How to Drill & Tap Threads Into Metal

Written by christian mullen
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Whether you are tapping aluminium or steel, you must be careful and use the correct drill to the right depth to properly tap a hole in metal. By using ample tapping fluid and assuring that the tap is going in straight, you can tap a hole perfectly without damaging the part or the tap. Removing a broken tap can mean hours of extra work.

Skill level:
Moderately Challenging

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Things you need

  • Drill
  • Tap handle
  • Tapping fluid

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  1. 1

    Determine the appropriate drill bit for the tap size you are using. If you make the hole too narrow, the tap will break under pressure and if you make it too big, the thread will be too loose for any screws that you use after you are done tapping. The Machinists' Handbook has all the tap and hole sizes available, so consult with the book for the correct diameter for the hole.

  2. 2

    Drill the hole to be tapped using a manual mill, drill press or CNC machine. It is important to look at the print and note the tapped hole's depth. Tapping holes requires that the hole be deeper than the tap depth, so go about an extra 1/4" so that the tap does not bottom out and break.

  3. 3

    Clean the hole thoroughly by blowing out the chips from the drilling process. Any chips in the hole may cause the tap to snag while you are creating the threads. By clearing the hole, you can help ensure the threads are cut properly during the tapping process.

  4. 4

    Start the tapping process using a gun-style tap if it is a through hole or a bottoming tap if you will not be going all the way through. Lubricate the tap itself with tapping fluid before starting, which will help the tap go in smoother. Make sure you start straight -- if it starts crooked, the screw will also go in crooked after you are done. Straight tapping can also prevent tap breakage. Taps are very difficult to remove if they break in the metal.

  5. 5

    Lower the spindle to keep pressure on the tap if you are using a manual mill like a Bridgeport. This will help you guide the tap and allow you to use the machine for pressure so you can slowly turn the tap handle without having to worry about applying pressure as well.

  6. 6

    Slowly remove the tap from the hole when you have reached the proper depth. Taps often have no threads on the end, so you want to go as deep as possible within the tolerance of the print. Blow the chips out before inserting a screw to check the threads, or use a thread gauge to see if the threads are correct. You can also measure a screw and insert it into the hole to see if you have reached the proper depth.

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