Tap Drill Size For Standard Threads: How to Choose the Ideal One


Tap drills come in different sizes to fit various standard threads available. However, you can always get the right size for your thread by using simple formulas. You can calculate the tapping drill size for metric threads by taking the thread’s diameter minus the pitch. The tapping drill size for the imperial thread is calculated the same way, i.e., subtracting pitch from the thread’s diameter.  

A tap drill is an object used to drill holes of exact diameter for tapping. It is used for drilling and tapping gem boxes, electrical panels, and many others. A tap drill is used to cut out a nut, i.e., the female portion of a pair, while a dye is used to create the bolt, i.e., the pair’s kale portion. Tap drills have different sizes, and here, you will get to know how to pick the ideal one for your thread. Read on!

Tap Drill Size For Standard Threads

What Are Tapping Tools?

A tap is a tool that looks like a screw and is used to thread out nuts. A die is used for threading bolts.

What Is Tapping Drill Size?

Calculating the sizes is easy but may vary. There is a distinct formula to determine the size. For a metric thread, the rule of thumb states that the tapping drill can be calculated by subtracting the pitch from the diameter of the thread. e.g., the threading drill for an M5 × 0.82mm is calculated by  5-0.82mm = 4.18mm. For Imperial threads the tapping drill size is calculated in the same way, diameter minus pitch for 3/4.10 UNC pitch = 0.1, diameter = 0.75, tapping drill = 0.65 = 16.5mm.

The formula: Drill Size = Major Diameter – (. 975 divided by the number of threads per inch) can be used to get the accurate drill size for 75% thread on a special pitch tap. 

How To Create A Thread?

To create a thread, you require a set of three taps that correspond to bits of the same size; these are the bottoming, intermediate, and taper tap. The plug or intermediate tap is used first to start cutting out material from a non-threaded hole. The fact that it has a tapered bit allows the machinist to position it right. However, the shape cannot create a thread to the end of the hole.

The bottoming tap can do this and is the next. However, it cannot start the threading on an uncut hole but useful during the cutting process as the second or third tap. The taper tap is only used in specific situations like when the material is very hard or the hole is too small. The taper tap has a smooth diameter transition. The dimension allows for less aggressive cutting action and thus minimizes the risk of damaging the drilled hole. In case the taper needs first to go, the intermediate tap follows second, and the thread is finished with the bottoming tap.

What Is The Function Of A Bottoming Tap?

Bottoming tap is usually used to cut threads in a hole that has already been threaded to a point. Note that the tapered end of a bottoming tap is too short to start  and finish through an unthreaded hole successfully

Which Tool Is Used To Turn Taps?

A tap wrench is an ideal tool for turning taps. It is a hand tool used to turn taps or other small tools, such as screw excavators and hand reamers.

How Do I Choose A Drill Bit For A Tap?

A good tap drill is 90% of the major diameter for fine threads. The concept of major diameter minus pitch gives a great tap drill diameter for metric V threads.

How Do I Measure Screw Size?

Length is usually measured from the point at which the head sits flat with the surface to the thread tip. Hex, truss, pan, socket cap, button, and round head screws are measured from right under the head to the end of the threads. You measure flathead screws from the head top to the tip of the threads.

What Do Tap Sizes Mean?

Thread size refers to the number of threads in each inch. It is measured along the length of the tap. Thread pitch is the metric size. Chamfers are the measured length of the tapering threads at the tap’s front.

How To Measure For A Tap And Die

Follow the steps below to measure tap and die:

1. Look at the threaded fastener you are repairing and measure the outside diameter of the thread.

2. Count how the number of threads there are per inch.

3. Find out whether your thread is National Fine or National Coarse.

4. Compare the numbers and thread type you just gathered to the numbers written on the taps and dies.

How To Size A Drill Bit?


First, you need to check the markings on the drill bit and then use some of the following ways and tools to measure the size:


These are small tools with a flat handle with two jaws at the top and bottom. To determine the screw’s size, place the body of the screw between the jaws and push it to shut with your fingers.

Test Hole 

You can use the bits with exact known measurements to measure others. For example, a new set of bits, or identify the ones you measured with calipers. First, you drill a series of holes through a thin, flat piece of metal. Mark each hole with the drill bit’s size. To ensure you right, you can check the fit by inserting the bit into one of the drilled holes.

The Shank

Typical drill bits have markings on the shank, usually above the threads. The size or number is written in fraction form (1/4 inch or 25/32 inch) depending on their graduates. According to Fastener Superstore’s Fastener Guide, you can measure the diameter of a bolt or screw from the outer strand on one side to the other side’s outer thread.

How To Use A Die And Tap Set?

You can use a tap and die set by following the simple steps below: 

Step 1: Threads Per Inch

A die set is for cutting threads onto a bolt or a cylinder. To use a tap or a die, determine the number of threads per inch of the part to be fixed. You can use a gauge system with several different pins to calculate a bolt’s TPI. Once you know the TPI of a bolt, choose the die that is compatible with it. It is indicated on which side to start when using a tapered die. The die fits into a specific wrench that holds and directs the die.

Step 2: Create New Threads

 Create new threads on a worn-out bolt using a die. First, place the bolt into a vise to hold it as you turn the wrench over it. Lubricate the bolt by putting some cutting oil on it to prevent the heat caused by the friction created when you cut metal with metal. Horizontally put the die over the bolt. Turn the wrench. The die is expected to catch on the threads cut into the bolt. After every few turns, reverse the wrench about a half a turn. This is to clear the threads so that the die will cut better. Ensure you keep re-applying cutting oil. Continue turning the wrench until the bolt moves past the die top.

Step 3: Fit A Steel Rod Into A Bolt

The rod must have a beveled end for it to fit well into a bolt. If it does not, you can make a symmetrical sloping edge at the end of a rod on a grinder to get an angle. The die will not function efficiently very well with a flat-ended rod. Remember to lubricate the rod often, as when cutting into a worn bolt. Make turns slowly and meticulously. Patience is crucial to getting the best results.

Step 4: Tap

Choose the suitable size for the bolt or the hole you are supposed to thread. Place the tap into the wrench and tighten it in. After placing the cutting end of the tap over the hole, turn it. Now lubricate the tap using the cutting oil and make a slight reverse turn on the die now and then.

What Is The Best Tap And Die Set?

Check below some of the best taps and die set:

1. GEARWRENCH 75 Pc. Ratcheting Tap and Die Set (overall)

2. Neiko SAE plus Metric Tap and Die Set, 76-Piece

3. Neiko SAE Pro-Grade Titanium Tap & Die Set (prograde)

4. GEARWRENCH 40 Pc. Ratcheting Tap & Die Set, Metric

5. Kings Country Tool Tap and Die Set (wood)

6. IRWIN Tap and Die Set SAE/Metric, 117-Piece

Number Of Threads That Are Partially Formed When A Bottoming Tap Is Used To Cut A Hole 

A bottoming tap has barely any taper at the end. It is designed to thread to the bottom of its reach, with only 1 to 1.5 threads being tapered. Bottoming Taps are useful for threading blind holes.

In Conclusion,

Tap drills help to bore the exact diameter of holes you want. Getting the right size is vital for accurate results. There are many ways of determining the size of the screws, bits, and taps. It cuts in only clockwise for standard threads. Spinning the tap will progressively work its way deeper into the hole as it cuts new threads. When you spin counter-clockwise, it will back out of the hole.

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