you’re out of true position, a commonly applied callout of hole accuracy.

Cylindricity—closet rods, toilet paper rolls, the barrel of a pen: cylinders are both round and straight, the two primary attributes of cylindrical forms.

Surface finish—rough like a cob or smooth as a baby’s…arm, roughness is determined by averaging the part surface’s tallest peak to its deepest valley.

Unfortunately, drills sometimes walk, making hole centrelines droop and wander. Workpieces move, causing problems with true position. Chuck a part too tightly or let the drill get dull and out-of-roundness may occur. Gary Kirchoff, product specialist at Elgin, IL-based cutting tool manufacturer Sumitomo Electric Carbide Inc. notes that achieving hole integrity is largely determined by the quality of the machining process, and the tooling that stands behind it.

Achieving hole integrity depends on many factors, including accurate tool alignment,  part setup, toolholder rigidity, and proper  feeds and speeds. Image: Dormer PrametMaking a plan
“The choice of drill and how it’s applied are critical to hole accuracy,” Kirchoff says. “When selecting conventional drills, high speed steel (HSS) is the least accurate, often leading to oversized holes. Cobalt is a little stiffer and more wear-resistant, so makes for greater accuracy. After that comes powdered metal and then solid carbide, which—aside from gun drilling—provides the best drilled hole quality possible.”

Kirchoff says surface finishes of Ra 32 µin or better are easily achievable with carbide drills, as long as the setup is rigid, there’s no runout in the drill or toolholder, clean lubricious cutting fluid is used, and the feeds and speeds are appropriate for the material being machined.

“And purchase the right tool for the application,” he adds. “A 140° split-point drill like our MP-X series does a great job in most applications, but many cutting tool manufacturers—Dormer Pramet included—offer drills that are fine-tuned for a specific material, hole depth, chip control, and so on. These give better hole quality and tool life than a general purpose drill.”

Dormer Pramet product specialist Jesus Nava agrees, adding that indexable drills shouldn’t be removed from the hole-quality equation. “This is especially true on CNC lathes,” he says. “I’ve worked with a number of shops that rough the hole with an indexable drill, then offset the tool a small amount in the X-axis and use it as a boring bar. This makes perfectly straight, dimensionally accurate holes and, depending on the feedrate, can provide excellent surface finishes as well.”

Keep cool and concentrate
Dan Habben, applications engineer at another Illinois-based cutting tool supplier, Sumitomo Electric Carbide Inc., says boring does indeed cut a clean hole, but at a cost. “You can easily change the tool nose radius, edge properties, and feedrate, so boring offers a great deal of control, but it’s still a single point process and therefore much slower than other methods of hole finishing.”

Bored or no, Habben says drilling should be performed using clean cutting fluid that’s been mixed to the proper concentration—with holes much above 12 mm (0.5 in.), lubrication should be increased to around 10 per cent or more, particularly in exotic materials. This improves surface finish and extends tool life, which in turn keeps the drill cutting round, straight holes longer and more predictably.

The toolholder is also important. “A lot of people don’t realize their holders have limited life spans,” Habben says. “It doesn’t really matter what style, type, or brand; with average use you can figure around four years before run-out begins to affect performance.”

Iscar’s HCP-IQ SumoCham self-centering drilling heads achieve hole roundness and cylindricity to 0.02 mm (0.0008 in.) or better in a number of materials.Even with the best toolholder and a shiny new carbide drill, reaming is often the best way to meet hole tolerance and finish requirements. Habben recommends an indexable reamer for maximum productivity and low tooling cost per part. “These are much more effective than blade-style reamers,” he says. “Sumitomo’s SR-series reamer, for example, can achieve feedrates up to 7,500 mm/min (295 ipm) and still generate mirror finishes on many materials.”

Sumo wrestling
Reamers need extremely precise alignment to cut properly, far more so than drills or boring bars. David Vetrecin, holemaking product manager for Iscar Tools Canada, says hydraulic toolholders—and in some cases shrinkfit—are a fine choice unless the spindle has been bumped.

“Easily 80 per cent of hole integrity problems in any machine tool are due to misalignment,” he says. “You might have the best tool and toolholder in the world, but put it in a machining centre with even a tenth or so (0.002 mm) runout at the spindle and you’re looking at a couple thousandths (0.05 mm) error out at the reamer end. That’s why we offer adjustable holders. They have radial and angular adjustment screws so you can dial in runout to 0.001 mm (0.0004 in.) or less.”

For CNC lathes, Vetrecin says the turret and spindle should also have excellent alignment—if not, a floating reamer holder might be needed to attain hole integrity, but warns these do not handle the high feedrates of indexable or replaceable head reamers, so should only be used with solid carbide or HSS tools.

If possible, Vetrecin suggests skipping secondary reaming and boring operations altogether, and just drill to size. He says the company’s SumoCham IQ drill is equipped with a wiper-style insert that burnishes the hole as it cuts, and has a self-centering tip that prevents drill walk. “It depends on the material, but we can usually achieve a 32 µin Ra and good straightness, at a much lower cost than solid carbide.” SMT

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