VÖLKEL Guide
Cut Clean Threads: How to Produce Internal and External Threads
A thread often only attracts attention when it does not run smoothly. Yet this small detail determines whether a connection holds, engages easily or causes problems later. That is why it is worth checking which thread is required and which tool is suitable before cutting the thread.
There are basically two applications: an internal thread is cut into a component, while an external thread is produced on the outside of a round workpiece. A tap is used for the internal thread and a die for the external thread.
For the result to be right, three things matter above all: the correct dimensions, the right tool and a clean, controlled cut.
Clean thread cutting therefore does not begin with the first cut, but with the preparation.
“Many thread-cutting errors do not occur during cutting itself, but during preparation. An incorrect tapping drill size, a misaligned start or insufficient lubrication can be enough to make a thread bind or tear, or cause the tap to break.”
Internal or External Thread: What Is the Difference?
Before cutting a thread, you need to know what type of connection you want to create. Internal and external threads work together, but they are produced in different locations.
Internal thread
Tap = internal thread
An internal thread is located inside the component. It is needed wherever a screw will later be inserted, for example in a nut, housing, mounting plate, bracket or repaired component.
External thread
Die = external thread
An external thread is located on the outside of a round workpiece. You will find it, for example, on screws, bolts, threaded rods, pipe ends or custom-made connecting parts.
The basic rules are similar for both: the dimensions must be correct, the tool must be guided accurately and the cut should be smooth, lubricated and controlled.
The difference lies in the preparation. For an internal thread, the tapping drill size is decisive. For an external thread, the prepared outside diameter is decisive. We will therefore look at both cases separately.
How Do You Cut an Internal Thread?
For an internal thread, everything starts with drilling the tapping hole. It forms the basis for the thread and must match the required thread diameter.
If the hole is too small, the tap has to remove too much material. This makes it harder to turn, can cause it to jam or, in the worst case, break. If the hole is too large, the thread flanks will be too weak and the screw will not hold reliably.
For metric coarse threads, the following simple rule of thumb is useful:
Rule of thumb for metric coarse threads
Tapping drill diameter = nominal diameter − pitchExamples:
- M6 × 1,0 → tapping drill approx. 5,0 mm
- M8 × 1,25 → tapping drill approx. 6,8 mm
- M10 × 1,5 → tapping drill approx. 8,5 mm
Important: This rule of thumb applies only to metric threads. For inch threads such as Whitworth, UNC or UNF, the pitch is specified in threads per inch (TPI). The thread angle also differs: 55° for Whitworth threads and 60° for UN threads. Therefore, use the appropriate table or calculator for the relevant thread system.
Tapping Drill Calculator
Select thread
Recommended tapping drill
Select a thread system and size.
Source: VÖLKEL GmbH
This lets you immediately find the correct tapping drill for every common thread—metric or inch, including fine threads and special series. For safety-critical components or special materials, however, it is still advisable to cross-check the value against the component manufacturer’s specifications.
To cut the thread itself, you need the correct tap. Depending on the application, this may be a hand tap or a machine tap. Hand taps are particularly suitable for individual parts, repairs and controlled manual work. Machine taps are useful when threads need to be cut by machine, with repeatable accuracy or in larger quantities.
After drilling, lightly countersink or deburr the hole. This helps the tap enter the hole cleanly and prevents immediate damage to the first thread.
How to cut an internal thread:
- Determine the thread size
- Select the correct tapping drill diameter
- Drill the tapping hole
- Countersink or deburr the hole
- Align the tap squarely
- Cut with cutting oil and controlled pressure
- Break the chips by briefly reversing, if necessary
- Clean and inspect the thread
What matters most is the combination of the correct tapping drill size, the right tap and accurate alignment. If the tap suddenly becomes difficult to turn, do not force it. Instead, reverse it, remove the chips, apply fresh lubricant and continue cutting in a controlled manner.
How Do You Cut an External Thread?
For an external thread, the starting situation is different. There is no tapping hole; instead, there is a round workpiece or bolt on whose outside the thread is to be cut. The first priority is therefore the correct outside diameter.
If the bolt diameter is too large, the die has to remove too much material and is difficult to start. If it is too small, the thread will not be fully formed. The surface should also be clean so that the die can engage evenly.
