Twist Drill Length: Which is Right for You - And Why it Matters

Twist drill bits bore holes in steel and other metals using balanced cutting edges and spiral grooves, or “flutes” that expel chips and shavings from the work piece. Like most tools, twist drill bits come in all sizes, specifications, and preferred uses. In most cases, machinists must choose among several important characteristics, including: 
  • Versatility 
  • Precision 
  • Strength 
  • Performance 
The proper combination depends on the material and shape of the workpiece, diameter and depth of the hole, and other factors.
Choosing the correct twist drill bit for a particular application can make the difference between efficient production and excessive material waste and downtime.

Jobber-Length Drills

Jobber-Length Drills

These drill bits are the most common because they can be used for many “run-of-the-mill” jobs encountered on most projects. Jobber bits have fairly long flutes, generally 9 to 14 times their diameter, allowing them to drill holes deep enough, again, for most common needs. While they may be acceptable for many uses, more specialized options may pay dividends for processes which require:

  • Higher Speeds
  • Deeper Holes
  • Longer Life
  • More Precision
  • Greater Durability

Screw Machine-Length Drills

Their shorter flutes give screw machine drills the increased toughness, and precision required for use in screw machines and other automatic drilling equipment. They feature shorter total overall and flute length, as well, allowing them to be used in tight spaces and for portable drilling applications that require rigidity. The shorter length also mitigates deflection, breakage and runout, often making them the correct option for drilling into harder metals and alloys. The trade-off is that screw machine bits can drill holes only as deep as their flutes.

Taper-Length Drills

Conversely, Taper length bits have longer flutes and generally can drill deeper holes than jobber bits. In fact, their flute lengths are the same as comparable-sized taper shank bits, and they can be used with standard chucks and collets, making them low-cost substitutes in some applications. Taper length bits also use standard helices for drilling steel and other hard metals. Available high-helix bits are more effective on nonferrous metals, low tensile steels and cast alloys because they aid in expelling chips from the deep holes. For the best results, drill partial holes and retract the drill from the hole to clear chaff out of the flutes.

In most cases, the shorter the drill, the better the outcome. All else being equal, screw machine drill bits will deliver more precision and better value because they will remain more rigid and straighter than their longer counterparts. On the other hand, shops that perform various standard drilling processes may prefer the versatility and cost savings that come from using jobber drills. Jobs which require the drilling of deep holes may require taper length drills.

Contact us for more information on which twist drill length, material, flute configuration, and shank style is best for your shop’s needs. Our staff is ready to demonstrate which of Regal’s extensive line of twist drill bits will maximize your operation’s performance and efficiency.