2 instead of 15 minutes: High-precision honing on CNC machining center or lathe

Honing process integrated into machining center

With RH reamhone, URMA offers a combined reaming/honing tool for ultra-fine machining. By integrating reaming and honing in one tool, RH reamhone is ideal for process-reliable bore machining in your own company.
This report shows the effort required to drill a hole 26 mm in diameter and 162 mm long into the base housing of an ophthalmic slit lamp. The production engineers at Haag-Streit, together with URMA and DIHAWAG, decided to use a specific honing tool technology to integrate the honing process into a machining center.

The Haag-Streit Group, headquartered in Köniz near Bern, is an internationally active Swiss medical technology company in the field of ophthalmology. The Group develops, produces and distributes state-of-the-art medical solutions and devices such as slit lamps, tonometers, examination units and chairs for the refractive workplace as well as biometers, perimeters, surgical microscopes and other accessories worldwide. Based on virtual reality, the Haag-Streit Group also offers solutions and simulators for medical training. Haag-Streit has been working closely with renowned scientists and doctors worldwide for over 160 years. Following the motto: “Look closer, see further”, they succeed in developing and producing innovative and advanced total solutions of the highest quality for eye specialists through precise work.

Thomas Rotzetter, Head of Cubic Manufacturing, Haag-Streit AG: “We develop and produce medical devices, with slit lamps forming a core segment. These are indispensable for basic eye examinations and we are positioning ourselves in the premium segment.” These slit lamps are based on both high-precision mechanics and exact optics, which requires the highest level of manufacturing expertise from Haag-Streit AG. Of the more than 300 employees at the headquarters in Köniz, 45 work in machining, supported by 14 trainees. A large proportion of the precision components are manufactured in-house using the turning and milling centers.

Thomas Rotzetter: “This enables us to achieve a vertical range of manufacture of around 70 percent. In general, our aim is to further develop production strategies in such a way that the components can be processed fully automatically on one machine. With this automated production, we achieve competitive manufacturing costs through unmanned production hours.” The main materials processed at Haag-Streit are aluminum, brass and chrome steel. The cross slide (cast iron) for the slit lamps, in which one of the reference axes is integrated and runs in a ball cage, is a material-related exception. This ball cage in turn is guided in the reference bore in the cross slide, the production of which is complex and previously had to be honed in an external process. The bore in the cross slide, in which the ball cage runs, has a nominal diameter of D = 26 H6, with a length of 162 mm. The batch size is around 80 components per week, which are now completely finished on a fully automated Chiron FZ 18.

The production process to date

Stefan Oetliker, AVOR programming CAM at Haag-Streit AG: “In the past, we honed the bore in an additional machining step. Pre-honing and finish honing took around 15 minutes per bore. The disadvantage was that the honing process was carried out on an external machine and was not integrated into the main process.” Bernhard von Gunten adds: “In order to optimize the production process of the cross slide, we were looking for a solution to completely machine the bore with the corresponding specifications in the existing machining center. We spoke to Kim Arnold (Senior Sales & Application Engineer, URMA AG) and Dominik Läng (Product Manager Turning Technology, Technical Sales Consultant, DIHAWAG) and discussed possible solutions. Both URMA and DIHAWAG are highly specialized in the field of bore machining and know the challenges of our components, not least because we have already optimized several reaming processes together.”

Kim Arnold from URMA adds: “We, URMA, are both a manufacturer of precision tools for bore machining (boring and reaming) and a distributor of Ecoroll and Diahon products. We have been distributing Ecoroll tools, i.e. smooth and deep rolling tools, for over 25 years. In the past, we have been able to achieve very good results with roller burnishing tools for requirements such as those placed on the bore by Haag-Streit. As roller burnishing is easy to integrate into a machining center, we naturally tested this process first, but this did not work optimally due to the material behavior of cast iron.”

Honing tools for use in machining centers

This is where a new tool technology comes into play. Since 2022, URMA has also represented DIAHON Werkzeuge GmbH & Co. KG, which specializes in the development of honing tools that are used in turning and milling centers. “We have integrated these tools into our URMA portfolio because they complement our range of expertise in terms of technological depth. They are not catalog tools, but application-specific tools that are designed for the respective production situation,” says Kim Arnold.

Bernhard von Gunten explains the procedure: “In order to outline the framework conditions of the production situation, we defined the key data of the bore, i.e. tolerance zones, surface quality, diameter, shape and position tolerances, length of the bore, material and the machine specifications. Finally, we specified the required cycle time.” The development of a tool solution for such a bore is carried out in close cooperation between the customer’s technicians and the URMA tool specialists as well as the specialists from DIAHON.

The new production process

“Honing tools that can be used in machining centers were new to us. We were not yet familiar with this process. We were all the more curious and approached the project positively,” says Stefan Oetliker. The first tool to be used is the FH fixedhone fixed mandrel honing tool, which is responsible for the fitting diameter. The second honing tool is responsible for the very high surface quality requirements. The tips above the plateau surfaces are removed with a hydraulically actuated AH actuatedhone tool, so that we currently achieve a surface finish of Ra 0.3 and better.

In this application, the honing stones of both honing tools are made of diamond, which enables particularly clean separation of the surface tips, which can then be rinsed out by the cooling lubricant.

Pre-honing

The honing stones of the FH fixedhone fixed mandrel honing tool are firmly connected to the base body (soldered). The diameter is adjusted by expanding the base body and is adjusted in 1 µm increments by an axially mounted adjustment unit. These tools work in a double stroke.

Finish honing

On the hydraulically actuated AH actuatedhone tool, the honing stones are mounted on floating bearings. These are held in position by spring washers and pressed against the bore wall by the coolant pressure of the internal cooling system. The surface achieved and the removal rate are defined by the number of strokes.

The entire process, consisting of reaming, pre-honing and final honing, takes less than 2 minutes per bore. Bernhard von Gunten is delighted: “With the new honing tools, we have succeeded in integrating the honing process into the Chiron FZ 18. This means that the workpiece comes out of the machine completely machined.”

Honing tools available from D = 4 mm

Soft materials such as aluminum are generally more difficult to hone, while steels and hardened materials up to HRC 70 can be honed excellently. In general, the honing stones can be adapted to the material to be processed. The tooling is always adapted to the customer’s application. For such requests, the tool specialists always carry out a feasibility analysis, which also includes test runs. Kim Arnold adds: “If the feasibility analysis is positive, we provide a functional guarantee in advance as part of the order. This also includes tool optimization during the test phase until the process is stable.”

The process development was time-consuming. Bernhard von Gunten says: “We had around two years to develop this process, albeit with interruptions because the machine was not always available for the tests. This shows how complex it is to integrate a manufacturing process with very high surface quality requirements and very tight tolerances into automated production.”

Conclusion: 10,000 drillings per honing tool – ROI of 18 months

The service life of the two honing tools is 10,000 bores. In this application, it takes two to three years before the tool needs to be overhauled by the manufacturer. Due to the long service life, the acquisition costs of the tools are put into perspective and quickly lead to economical production. Bernhard von Gunten is delighted with the new honing process: “Compared to our previous manufacturing processes, we save over 13 minutes of machining time with the integrated honing process. The biggest advantage is that the honing tools can be used on a machining center, i.e. the component comes out of the machine finished and the costs for an external honing machine and additional operating costs are eliminated.”

More information:

URMA AG
Obermatt 3
5102 Rupperswil
https://www.urma.ch/en/tools/honing/reamhone.php

Obermatt 3
5102 Rupperswil
Switzerland