Some holes are in places drill presses can't reach. A bolt pattern on the inside of a weldment, holes in a deep pocket, fasteners inside a cabinet frame, or locations behind an obstruction — these require either a different machine, a different fixturing approach, or a drill extension. This article covers when and how to use drill extensions correctly.
The most common type for machine work. A solid steel bar with a chuck or collet on one end and a shank on the other — Weldon flat shank, straight shank, or Morse taper depending on your spindle. Available in 6″, 10″, 12″, and longer. They add reach while maintaining concentricity — as long as the extension itself is running true.
Solid extensions add significant runout potential. Every additional coupling is another place for accumulated error. Use the shortest extension that reaches the hole; never stack extensions when one longer unit would work.
Systems like the Jacobs quick-change or similar hex-shank systems allow switching drill sizes without returning to the machine chuck. These are popular for portable drill use and repetitive setups. They add more runout than a collet-based system but are fast and practical for non-precision work.
Flexible shaft attachments and right-angle drill attachments address a different problem: not depth but angle. A right-angle drill head lets you drill in a direction perpendicular to the spindle — essential inside tight cabinets or enclosures. Flexible shafts allow the drill to follow a curved path, though these are generally for light-duty work only.
Right-angle attachments introduce significant runout and should not be used for tolerance-critical holes. They're for clearance holes, pilot holes, and installations where a tight fit isn't required.
Every interface between shank and chuck introduces potential runout. A drill in a quality collet might run at 0.001″ TIR. Add a 12″ solid extension: now the drill tip — 12 inches from the last clamping point — amplifies that runout. Even 0.002″ of runout at the coupling becomes 0.005–0.010″ at the drill tip when magnified over extension length.
Extension length and drill diameter have an inverse relationship to hole quality. Longer extension + smaller drill = more deflection. The deflection formula is simple in concept: a longer, thinner column deflects more under the same cutting load.
For extended-reach precision work:
Spot drilling is especially important when using extensions. A clean center punch or spot drill dimple prevents the drill from walking at entry — at full depth, walking is very hard to correct and the extension has no rigidity reserve to fight it. If the application allows any access for a standard spot drill before the extended drill goes in, use it.
The workpiece must be held rigidly. A flexible part or workholding setup compounds extension flexibility — now you have two flexible members in the system. For weldments and assemblies, C-clamp the work to the table or fixture plate. Don't rely on weight alone to hold large assemblies while drilling with extended reach.
If possible, use a drill jig or guide bushing at the hole location. A hardened bushing pressed or clamped into a plate at the drill location constrains the drill's radial movement near the cut. This is the professional solution for production extended-reach drilling.
Coolant delivery gets complicated with extensions. For through-spindle coolant machines, extended holders with coolant-through capability exist — more expensive but maintain flood cooling at the drill tip. For manual setups, a coolant stream directed at the drill entry and frequent retract-and-flush cycles are the practical solution. Dry extended drilling in steel leads to heat, built-up edge, and broken drills.
Sometimes an extension is solving the wrong problem:
Extensions are a practical solution for access problems in lower-precision work. For tight-tolerance holes, address the access problem at the fixturing or machine selection stage rather than trying to make extended drilling hit specs it can't reliably achieve.
A dull drill deflects more under the reduced rigidity of an extension. MachinistPost sharpens HSS drills back to spec — mail them in.
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