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Vol. 21
July Issue
Year 2020
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Good Vibrations


in Vol. 21 - July Issue - Year 2020
Fully Automatic Surface Finishing of Complex Parts with the "Surf Finishing" Technology



Picture 1: Turbine blade treated in a Surf-Finisher


Picture 2: “Multi”-Surf-Finisher equipped with 4 spindles





Picture 3: Raw workpiece


Picture 4: After 4 minutes drag finishing


Picture 5: After 1-minute surf finishing


Picture 6: Robotic handling of the work pieces allows targeted finishing of precisely defined surface areas


Camshafts (picture 7) Surface smoothing from Rz = 3 µ down to 1.3 µ


Steering column bush (picture 9) Light deburring


LP turbine blades (picture 11) Deburring of the blade root and shroud section


Pliers (picture 8) Surface smoothing from Rz = 3 µ down to 2 µ


Knee femorals (picture 10) Surface smoothing, high gloss polishing


Worm gear (picture 12) Surface smoothing down to Ra = 0.5 µ


Picture 13

Nearly 90 years ago, with the introduction of the first finishing vibrator, mechanical mass finishing was officially “born”. Since then, this technology has continuously been adapted to the growing demands for improved surface finishes. The first vibrators were mainly used for replacing manual deburring by mechanical means. Centrifugal disk finishing, developed in the early 1960’s, allowed tackling more demanding finishing tasks. For example, coins most of us carry in their wallet have likely been polished in a centrifugal disk finisher. The introduction of drag finishers in the 1980’s opened entirely new possibilities for finishing complex, high-value components like orthopedic implants – from cut-down and surface smoothing to high gloss polishing. “Surf Finishing”, has elevated mass finishing to an entirely new level: It offers a high degree of processing flexibility, extremely short processing times and a degree of automation so far considered impossible.

"Surfing" for surface finishes never achieved before

At the center of surf finishing is a rotating work bowl filled with grinding or polishing media. The work pieces, attached to special spindles or a robot, are immersed into the moving media bed. The work bowl rotation of up to 170 RPM creates linear speeds of more than 5 m/sec! The resulting high pressure exerted by the media on the stationary workpieces results in excellent finishing results within unheard-of short cycle times (Picture 1).
Surf-Finishers are excellent for aggressive deburring, edge radiusing and cut-down, but with a wide range of possible machine settings and the right finishing media, they are equally effective for surface smoothing and polishing. But as we will learn later, Surf-Finishers can do a lot more: They allow the targeted finishing of specific surface areas as well as difficult-to-reach internal passages on the workpieces.

A clever equipment concept offers abundant processing possibilities

Surf-Finishers are available in different versions. The "Multi"-Surf-Finisher is equipped with a carousel with 4 adjustable rotating spindles onto which individual workpieces are mounted. By lowering the carousel, the workpieces mounted to the spindles are immersed into the rotating work bowl filled with media.
Instead of the carousel with attached spindles, Surf-Finishers can also be supplied with 6-axis robots outfitted with special workpiece grippers. In this case, the robots are immersing the workpieces into the moving media while simultaneously rotating and/or holding them at different angles (see more details below).

Technical features of the 4-spindle "Multi"-Surf-Finisher (picture 2) Carousel

To provide the greatest possible processing flexibility and enhance the finishing effect, the carousel offers a rotational as well as a vertical dimension (up/down movement). The latter allows different immersion depths resulting in different pressures between the workpieces and the media. 
A special feature of the "Multi"-Surf-Finishers is the rapid up/down movement of the carousel at variable speeds. The rapid pressure change between workpieces and media increases the finishing speed, but also creates extremely homogeneous surface finishes. This variable speed also helps to minimize cycle times: for example, the carousel can move downwards very fast and then slow down, before the workpieces are immersed in the media.

Spindles

The spindles offer a wealth of different settings, where each is equipped with its own rotational drive and angle adjustment. A frequency inverter allows spindle speeds between 0 and 90 RPM, whereas the angle can be altered between vertical position and 25°. Both – rotational speed and angle adjustment – are extremely important to adapt the machine settings to the workpieces and the respective finishing task!
The spindles also permit a fast oscillating (left/right) rotation between 0 and 360° at speeds of up to 90 RPM: Because of the comparative inertia of the media in the rotating work bowl, these rapid spindle speed changes create a highly homogeneous relative movement between the workpieces and the media. This speeds up the process and improves the finishing qualities. The same oscillating (left/right) movement is also possible with the carousel.
Since each spindle has its own drive and angle adjustment, they can be operated synchronously with identical settings or entirely independent from each other. This provides tremendous operational flexibility in the manufacturing process.

"Multi"-Surf-Finishers help reduce finishing costs and improve the finishing quality

"Multi" Surf-Finishers offer significant advantages over other finishing systems:

The high pressure between media and workpieces produces higher process intensities and, therefore, shorter finishing times. Compared to vibratory systems. Surf-Finishers generate a 50-fold higher intensity. For a comparison of the various finishing systems, please refer to the chart below: 

Remarkable is that surf finishing is at least 2.5-fold more intensive than drag finishing. This is impressively demonstrated by the following example (pictures 3 - 5 ):

The adjustable spindle settings (speed, angle adjustment and left/right oscillation) relative to the media movement in the rotating work bowl allow the targeted finishing of internal workpiece passages. 
For example, after machining, grey iron castings (housings for exhaust gas control) have heavy burs in difficult-to-reach work piece areas. These burs must be completely removed, and all internal and external surfaces must be cleaned and completely free of oil. This difficult job is completed in a "Multi"-Surf-Finisher in less than 5 minutes .
The high processing intensity of Surf-Finishers allows the use of smaller finishing media, resulting in finer surface finishes compared to other mass-finishing systems.
Individual spindle settings (RPM, spindle angle, reciprocating rotational movement) permit the processing of different workpieces with different process parameters in one single work piece batch. This provides valuable flexibility to the manufacturing process and keeps work-in-process inventories low.

A special Surf-Finisher: Robotic handling of the workpieces in the work bowl

For special applications requiring a higher degree of workpiece movement in the media mass than possible in spindle units, the Surf-Finishers can be supplied with one or multiple 6-axis robots equipped with special workpiece grippers. The robots guide the workpieces through the media at pre-programmed tilting and/or rotary movements, thus providing a higher or lower exposure of specific surface areas to the media flow. This allows the targeted finishing of precisely defined areas on the workpieces (picture 6).

A wide variety of finishing possibilities

Surf-Finishers can be utilized for practically any finishing task, be it aggressive deburring & edge radiusing, intensive surface smoothing or high gloss polishing. Here are some workpieces that have been successfully finished in a Surf-Finisher:

Automation 

Surf-Finishers were specifically designed with automation in mind:
The carousel of the "Multi"-Surf-Finisher allows an extremely precise positioning of the spindles so that a robot can easily attach and remove the workpieces mounted to special fixtures to and from the spindles.
For this purpose the spindles are equipped with quick connect couplings with automatic clamping and release mechanism.
All work piece transport & transfer systems are designed to allow easy handling of the work pieces by robot
A bar code reader permits fully automatic calling up of processing programs from the PLC for workpieces arriving at the Surf-Finisher
Picture 13 shows a fully automated, recently implemented "Multi"-Surf-Finisher system that is at the heart of a complete production line:




Author: Eugen Holzknecht
Good Vibrations
by Eugen Holzknecht, Contributing Editor MFN and 
Rösler Oberflächentechnik GmbH
E-mail: holzknecht.usa@gmail.com