Although it is quite hard to get the perfect tap drill for your screws, having a good chart for their sizes helps out a lot. It is because of the various variables you need to consider to match the screw and tap.
From screw size to thread pitch, from the substrate to fit connection, everything needs to be considered for using the correct sized tap drill. You’ll need a quick and handy reference chart that will take all of these into consideration and show you the required size.
On the first look, the tap drill size chart can look a bit complicated, but with proper guidance, you can understand it quickly as well.
Table of Contents
What is the purpose of a tap?
A tap is a screw-like tool with threads that look like both. There may be two, three, or four flutes between the to cut screw threads in a hole or a nut. The interruption between the threads and flutes creates the cutting edge. The threads just behind the cutting edge may be relieved or backed off to produce sharpness.
What are the 3 types of taps?
The three tap types are taper, plug, and bottoming tap. Let’s discuss a little about all of them.
Taper tap – you can identify a taper tape by visible and pronounced tapering at the cutting edges. There are almost 8 to 10 threads tapered from the tip. This provides a less aggressive cutting action.
Plug tap – its cutting edges have a less pronounced taper and give it a bit less gradual cutting action than taper tap. It is also a bit more aggressive than taper tap and typically has between 3 to 5 tapered threads at the tip.
Bottoming tap – this is the most aggressive top of them all. It has only 1 or 1 tapered cutting edge before you engage the full cutting force. These taps are best used after the taper or plug tap has been used to cut the initial thread. The best thing about this tap is that it can cut threads all the way to the bottom of the hole.
What are tap size and drill size?
People use taps to cut threads for bolts or screws into materials such as plastic, metal, or even wood.
A tap can remove material from a punched hole or a pre-drilled hole. It has several flutes in its design and a higher number of flutes increase its strength. It helps reduce chip flow. There are grooves or valleys cut into the tap’s body.
When you tap a hole, it creates female threads within the hole. Every tap is tapered for the first few threads that help it to start the cutting process easier.
Each tap is marked with three distinct numbers:
2) Threads per inch
3) Type of threads it cuts
You need a specific-sized hole for the tap. If the hole is not of the appropriate size, it won’t fit the tap. So, you need the proper sized drill to pre-drill the hole first.
So, how’d you found out what sized drill to use?
Well, with the help of a basic formula you can find the correct drill bit for the thread tap.
Tap Drill Bit Size Chart
|Tap Size||Drill Bit Size||Dill Diameter|
|1 1/2-6 NC||1 11/32″||1.3438|
|1 1/4-7 NC||1 7/64″||1.1094|
|1 1/8-7 NC||63/64″||0.9844|
|1 3/8-6 NC||1 7/32″||1.2188|
|1 1/2″-12 NF||1 27/64″||1.4219|
|1 1/4-12 NF||1 11/64″||1.1719|
|1 1/8-12 NF||1 3/64″||1.0469|
|1 3/8-12 NF||1 19/64″||1.2969|
Tap Drill Size Formula
There are threads of two sizes; metric and imperial. Both are calculated the same way by subtracting the pitch from the diameter of the thread. For example,
If the thread is M5x0.75 then the tapping drill is 5-0.75 = 4.25mm
If the thread is 3/4.10 UNC pitch = 0.1, diameter = 0.75, tapping drill = 0.65 = 16.5mm.
We’ll describe the way of calculating the drill size with a few steps below:
First, you need to measure the overall diameter of the tap. It will require a micrometer. If you want a drill bit for a ¼ – 20 tap, where ¼ is the diameter and 20 stands for the thread pitch, then the overall measurement of the tap’s diameter is 0.252 inches.
The next step is to use the formula Dh = Dbm — 0.0130 (percentage of the full thread desired/ Ni) to find the correct drill bit size. Here, Dh means the drilled hole size in inches; Dbm means the basic major diameter of the thread; Ni is the number of threads per inch.
In our case, Dbm = 0.252 inch and Ni = 20; percentage should be between 70 to 85.
Now apply for the numbers in the formula with 84 percent. So, the formula becomes,
Dh = 0.252 – 0.0130 (84/20) = 0.0130 (4.2) = 0.252 — 0.0546 = 0.1974 inches.
This is the final step. You need to find the nearest drill bit size in 32nds of an inch. Multiply 0.1974 times 32 and the answer comes 6.32. It corresponds with a 7/32 drill bit which is also recommended within the chart.
How To Read A Tap Drill Chart
Now that we know how to calculate the drill size for tap drill, it is better to have an understanding of how to read the tap drill chart as well. This will help us work faster without needing to calculate ourselves.
