![]() ![]() For most applications, you can get longer tap tool life and lower likelihood of breaking a tap off in the hole by using a lower percent of thread. Most tap drill charts call out only one tap drill size, and that will produce an approximate 75 percent thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread. For example, American National and Unified thread specifications provide for a maximum of 83 1/3% thread. But is it okay to have less than 100% threads? In fact, many standards bodies insist on it. Now we all know what happens when we break a tap, right? Sailors would go running out of the shop if they heard the language we use in that case. Getting 95% of the strength with 1/3 the force on the tap means you’re dramatically less likely to break the tap off in the hole. But here is the real kicker: That 100% thread requires 3 times the power to tap! Why do you care? because it is the power to tap that breaks taps, for one thing. Kennametal says a 100% thread is only 5% stronger than 75% thread. In fact, they contribute surprisingly little strength. They’re pretty delicate anyway and will wear off quickly. Now suppose you ran a twist drill down the hole and shaved off some of the peaks. Imagine the fully formed internal thread. To know which drill bit size is right for your particular tapping application, you need to consider what thread percentage you want when you’re done tapping. How can that be? Well, it’s actually pretty simple. Anytime you see just one size being recommended, you know you can do better–one size does not fit all when it comes to tapping. The same is true of the size recommended on the tap and even the recommended size from the tap manufacturer. Moreover, the size recommended in most drill bit size charts is very often not the best size to use. Let’s start with an important fact–there isn’t just one drill bit size to use for a given tap size. Newsflash: There Isn’t Just One Drill Bit Size for a Tap But in this age of computers and the Internet, you can do better. If it has information on which twist drill size to use for tapping, so much the better. Make no mistake: having a good drill bit size chart (sometimes called a “drill index”) is a very handy thing indeed to have around. ![]() What’s better than a drill bit size chart? Let us help, and BTW, it’s completely free too! If you just want a simple tap drill chart in Metric and Imperial sizes, or a pdf you can print and post in your workshop, scroll on down.īut, before you do, wouldn’t you like to hear about something that’s much better in every way? Something that’ll make it much less likely you’ll break a tap? Something a little more modern than a simple chart? The result is 4.0 mm - 0.8444 mm = 3.1666 mm, which is the necessary diameter for the drilled hole.Tap Drill Chart: Metric & Imperial Size.Subtract the result from the thread diameter (4.0 mm).Match this row to the 65% thread percentage column.Look for the 1.00 mm pitch in the chart's first column.For metric threads, the recommended thread percentages are 65% and 70%.įind the drilled hole size for a 65% thread on a 4 mm x 1.00 (M4) tapped hole:.Similar to the standard tap and drill section (see above), use the two formulas below to calculate the necessary size of the drilled hole.Look up the desired pitch and thread percentage to find the corresponding outer diameter.The tap and drill chart below is for Metric threads. 5 millimeters, the double thread depth is 1.0 millimeter. In the second method, double thread depth means twice the depth of the thread.For either formula, when inputting thread percentage, use, for example, 65% instead of 0.65. Use the two formulas below to calculate the necessary drilled hole size in inches.The recommended thread percentage is 65%, 70%, and 75%.Thread percentage is the percentage of a bolt's shank that has threads.Look up the TPI and desired thread percentage to determine the corresponding outer diameter.The chart below is for standard gauge threads (inch). ![]() Based on the desired nominal size, look at the desired standard column to determine the minimum outside diameter of the component for the threads.Use the thread nominal size column to identify the desired thread designation.The following chart describes how to determine the size of standard male threads when making a bolt or threaded pipe. Use the corresponding drill size to drill the hole.Identify the screw designation, which correlates with the tap size on the chart.The chart below is used for tapping female threads into a component. Tamesons selection of thread cutting tools ![]()
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