Machining of Composite Materials(2), hole-making

This article discusses the hole-making of composite processing, describing the basic requirements of composite hole-making, possible issues, and the measures to improve machining results.

Hole-making, which accounts for 60-80% of composite machining, is challenging because it usually results in delamination and splintering, especially at the entrance and exit of holes. The design of the drill bit geometry is critical for composite drilling.

The characteristics and critical points of hole-making in composite material:

  • The composite material is brittle
  • The cutting force required is minimal
  • Little cooling required

Because the hardness of the composite material is very high, the wear resistance of the tool should also be very high

  • The strength of the substrate resin is relatively weak, quickly resulting in delamination between each fiber fabric of the composite.
  • Elastic difference, resulting in an irregular surface of the composite
  • During processing, when the resin fails to cement the fiber, the fiber is pulled out, resulting in splintering
  • In composite drilling, the defect holes drilled usually appear before the drill bits wear out. Therefore, whether to change the drill bits depends on the quality of the hole drilled. 

The basic requirements of composite hole-making

Drill holes on a single layer plate

    • Roughness Ra < 4.8 μm
    • Delamination range <1 mm (based on aperture)
    • No splintering
    • Cp >1.6

Drill holes in composite + metal laminates

    • Cpk>1.33
    • Ra<3.2 (Carbon fiber layer)
    • Ra<1.6 (aluminum, titanium layer)
    • Hole tolerance: +/ -20 to +/ -40 μm
    • No lamination or splintering at the hole outlet
    • The metal layer chips cannot remain on the carbon fiber layer

Qualified hole

Unqualified holes: fraying, delamination, splintering

Standard method of measuring hole size

Measuring position:

  • Diameter direction, measuring diameter every 90°sampling points
  • Measure the diameter of the hole in the axial direction (depth direction). Measure the diameter of the hole at 2mm from the outlet and inlet, and at the middle position respectively

Control of hole size

  • Pre-drilling is usually used to ensure that the hole position is correct
  • The final size of the hole is best achieved using a one-shot machining solution
  • Reamer drills should be used if high precision holes are required

About machining efficiency

Increasing the RPM and feed rate can undoubtedly increase the drilling speed, but the premise is to ensure the quality of the hole.
If the quality of the hole is qualified, it must be reworked, which will increase the machining cost and save time.
Therefore, improving the quality of the hole drilled and avoiding rework is the best way to improve machining efficiency and productivity.

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