As with the tapping drill for an internal thread, it is worth determining the correct diameter precisely in advance rather than estimating it. With our Outside Diameter Finder, you can immediately find the recommended blank diameter for recutting your thread—whether metric, inch or a special thread such as a steel conduit or pipe thread.
Outside Diameter Finder
Select thread
Recommended outside diameter
Select a thread system and size.
Source: VÖLKEL GmbH
An external thread is cut with a die. The die stock allows the tool to be guided safely. It is important that the thread size, thread type and material match the tool.
Before you start, lightly chamfer the end of the workpiece. This chamfer helps the die engage more easily and produces cleaner first threads.
How to cut an external thread:
- Determine the thread size
- Prepare the round stock to the correct diameter
- Clamp the workpiece securely
- Lightly chamfer the end
- Insert the die into the die stock
- Position the die squarely
- Cut evenly
- Clean the thread and check it with a nut or gauge
With external threads, the first few turns are especially important. If the die starts at an angle, the error will continue through the entire thread. Start slowly, guide it steadily and only continue once the tool engages cleanly.
When the Thread Binds: Typical Warning Signs
When cutting a thread, you can often tell quite quickly that something is wrong. The tool suddenly becomes difficult to turn, starts at an angle or the finished thread feels rough. At that point, do not simply keep turning. In most cases, the problem is not the thread itself, but the preparation or guidance.
Typical warning signs include:
- The tool does not engage cleanly.
- The resistance suddenly increases sharply.
- The thread starts at an angle.
- The surface looks rough or torn.
- The screw or nut is difficult to turn later.
- The tool squeaks, chatters or jams.
In such cases, stop, reverse and inspect. With an internal thread, the tapping drill may be too small or chips may be trapped. With an external thread, the blank diameter may be incorrect or the die may not be starting squarely. Insufficient cutting oil is another common reason why cutting becomes unnecessarily difficult.
Thread cutting requires control, not force.
If something binds, applying more pressure usually does not help. It is better to reverse the tool, remove the chips, apply fresh lubricant and only then continue cutting in a controlled manner.
Cut the Right Threads Cleanly with VÖLKEL
Thread cutting is a good example of how reliable connections begin with proper preparation. The aim is not simply to cut any thread. The aim is to produce a thread that matches its mating part, runs smoothly and holds reliably.
For internal threads, we offer suitable taps. Dies are used for external threads. Die stocks, tap wrenches and thread-cutting sets additionally ensure that the tools can be guided accurately and used flexibly. Thread-cutting sets are particularly practical when internal and external threads in several sizes need to be produced, reworked or repaired.
Accurately identifying the thread before machining is always important. The right tool can only be selected once the thread size, pitch, material and application are clear. And when time matters: All VÖLKEL products are available from stock for immediate delivery.
The advantage is greater process reliability: using the correct tool helps prevent angled starts, damaged thread flanks and unnecessary tool breakage. The result is a thread that runs smoothly and holds reliably.
Conclusion: Clean Threads Begin Before the First Cut
Good threads are not produced by force, but by correct dimensions, accurate guidance and the right tool. For an internal thread, the tapping drill size is the decisive starting point; for an external thread, it is the prepared outside diameter.
Careful preparation, controlled cutting and selecting the right tool for the application help prevent many typical problems. VÖLKEL taps, dies and thread-cutting sets can be used to produce or rework internal and external threads reliably.
Thread Cutting FAQ
What is the difference between an internal thread and an external thread?
An internal thread is located inside a component and is cut with a tap. An external thread is located on the outside of a bolt, pipe or round stock and is produced with a die.
What tapping drill size do I need for an internal thread?
For metric coarse threads, a simple rule of thumb applies: nominal diameter minus pitch. For M6 × 1,0, for example, this gives a tapping drill size of approximately 5,0 mm.
Why do you need to reverse when cutting a thread?
Reversing breaks the chip. This reduces the risk of the tool jamming and produces a cleaner thread.
Do I need to use cutting oil when cutting a thread?
For metal, cutting oil is almost always advisable. It reduces friction, improves the surface finish, helps remove chips and protects the tool.
Why does a tap break?
Common reasons include a tapping drill that is too small, an angled start, insufficient lubrication, trapped chips or excessive force.
