As a general rule, the tougher the material, the less the percentage of thread required to meet design requirements. But that two per cent reduction in thread percentage will reduce torque on the cutting tool and increase tap tool life. The slightly larger drill diameter still provides a 73 per cent of thread value, which is more than acceptable. Tap life is greatly reduced in an effort to theoretically increase thread strength.Īs an example, a 7/16-14 UNC cut thread is usually denoted as a letter “U” 9.3 mm (0.3680 in.) diameter drill on most older tap drill charts that equates to a 75 per cent value for percentage of thread, but a 9.4 mm (0.3701 in.) drill might be a better choice. According to some, 100 per cent thread specification is only five per cent stronger than a 75 per cent thread specification, but requires three times the torque to produce. It is important to realize thread strength is not directly proportional to per cent of thread.

By increasing the pre-drilled hole diameter, the machinist can increase the life of the tap by reducing the amount of force required to form the thread. Many tool manufacturers suggest using a percentage of thread values between 60 and 70 per cent for most pre-drilling applications. Making the correct tap drill size choice will affect the machining operation. High speed drills typically cut larger actual hole sizes than carbide drills. Many tap drill holes are now being created with high performance carbide drills and these carbide drills generate more accurate holes than high speed steel drills. Another reason why some tap drill charts are outdated is that most drills for producing tapped holes were high speed steel or cobalt when the charts were created. The percentage of thread values that older tap drill charts provide is higher than needed in most cases. To reduce the risk of thread failure, the design engineer was often cautious and specified high percentages of thread height in tapped holes. Not intentionally of course, it is just that most machinists are using outdated charts designed back in the 1950’s when high speed drills were the norm. One of the most common mistakes a machinist makes when tapping a hole is using the wrong size drill. Are you really getting the best results from your taps? Avoid common mistakes.

Drill size for 5mm tap