#FORM TAP DRILL SIZES CODE#

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.

#FORM TAP DRILL SIZES CODE#

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%. 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

Form tap drill sizes