How To Drill And Tap A Hole
Let's work together to drill and tap a hole in a piece of steel. This will create threads in the metal that accept a bolt--and really opens up your options as far as projects go! These instructions are for steel but you can do this in aluminum as well.
First step: Lay out your hole on the metal and use a center punch to make a dimple in the center of where the hole will go. Decide on what size bolt and thread pitch you want to use. We chose a 5/16"-18 bolt for this project which means it's 5/16" OD (outside diameter) and 18 threads per inch.
Next, select your drill bit. Now, drilling a hole that will be tapped requires a specifically sized drill bit (called a "tap drill") and you can find out what size you need by checking google for a "tap drill chart" and then selecting your thread size from the chart. I used this one. In our case, since we are tapping 5/16-18 we needed an "F" drill. Yes, drill bits come in more than fractional and MM sizes! There are number drills, letter drills, and fractional drills.
Now it's time to drill the hole. Look at your drill bit--if it's a "split point" bit that means you can drill right in to the metal without a pilot hole. If you have a traditional drill bit and your hole is larger than roughly 3/16" you should drill a pilot hole which is a smaller hole about the thickness of the web of your drill bit. I frequently use 1/8 drill bits for pilot holes. Drill the pilot hole if necessary and then the main hole. It's important to keep the drill as square as possible to the metal you're drilling in to because this will determine how straight the bolt threads into the hole later.
Here is a link to the site that provided the picture above with more info on drill bits
OK, the hole is there. Next, put a countersink in your drill and countersink the hole until the mark from it is roughly 20-30% larger than the hole you just drilled.
Now you're ready to tap the hole. Take your tap (sized by the thread size and pitch you want to tap) and put it in your tap handle. Add a bit of tapping fluid to the tap and turn it slowly in to the hole. Again, make sure it is as square as possible to the metal to prevent a crooked screw later. After about every second or third full revolution I like to turn the tap backwards by about a quarter turn to break the chip off. Once you feel the resistance decrease you're through the hole and can back out the tap.
Blow out the hole with your compressed air (if you have it; watch your eyes!) and then test the bolt in the hole. Congratulations--you're done!
First step: Lay out your hole on the metal and use a center punch to make a dimple in the center of where the hole will go. Decide on what size bolt and thread pitch you want to use. We chose a 5/16"-18 bolt for this project which means it's 5/16" OD (outside diameter) and 18 threads per inch.
Next, select your drill bit. Now, drilling a hole that will be tapped requires a specifically sized drill bit (called a "tap drill") and you can find out what size you need by checking google for a "tap drill chart" and then selecting your thread size from the chart. I used this one. In our case, since we are tapping 5/16-18 we needed an "F" drill. Yes, drill bits come in more than fractional and MM sizes! There are number drills, letter drills, and fractional drills.
Now it's time to drill the hole. Look at your drill bit--if it's a "split point" bit that means you can drill right in to the metal without a pilot hole. If you have a traditional drill bit and your hole is larger than roughly 3/16" you should drill a pilot hole which is a smaller hole about the thickness of the web of your drill bit. I frequently use 1/8 drill bits for pilot holes. Drill the pilot hole if necessary and then the main hole. It's important to keep the drill as square as possible to the metal you're drilling in to because this will determine how straight the bolt threads into the hole later.
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| Standard on the left, split point on the right |
OK, the hole is there. Next, put a countersink in your drill and countersink the hole until the mark from it is roughly 20-30% larger than the hole you just drilled.
Now you're ready to tap the hole. Take your tap (sized by the thread size and pitch you want to tap) and put it in your tap handle. Add a bit of tapping fluid to the tap and turn it slowly in to the hole. Again, make sure it is as square as possible to the metal to prevent a crooked screw later. After about every second or third full revolution I like to turn the tap backwards by about a quarter turn to break the chip off. Once you feel the resistance decrease you're through the hole and can back out the tap.
Blow out the hole with your compressed air (if you have it; watch your eyes!) and then test the bolt in the hole. Congratulations--you're done!
