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Development of a combined and automated hard turning and polishing production system – effective, automated, safe and integrated hard turning and polishing

Final Report Summary - EASITAP (Development of a combined and automated hard turning and polishing production system – effective, automated, safe and integrated hard turning and polishing)

Executive Summary:
European SMEs which produce high precision parts by means of hard turning are facing a steady loss of competitiveness in their growing markets since existing manufacturing technologies do not fulfil increasing demands on part quality and variety. During the last years the competition has worsen due to up-coming low wages countries outside Europe and an increasing cost pressure. Additionally these days the economic crisis causes a tremendous drop in orders. To sustainably ensure the survival and competitiveness of the SMEs within easiTAP a system was developed which enables a faster part-production with concomitantly better part qualities. This system consists of an automated production system integrated in one lathe which ensures a reliable flexible production of high-precision parts of hardened steel. This system consists of several components:

- A automated polishing device / spindle which enables the fully automated polishing of turned parts in one machine and one clamping
- An optical measurement device which is able to calibrate a new tool directly and measure the cutting edge geometry and wear of the tool in order to compensate form errors in conjunction with the controlling of the lathe
- Monitoring systems which enable force measurements and acoustic emission measurements
- New “hard” burnishing tools made of diamond, which enable the burnishing of workpieces up to a hardness of up to 70 HRC
- Automated Y-Axis which enables a fast and reliable adjustment of the tool height
- Automated polishing tool changing system for fast and automated tool changes

Due to these machine modifications it is possible now to manufacture high precision parts with high surface demands fully automated in one machine. Time and cost intensive polishing operations can be conducted fully automated now without the need of manual labour. By the combination of hard turning, hard burnishing and polishing in one machine and one clamping it is possible to achieve highest surface qualities within an extreme short amount of time and without a loss in form accuracy.

Project Context and Objectives:
When cubic boron nitride (CBN) was discovered as a perfect tool material 40 years ago, it became a revolution in hard machining. Hitherto the only way to machine very hard surfaces (>50HRC) was by applying grinding process. Cutting processes like turning or milling could not be used because of missing suitable tool materials. CBN material has the same hardness like diamond but is several times more temperature resistant. Thus, now it was possible to machine even materials with hardness about 65 HRC by turning or milling. This gave completely new potential to hard machining, and processes like hard turning or hard turning were developed.

Today hard turning is a wide used technology with direct competition to grinding. The major benefits of hard turning are the high flexibility, the high material removal rates and that hard turning can be performed on every standard turning machine. Thus, hard turning became popular especially at SMEs companies. These could use their already available turning machines to machine hard materials without the need to buy new grinding machines. However, hard turning brought disadvantages as well: Due to the extreme cutting forces and temperatures even the CBN tools do wear. After a certain tool wear limit is reached, surface roughness becomes worse significantly and surface damages do occur. While in grinding the tools can be dresses and reused, in hard turning the exchange of the entire tool is necessary. This does not just take a lot of time, especially for the subsequent tool setup, but is also very cost intensive (one tool costs 30-50 €, for comparison: a common cemented carbide tool cost 5-10 €). Thus, ways need to be found to:
- Use the tools longer
- To decrease the tool setup times

Hard turning is usually used to generate functional surfaces. This can be stamps, pressing tools, bearing seats and especially bearing rings. Hereby very high surface qualities / roughness need to be achieved. Thus, in many cases after hard turning a subsequent polishing process takes place. These are conducted manually and are therefore very time intensive and cost intensive. A manual polishing process can take e.g. between 30 – 120 Minutes. Therefore it is the goal to:
- Automate the polishing process

Within the project easiTAP it was the goal to meet the above mentioned requirements of the SMEs. Thus the objectives were:
- Developing of an automated polishing device / process for hard turning machines
- A tool measurement system for a faster tool setup
- Systems and Mechanisms to use the tools longer and reduce tool costs by this.
Project Results:
When cubic boron nitride (CBN) was discovered as a perfect tool material 40 years ago, it became a revolution in hard machining. Hitherto the only way to machine very hard surfaces (>50HRC) was by applying grinding process. Cutting processes like turning or milling could not be used because of missing suitable tool materials. CBN material has the same hardness like diamond but is several times more temperature resistant. Thus, now it was possible to machine even materials with hardness about 65 HRC by turning or milling. This gave completely new potential to hard machining, and processes like hard turning or hard turning were developed.

