Previous post from the series: Technological process of sleeve, part 3.

Operation 30 – drilling

Figure 1 shows a technological sketch for this technological operation – drilling through holes. According to the principles of the technical drawing, very thick lines mean surfaces machined in a given technological operation.

Fig. 1. Sleeve technological process – operation 30.

In this operation 30, only 8 through holes Ø8 are made. The holes are machined on a floor-model drill press and requires the use of a special drill chuck – developed specifically for this operation for this workpiece.

Drilling is a roughing operation in which the tool is a twist drill (monolithic, steel, cemented carbide, with insert inserts – so-called folding drills). Today, drilling is done mainly using solid carbide drills, with replaceable inserts or with soldered carbide tips (steel drills). When drilling under high-speed machining (HSM), or only with elevated machining parameters (rotational speed and feed rate), use solid carbide drills instead of high-speed steel drills. High pressure coolant provides (more efficient) chip removal from the cutting zone and extends tool life (pressure 4 to 8 MPa, capacity 10 to 25 l / min). Drills have internal coolant supply channels which requires a proper machine tool spindle construction.

The following types of drilling are distinguished:

• general,
• gradual / with chamfer – tools specially adapted to this type of machining,
• other methods: radial adjustment, boring, helical interpolation, plunge drilling, milling, deep hole drilling.

Qualitative parameters

Classic literature references state that when using a twist drill, holes are obtained in 12th and 13th ISO accuracy classes with roughness of Ra = 20. The specified configuration of MtWTS (Machine Tools Workpiece Tool System) and machining parameters will be provided by Ra = 5. When making shallow holes with a rigid tool it is possible to achieve accuracy classes 10 and 11. Modern tools are an example of technological progress of a qualitative nature, which allows achieving holes in better quality classes (IT8 and IT9 – along with high pressure coolant). The final quality parameters (geometric tolerance and surface roughness) depend on:

• tools,
• machine tools (tool clamping),
• technological instrumentation.

It is recommended to use as short drills as possible with the shortest overhang. Achieving high quality requirements for the hole (narrow diameter and straightness tolerance, low roughness with the longest possible tool life) requires minimizing runout, which should not exceed 0.02. The concentricity of the drill and spindle should be maintained.

Figure 2 shows the basic concepts of drilling.

Fig.2. Basic concepts related to drilling.

Cutting parameters

The cutting speed vc as the machining parameter depends on the properties of the workpiece (hardness). The cutting speed significantly affects the tool life, the power demand P_c [kW] of the main drive and the torque M_c [Nm]. Increasing cutting speed leads to higher temperature and higher flank wear. For soft materials, increased v_c promotes better chip formation (e.g., long chips through low carbon steels).

The working feed fn plays a role in terms of machined surface parameters (roughness), but may affect diameter tolerance and straightness. Like the cutting speed, it affects chip formation, power and moment. Increased working feed reduces the main machine time tg (standard of technological times), less wear in relation to the drilled length, but the negative side is the greater possibility of CB (so-called catastrophic blunting).

Technological instrumentation

Technological equipment plays an important role in the proper performance of drilling operations. Depending on the drill chuck design, different workpiece accuracy can be achieved. The handle must also implement the so-called down force, which plays a special role during the return movement of the drill preventing the object from rising in the event of beating the drill. Depending on whether the chuck is equipped with fixed or exchangeable drill sleeves, more or less flexibility of use is ensured. If a change in the position of the sleeve is no longer possible, it is possible to make changes within a certain range of diameters of the holes made.

Sources

1. Feld M., Podstawy projektowania procesów technologicznych typowych części maszyn, WNT 2000
2. Kapiński S., Skawiński P., Sobieszczański, Sobolewski J.Z., Projektowanie technologii maszyn, OWPW2002
3. Puff T., Technologia budowy maszyn, PWN 1985
4. Rudaś T., Horczyczak M., Morek R., Materiały dydaktyczne – ZAOiOS WIP PW
5. Machining guide, SANDVIK Coromant