We’ll describe the ways to chart for tap drill sizes in the instruction below. Our goal is to make it easy for you to read the chart whether you’re a casual DIYer, hobbyist, or mechanist.
Let’s start with the basics. A good tap drill chart will usually have three columns where the first column will include the size of the screw you’re using. The second column will suggest the tap drill size you’ll need to use. And the final column tells you the size of the drill bit to use to cut a hole.
Step 1 – Start with Bolt and Screw Size
To use a tap drill conversion chart, you ought to know what size fastener you’re using. As we’ve mentioned earlier that screw sizes may come in two different measurements; Standard (SAE) and Metric. Most of us are probably more acquainted with standard sizes in the US. So, we’ll have a breakdown of standard sizes here.
Know that there are two different classes of screw sizes; small diameter set and large diameter set.
Small diameter set – it denotes sizes less than ¼”. However, such fractional measurements aren’t used. Instead, numbers between 0 to 12 are used where 0 means the smallest and 12 means the size just before ¼”.
Large diameter set – sizes greater than ¼” are known for large diameter sets. And unlike the small ones, fractional sizing is used till 1”.
Read More: How to Determine Drill Bit Size for Screw
Step 2 – Know the Major Diameter and Pitch
Major Diameter – The next part is to know about the corresponding major diameter of the screw. Each of the screws has only one major diameter. It is the distance between the two outer ridges of the threading. You can find it in the tap drill chart as well.
Pitch – another important thing to understand is the pitch of the screw as well. The pitch is the thread count per inch (TPI) of the screw. There might be more than one thread count and a good chart will show all the possible counts. This helps your match the appropriate drill size.
You need to know that, while calculating in the Standard specification, each of the fastener sizes has two possible pitch (threads per inch) values for screw sizes between 1 to 10. As for sizes 12 and larger, there are three thread count sizes. These values show you the coarseness of each threading.
- The smaller thread per inch value is known as UNC (Unified National Coarse) and it is the course of the two/three values.
- The greater value than this but smaller than the biggest value is known as UNF (Unified National Fine) pitch.
- And finally, the highest number is the UNSF for the “Super-Fine” value.
Step 3 – Mastering the Drill and Tap Chart
When you need to understand something critical, sometimes the best way is to do it with an example. Suppose you need to bolt together two pieces of metal. The first thing you need to know is what bolt you’re using.
Suppose you determine that the bolt is “12-28”. Now check the chart and find the #12 size bolt on the Screw Size column. From there you can simply glide over to the right and find that for heavy-duty materials such as steel that tap size you need to use is size 10.
For the tap drill size chart metric, you can follow the same procedure as well. You just have to remember that in metric drill the thread pitch isn’t denoted by thread per millimeter. Instead, they follow the formula 1/number of threads per mm.
Can you use a drill to tap threads?
We’ve discussed previously that tapping means a process by which you add threads to a hole to accommodate a screw there. However, it can be a major hassle if you have to make a lot of tapping holes at a go. Fortunately, instead of using hand tools, you can use cordless power drills to tap much quicker.
Step 1 – Clamp the Metals
Use a vise or a C-clamp to clamp the metal in place so it doesn’t move. If the metal moves the drill may slip off and potentially cause injury to yourself. Besides, keeping metal steady makes the drill penetrate without a problem and much quicker.
Step 2 – Make a Divot
With the help of a center punch, you can make a pre-hole or a divot in the place you want to drill. This divot helps drill to grip the surface better and penetrate more efficiently.
Step 3 – Insert the Tap into the Drill
Now insert the tap drill bit into the end of your drill machine and tighten the chuck around so it is held securely.
Step 4 – Apply drilling oil
It is necessary to apply some drilling oil into the divot. This oil is a type of lubricant that helps prevent the bit from overheating. It also makes the tap drill easier to cut through the metal.
Step 5 – Start drilling
Now place the end of your tap drill bit into the middle of the divot and start your drilling very slowly at first.
Some Tips Regarding Tap Drilling
- The drill must be pointing straight down so that the hole becomes straight for your tap.
- It is important to start the drilling slowly at the beginning so that the bit can grip the metal.
- Always apply consistent pressure after bringing the drill up to medium speed. But don’t push too hard or else the bit could snap.
- While drilling, remove the drill every 1 or half of an inch to blow out the flakes. Otherwise, these shavings will create more friction inside the hole and cause the bit to heat up. It can also cause the hole to go uneven.
- Apply more drilling oil if you notice the drill having trouble or getting too hot.
There you have it. We’ve shown you the perfect example of a tap drill size chart and explained the process of reading it. Hopefully, our article will help you to tap any metal from now on without having trouble selecting the drill bit and screw size.