PROCESS RELIABILITY AND REPRODUCIBILITY:ROUNDING FIR-TREE SLOTS

REPEATABILITY – WHY ROBOTIC PROCESSES ARE NOT THE BEST CHOICE

Standard engineering practice for this process step is mechanical, semi-robotic deburring and rounding with grinding wheels or brushes. These tools used wear over time, producing inconsistent machining results over their life span. For ex-ample, sharp edges can slice off brush tips, make brush wear hard to predict, and therefore lead to varying results as well as heavy process contamination due to the brush residues. Likewise, grinding wheels are subject to uneven material re-moval, which the robot movements have to compensate for, and the abrasion causes process contamination.
 
On complex workpiece geometries, rounding results are often asymmetrical because the grinding wheels or brushes cannot be fine-tuned to the exact geometry of the fir-tree slot, or because they cannot get into hard-to-reach areas at all. These asymmetrical and inconsistent rounding results often fail to meet tolerances, cause shape variation and therefore increase both your reject rate and your costs. 

COMPARING ALTERNATIVE PROCESSES

In the quest for an alternative process with convincing finishing results, an engine manufacturer worked with OTEC to evaluate two different mass finishing technologies.

The first process, in a round vibratory trough, proved inexpedient for the application: even after several hours of processing, it was unable to bring all the edge breaks within the tolerance range. The rounding produced in the tests was neither sym-metrical nor repeatable. Furthermore, the increased noise level and associated vibrations were a problem given that sensi-tive measuring devices are used close by.

The second process, in the OTEC stream finishing machine, quickly achieved very promising results at the first attempt. By positioning the component precisely, all the edge breaks were within tolerance in just 15 minutes, with highly symmetrical rounding along the entire edge. Additional runs confirmed that the process was repeatable and stable. Shape variation was less than 1 µm – so low that not a single adjustment was required in upstream broaching. Lastly, the low-noise, low-vibration process was suitable for use near sensitive measuring devices. 

These advantages have already convinced several engine manufacturers:

• Minimal process time of just 15 minutes
• Highly symmetrical, uniform rounding
• Reliable, repeatable process results
• Residue-free burr removal
• Minimal shape variation in fir-free slot pressure surfaces
• Reliable, measurable compliance with narrow tolerances of less than 1 µm
• Purely mechanical material removal, without the use of chemical process accelerants

The relative motion between the component and the media produces an even finish. The enormous machining forces of up to 9 tons used in stream finishing can remove material faster and more precisely than any other type of surface finishing process: 
depending on component size, initial state and preparation (e.g. pre-chamfering) they can produce a radius of up to 700 µm within 15 minutes. And because the media granules are so fine, the rounding process is also suitable for complex, hard-to-reach geometries. 

OTEC is the only supplier of stream finishing machines with integrated automation for large and heavy components: the newly developed SF-HP is suitable for workpieces up to 650 mm in diameter and weighing up to 200 kg. 

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