Extend Tap Life with Proper Drill Selection

Attaching two metal plates or components with screws, bolts, or other threaded fasteners requires a two-step process. First, corresponding holes must be drilled into the workpieces. Then the holes need to be threaded. Craftspeople have to carefully coordinate these separate processes to ensure the hole matches the internally tapped threads, and that both are appropriate for the chosen fastener size.

Percentage of Thread

Most tap drill selection charts and calculators suggest drills that will deliver 75 percent of thread, that is bits that will

tap and drill sets

cut internal threads deep enough to “capture” 75 percent of the fastener’s external threads.
Often, that is overkill, and forces taps to work harder than necessary. Holding power increases only marginally as thread engagement rises above 60 percent, while tap torque required to reach those higher percentages increases exponentially.
The smaller the thread percentage needed, the larger the hole can be, and the larger the hole, the easier it is to tap. That equates to longer tap life, faster production, and greater profits.
The proper percentage of thread for a given application can be attributed to several factors:

  • Depth of the hole – Deeper holes have more threads, so percentage can be reduced.
  • Threads per inch – Fine threads have more surface area engaged than do coarse threads, so percentage can be lower.
  • Workpiece material – Stronger, tougher metals need less thread engagement in order to achieve sufficient hold.
  • Criticality – Safety or critical-to-process components may demand higher thread percentages as fail-safe measures.

Calculations and Regal Tap Drill Charts


Drilled Hole Size (inches) = Basic Major Dia. of Thread (inches) – .013 x % of Full Thread
No. of Threads per Inch

Note: Threads per inch must be in fraction form (i.e. 16 threads per inch = .0625 pitch)


Drilled Hole Size (mm) = Basic Major Dia. of Thread (mm) – % of Full Thread x mm Pitch

Note: Enter % of full thread in percent form (75% = 75, not 0.75)

Basic major diameter is the screw’s widest diameter (from thread point to thread point); the screw’s pitch is measured in millimeters.
Regal Cutting Tools has developed charts for selecting both imperial and metric tap drills for each drilled-hole size.

Optimal Tap Selection

Choosing a tap that cuts between 55 and 75 percent of thread delivers several advantages:
1.     Clearance between the tap hole and the tap’s diameter reduces tap wear and the effort required.

2.     Protection for the screw’s threads. A tap drill that is too large (or a tap that’s too small for the hole created) results in shallow internal threads and the potential for stripping.

3.     Better internal threads. A hole that is too small (or a tap that is too big for hole) forces the tap to attempt to remove too much material per pass, increasing the potential for binding, breaking, and tearing the threads.
Tapping softer metals such as aluminum and copper, is relatively easy. A higher percentage of thread allows for greater thread engagement with minimal effort. This is important, because the softness of the material dictates deeper thread cuts to ensure a proper hold – especially when few threads must bear the burden, as when tapping thin sheets.
Regal carries an extensive line of taps and drills in every machine screw, fractional and metric size imaginable. Custom orders are also available. For more information, contact our experts today.