More accurate countersinking and rivet shaving
 Temperature is measured on a non-rotating spindle body rather than the tool holder, allowing for thermal growth compensation. Click to enlarge |
Wing skin riveting and bolting requires the surface to be flush to +/-0.025 mm to produce an acceptable finish. Airbus and Electroimpact have employed a new method enabling automated wing riveting technology and panel assembly techniques to achieve better shave height and countersink accuracies than have previously been possible in production.
Wing panels are assembled by clamping the wing skin to the stringers or ribs. A countersunk hole is then drilled and a rivet or bolt installed. In riveting applications, the rivet is subsequently shaved flush with the skin. In many instances the automated fastening equipment is set to leave shaves high, rather than risk deep shaves; a manual operator removes the rest of the material. In bolting applications, bolt-head height variation is often larger than desired.
The process for achieving high tolerance countersink and shave heights involves taking advantage of the parallel axis clamp-drill arrangement, which has become the standard on C-frame fastening equipment. This process can be used on any parallel axis clamp-drill arrangement. This includes post mills, C-frames, and, most importantly, robotic drilling/fastening equipment. Several five-axis machine tools are using this process to meet stringent production tolerances. This process allows lightweight machines, which may deflect while clamping, to achieve the required tolerances. An end effector mounted on a Kuka robot uses this process and achieves extremely tight (+/- 0.018 mm) countersink tolerances.
In the typical arrangement, cutting operations are performed by servo-controlled spindles, which stroke normal to the skin and cut as they feed toward the skin. The pressure foot is pressed up against the skin. The spindles stroke relative to the pressure foot and toward the skin. Typical rpm is 6000-20,000 and the typical feed rate of the spindles when cutting is 0.1-0.25 mm per revolution. Typical of most clamp-drilling machines used in aircraft manufacture, this method requires that the part be stabilized by clamping and that the pressure foot face and skin share the same plane. The apex of the stroke of the spindle determines the depth of cut. Using linear scales on the spindle feed axis to close the position feedback loop increases accuracy. Along with temperature compensation and the above assumption that the pressure foot face and skin share the same plane, theoretically all drilling equipment should be able to achieve this +/-0.013 mm countersink and shave tolerance. In practice, however, much larger variations are experienced during the manufacture of wing panels.
Thermal growth of the spindle is a major source of inaccuracy. For practical reasons, temperature is measured on the non-rotating spindle body rather than the tool holder. This enables compensation for most of the thermal growth. The tool holder temperature did not follow the temperature of the spindle body precisely. The tool holder is cooled while spinning, and when stopped it grows as the spindle shaft transfers heat into it. While the tool holder is warming up and growing, the spindle body (where temperature is being measured) is actually cooling. In practice, the nonlinear aspect of this temperature variation causes a 0.038-mm spread across the median temperature compensation curve. It is undesirable to leave the spindle running because of the long time to reach steady state after changing tools and the danger if an operator must work around a spinning tool.
On the A340-600 wing panels, stacks vary from 6.35-25 mm and larger. Because of the stiffness of the panels, the part will not conform to the pressure foot surface if there are normality errors. Normality of the tool is usually driven by sensors. In some areas, where normality sensors cannot be used, assemblers rely on the programmed angles of the machine tool. A normality error of 20 ft with a pressure foot 25 mm in diameter produces an error at the center of 0.073 mm, exceeding the desired tolerance.
Chips or contamination between the clamp pad and the panel will place the panel further from the drill apex, resulting in high fasteners.
 The rivet shaving process employed at Airbus. Click to enlarge |
The pressure foot will often press on the panel with a force exceeding 9000 N to eliminate gaps between the pieces being fastened. Variations in this force will cause the clamp pad to deflect differently. This changes the relationship between the spindle and the clamp pad, causing errors in fastener height.
A method of measuring these variations and compensating for them has been developed. A touch probe is used to measure the location of the panel after clamp-up. The best probe typically is the drill bit, for the following reasons: it is accurately positioned by a linear scale, it is the first tool used in any fastening process, and the error from the tool holder temperature change will be measured at the same time. The measured panel position is then compared to a stored position that is found during setup and calibration.
The cycle works as follows:
- As the clamp table comes forward drive the drill bit out proud of the pressure foot plane.
- Before contact is made with the panel, reduce torque to nearly zero on the spindle feed axis.
- Clamp as normal. The panel pushes the drill back.
- Measure the drill position and subtract from it the known position of the pressure foot plane (call this deltaP).
- Subtract the temperature com (deltaT) from deltaP to get the change in length due to the above variations (deltaL). deltaL=deltaP-deltaT.
- To the known position of the pressure foot plane, add deltaT and deltaL to achieve the correct apex of the drill spindle.
- Back up the drill and start spindle.
- Return drill feed torque to full.
- Drill the hole as normal using the apex calculated with the measured errors above to achieve the correct countersink depth.
For bolting applications, the bolt is then driven into the hole, and countersink depth sets the head height.
For riveting applications, the rivet is shaved flush after the rivet is formed. This is done with a separate spindle that also must be compensated. The shaving bit cannot measure the panel position because the rivet is now in the panel. The panel location found by the drill can be used to calculate the shave depth. The only difficulty is that the two spindles will typically run at different temperatures, so using just the position measured by the drill would cause an error equal to the difference in thermal growth of the spindles. To compensate for the thermal growth, the temperature compensation for the shave spindle is added to deltaL, which already has the drill temperature compensation subtracted (as shown in step 5) to obtain the correct position. Both spindles have the same duty cycle, so any nonlinearities in the growth of the tool holder will be similar. The two spindles have unique temperature compensations this way and can still take advantage of the measured panel position. Using this method on the A340-600 panel assembly line, a total shave height variation of 0.025 mm was achieved.
Information was submitted by Todd Rudberg and Scott Smith of Electroimpact, Inc., and Andy Smith of Airbus UK, Ltd.
