Threading is a process of both external and internal machining of threads into a workpiece. Standardized specifications are available to determine the desired results of the threading process.
Threading is often performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut.
To perform internal threading, the exact diameter to bore the hole is critical before threading.
- Click on the links to learn more!
Thread milling
Thread milling is considered the most cost-effective method for threading holes greater than 1" (25.4 mm) in diameter and holes in odd-shaped parts. The method is also preferably applied in the area of metric threads with pitch value of approximately 1-6mm. For imperial threads the application area starts with about 19 threads per inch.
The technique employs a cutter used with a formed profile. Replaceable inserts are typically used, because they are more economical than solid tools. They allow precision threads to be quickly and efficiently milled in holes from 3/8" (9.5 mm) in diameter up to almost any size within the machine’s work envelope.
Tapping & dicing
When machining internal and external threads with a smaller diameter, tapping and conventional tapping tools are used.
Dicing is pretty much used only in low volume, hand-finished processing.
Thread turning
Compared to normal turning operations, thread turning is more demanding. Cutting forces are generally higher, and the cutting nose radius of the threading insert is smaller and therefore weaker. Due to the importance of precise correspondence between the feed rate and the pitch of the thread, the strain is larger on threading inserts than what is typical for a turning insert.
As for turning tools overall, turning threads have benefited from improvements in coatings and material grades. In addition there have been design improvements in thread turning inserts resulting in better chip control.
Partial/full profile inserts
Partial profile inserts, (also known as “non topping” inserts), cut the thread groove without machining the crest of the thread. One insert can produce a range of threads and the nose radius is designed to be small enough that the insert can machine various pitches.
For small pitches, the nose radius will be undersize. This means the insert will have to penetrate deeper. The full profile insert therefore produces a stronger thread.
Full profile
Partial profile
Advantages:
- The profile fits the size of the actual thread standard.
- More economical due to less operations.
- Components with internal and external threads are concentric.
Disadvantages:
- The insert doesn´t fit different pitches and standards, you therefore need to hold a larger stock.
Advantages:
The insert may be for different pitches within a limited pitch range for a common thread angle.
- You only need to keep a smaller stock of inserts.
Disadvantages:
- The radius at the profile crest corresponds to the radius of the smallest pitch in the profile group (A, AG, G etc).
- You need additional turning for finishing the thread crest.
- The thread form deviates slightly from the standard.
Semi partial inserts
For large Trapez profiles.
Advantages:
- By making the shoulders relief, the insert reduces the load from the cutting corner.
- Deburring the sharp corner on the major diameter.
Disadvantages:
- You need additional turning for finishing the thread crest.
Multi-tooth inserts
Multi-tooth inserts, a threading insert that has a number of teeth in series along the cutting edge. The height of the teeth varies so that the actual depth of cut when machining is graduated. Therefore with multi-tooth inserts it is possible to reduce the number of passes up to 80%. Tool life of multi-tooth inserts is generally longer than that of single point inserts (general threading inserts) as the cutting force is spread over a wider area.
Due to high cutting forces, multi-tooth inserts are not recommended for thin-wall parts as chatter can occur. Also, the design of a work piece machined with one of these inserts needs to have a sufficient amount of thread relief to allow all of the teeth to exit the cut.
External & internal right hand thread
External & internal left hand thread
When machining an external right hand thread, you can either use a left or right hand holder.
The top picture shows machining using a right hand holder machining towards the chuck. You can also turn the toolholder upside down and place it on the opposite side of the work piece.
The bottom picture shows machining of an internal right hand thread using a right hand holder machining towards the chuck.
When machining an external left hand thread, you can either use a left or right hand holder.
The top picture shows machining using a right hand holder machining away from the chuck.
The bottom picture shows machining of an internal left hand thread using a right hand holder machining away from the chuck.
x