metalworker in coveralls and safety gloves holding large drill bit

Finding the Right Drill Bit Size for a 3/8" Tap

Introduction to Drill Bit Sizing for Taps

How should you choose the right drill bit size for varying-sized taps? The answer is pretty straightforward yet includes a host of factors and considerations. Most drill bits nowadays are between 1 mm and 20 mm in diameter and will align well with the same diameter for drilling holes.

Another way to determine the right drill bit size is by aligning it with the corresponding-sized screws. The screw’s diameter should be slightly larger than that of the hole.

It’s also important to understand just what a tap is. Tapping creates threading in the hole's circumference to be drilled. Essentially, taps create the grooves through which the screws of the drill penetrate the material.

metal drill bit on white background

What Size Drill Bit for a 3/8 Tap?

We’ve already gone over what the correct size drill bit is for this particular tap, as well as the factors you need to consider when selecting this particular drill bit size: align the screws with the taps so that the screws are just a bit wider in diameter, and align the bits themselves with the size of the hole.

Now, it’s essential to know how to measure the tap. The equation is as follows: tap-drill diameter equals nominal diameter minus thread pitch.

Tips for Successful Tapping

The following are tips for successful drilling and tapping:

  1. Clean the drill chuck to minimize metal chips and ensure a proper match. The drill chuck is simply the apparatus that holds the drill.
  2. Use oil. This is necessary to prevent overheating and maintain the cleanliness of the tapping process.
  3. Center the drill. This ensures that larger drill bits will drill with precision.
  4. Peck. This is the “one step forward, one step back” mechanism. Drill partway, then back out a bit, then continue forward again.
  5. Don’t apply too much force; use rotational force (torque) to prevent tap breakage.

drill bit

Common Mistakes to Avoid

Just as there are proper ways to drill, there are also incorrect ways that can get you in trouble from the perspective of effectively getting the job done and managing safety risks.

The first of these is choosing the wrong size drill bit. If you do this and the drill bit is too big or too small, you will not effectively get the job done.

Another incorrect move would be to use a dull or damaged drill bit. This could lead to unclean cuts and overheating. The force/speed/efficiency equation would also be offset, which could pose a safety liability.

Additionally, if you over-tighten the tap, the drill bit could break on you, and the project would go down in history as ruined! Well, maybe not that monumental, but it would ruin the job.

Lastly, if you don’t use the right lubricant for the drilling job, the hole won’t be consistent, and you could get stuck mid-drilling.

tap and drill bit


Can I use a cobalt drill bit for tapping?

Absolutely! Cobalt drill bits are highly effective for drilling jobs with thicker metals.

How do I know if I am tapping straight?

You’ll need to center your drill to do so. If you aren’t tapping straight, it means your drill isn’t centered.

Is it necessary to use a lubricant when tapping?

The answer is yes – you’ll need to use an oil and the right one at that.

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