Thread forming tap size11/29/2023 ![]() ![]() Red: Greater than 3%, Small or Tight Fit … Might gage incorrectly, will be difficult to tapĪs seen in the Far Right column, “Closest MM Drill,” by utilizing metric drill diameters you will be able to go between basic inch drill sizes to achieve the 75% target for inch cut taps.Pink: Greater than 3%, Large or Loose Fit … Might gage incorrectly, should tap easily.Yellow: +/- 2% to 3% of the Target … Could cause issues, especially on the tight side.Green: +/- 1% of the Target … Should be fine.The color code of Green/Yellow/Pink/Red means the following: The column titled “Drill Chart Percentage” is the actual percentage when the suggested diameters are calculated for each thread size.Įach of those percentages has an associated color code. The drill chart I used suggests the drills in the column titled “Drill Chart Drill Choice”. But WAIT !!! Did you calculate the TRUE HOLE SIZE by the suggested effective diameter percentage? Did you assume the recommended drill size will provide the correct percentage? This might be why you have reoccurring issues in certain thread sizes you tap.īelow is a chart I created that shows a range of some popular fine and coarse “inch cut tap” specifications. So you pull out your favorite drill company’s decimal equivalent/metric conversion/ tap drill chart and by their guidance, you use the tap drill they recommend for the thread you are making. The effective thread percentage of a roll form tap is 65%. ![]() A roll form tap requires the hole to be drilled to the pitch diameter, basically in the middle of the minor diameter and the major diameter of the thread specifications.The effective thread percentage of a cut tap is 75%. ![]()
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