Boring is a machining process that works on holes in parts produced by other processes (turning, milling and so on), with the aim of achieving greater dimensional and geometric accuracy or reducing surface roughness. Boring tools can produce cylindrical and tapered holes, and also internal threads.
Machine tools for boring
Boring can be carried out on many machine tools, such as the lathe. To bore a part on a lathe, the part must be a small solid of revolution with a hole along its axis of symmetry. The part is held in the rotating chuck and turned, while one of the boring tools is fixed in the lathe tailstock. In this case, machining is done with a boring bar.
For parts with one or more holes, milling machines, machining centres (with rotating boring tools) and dedicated machines known as boring mills or boring machines are used.
What is a boring mill?
A boring mill (or boring machine) is used to machine holes in prismatic parts, with tight tolerances and a requirement for high-quality machining. These machine tools must offer high precision and tight tolerances, as one of their features is the ability to work on narrow surfaces. A boring mill can carry out many operations, all of them related to enlarging existing holes. These include:
- Reaming, with a multi-edge tool that produces more accurate holes, with a limited depth tolerance and a small radial depth of cut.
- Single-edge boring, for roughing, finishing and finishing with chip control; it is also used when machine power is insufficient.
- Step boring, with boring tools called inserts, set at different axial heights and diameters.
- Multi-edge boring, for roughing with feed per revolution.
As already mentioned, lathes and milling machines can easily be turned into boring machines as long as the right boring tools are used. However, if you want a dedicated machine for these operations, there are three types: vertical, horizontal and jig boring machines.
Vertical boring machines take their name from the rotation that takes place on a horizontal table; on horizontal boring mills, the workpiece remains stationary while the spindle rotates on a horizontal axis. Finally, jig boring machines produce holes in parts that require extremely high precision, such as dies.
Which boring tools should you choose?
The choice of boring tools depends on the machine used for the operation. A boring mill, for example, uses a micrometric boring head (which is not the same as a lathe chuck). This micrometric head is designed specifically for this work on this type of machine tool and is also known as a boring spindle or boring head.
The steel boring head is mounted on a boring tool holder, and its rigidity limits the cutting conditions that can be used. During boring, excessive vibration must also be avoided, as it can compromise the accuracy and quality of the holes. So how do you choose the right tools for boring? What should you take into account?
Features of boring tools
Whatever machine tool you use for boring, there are certain points to bear in mind. Look for tools that speed up the process and also make it easier. In the case of boring mills, it is important to choose a machine that can carry out roughing of the parts, has suitable clamping systems and a low tendency to vibrate. The tool must be made of durable materials – cast iron, forged steel, stainless steel and so on – as a boring tool has to withstand high speeds and high temperatures and be hard enough to machine very hard parts.
Beyond these features, there are other very specific aspects to consider when choosing the most suitable tool. It is advisable to select the tool with the largest bar diameter that can evacuate swarf effectively. It is also important to choose boring tools with high toughness and wear resistance.
On boring mills and machining centres, choosing the right tools is very important; this is because the production cost of this type of work is relatively high, so machining must be done in the shortest possible time while always aiming for precision and quality. For optimum boring, consider the part design, the material and its limitations, that is, whether the boring is internal or external, or whether it involves grooving, roughing or finishing. You should also consider the stability and machining conditions and, of course, the type of machine used and its power.