Finding The Right Drill Size For Tapping

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by Marlon Blandon

Are you really getting the best results from your taps? Avoid common mistakes.

One of the most common mistakes a machinist makes when tapping a hole is using the wrong size drill. Not intentionally of course, it is just that most machinists are using outdated charts designed back in the 1950’s when high speed drills were the norm.

To reduce the risk of thread failure, the design engineer was often cautious and specified high percentages of thread height in tapped holes. The percentage of thread values that older tap drill charts provide is higher than needed in most cases. Another reason why some tap drill charts are outdated is that most drills for producing tapped holes were high speed steel or cobalt when the charts were created. 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. High speed drills typically cut larger actual hole sizes than carbide drills.

Making the correct tap drill size choice will affect the machining operation. Many tool manufacturers suggest using a percentage of thread values between 60 and 70 per cent for most pre-drilling applications. 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. It is important to realize thread strength is not directly proportional to per cent of thread. 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. Tap life is greatly reduced in an effort to theoretically increase thread strength.

Tap drill card.As 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. The slightly larger drill diameter still provides a 73 per cent of thread value, which is more than acceptable. But that two per cent reduction in thread percentage will reduce torque on the cutting tool and increase tap tool life. As a general rule, the tougher the material, the less the percentage of thread required to meet design requirements. In some harder materials such as nickel alloys, stainless steel, and hardened steels, it is possible to tap with as little as a 50 per cent of thread value.

Roll form threads require tap drill sizes that are larger than those specified for cut taps. A 7/16-14 UNC roll form thread will require a 10.25 mm (0.4035 in.) drill size. Material is being displaced and formed instead of cut, requiring the pre-drilled hole to maintain the correct amount of material to be formed into the taps thread profile. Roll form tapping displaces metal from the hole walls to form the threads so cutting tap drill will leave too much material in the hole

Choosing the correct tap drill size for an internal threading application is not as simple as looking at an outdated tap drill size chart. Understanding how the values can affect the manufacturing process is an important consideration. SMT

Marlon Blandon is thread mills product manager with Emuge Corp.


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