Tapping Screw Threads





Introduction: Tapping Screw Threads

About: I run Neal's CNC in Hayward, CA, an expert CNC cutting and fabrication service. Check out what we do at http://www.nealscnc.com/. I'm a founding member of Noisebridge, a hackerspace in San Francisco, and Ac...

From time to time it is needful to screw something into something else, which may not yet accept a screw.  If the something else is metal, screw threads can be cut into it using a handy tool called a tap.  (For rods, the screw thread cutting tool is called a die).

For a long time this seemed to me like an esoteric and scary task, but when I had to do it, I discovered that it is not difficult at all.  It requires a bit of time, a bit of fiddling, and a tap & die set, but is well within anyone's ability who has full use of their hands.  I do not even own this tap & die set, I borrowed it from my cousin!  You may have a cousin with a tap & die set too.  Many people do.  

Cutting oil is available at any hardware store.  You can if necessary get away without this but I recommend it.  We found, having tried it, that lubrication did two things: it made starting the tap a lot easier, and it made the threads a little tighter so that the screw was not quite as loose in the hole.  In fact in a couple cases we needed to use a wrench to get the screw all the way tight!  This is not a very big deal either way but I suspect that the tighter screw will last longer.

Step 1: What Size?

Screws are labeled by the diameter of the outside of the threads.  To make a threaded hole that fits a particular screw size, you need to start with a hole that is slightly smaller: just a bit bigger than the size of the inner core of the screw (as if the threads were all removed).  Additionally, threads come in several different gauges, although usually there are only one or two standard options for any given screw diameter.

In our case we had 3/8" screws, and therefore needed to drill holes of 5/16".  This information is helpfully printed on the tap itself!  The thread gauge for our screw is 16 threads per inch.  The tap set had a handy little tool for measuring this, but it is also easily possible to count threads next to a ruler.

Step 2: Preparation

The tap & die set should have two handles, one to hold taps (for inside threading) and one to hold dies (for outside threading).  Select the tap handle and unscrew it until the tap you've chosen will fit in, then tighten it again.

Clamp your metal solidly to something that won't move, or otherwise fix it in place.  Run a light coating of oil over the edges of the tap screws.  You're ready to go.

Step 3: Twist

Insert the end of the tap into your hole.  Keeping the tap as perpendicular to the face of the metal as possible, and exerting some downward pressure, start turning the tap slowly clockwise.  Very soon you will feel the teeth of the tap start to cut into the metal.  For a short while the turning will be wobbly but as the tap progresses downward into the hole, it will stabilize.

I found that a good starting technique was to hold the handle close to my body, and turn with my hips, keeping my hands in the same relation to my body the whole time.  I kept my eye on the end of the tap where it was held in the handle, and concentrated on that not moving sideways.  After the tap has set it was fine to ease off on this and turn by hand.

The tap works by scraping a spiral groove on the inside of the hole.  The metal displaced from the groove in the hole is pulled into the grooves on the tap.  You're doing this by hand, and it gets hard to cut the metal pretty quickly -- after about a quarter turn the tap gets very stiff.  This is because the piece of metal already scraped out is too big now.  To cut it off so it can drop out of the way, reverse the tap direction for a quarter or even a half turn.  You will hear and feel the other side of the tap screw edge knocking off the metal scraping.

Keep doing this until the tap is all the way through, and turns freely.  The bottom 1/4" or so of the tap, called the chamfer, should be entirely sticking out.  Then simply unscrew the tap out of the hole again. Don't forget to clean off the oil and metal shavings!

Step 4: Screw

Wipe off the metal bits and test it out!  You should be able to screw your screw in now, possibly with a bit of difficulty if your tapping was uneven at all.  I advise one practice hole if that's feasible, for your very first time; it's an easy enough process but there is no substitute for actually doing it to make the explanation all fall into place.



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    11 Discussions


    11 months ago

    Nice explanation. Didn't know about backing out the die to clear the cuttings. Now i need to go tap something...

    SpeedTaps now offers taps that let you use a impact gun. Check them out at www.speedtaps.com

    Thanks for sharing! Can you recommend a good tap and die set for a reasonable price?

    Thanks for sharing! Can you recommend a good tap and die set for a reasonable price?

    I found that using a specialized lubricant called "cutting fluid" really helps when cutting threads. And it makes a big difference compared to general-purpose thin lubricating oil, with the cutting fluid the process is almost silent while with the oil the tap makes a lot of noise when turning it in.

    Personally I used Park Tool's cutting fluid which I got from a mountain bike shop along with other tools.

    Good and useful instructable, rachel. I have the same problem as Bill WW, but use a self-blocking pliers.

    Regarding step 1, I use a vernier caliper to measure the smaller diameter, in the valleys between threads. Then I choose a drill bit a litlle greater. Theoretically you can use the exact measure, but then will be more difficult to tap it.

    Instead of a commercial cutting liquid I use liquid soap or animal grease, satisfactorily. The advantages are: it is cheaper, easier to find, and you don't need to care where you keep it.

    1 reply

    I use a tap and drill chart. Although depending on the material I'm tapping and the tap I'm using I have to make adjustments sometimes. I was tapping heavy stainless steel angle with 8-32 and I had to go way up in drill size. At one point I almost thought it couldn't be done! I was breaking so many taps doing that. Good thing I had a couple boxes of those taps. There are over 50 tapped holes in this project:


    The stainless angle I had problems with is in there too.

    Nice Instructable, just one thing ,I myself nave NO cousins.

    Nice instructable, Rachel. You sure have the right tools (photo #1)!

    I need the proper wrench that holds the tap or die. I have been struggling, even today, using Crescent wrench or Vice Grips.

    My problem now is threading really small diameter material, like 1/8".

    I have wondered about the cutting fluid - I have just used oil.


    Nice screw tapping project Rachel. Hope our path's cross in the near future.