Designing and implementing a monitoring system for a portable drilling system to enable adaptive drilling

Lead Research Organisation: University of Manchester
Department Name: Mechanical Aerospace and Civil Eng

Abstract

Airbus are currently in the process of upgrading some of their equipment used for assembly machining, across various sites. One of these changes is that the new generation of portable drilling systems will be electrically powered, in contrast to pneumatically powered as in the case of the current version. These electrically powered systems will use DC motors to drive the cutting tool (for both cutting rotation and feed motion). In conjunction with this, Airbus are keen to take the opportunity to introduce what they refer to as "adaptive drilling". Adaptive drilling in this context means that instead of using pre-set process parameters for the drilling cycle, e.g. spindle speed and feed rate, fixed on/off settings for coolant based on pre-set timer intervals, to name just a few, the new system would be equipped with a process monitoring system. This process monitoring system would automatically make adjustments to the drilling process in response to information extracted from signals obtained from a number of sensors integrated into the drilling device. As for example, depending on what material the drill is cutting, both spindle speed and feed rate can be adjusted. Or, upon noticing that the drill is about to break through the back of the material or enters a layer that is supposed to be drilled without any cutting fluid, the cutting fluid supply is stopped, thereby avoiding the system to spray cutting fluid out the back of the workpiece or contaminating certain stack layers.
This provides a number of major advantages over the currently employed non-adaptive drilling approach: Firstly, process parameters can be used that are optimal for the individual material layers the drill has to penetrate through, rather than using one fixed set of speed and feed for the whole stack. The system would automatically switch to a different set of cutting speed and feed rate when exiting one layer and entering the next. This will not only have huge implications on the quality of the holes produced (using optimal cutting parameters will result in better surface finish; less burr formation and delamination, thereby reducing the amount of re-work necessary), but will also be beneficial to the overall machining costs as well as the work environment (reduced use/wastage of cutting fluids; reduction of cutting fluid sprayed into the work environment). Secondly, the adaptive system would make the time-consuming re-programming of the drilling device obsolete. At the moment, every time a change to the drilling application is made, as for example a change in material stack configuration (e.g. layer thickness, layer arrangement), the system has to be taken out of operation and its timer re-programmed, all of which adds huge costs to the assembly machining process. The new system would have the capability to change and adjust parameters in-situ, by using information obtained from the signals recorded during the actual drilling operation by a number of built-in sensors. Thirdly, with the monitoring system being upgraded to a tool and process condition monitoring system, which will require additional research and is the final stage of this PhD project, the system will be able to not only monitor the condition of the cutting tool (i.e. determine whether the tool has reached a wear stage at which it will fail to produce holes that meet the quality requirements), but also to detect whether during the drilling process certain defects have occurred, e.g. delamination of composite layers. This information can then be used to, as for example, either adjust the cutting parameters (so that a lower feed might induce less damage) or inform the operator to change the tool (as it might have reached its end of life).

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509565/1 01/10/2016 30/09/2021
1960952 Studentship EP/N509565/1 12/09/2017 30/09/2021 Andrea Pardo
 
Description This research project allowed for a fundamental understanding of the drilling of multi-material aerospace stacks in view of, firstly, the effect of different parameters on the cutting process and, secondly, the relationships between process signals and process incidences and how they are affected by tool wear.

In the first part of the research, the impact of the interlayer gap width, tool point angle and parameter changeover position on the resulting borehole quality was investigated. Adapting the cutting parameters and cooling strategy based on the material being machined was found to result in a stable cutting process and generate boreholes exhibiting small interface damage. Introducing an interlayer gap to a stack comprising aluminium layers resulted in an increase in interlayer burr formation, as opposed to drilling a stack with no interlayer gap. However, the presence of an interlayer gap can ultimately be detrimental to the final burr size, due to the sliding action of the upwards-travelling chips over the borehole edges. When drilling stacks comprising a composite layer above a titanium one, the damage on the composite interlayer surface was caused by the drilling of the metal layer below, as a result of the upwards-travelling metal chips and heat accumulation in the tool and stack interface. The wider the interlayer gap, the easier it becomes for metal chips to penetrate the interface and damage the composite surface. Tools with a point angle of 118 and 180 degrees resulted in significant thermal damage in the CFRP layer, which is attributed to the temperature being generated when drilling the titanium layer as well as the restricted flow of chips through the drill's chip flutes. Tools with point angles of 140 and 150 degrees were did not lead to such large thermal damage, which can be explained by the noticeably smoother chip evacuation. The changeover position was found to have a noticeable impact on the burr formation, extent of delamination and borehole diameter. By delaying the changeover position, the burr height increased whereas the extent of delamination was reduced.

The second part of the research dealt with establishing relationships between signals recorded from different sensors and process incidences, using both a machining centre and examples of the latest generation of aerospace portable drilling units. Cutting forces, motor current, acoustic emission and acceleration were found to be suitable process signals for the detection of tool engagement, material transition and tool disengagement, with the thrust force being the most responsive signal to the occurrence of these incidences. Based on the knowledge gained, a decision-making strategy was designed, and its performance was tested and compared to the strategies used in two drilling units available on the market. The proposed strategy, based on gradient monitoring, yields substantial improvements in terms of both reliability and responsiveness when compared to the magnitude-based approaches used by the systems available on the market.
Exploitation Route The obtained knowledge and understanding will aid both the manufacturing industry and academia in the following areas:
-General understanding of the stack drilling process and process signals related to it.
-Understanding of the interlayer burr formation process and its relationship to interlayer gap width.
-Impact of tool point angle, parameter changeover position and interlayer gap width on interface quality in drilling of CFRP/Ti stacks
-Impact of tool wear on process signals in stack drilling
-Development of decision-making algorithms for adaptive drilling.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology

 
Description The knowledge and understanding gained from the research has made the project partner (Airbus UK) reconsider key parameters that define part of their assembly machining operation, e.g. cutting speed, feed rate, tool geometry, stack clamping force and parameter changeover position. The knowledge also aids the design and development of a more advanced algorithm to be employed on adaptive drilling units taking into account the impact of tool wear on the signals used for decision making.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Description 13th CIRP Conference on intelligent computation in manufacturing engineering 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Presentation at international conference on 'Process signals characterisation to enable adaptive drilling of aerospace stacks'.
Year(s) Of Engagement Activity 2019
URL http://www.icme.unina.it/ICME%2019/ICME_14.htm
 
Description 14th CIRP Conference on intelligent computation in manufacturing engineering 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Presentation at international conference on 'Assessment of decision-making algorithms for adaptive drilling of aerospace stacks'.
Year(s) Of Engagement Activity 2020
URL http://www.icme.unina.it/ICME%2020/ICME_20.htm
 
Description Project presentation to industrial partner (Airbus) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Annual project presentation and review delivered to representatives of industrial project collaborator (Airbus).

Points of discussion:
- Progress to date
- Review and discussion of findings
- Discussion on work to be carried out for following year
Year(s) Of Engagement Activity 2017,2018,2019,2020,2021