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An Introduction to Tapping

by MadReptillian

The Three Basic Types of  Hand Taps

A Tap is used for cutting an internal thread to create a female thread that a bolt can screw into. There are a wide range of taps available, and for beginners it can be quite overwhelming. This is a guide to give the new machinist an introduction to taps. I will also cover the tools for using the taps correctly to ensure you get a square thread – how often do you screw a bolt in to find it’s been tapped on an angle? Then I will explain the procedure for tapping a perfect thread.

Tapping begins with the drilling of a hole and then chamfering the top edge of the hole – this makes it easier to locate and start the tapping process. Ideally you want a chamfer that is slightly larger (+0.020″ or so) than the final internal diameter of the threads. If you discover that your chamfer isn’t large enough after tapping, you can always re-chamfer the hole.

There are 3 basic types of hand taps per thread size: (image source)

Taper Tap (also known as starter tap)
A taper tap is the first tap you want to use to start your thread. You will notice that the first 5-7 threads have been ground away resulting in a long taper which makes it easier to start your tap square, and it also makes the cutting action easy and gradual. In a way you could say a taper tap ‘paves the way’ for the plug tap to follow. If you are tapping a through-hole (a hole that passes all the way through your part) a taper tap is all you will need to complete your hole. However, if you’re tapping a blind hole (a hole that doesn’t pass all the way through your part) you’ll need to move on to a plug tap, and then to a bottoming tap if you intend to achieve fully formed threads all the way to the bottom of your hole.

Plug  Tap (sometimes referred to as second or intermediate tap in Australia and Britain)
The plug tap is the second tap you want to use to increase the thread depth inside your hole. This tap has a smaller taper with only 3-5 threads ground away which allows you to cut fully formed threads to within 3-5 threads of the bottom of the hole. If you’re tapping softer materials you can sometimes advance from a taper tap directly to a bottoming tap and skip the use of a plug tap. 

Bottoming Tap (sometimes referred to as a plug tap in Australia and Britain.)
The final tap is known as the bottoming tap. This tap has had only 1-2 threads ground away resulting in a very small taper at the tip of the tap. By finishing a blind hole with a bottoming tap you will achieve fully formed threads all the way to the bottom of your hole.

Different Tapping Tools used while Hand Tapping

If you want to use a tap you’re going to need a way to hold it. This is where tools known as Tap Handles or Tap Wrenches come in handy.  These tools are also sometimes referred to as a “T Bar” or ‘T Wrench”. It is fairly simple to work out how your tap fits into these tools.

Specialty Taps

The taps shown in the diagram above are known as Straight Flute Taps and are for general purpose and tapping by hand. There are also Power Driven taps that you can use with your machines (lathes, mills, CNCs).

The most common type of Power Driven tap is the Spiral Point tap – also commonly known as a Gun Tap (above). The cutting edges in these types of taps are angled ever so slightly so that the chip breaks away into the flute of the tap which prevents the tap ‘tightening’ – this is most commonly known as Crowding.

Above is the Spiral Flute Tap. It is designed primarily for the machine tapping of blind holes. The cutting action of the spiral flute curls the chips up out of the hole instead of them falling to the bottom and seizing your tap. These type of taps are most suitable for soft metals like aluminium and steel.

This is the Interrupted Thread Tap and you will notice it has large gaps between cutting teeth. This is to reduce friction which is ideal for threading tough materials such as Stainless Steel and Bronze. It also makes it easy for coolant to reach the cutting edges.

There are two types of taps in this photo, the top two are Fluteless Taps without oil grooves (also called “roll taps”) and the bottom tap you will see oil grooves – Fluteless Tap with oil grooves. The yellow coating is a protective layer called Titanium Nitride and this prevents the wearing of the tap. Fluteless taps are designed for soft materials such as Nylon. Taps with oil grooves give you the option of delivering lubricant to the cutting edges.

This is the Taper Pipe Straight Flute Tap. It is primarily designed for threading pipes and fittings. The tapered cutting edges create a tighter fit when fitted together. You wouldn’t want the pipes in your house to leak would you?

Please remember that Power Driven taps are recommended for the more experienced machinist – if they are not set up correctly as per manufactures instructions you could potentially harm yourself or damage your machine.

The correct way to tap an internal thread

There are two ways to tap a thread – by hand or by machine. If you do it by hand there is a strong possibility it won’t be square to the hole.  However if you do it by machine you will get it precisely square 99.99% of the time. Let me explain to you what I mean.

