Have you ever wondered how we make screw threads in the machine shop? You may know about our tap and die set, which are convenient tools for making standard thread types. Taps and dies are easy to use, but sometimes it’s nice to know how to make threads without the convenience of a tap or die, so what do we do?
The most common method is to use a threading tool on the lathe. The lathe has two ‘automatic’ functions on board; the feed bar and the lead screw. The feed bar allows the carriage to move at certain rate per revolutions of the chuck. Meaning, we can change the gears so that the carriage moves, say, 0.009 inches per revolution. This constant speed is good for facing operations.
In addition to the feed bar, we have the lead screw. By changing the gearing on the lathe, we can calibrate the lead screw to turn at a given rate to provide standard threading operations. Meaning, by changing the gears we can specify if we want to turn 20 threads per inch, 32 threads per inch, etc.
The standard method for turning a thread on the lathe is to face some round stock to the desired diameter, then add some blue layout fluid. Put the threading tool in the tool holder and to do a spring pass towards the chuck. A spring pass just barely scrapes away the layout fluid, and allows you to see if the threads are correct. You then repeat this process over and over until you’ve got deep enough threads. This method works perfectly fine, but can be a little scary as the carriage approaches the chuck.
A method to avoid the fear of crashing the carriage into the chuck is to feed away from the chuck. In order to do this a few changes need to take place. First, you need to place the threading tool upside down in the tool holder. Additionally, we need to run the headstock in the opposite direction and the lead screw back away from the chuck. By doing this, we can create the same threading operation that moves away from the chuck, which eliminates any risk of a crash during threading operations.
Are you interested in threading on the lathe? Making a bolt and nut is a great way to learn this skill and I’d be happy to show you how.
Also, check out the video below to learn more about threading operations on the lathe.
Thank you for not reading this.
MVTHS Engineering 