in Vol. 5 - September Issue - Year 2004
Vibratory Finishing Process for stainless Steel Spectacles
Author: Dirk Gather, Plant Manager
The Gleitschliffzentrum Oranienburg (GZO) and the manufacturer ic!-berlin gmbh realized the conclusion of a long-term cooperation contract to optimize the development and process assurance for the surface finishing of spectacle frames.
Out of several years of cooperation in the development of usable technologies for the surface treatment of spectacle frames manifold synergetic effects could be gained. Technical requirements and designers´ wishes could be connected in a very impressive way. Involved are researchers of metal materials, specialists for cutting and metal-forming, technicians for surface treatment as well as coating companies and designers.
As a surface finishing service-center, the GZO offers a broad spectrum of technologies. Due to the main competence, vibratory finishing and shot blasting, the company has wide appeal to nearly all branches of the metal working industry. The automotive industry, manufacturer of surgical instruments, glasses and electronic parts and the aviation industry are just some fields of the companies´ activities.
ic! berlin gmbh, with design, production, sales and head office at Berlin, is a manufacturer of high-value and trendy sunglasses and optical spectacles.
Founded in 2000, the young company has meanwhile stretched their sales activities all over the world.
The speciality of the ic! glasses is a patented innovative hinge system, that is so unique, that everybody wants to copy it. Furthermore the glasses are very light (total weight just 13-18 grams), and very attractive thanks to the extraordinary optic of the metallic surfaces.
The metal components of the spectacle frames will be cut out by laser or etching technology and afterwards mechanically formed.
The character of these processing steps leads to the emergence of very sharp edges. These edges are unwelcome, because the spectacles may be touched several dozen times per day by the wearer. Just to break the edge by a mechanical process (as required for functional parts) is not enough in that case – the edges have to be rounded to receive a touchable part. Referring to the process it has to pay special attention to the sensitive area of the ear piece connection. To keep the function in order, it is important not to remove too much material.
A further requirement treating the spectacle frame is to establish a very smooth and decorative surface, to be finished in a polished or a satin blasted appearance.
Some of the spectacle components will be delivered in this look, another part will be coloured afterwards by a decorative galvanic plating process. The colours of the actual collections are mainly black and grey.
After finding a definition for the requirements to the surface finishing process as seen above, a usable technology for serial production had to be found. In 2000, when the cooperation between the companies started, the GZO was able to develop an economical and very effective process to satisfy the high quality requirements of ic!
Some different mechanical and (electro-) chemical technologies have been tested and examined. Electrolytic polishing, Dynaflow – grinding and polishing technology, brushing and similar proceedings did not deliver satisfying results. After an extensive series of tests and analyse at the research department of the GZO it turned out that a multi-step process combining several vibratory finishing and shot blasting steps is the best way to achieve good results.
All the quality and economic requirements and aspects could be fully satisfied applying that process combination.
Going into Detail
The first step: The sharp edges will be rounded using a rotary vibrator system. A charge of several dozen spectacle frames (quantity dependent on machine volume) will be treated in a mixture of ceramic grinding media adding a fine-grinding paste to support the process.
In that step the edges will be rounded to 0,2 – 0,25 mm (required by drawing) and the surface will be smoothed out to Ra 0,16 µm.
The second step: The surface will be polished using a rotary vibrator system. Again a charge of several dozen spectacle frames will be treated in a special porcelain media adding a polishing paste. In that step the roughness of the surface will be reduced to Ra 0,05 µm.
The decisive parameters for these steps are:
Amplitude (vibration/oscillation) of the rotary vibrator
Speed (rpm) of the motor
Grinding capacity, hardness and geometry of the grinding and polishing media
Condition and structure of the polishing media
Condition and quantity of the paste
The difficulty is, to treat an acceptable quantity of parts without damages like scratches, buckling or breaking. Naturally it comes to touches between the parts during a vibratory finishing process. Unfavourable geometries of the parts may lead them to get caught and damaged that way. Spectacle frames normally are predestined for that!
The problem could be solved by using a special configuration of the rotary vibrator type Rösler R 420 E in combination with a frequency converter to regulate the speed which is infinitely variable.
Due to that system the spectacles could be moved to spread evenly among the grinding / polishing media. The touches of the parts during the process can be minimized and a steady quality of treatment can be ensured.
Some types of spectacles with a really filigree geometry, run a high risk of being destroyed during the process, and have to be stabilized with a clip for protection. Sometimes it can be necessary to run the process in a smaller type of rotary vibrator.
The decisive quality characteristic of the polishing media is the best possible preparation. The media has to be “pre-rumbled” for several days to get a perfect surface condition.
During the polishing process the motor speed has to be reduced to get a soft and smooth movement of the media.
Some more changes of the machine settings are possible to have an effect on the process. Dependant on size, material and geometry of the spectacles, the best possible adjustment has to be found from case to case.
The next step of the proceeding is the cleaning / burnishing of the parts into the rotary vibrator. A continuous dosage of water and burnishing compound for approx. two hours leads finally to an absolutely clean and shiny surface.
A part of the total production quantity has to be shot blasted with special glass granules after the above described treatment. Therefore a modified edition of the injector will be used – blasting cabinet type Rösler ST 1000 PS.
The main component of the processing is the final quality control using a measuring microscope to check the rounding of the edges and furthermore a measuring instrument to check the roughness of the surface (µm)
A place of work has been installed at the GZO exclusive for the visual control and the packing of the spectacle components. That way the quote of rejects could be noticeably reduced.
The close and constructive cooperation between the two companies and their research & development teams will lead to fast growing production capacities and optimized processing.
Thanks to the high quality standards the successful expansion of ic! spectacles can hardly be stopped.