Anyone who has tinkered around with machines knows: Guessing at drill sizes when tapping a screw hole almost always result in broken taps or damaged parts. And even if you’re careful with your technique, your threads will suffer unless you have the proper diameter.
This chart contains actual diameters required to tap each standard bolt, ranging from #4 screws up to one inch bolts. Don’t worry; you won’t have to remember all of those fractions on the charts. Just learn how they make sense (so you’ll know which tool to grab).
Drill Bit Sizes for Tapping Screw Holes
The size of the final hole isn’t what makes room for the tap. It’s drilling a hole that give the metal room to move. Drill too big and your threads won’t hold well, they’ll be too shallow. Too small and you’re fighting so hard that the tap snaps.
The infographic provides a clear reference for which drill bit pairs with which tap. Bolt sizes is on the left. Find your bolt size then find its pair on the chart. A one-quarter inch by twenty thread bolt need a number seven drill bit (shown here). That will leave enough material for tapping strong threads without binding or stopping the tap. This chart help you get close to a balanced result.
Clearance holes come in both free and close fits. The close fits has just a few thousandths more than minimum required size, typically about 16 thousandths. These allow parts to be fitted snugly together so they do not move like they would with a free fit hole. A close fit is used in any area subject to shearing force, or where structure joint alignment are critical. For most general applications free fit will do just fine. However, if your application require rigidity and squareness, close fit is the way to go.
How to do it depends off material. Aluminum is soft, so it’s easy to tap into. Stainless steel is harder. You can break a tap if you don’t have the right drill size for stainless. Because stainless work hardens fast, it becomes increasingly tough with each cut. So always use lots of cutting fluid with stainless steel. And back the tap off half a turn after each complete rotation. That breaks the chips which allows them to clear out and keeps the tap from binding. Tedious, yes; saves your tap, yes.
Most folks don’t know this: speed is key. Going too fast heats up the tap, ruins its temper and cause an early demise. According to the graphic, aluminum requires higher speeds while harder metals is better off with slower speeds. It’s not an exact science, but it will keep you within a safer operating zone.
Another good habit to get into is deburring the tapped hole prior to insertion. This gives the tap something to follow and prevents the bit from wandering around or tearing away at the initial thread. Little things like that add up over time and result in less ruined part with cleaner threads.
The right bolt drill size chart is reassuring. You can focus on the technique rather than worrying about the math in a procedure that requires precision. Knowing exactly what size drill bit to use lets you focus on the details instead of the math.
Have it handy, go slow with the feed rate, and let ‘er rip. When you glance over a properly threaded hole, you’ll be glad you should of did your homework.
