Guide to Design and Machining Thread: How to Cut an Internal & External Thread? | CNCLATHING

2020.5.30

Screws still play an important role in the mechanical industry. To connect a screw into another component, an internal thread is required. Thread also can be cut in the outside surface of a machined part. In this article, we’ll introduce what is a thread in manufacturing, thread types, tips of thread design and machining, as well as how to cut thread on a lathe machine, including internal thread cutting and external thread cutting.

What is a Thread in Engineering and Manufacturing?

A thread is a continuous helical ridge with a uniform section formed on the internal (nut) or external (screw or bolt) surface of a cylinder or a cone. The profile of the thread is composed of the crest (the top of threads), root (the bottom of threads), and flanks (joining crests and roots). Thread pitch is the distance measured parallel to the thread axis, between corresponding points on adjacent threads. Pitch diameter is the diameter of the theoretical cylinder. When the theoretical cylinder passes through the thread, the distance between the crest of the thread and the root of the thread is equal. In an ideal product, these widths are equal to half the pitch.

Types of Threads:

There are many types of threads according to different classification standards. Machine screw threads and spaced threads are in common use on fasteners. UNF (fine) thread and UNC (coarse) thread are standard series in the Unified screw thread system. Here we’ll talk about the internal thread and external thread.

– Internal threads (female): a screw thread on an inner or concave surface.

– External threads (male): a screw thread on the outside of a cone or cylinder.

Internal Thread Design Tips

In daily life, the nut is a type of typical internal thread. A tap is a metal threading tool used to cut the internal threads on a CNC machining part, thrilling is the process of threading and drilling internal threads using a specialized cutting tool on a CNC mill. 

– Threaded products should have a countersink at the end of the internal threads. 

– The surface of the starting end of the thread shall be flat and consistent with the central axis of the thread. 

– Use lower height thread instead of higher if it’s not required. 

– The wall thickness of tubular parts must be sufficient to withstand the pressure of cutting or forming. 

– Use standard thread forms and sizes is more economical.

– Coarse threads are more economical than fine ones, unless for the largest size.

– Sharp corners should not be specified at the root of the thread to be ground.

Internal Thread Machining - How to Cut an Internal Thread

For machining an internal thread, you need twist drill, 90°countersink, internal hand or machine tap, adjustable tap wrench (for hand) or hand drill (for machine), and safety goggles. Before threading, you need to know which tap to choose and what diameter of the hole needs. Internal taps are designated according to nominal size. The diameter or the twist drill for tapping can be calculated using the formula: Core hole diameter = Tap diameter – Thread pitch. In our chart, you can find the standard Metric and English drill bit and tap size for your thread holes.

1. Centering with a punch and then drill a core hole with the twist drill

2. Use the 90°countersink to countersink into the core hole and make a chamfer

3. Put the tap into a tap wrench and turn it into the core hole to cut the thread

 

Internal threads are also available on CNC turning parts, lots of rules of cutting a thread on a lathe machine are the same as machining threads, but the threads specifications have more limits. 

External Thread Design Tips

The common external thread including screws, bolts, studs or plug gage. When designing external thread, try not to terminate near shoulder or adjacent larger diameter. The width of this convex surface depends on the size of the thread, the thread is coarse or fine and the throat angle of the thread cutter. In case of no need for high thread strength, it is recommended to use low height thread form. The external thread end of all threaded products should have chamfer. The external thread is cut with a round die which is fixed in a die stock, the round die has three or more cutting edges which are are serrated and represent the nut thread. 

External Thread Machining - How to Cut an External Thread

If you have a turning lathe, turn the diameter about 0.1 times the pitch smaller than the thread outer diameter. You need the threading tools for external threads are file, round rod, vise for clamping, round die, die stock, flat-tip screwdriver and cutting spray. 

1. File the edges of the round rod and chamfer it at 45°, which should be slightly larger than the thread depth.

2. Clamp the round die into a die stock and fix it firmly, turn the round die clockwise onto the round rod with sufficient pressure to cut the thread. Make sure to cut straight thread. The cutting spray can be used here to improve the surface quality and extend tool life.

How to Cut a Thread on a Lathe Machine?

Here is a video shows how to cut a find thread on a turning lathe.