Drill Wander: A Common Problem With Longer Deep Hole Drills

So-called drill wander is a common and frustrating problem in many deep hole drilling operations.

So long as the drilling machine is set up correctly, the drill should – in theory – both enter and exit from the centre point. In practice, however, this rarely happens on deep holes; defined as any hole with a depth to diameter ratio greater than 10 to 1.

The issue occurs due to the drill’s tendency to wander off centre when carrying out deep hole drilling, especially when gundrilling long, small diameter holes. The hole itself is perfectly straight (gundrilling rarely creates a curved bore) but the exit point deviates from the starting point.

The deviation of the exit hole from the centre – or level of drill wander – is approximately 1 mm per metre, or 0.001” per inch drilled.

What Causes Drill Wander?

Several factors can cause drill wander or worsen its effects.

• The material: Stainless steel is vulnerable to drill wander because of the way the metal gradually hardens during cold machining, driving the drill tip off course and sometimes creating an oversized bore.
• Low-grade metals or poor-quality, rough bars often contain irregularities that worsen the effects of drill wander.
• Longer drills have a higher propensity to wander and are at greater risk of breakage during the drilling process, especially if the drill and bore protrude from the side of the piece.
• Swarf accumulating in the hole can clog the drill and cause it to wander.
• Faulty equipment and warped or bent drills are likely to wander and may also break.
• An inaccurate starting position will make drill wander inevitable.
• Using the wrong feed pressure and speed, or inconsistent drilling, can make drill wander more likely.

How To Prevent Drill Wander

While the risk of drill wander can never be eliminated entirely, the risk and severity may be reduced through a combination of quality equipment, thorough surface preparation, and good drilling practice.

• Use a shorter, stubbier or more rigid gundrill tool when possible.
• Use counter rotation, in which the part and the drill are rotated in opposite directions. This is the most accurate and effective way of minimising drill wander, as the opposing forces counteract the drill’s tendency to wander and force the bore to remain straight.
• Always drill on a clean, perpendicular surface, using faced and chamfered billets.
• Don’t allow swarf to accumulate in the bore.
• Maintain the correct feed rate and drill speed when gundrilling.