Today hard turning is a wide used technology with direct competition to grinding. The major benefits of hard turning are the high flexibility, the high material removal rates and that hard turning can be performed on every standard turning machine. Thus, hard turning became popular especially at SMEs companies. These could use their already available turning machines to machine hard materials without the need to buy new grinding machines. However, hard turning brought disadvantages as well: Due to the extreme cutting forces and temperatures even the CBN tools do wear. After a certain tool wear limit is reached, surface roughness becomes worse significantly and surface damages do occur. While in grinding the tools can be dresses and reused, in hard turning the exchange of the entire tool is necessary. This does not just take a lot of time, especially for the subsequent tool setup, but is also very cost intensive (one tool costs 30-50 €, for comparison: a common cemented carbide tool cost 5-10 €). Thus, ways need to be found to:
- Use the tools longer
- To decrease the tool setup times

Hard turning is usually used to generate functional surfaces. This can be stamps, pressing tools, bearing seats and especially bearing rings. Hereby very high surface qualities / roughness need to be achieved. Thus, in many cases after hard turning a subsequent polishing process takes place. These are conducted manually and are therefore very time intensive and cost intensive. A manual polishing process can take e.g. between 30 – 120 Minutes. Therefore it is the goal to:
- Automate the polishing process

Within the project easiTAP it was the goal to meet the above mentioned requirements of the SMEs. Thus the objectives were:
- Developing of an automated polishing device / process for hard turning machines
- A tool measurement system for a faster tool setup
- Systems and Mechanisms to use the tools longer and reduce tool costs by this.

Description of the main S&T results/foregrounds
In easiTAP several systems were developed to meet the above mentioned objectives:
- Automated polishing device integrated in the turning machine
- Automated polishing tool turret for fast polishing tool changes
- Optical tool measurement system for faster tool setups
- Automated y-axis for faster and more reliable tool setup
- Novel “hard burnishing” tools for surface improvement and longer tool service life
- System for monitoring of the hard turning process and detecting tool critical tool wear

Automated polishing device integrated in the turning machine
To be able to polish workpieces reliable and fully automatic in one machine and one clamping, a polishing device/spindle was developed and integrated into the turning machine. The polishing device is mounted on the support of the machine and can be moved in x- and z-direction by this. Furthermore the device can be rotated by an additional servo motor in b-axis direction (see figure 1 below).

Figure 1: Developed polishing device integrated in the turning machine

The polishing spindle is able to revolute with up to 6000 rpm. By this it is possible to reach optical surface qualities in the machine below a surface roughness of Ra = 0.05 µm. The B axis can be moved in an angle of 90°. Thus it is possible to polish even complex workpieces in the machine. The figure 2 below is showing such a polishing operation.

Figure 2: Polishing of a workpiece right in the turning machine

Automated polishing tool turret for fast polishing tool changes
The grid size of the grains within the polishing agent has significant effect on the generated surface quality. In manual polishing processes the grit size is changed frequently (e.g. D=6µm for roughing, D=3µm for prefinishing, D=1µm for finishing). To realize this with the automated polishing device, frequent manual polishing tool changes are required. This is standing against the idea of a fully automated combined turning & polishing in easiTAP. Thus, a polishing tool changer was developed and integrated into the machine. This tool changing system is able to hold 12 different tools and change between them fully automatic without the need of manual workforce (see figure 3). One tool change takes about 20 seconds. A video of the tool change can be found on YouTube under:
http://youtu.be/4Pu2g8sSJTc

Figure 3: Automated polishing tool change

Optical tool measurement device
To be able to measure tools fully automated right in the machine, an optical tool measurement system was developed and integrated in the machine. The optical tool measurement system consists of a high resolution camera which takes pictures of the tool. A image system analyses the tool picture then and is giving the relevant information (x-, y- ,z-position, tool radius) to the machine controls. The machine control can do a tool setup/compensation then according to the information of the measurement system. Thus it is not just possible to do a fast tool setup it is also possible to compensate form errors due to tool wear. Figure 4 shows the tool measurement system in parking position as well as the graphic user interface of the system.

Figure 4: Optical tool measurement system in parkin position (left);
graphic user interface for tool measurement (right)

Automated y-axis
While tool errors in x- and z-plane can be compensated by the machine, this is not possible in
y-plane. Reason for this is that state of the art turning machines do not have an automated y-axis. This is a major problem, especially when it comes to face turning. Due to e.g. tool deviation in
y-plane, it can happen that the surface is machined only partly and a small pin is left behind on the surface (see figure 5a). This cannot be tolerated. Therefore, tool holders are available on the markets which allow a manual height adjustment (see figure 5b).
.
Figure 5: Pin generation due to tool deviation (a); Tool holder with manual high adjustment (b)

The manual adjustment is extremely time consuming. A skilled worker needs in average 20 minutes to adjust a new tool exactly. The reason is that the adjustment happens iterative: The worker needs to machine a part, take it out to measure it, adjust the tool, machine the part again… In industry this is a major problem which causes high non-productive times. To make the tool setup faster and independent of manual workforce y-axis was designed and integrated in the machine, see figure 6).

Figure 6: Picture of developed and integrated y-axis

Due to this machine modification it is now possible to set the high of the tool fully automatically by the machine controls and NC-commands. Figure 7 is showing the tool in different y-axis positions.

