When to Choose and Use Long Drill Bits

You know the drill…or do you? You may have read our previous posts on selecting the correct tap drill diameter as well as reviewed our tap drill sizes chart, but diameter isn’t the only dimension you should consider. Twist drills’ design incorporates balanced cutting edges and spiral flutes, offering the most efficient method for drilling wood, steel, and other non-ferrous metals. When it comes to selecting the right twist drill, it pays to understand the advantages and drawbacks of shorter and long drill bits.

Screw Machine-Length

Originally built for screw machines, these shorter bits are typically 4 to 10 times longer than their diameter (smaller sizes are proportionally longer). Also called “stub-length” bits, they can also be used in portable drills where tight clearance is required. Their shorter flute and overall length give them a rigidity often needed when drilling into harder metals and alloys. Rigidity keeps the holes straighter, with reduced runout, skating and deflection. Their main drawback is that they can drill only as deep as their flutes – 2¼ inches for a ½-inch bit.

Jobber-Length

A popular choice based on length is the jobber-length bit. It’s not too long and not too short, but just right for most applications. That’s why it is jobbers are the most commonly used bits in the workshop. They are perfectly fine for drilling into wood and soft metals. With lengths ranging from 9 to 14 times their diameters, jobber drills may be all you need for all but the deepest or most precise holes.

Taper-Length

Taper-length drills, on the other hand, are longer than jobber-lengths of the same diameter. Their correspondingly longer flutes – the same size as taper-shank bits, hence their name – allow them to drill deeper holes. They can even be substituted for taper-shanks on many jobs, making them a low-cost long drill bit option since they work with standard chucks and collets. Because chips from deep holes have farther to travel, specialty high-helix taper-length drills may be used on cast alloys and non-ferrous metals to aid in chip evacuation.

Extra-Long

Measuring up to 18 inches, extra-length or extra-long drill bits are often the only option for drilling through or deep into thick workpieces or reaching hard-to-access components in construction, repair, and manufacturing applications. They are often required when drilling lengthwise into roof beams or house framing or for boring out channels through which wire or cable will be run.

Aircraft extension bits are a type of extra-long drill bits with jobber-length flutes. They are used when reach, rather than hole depth is needed, such as attaching the extruded aluminum panels of a hollow airplane wing. You can achieve similar results by attaching a shorter bit to a long drill bit extension or extender. Because their length makes extra-long drill bits less rigid and steady than shorter bits, they are usually found in smaller diameters, to keep the required rotational power low to reduce runout and imprecision. For best results and to avoid breakage, start the hole with a shorter bit, then use the extra-long drill bit to continue drilling the pilot hole to the desired depth.

In nearly every instance you should use the shortest, most rigid drill bit available that can reach the drilling depth you need. If ultra-precision is not required, your operation may opt for jobber bits rather than screw machine-lengths. But when deep blind holes are called for, there is no substitute for extra-long drill bits.

The professionals at Regal Cutting Tools can advise you in selecting the best combination of productivity and economy in selecting drill lengths that will work best for your application. Contact them today.