Orbital drilling for delamination-free holes
A series of holes created using the programmable orbital drilling unit. |
Sandwich constructions for space applications are typically made up of thin layers of carbon-fiber composite skins separated by an aluminum or carbon honeycomb core. A common and significant problem occurs when drilling such structures using conventional methods: delamination on the exit side of the composite layers.
In a study by Saab AB and Novator AB to determine the feasibility of a portable drilling production tool aimed specifically at space applications, orbital drilling was examined as a possible solution to delamination and other frequently encountered problems with conventional drilling. Both automated (CNC) and semi-automatic (portable) orbital drilling methods were investigated. It was concluded that this technique effectively eliminates delaminations and other types of damage in composite skins, such as chip out and fraying. This type of damage severely reduces strength and fatigue life.
Conventional drilling is based on rotating the cutting tool around its own axis while feeding the tool through the material. As the tool advances through the material a great amount of pressure builds up locally at the tool center. A high thrust force causes delaminations. It also causes extensive heat build-up and a rapid tool wear-out.
Hole no 3., magnified five times, shows no delamination. |
In conventional drilling the precision of the hole is dependent on the precision of the tool, and a specific hole size requires a specific tool size. Moreover, the number of times a conventional drilling tool can be reground is limited.
Orbital drilling, on the other hand is based on machining the material both axially and radially by rotating the cutting tool about its own axis as well as eccentrically about a principal axis while feeding the tool through the material. Saab and Novatel found that orbital drilling offers a number of advantages over conventional drilling. Among other things, orbital drilling:
- Eliminates a stationary tool center, thus substantially reducing the axial force compared with conventional drilling. This reduces the risk for delamination in composite laminates substantially and the risk of burrs in metal drilling.
- Allows the tool diameter to be smaller than the hole, providing for efficient chip and heat extraction.
- Keeps the cutting edge only partially and intermittently in contact with the hole surface, allowing for efficient cooling of the tool.
- Allows for a tool of one diameter to be used to machine holes of different diameters. Moreover, the tool can be reground many more times compared to a conventional drilling tool.
- Prevents the cutting tool from drifting when entering a curved, inclined, or irregular surface.
- Allows for efficient repair of misaligned or damaged holes by opening them up.
The test specimen for the programmable orbital drilling unit was a sandwich construction consisting of a 0.3-mm-thick carbon-fiber composite laminate. The core material was of aluminum honeycomb 20 mm thick. For the prototype portable orbital drilling unit, a carbon-fiber reinforced composite material 4.24 mm thick was used.
The prototype portable orbital drilling unit used in delamination tests. |
Of the 25 holes drilled with the progammable orbital drilling devices, only one experienced a delamination--and that was attributed by Saab and Novator researchers to a defective material, not the drilling proccess. No difference between the entrance and exit hole diameters in the composite skins could be detected by a micrometer. Ovality of the holes ranged from 20-40 uum.
No delaminations were detected in the six holes drilled with the portable unit.
