Multiaxis Machining: PKMs and Traditional Machining Centers

Portable robotic machine tools potentially allow feature machining processes to be brought to large parts in various industries, creating an opportunity for capital expenditure and operating cost reduction. However, robots lack the machining capability of conventional equipment, which ultimately results in dimensional errors in parts. This work showcases a low-cost hexapod-based robotic machine tool and presents experimental research conducted to investigate how the widely researched robotic machining challenges, e.g. structural dynamics and kinematics, translate to achievable tolerance ranges in real-world production to highlight currently feasible applications and provide a context for considering technology improvements. Machining trials assess the total dimensional errors in the final part over multiple geometries. A key finding is error variation which is in the sub-millimetre range, although, in some cases, upper tolerance limits < 100 μm are achieved. Practical challenges are also noted. Most significantly, it is demonstrated that dimensional machining error is mainly systematic in nature and therefore that the total error can be dramatically reduced with in situ measurement and compensation. Potential is therefore found to achieve a flexible, highperformance robotic machining capability despite complex and diverse underlying scientific challenges. Overall, the work presented highlights achievable tolerances in lowcost robotic machining and opportunities for improvement, also providing a practical benchmark useful for process selection.

Section: Robotic Machine Toolmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Achievable tolerances in robotic feature machining operations using a low-cost hexapod

Barnfather

Goodfellow

Abram

2017

“…They shared the results from users and manufacturers of such machine tools and suggested ways to improve the use of PKM in machining. Later, Fassi and Wiens [8] reviewed the use of PKM-type machine tools in machining industry. In this review paper, they mainly pointed out that the unfavourable transmission of motion hinders both positional accuracy and dynamic stiffness.…”

Section: Introductionmentioning

confidence: 99%

Investigation of the effects of Stewart platform-type industrial robot on stability of robotic milling

Tunç

Shaw

2016

Mobile machining with industrial robots is proposed as a cost-effective and portable manufacturing alternative to large scale CNC machine tools in large-scale part manufacturing. Robotic milling, one of the widely used mobile machining approaches, involves several technical challenges and distinct characteristics in terms of machining dynamics and stability due to completely different structural build up. In this paper, distinctive effects of Stewart platform-type of hexapod robot on stability of robotic milling is investigated based on characterisation of its structural dynamics, simulation of stability limits and experimental validation. Three aspects are demonstrated: (1) the positiondependent stability diagrams due to the position-dependent dynamics of the hexapod platform, (2) the effects of cross transfer function due to the complex kinematic chain on milling stability and (3) the role of feed rate direction in stability of robotic milling. The conditions for minimised positiondependent stability through appropriate tooling are also illustrated through simulations and experimental verification. The cases where process stability may be governed by either the hexapod robot or the cutting tool modes are discussed and identified through stability analysis. It is shown that the feed rate direction becomes a significant parameter for stability limits in robotic milling. The conditions at which the cross transfer function becomes significant on milling stability are discussed through simulations and experimental results. It is shown that cross transfer functions may significantly affect milling stability especially when the radial depth of cut is less than 50 % of the tool diameter. As one of the important outcomes of this research, it is found that appropriate tooling may decrease the reliance of milling stability on robot position.

“…The hexapod robot format is of interest for machining applications because of stiffness benefits and noncumulative error stack up in independent links and joints, which provides improved chatter resistance and accuracy compared to serial-arm alternatives [1][2][3][4][5][6]. A limitation to robotic machining using hexapod industrial robots is that they are less able to achieve the higher machining tolerances of conventional machine tools due to relative differences in structural rigidity and tool deflection and challenges associated with control algorithms, encoder capability and component misalignments [7][8][9][10][11][12][13][14].…”

Section: Introductionmentioning

confidence: 99%

Positional capability of a hexapod robot for machining applications

Barnfather

Goodfellow

Abram

2016

In large volume manufacturing, moving heavy components around manufacturing facilities to machine features on them can represent a significant proportion of the parts final cost because the tools used for this require a high initial investment and operating costs. This motivates interest in robotic machining so that the "process-to-part" concept can be employed. However, typical industrial robots lack the positional capability of conventional equipment, which ultimately results in dimensional errors in the features machined. This research investigates accumulation of error originating from non-cutting stages of robotic machining programs, using a hexapod robot. This is done using a procedure adapted from ISO 9283-Manipulating Industrial Robots-Performance Criteria and Related Test Methods to determine positional accuracy and repeatability, i.e. systematic and random errors. This concludes that, although the robot encounters high levels of error prior to cutting, a portion of these may be offset with in-situ condition monitoring to facilitate higher tolerance machining. Potential is therefore found for using robot machining for manufacturing cost reduction in the large-scale manufacturing industries.