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%. A cut tap will require the hole to be drilled to the minor diameter of the thread specifications.How much material should be left for a tap? The amount to leave in a hole that will be tapped is based on the type of tap (cut tap or roll form tap) which have their own effective thread percentage guidelines. Obviously, the straighter the hole is the more consistent the reamer will cut as it goes into the hole, producing the diameter you expect it to. The amount to leave is based on the material being reamed in a diameter range such as 1/8” to 1/4”, 1/4” to 3/8” and so on. Manufacturers of reamers will suggest how much material to leave for the reamer to cut properly. There, I mentioned subsequent tools (taps and reamers) that enter the hole created by the drill and the typical issues these tools will run into if the hole isn’t straight. This article is a fitting follow-up to the previous one, the topic of which was utilizing the proper spot drill in order to cut a straight hole. By Jim Rowe with Arch Global Precision, a Neill-LaVielle valued supplier
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