What is thread milling?
Thread milling is a method of producing threads on a workpiece by using a thread milling cutter.
Thread milling has following advantages over threading taps and threading dies.
- It is a faster operation
- Risk of tool breakage is less
- Left and right hand threads can be machined with a dame tool
When is thread mill used?
- Thread milling is used when the threads are not possible to cut with a die head during the turning operation.
- Thread milling is preferred for cutting internal threads which are not suitable to cut with a tap.
- It is also used to cut threads on a larger diameter workpiece.
Types of thread milling
There are two types of thread milling
- Form milling
- Thrilling (threading + drilling)
In form milling, a single form thread cutter or a multiple form thread cutter are used.
In single form cutter, the cutting edges of the tool lie in the single plane.
In thread milling operation by using a single form cutter, the workpiece is rotated slowly and then it is moved precisely along the axis of workpiece.
Because of this, the threads are cut on the entire length of the workpiece.
This operation can be done in a single pass or a multiple pass.
If the full depth of cut is provided, then the thread milling can be done in a single pass. But if the depth of cut is not full, then the multiple passes are required to cut the threads on the workpiece.
Single form cutter is generally used for milling larger threads.
Larger and long diameter screws (for example, lead screw) can be cut by using a disc type form cutters.
Multiple form cutters have multiple rows of cutting teeth (see above image).
In thread milling by using a multiple form cutter, the spindle of workpiece and cutter are set parallel to each other.
Generally the thread cutting by multiple form cutter is used for cutting threads on a workpiece such as boss of a valve body or a hole in a metal block.
Thrilling is a word derived from two processes i.e threading and drilling.
So in thrilling operation, the threading as well as drilling are performed with a single cutting tool.
This operation is generally performed on a CNC mill by using a specialised cutting tool.
In this cutting tool, the shape of the tool tip is like a drill and the body has a thread shaped form with a counter sink cutter from the shank portion.
In this operation, the drilling is done first and then the threading operation is performed.
By this method the drilling, chamfering and threading operations are performed in a single cycle.
The disadvantage of this method is that it is limited to hole depth upto three times the diameter of the tool.