Machinery's Handbook, 31st Edition
2176 Diameter of Tap Drill Table 10. Tap Drill or Core Hole Sizes for Cold Form Tapping ISO Metric Threads Nominal Size of Tap Pitch Recommended Tap Drill Size 1.6 mm 0.35 mm 1.45 mm 1.8 mm 0.35 mm 1.65 mm 2.0 mm 0.40 mm 1.8 mm 2.2 mm. 0.45 mm 2.0 mm 2.5 mm 0.45 mm 2.3 mm 3.0 mm 0.50 mm 2.8 mm a 3.5 mm 0.60 mm 3.2 mm 4.0 mm 0.70 mm 3.7 mm 4.5 mm 0.75 mm 4.2 mm a 5.0 mm 0.80 mm 4.6 mm 6.0 mm 1.00 mm 5.6 mm a 7.0 mm 1.00 mm 6.5 mm 8.0 mm 1.25 mm 7.4 mm 10.0 mm 1.50 mm 9.3 mm a These diameters are the nearest stocked drill sizes and not the theoretical hole size, and may not produce 60 to 75 percent full thread. The sizes are calculated to provide 60 to 75 percent of full thread. Removing a Broken Tap.— Broken taps can be removed by electrodischarge machining (EDM), and this method is recommended when available. When an EDM machine is not available, broken taps may be removed by using a tap extractor, which has fingers that enter the flutes of the tap; the tap is backed out of the hole by turning the extractor with a wrench. Sometimes the injection of a small amount of a proprietary solvent into the hole will be helpful. A solvent can be made by diluting about one part nitric acid with five parts water. The action of the proprietary solvent or the diluted nitric acid on the steel loosens the tap so that it can be removed with pliers or with a tap extractor. The hole should be washed out afterwards so that the acid will not continue to work on the part. Another method is to add, by electric arc welding, additional metal to the shank of the broken tap, above the level of the hole. Care must be taken to prevent depositing metal on the threads in the tapped hole. After the shank has been built up, the head of a bolt or a nut is welded to it and then the tap may be backed out. Tap Drills for Pipe Taps Size of Tap Drills for Briggs Pipe Taps Drills for Whitworth Pipe Taps Size of Tap Drills for Briggs Pipe Taps Drills for Whitworth Pipe Taps Size of Tap Drills for Briggs Pipe Taps Drills for Whitworth Pipe Taps 1 ∕ 8 11 ∕ 32 5 ∕ 16 1 1 ∕ 4 1 1 ∕ 2 1 15 ∕ 32 3 1 ∕ 4 … 3 1 ∕ 2 1 ∕ 4 7 ∕ 16 27 ∕ 64 1 1 ∕ 2 1 23 ∕ 32 1 25 ∕ 32 3 1 ∕ 2 3 3 ∕ 4 3 3 ∕ 4 3 ∕ 8 19 ∕ 32 9 ∕ 16 1 3 ∕ 4 … 1 15 ∕ 16 3 3 ∕ 4 … 4 1 ∕ 2 23 ∕ 32 11 ∕ 16 2 2 3 ∕ 16 2 5 ∕ 32 4 4 1 ∕ 4 4 1 ∕ 4 5 ∕ 8 … 25 ∕ 32 2 1 ∕ 4 … 2 13 ∕ 32 4 1 ∕ 2 4 3 ∕ 4 4 3 ∕ 4 3 ∕ 4 15 ∕ 16 29 ∕ 32 2 1 ∕ 2 2 5 ∕ 8 2 25 ∕ 32 5 5 5 ∕ 16 5 1 ∕ 4 7 ∕ 8 … 1 1 ∕ 16 2 3 ∕ 4 … 3 1 ∕ 32 5 1 ∕ 2 … 5 3 ∕ 4 1 1 5 ∕ 32 1 1 ∕ 8 3 3 1 ∕ 4 3 9 ∕ 32 6 6 3 ∕ 8 6 1 ∕ 4 All dimensions are in inches. To secure the best results, the hole should be reamed before tapping with a reamer having a taper of 3 ∕ 4 inch per foot. Power for Pipe Taps.— The power required for driving pipe taps is given in the following table, which includes nominal pipe tap sizes from 2 to 8 inches. The holes to be tapped were reamed with standard pipe tap reamers before tapping. The horsepower recorded was read off just before the tap was reversed. The table gives the net horsepower, deductions being made for the power required to run the machine without a load. The material tapped was cast iron, except in two instances, where cast steel was tapped. It will be seen that nearly double the power is required for tapping cast steel. The
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