Figure 7: Y-Axis in different positions

By the y-axis it is now possible to compensate the height deviations of the tools within a short amount of time. Time intensive manual positioning operations are not required anymore. Thus, the long unproductive tool setup times can be reduced significantly. While the tool setup took at least 20 minutes before, it is now possible to do it in less than 7 minutes.

Novel “hard burnishing” tools
To improve the surface quality in common turning subsequent burnishing operations are usually used. For hard turning this is not possible because the workpiece material is too hard for common burnishing tools. Thus, during easiTAP burnishing tools were developed with diamond tips. Investigations do show that these diamond tips can be used for burnishing of materials with hardness up to 65 HRC, as long a sufficient cooling is applied. Figure 8 is showing these burnishing tools and the diamond tips.

Figure 8: Modified burnishing tools with diamond tip for “hard burnishing”
With these hard burnishing tools it is possible now to burnish even hard materials, without a significant wear effect on the burnishing tip. Figure 9 does show a bearing steel with hardness of 63 HRC which was hard turned and burnished afterwards (1 time / 5 times). By burnishing only once (20 seconds process time) the surface roughness can be improved by 80%. With four further overruns (80 seconds process time) the surface can be improved by further 47%. This shows that hard burnishing is a powerful potential finishing process for hard turned workpieces.

Figure 9: Effect of hard burnishing on hard turned parts (M50 with 63 HRC)

System for process monitoring
To monitor the status of tool wear, force and acoustic emission systems were integrated into the turning machine. In investigations it was shown that the force correlates with the tool wear. Thus, the force signals can be used to detect critical tool wear. By setting a force limit the system is able to give an automatic warning that the tool needs to be changed. Figure 10 shows the measurement setup.

Figure 10: Setup of the monitoring system

>>>FOR FIGURES PLEASE SEE ATTACHED PDF FILE<<<

Potential Impact:
By the developed easiTAP systems it is possible to improve the surface quality on one hand, but also to save time and costs on the other hand. Figure 11 is giving an overview about the four main developed systems and their impact on production times.

Figure 11: Impact of developed easiTAP systems on the production times

By the optical tool measurement system it is possible to reduce the times for tool measurement / setup (X-, Z- Position and tool radius) from 19 Minutes (state of the art) to 2 Minutes. This is equal to a time saving of -89%. With the automated y-axis the setup times can be reduced from 20 minutes to less than 7 minutes. By the combination of hard turning, hard burnishing (with developed tools) and polishing (with the developed polishing spindle) it is possible to reduce polishing times from about 16.5 minutes to 3 minutes. This is a time saving of -82%. In addition, the automated polishing tool changer gives a time saving of -83% as well.
To evaluate the overall impact a case study was conducted. In this study high precision bearing rings were machined (batch size = 50) with a state of the art turning machine and with the developed easiTAP system. The overall machining time with the state of the art system was about 570 minutes. With the easiTAP system even better surface qualities could be achieved within 390 minutes. This is a time saving of -32%. Due to this, the cost could be decreased as well by -24%.

>>>FOR FIGURES PLEASE SEE ATTACHED PDF FILE<<<

List of Websites:
The address of the public webpage is:
http://www.easiTAP.eu

A video of turning/burnishing and polishing can be found on YouTube under:
http://youtu.be/EO5GRCU9deM

A video of the automated tool change can be found on YouTube under:
http://youtu.be/4Pu2g8sSJTc


Contact details of participants:
Fraunhofer IPT
Mr. Florian Degen
Steinbachstraße 17
52074 Aachen, Germany
florian.degen@ipt.fraunhofer.de
http://www.ipt.fraunhofer.de
+49 (0) 241 8904-289

Cerobear GmbH
Kaiserstr. 100
52134 Herzogenrath, Germany
http://www.cerobear.de

Direct-Line
Jedlik Ányos 14
2330 Dunaharaszti, Hungary
http://www.dldh.hu

B.V. Gereedschapswerktuigenindustrie Hembrug
H.Figeeweg 1a+b
2031 BJ Haarlem, Netherlands
http://www.hembrug.com

IMIX
Tweelingenlaan 63
7324 BK Apeldoorn, Netherlands
http://www.imix.nl

Mößner GmbH Diamantwerkzeugfabrik
Kelterstr. 82
75179 Pforzheim, Germany
http://www.moessner-gmbh.com

SN-Spindeltechnik
Schillerstr. 20
04720 Döbeln, Germany
http://www.sn-spindeltechnik.de

Torman Ltd. Sti.
KOSB, Kemalpasa Asfalti Cikmazi 1
35170 Izmir, Turkey
http://www.torman.com.tr

Werkzeugmaschinenlabor der RWTH Aachen WZL
Steinbachstr. 19
52074 Aachen, Germany
http://www.wzl.rwth-aachen.de