Cutting Left-Hand Threads – How to Make It Work Counterclockwise
We all know the right-hand thread. In almost all areas where threads are used, it is the standard. The world is simply made for right-handed people – left-handers can tell you a thing or two about that. But this uniformity also has its advantages. The rotation direction of screws and bolts is always the same and you don't have to worry about yet another factor when cutting threads. With thread type, diameter, pitch, chamfer, and so on, you already have enough to deal with.
But sometimes you really do need a left-hand thread. In such cases, we have two pieces of good news for you. First: It is not difficult at all to cut a left-hand thread. Second: As a manufacturer of high-quality threading tools, we at BAER have everything you need in stock – including all the information about left-hand threads that will help you complete your work successfully. So let’s get started.
Applications for a left-hand thread
You might be wondering: Why change direction at all? For most workpieces, right-hand threads work perfectly well. True, but left-hand threads are not intended for most workpieces, only for very special applications.
Roughly speaking, there are three cases:
- For some rotating components, right-hand threads may loosen or even detach on their own, such as on left bicycle pedals.
- Left-hand threads help prevent incorrect connections, for example on gas cylinders or in other potentially explosive situations.
- Components such as fittings that must be installed in an existing pipe system can sometimes be mounted more easily if they have both a left-hand and a right-hand thread.
You can recognize a left-hand thread by the abbreviation LH or, if you have a trained eye, by the orientation of the thread flanks. It becomes easier when you compare it directly to a standard right-hand thread screw.
These tools are required to cut a left-hand thread
To cut a left-hand thread, you need special taps and dies with the appropriate reverse cutting direction. Apart from the orientation of the thread flanks, these tools do not differ from their right-hand counterparts. Additional tools and accessories such as twist drills, countersinks, or die stocks for manual cutting can be used just as usual.
How it works – Cutting left-hand external and internal threads
Cutting left-hand threads works the same way as cutting right-hand threads – just mirrored. You only need to make sure you use the right tool and turn in the proper direction. Shape, dimensions, and tolerances do not differ from the thread cutting process you already know.
Cutting a left-hand internal thread
Tool selection
This means that your first step is to find the right tool. Left-hand threads can be produced manually or by machine and can be cut into many different materials. Make sure that your tap matches your material and the cutting application.
Preparing the core hole
Have you found a suitable tap? Then continue with the preparation of the core hole. Drill a core hole with a suitable diameter and – in the case of a blind hole – the proper depth. Then you can chamfer the edge of the hole slightly.
Cutting the internal thread
Clamp the tap into your holding tool or into the chuck of your drilling machine and apply plenty of cutting oil or cutting paste. Now you must turn the tool counterclockwise to create the thread.
Preparing the bolt
What the core hole diameter is for internal threads, the bolt pre-machining diameter is for external threads. So choose a bolt with the correct dimensions or machine it on a lathe. After that, bevel the end of the bolt to make starting the cut easier.
Cutting the external thread
Now place the die (and the die guide) on the bolt using enough cutting oil and turn counterclockwise. Once the desired thread length is reached, turn the die clockwise to remove it from the bolt.
Taps and dies for left-hand threads in the BAER online shop
As you can see, cutting a left-hand thread is not difficult and does not differ much from the usual method. With the right tools, nothing can really go wrong. In the BAER online shop, you will find a wide selection of taps and dies for left-hand threads.
All tools suitable for left-hand threads are clearly marked with the addition "Left" in the product name. In addition, our product overview highlights the relevant categories with a red counterclockwise arrow.
And if you want to make it especially easy, simply use the practical Toolfinder on our home page. Select any specifications that match your cutting application and then check the box for “Left” under direction. You will immediately see only the tools suitable for cutting a left-hand thread.
Do you still have questions about left-hand threads, need further advice, or are looking for a custom-made solution? Our customer service is happy to help with all concerns. Contact us anytime by phone or via our practical contact form.