Methods And Skills For Milling Large Diameter Threaded Holes

Why Use A Thread Mill To Mill Large Diameter Threaded Holes?

When a hole needs to be threaded, traditional tapping, cold forming tapping or thread milling can basically be done.

Thread milling is the first choice if it is a large hole, i.e. a hole with a diameter of 1″ to 1.5″ or more, or a tapered tube.

Part Of the reason is:

In larger threading applications, the total cost of applying a tap can be higher than thread milling. For example, larger taps may cost $1,000 or more.

If a special tap is required, the cost of the tap will be greater. You may have larger diameter tapped holes, but there is no standard tap size.

Although the initial cost of threaded hole milling is higher, it enables higher productivity and makes machining more economical.

For holes 1 inch in diameter or larger, multi-flute thread milling, which produces all the threads at once, is the fastest method.

Tapping And Thread Milling

Tapping is a common method of producing internal threads, but that doesn’t necessarily mean it’s the best method.

Because thread milling uses helical interpolation to cut threads on a CNC machine with three simultaneous axes, the tool is smaller than the diameter of the hole being machined. So if the thread mill breaks while threading, it doesn’t get stuck in the part and can be easily removed. This will definitely reduce rework costs! If it’s a broken tap, you have to burn the tap, and a lot of times, you have a lot of rework, or scrap the part.

Instead of engaging all sides of the hole like a tap, you can machine more freely when thread milling. This reduces the horsepower requirements of the machine tool while increasing productivity.

This is a freer cutting tool and therefore can operate at higher surface areas.

Another advantage of thread milling is that standard tools can produce non-standard pitch diameters to achieve the desired thread size prior to operations such as heat treatment or electroplating. This is achieved by understanding how the workpiece material reacts when processed, such as contracting or expanding by a specific amount, and compensating for this movement when cutting threads. For taps, it requires a special fabrication with a non-standard pitch diameter.

Thread milling has become increasingly common due to its advantages in appropriate applications. Because it reduces cycle time and increases tool life. It also provides cleaner, stronger threads than tapping.

These advantages are only enhanced when milling large threaded holes. For example, in these applications, there is more room for chip evacuation and sufficient coolant to effectively enter the tool/workpiece interface.

Cool Down

In most thread milling applications, coolant is very effective because there is only one point of contact between the tool and the workpiece and the chips are not trapped.

The indexable thread milling cutter adopts internal cooling, and the integral tool adopts the overflow cooling method.

Multi-edge thread milling and deep hole thread milling tools for milling larger hole diameters should have a cooling pressure of 1,000 pressure.

When machining aluminum threads with carbide tools, micro-cooling can be used.

Thread Mill Type

Thread Mill Type

Most large diameter tapped holes can be milled with indexable or replaceable inserts.

If the customer wants the flexibility to be able to machine multiple pitches with one thread mill, then a thread mill with replaceable inserts is more suitable. If it is dedicated to single-pitch machining, then solid carbide thread mills are more suitable.

For smaller indexable or replaceable inserts, single-edge, multi-segment inserts or multi-edge, single-segment inserts are available.

As the diameter of the threaded hole that needs to be milled increases, the space allows the application of multi-point indexable tools. These tools can be single-pitch or multi-pitch. Because of the smaller contact area between the tool and the workpiece, users can deliver single-pitch tools much faster than multi-pitch tools. However, a multi-pitch tool has less travel distance because it cuts multiple threads at the same time. If the total length of your part is 12 threads, you only need to do one circle and you’re good to go. A single-pitch tool, on the other hand, has to make 12 turns around the part, but it mills faster.

When using a multi-pitch tool, at some point the thickness and thread depth have too much contact with the workpiece surface, and cutting resistance can overwhelm the tool and machine.

The type of holder is also an important consideration when milling large diameter threaded holes. Similar to any milling operation, minimizing runout improves tool life and part quality, and a milling cutter chuck or hydraulic holder is recommended.

Milling chucks are characterized by shock absorption and high gripping forces, making them ideal for thread milling, which generates a lot of lateral pressure on the tool. When thread milling, retractable toolholders experience a “stroke” phenomenon, where high levels of vibration can dislodge the tool from the toolholder.

An engine manufacturer wanted to mill a 36mm thread into cast iron. The manufacturer only used shrink-fit holders, but had problems milling threads. Tried adjusting the speed and feed first, but to no avail. After changing a milling chuck, the problem was solved.

Hydraulic collets are not recommended when milling large diameter threaded holes, as they do not have the strength and grip to handle the high radial forces that occur when thread milling.

Threaded Hole Depth

Large diameter threaded holes are not necessarily deep, but thread mills can have long strokes due to the fixtures that are often required to hold large and heavy workpieces. If the depth-to-diameter ratio of the tapped hole is too large, you may need to take multiple operations.

In some cases, coarse-pitch thread milling requires one roughing and one finishing, while deep-hole threading requires one operation at one depth of the hole, followed by another operation at a deeper depth.

In thread milling, excessive side pressure can negatively affect thread quality. To compensate for this, lighter feeds and more processes are usually adopted.

The selection of a suitable thread mill depends on the desired thread shape and workpiece material.

Machining a depth of 6 times the threaded hole diameter may be beyond the capabilities of thread milling. Milling of such a long thread may cause offset problems during the machining process, and thread milling may not be a better machining method.

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