The right size of screw should be chosen depending on the size of the hole. It can be said that those who can successfully drill screw holes have a certain level of experience and skills in metalworking.īefore drilling a screw hole, the original hole must first be drilled with a drill. Without repeated practice, the screw hole may be slightly bent or the groove part of the screw hole may be crushed. The process of cutting a screw hole is quite difficult. Tap wrenches are used together with taps when you want to use screws, such as when repairing a screw that does not fit well in the original screw hole or when you want to connect metal parts with a screw. Of course, when cutting large screw holes, it becomes difficult to do so by hand. Threaded holes can be drilled manually in materials that are not so hard, such as steel. Tap wrenches are used for turning taps when cutting threaded holes. There are several types of taps, ranging from tapered types that are tapered to dimensional types that are almost the same in diameter throughout. A long, thin drill-like tool called a tap is attached to the square hole in the center and fixed firmly in place. The tap wrench is in the form of a long, thin rod, which is turned by placing one's hands on the left and right handles. Special tools are required when making screw holes, and one of these special tools is a tap wrench. Marlon Blandon is thread mills product manager with Emuge Corp.A tap wrench, also called a tap handle, is a tool used for manually cutting screw holes. Understanding how the values can affect the manufacturing process is an important consideration. Roll form tapping displaces metal from the hole walls to form the threads so cutting tap drill will leave too much material in the holeĬhoosing the correct tap drill size for an internal threading application is not as simple as looking at an outdated tap drill size chart. 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. A 7/16-14 UNC roll form thread will require a 10.25 mm (0.4035 in.) drill size.

Roll form threads require tap drill sizes that are larger than those specified for cut taps. 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. 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 1213 form tap