Threading Taps

By | August 16, 2016

A tap is a cutting tool used to produce threads inside a hole. In metalworking it is necessary to drill a hole prior to create female threads using a tap. This type of threading operation is called tapping. There are other methods of threading such as thread cutting and thread rolling.

Threading Tap Set

A Hand Tap Set with three taps

How to use a Threading Tap? (Tapping Procedure)

In soft material like wood it is possible to drive a threaded wood screw into the work without drilling a hole. But in metalworking it is important to form a hole to the correct size before doing the tapping. Here is how you would proceed.

Tapping – Step by Step

  1. Find out the size and type of thread you want to do the tapping and prepare the tools. The commonly used screws have metric or British threads.
  2. Calculate the core diameter drill size.
    It is important that you use the correct size drill to form the hole. If the hole is too small, the tap will not be able to penetrate into the work piece and will result in breakage. If the hole is too big, the depth of the thread may not be sufficient to hold the screw. See the tapping hole size formula
  3. Clamp the work rigidly using a vice or any of the other work holding devices that is appropriate for your job.
  4. Take the first tap (taper tap) and hold the tap using a tap wrench
  5. Insert the tapered end into the hole and align it straight making sure that the tap is perpendicular.
  6. Rotate the tap in clockwise (for right hand thread) while pushing it into the hole. Once a few threads are cut, you only need to rotate the tap to keep cutting.
  7. While doing the manual tapping using hand taps, make sure that you rotate in the opposite direction to enable the chips to break and clear. If necessary, remove the tap completely by turning in the reverse direction and clean the tool and repeat the process.
  8. Use the suitable cutting fluid.
  9. Once you have completed the tapping using first tap, repeat the procedure using the second and third tap.

Formula to Calculate drill diameter for tapping

For ISO metric threads the formula is as follows,

 Ød = ØD-P
where, Ød= Core diameter (tap drill size) ØD=Outside diameter of the thread P= Pitch
For example, if you want to do threading for a standard M10 screw where the pitch is 1.5mm the calculation will give us 8.50mm as the hole size.
That is, 10 – 1.5 = 8.50mm

For inch size threads the formula is, Ød = ØD-1/P
where, Ød is the drill size, ØD is the major diameter and P is the thread pitch.
*Note: In metric standard pitch is measured as the distance between two consecutive threads where as in American and British standards the pitch is measured as threads per inch (TPI).

Type of Taps

Taps are available in different material (HSS, carbide etc.), size, type of thread (right-hand or left-hand thread), standard (metric, inch) and the usage (manual or machine tapping).
Following are some of the most commonly used thread types and their uses.

Hand Taps for manual tapping

Hand taps are used to create threads manually. A set of hand-tap normally will have three taps.

First Tap (Taper tap): First tap is smaller is size with less thread depth and a lead taper (7 to 10 threads) at the cutting end. This lead taper serves two purposes. Firstly it makes it easy to align the tap straight to the hole axis since the end of the tapered tap can go inside the hole without cutting. Secondly the gradual thread formation on first tap makes it easy to start cutting on hard materials like alloy steels.

Second Tap (plug tap): The second tap which is also called plug tap has less lead taper (approximately 4 threads) than first tap and is the most used tap for through holes where you can pass the tap till the taper portion comes out from the bottom end of the hole.

Third tap (Bottoming tap): The third tap is the finishing tap which gives the correct depth to the thread inside the hole. It has full depth cutting edges till the tip with a very small chamfer at the cutting tip. This enables threading in blind holes.

Machine Taps

These are used on machines like drill press, lathe, machining centers and milling machines. Unlike hand taps, machine tap is a single tap with the finishing size. The tap rotates and penetrates into the previously formed hole and when the depth is reached (thread length); the rotation is revered to remove the tap.  Machine taps normally comes with helical flutes that enable easy chip removal.

Hand Tapping Tips & Techniques

We have already covered few important tips in tapping procedure section. But I thought it is worth reminding you again since tapping is one of the most frequently used operations in metalworking.

Avoid Tap Breakage:

Threading taps are harder than normal steels since they need to cut the material. Their cross sections are weak especially for smaller sizes, due to their design with cutting edges and relief grooves.  The extra hardness along with its weak construction makes it very brittle. If a tap is broken inside the hole, it is usually difficult to remove unless you can do some special machining operations like EDM. Hence handle them with care and take the following precautions to avoid breaking taps.

Drill the correct size hole
Refer the drill size chart or calculate the correct tap drill size to produce the required diameter hole. A smaller sized hole will most probably force the tap to break while an oversized hole will have shallow threads.

Check the Cutting Edges
Dull cutting edges will require extra force to cut the material and can cause breakage.

Use the correct wrench
It is important to hold the tap using a proper tool holding device such as t-handle wrench or adjustable tap wrench in case manual tapping. For machine tapping ensure that the tool clamped rigidly on a collet or chuck.

Align the tap axis with hole
One of the most common reasons for tap breaking is tap going at an angle. For thin work pieces it may not cause the tap to break, but it is still a problem since the screw will sit at an angle.  Take your time to ensure that the tap is aligned straight to the axis of the hole. Mostly the drilled hole is perpendicular to the surface of the work piece. You can use a simple trisqure to ensure the perpendicularity of the tap.

Remove Swarf
Taps come with grooves to ensure proper chip removal. In case of machine taps, the helical flutes will pull the chips out of the hole. While performing manual tapping you have to relieve the tap often to break the metal chips. Usually for every one rotation forward, turn at least half of thread in reverse direction. As you get more experienced you can feel the “click” when the swarf gets accumulated inside and that’s the time you have to slowly reverse the rotation to break the chips.

Blind Holes
Bottoming out at the blind hole is a common cause for threading tap breakage. While tapping the blind holes extra care must be taken when the tap reaches near to the bottom of the hole to avoid breakage.

Cutting Fluids for Tapping

The application of cutting fluids cools the threading tap, assist cutting and improve the finish of the threads produced.  In case of machine tapping, jet of cutting oil facilitates easy chip removal as well.
When performing hand tapping, use a paste such as Trefolex for the best results.

For materials like Aluminium use Kerosene or less viscous synthetic oils as cutting fluid.

Carbon Steel Taps Vs HSS Tps Vs Carbide Taps

I come across this discussion often on internet as well as while talking to my hobbyist friends. Let me tell you the pros and cons of each of these.

Carbon Taps

These are made out of high carbon steels. They are useful for tapping soft materials.

Pros: Inexpensive and less brittle.

Cons: Poor tool life. The cutting edges get dull very fast and hence not recommended for tapping harder materials such as steel and not suitable for batch production.

If you are a hobbyist mostly working on soft materials like plastic, aluminium, brass etc. get a carbon steel tap.

HSS Taps

HSS (high speed steel) is the most widely used material for making threading taps. They are hard and tough. They can retain sharp cutting edges much longer and are less brittle when compared to carbide.
Pros: Longer tool life. They can be used on variety of materials.
Cons: Brittle and more expensive when compared to carbon steel taps. However in the long run, cost of HSS taps will be nearly same as carbon taps due to higher tool life.

Carbide Taps

Tungsten carbide taps are harder and hence more brittle when compared to HSS taps. They are useful for cutting threads into harder tool steels. Their ability to retain sharpness for longer period, make them a good choice for batch component production.
Pros: Longer tool life. Ability to cut into harder materials.
Cons: Expensive and brittle.