In this picture you will notice the correct setup for tapping a hole using the mill (the mill is used to align the tap, it is NOT turned on during the tapping procedure).

In this example I drilled the hole to the correct diameter needed for the thread and then applied a chamfer. I then used a pointer in the drill chuck – one that I ground on the Tool & Cutter grinder (see below for picture and description). You could also use a spring center. The pointer or spring center is used to keep the tap and tap wrench square and aligned with the hole in the part being tapped. Most taps and tap wrenches these days come with a centre hole in the top of them.

Next I applied tapping fluid to my taper/starter tap and placed the tap on the chamfered hole while applying a slight pressure using the quill (Note: pressure is not needed if you’re using a spring center, as the spring inside the center provides all the pressure you’ll need).

Next, I put my mill into neutral and from then on it was just a case of simply keeping a firm pressure on the quill lever with one hand, whilst rotating the tap wrench with the other in a clockwise direction (assuming you’re tapping a traditional right-handed thread). This process is not to hard – even for the uncoordinated! Once the taper tap has cut it’s first two or three threads you’ll start to feel some resistance. At that point you’ll need to reverse the direction of your rotation (turn the tap counterclockwise) to break away the chips that have formed. If you listen you’ll actually hear the chips break away. It usually takes a quarter to a half turn to break away the chips. Once you hear the chips break away, return to your original direction of rotation (turn clockwise) and continue the tapping procedure, pausing to break chips every quarter to half turn in hard materials and every half to two turns in softer materials. If you find your progress is slowing your hole may be filling with chips. Back the tap out entirely and blow the tap clean of chips. Also blow out any chips from within the hole that may be impeding your progress. Re-apply tapping fluid and return to tapping the hole.

Once you reach the bottom of the hole with your taper tap repeat the process with your plug and bottoming taps.

Now this is the correct way to tap a hole in a lathe. Notice it’s extremely similar, except this time you need to put your chuck into neutral or high gear and apply pressure on the tailstock handle (or use a spring center) – make sure you lock the tailstock in place otherwise you might find you can’t tap.

If you don’t own or have access to a lathe or mill you can still accurately tap a hole by hand using a tapping block or an engineers square. A tapping block typically has several holes drilled in it. Find the hole that is just large enough for your tap to pass through without binding.

Next, align the hole in the tapping block over the hole you’d like to tap, and hold the tapping block in place with one hand while rotating your tap with the other. By keeping firm pressure on the tapping block you’ll ensure that the tap stays square to the hole you’re trying to tap (image source).

Another method is to use a machinists square to check your tap angle every 90 degrees (every quarter turn). If the tap starts to stray from 90 degrees you can rectify it by gently guiding the tap in the next quarter turn to the proper orientation needed.

So there you have it, a simple guide on the types of taps available to you and the correct way to tap your holes. It shouldn’t be too hard even for a novice. Experience is the best teacher – so give it a go!

Pete Woods (AKA “MadRep”)
Machinist

About the author

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About madreptillian

Aeronautical Machinist for the Royal New Zealand Air Force

22 comments

  1. comeausj said:

    Tapping is all well and good. I find it pretty easy compared to dies. Dies and external threads in general have been kicking my but since I got my new lathe about 3 weeks ago. I finally got a pair of passable internal and external threads, but they are pretty loose. I think I cut the Major diameter down too much. Anybody got any tips for threading on a lathe?

     

    Information that may be helpful:

    Harbor Freight 93212, trying to make 3/8″ Aluminum with 16 Tpi

    Check “Often” for your “fit” also when you get close take a few passes “without” advancing the tool post and check in between passes. I have the same problem of cutting too far when I needed to quit several thousandths back.Embarassed I have managed a few passable threads though. With taps and dies the “fit” tight or loose is determined at the factory and they come (those that are not standard) labeled as to their fit. 

    Someone chime in here with the proper terms.

    Also there are dies that are adjustable to change the fit slightly.

  2. Adjustable dies are called 'Split Dies'. They are mounted in a die holder which has three screws in it. The two end screws locate in dimples in the side of the die, and they act to close the die over, the centre one has a pointed tip, this wedges into the split and is used to open the die up.

     

    Split dies are certainly my preference, as they allow you to tune the fit to suit. There is a trade off however if you're using a tailstock mounted die holder. To allow the die to be opened up the holder has to have a recess for the die that is larger than the die diameter, this means that the die is held slightly off centre, and can result in threads that are not true, usually though they suffice for most things, as if you need true accuracy then single point cutting is the only way.