Thread Geometry And Drilling The HoleĪs is evident from the above, to create the thread we first need to drill a hole on the workpiece. If the taper needs to go first, the intermediate tap follows second and the thread is finished with the bottoming tap. This minimises the risk of damaging the drilled hole. The taper tap features a smooth diameter transition that allows for a less aggressive cutting action. The taper tap is only used in certain situations such as when the material is very hard or the hole is too small. However, the bottoming tap cannot start the threading on an uncut hole, so it can only take part in the cutting process as the second or third tap. So next comes the bottoming tap that has the capacity to do this. Unfortunately, due to the shape it can’t create a thread all the way to the end of the hole. That is why it features a tapered bit, allowing the machinist to align it correctly. The intermediate, or plug tap, is used first to start cutting out material from a non-threaded hole. To create a thread properly, one would need a set of three taps that correspond to the same bit size namely the bottoming, the intermediate, and the taper tap. Therefore the tap drill chart is extremely important. The result is 4.0 mm - 0.8444 mm = 3.1666 mm, which is the necessary diameter for the drilled hole.As you would expect, taps come in all forms and sizes in order to accommodate standardised screw threads.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 unified 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 unified 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 unified 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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