DAAAM INTERNATIONAL SCIENTIFIC BOOK 2008 pp. - CHAPTER ??

OPTIMISED MACHINING OF FIBRE REINFORCED MATERIAL

BERGER, D.; BLEICHER, F.; DORN, C. & PUSCHITZ, F.

Abstract: Fibre reinforced materials are getting more and more important for European manufactures. Coming from automotive and aviation industry nowadays also SMEs have to deal with these high tech materials. One of the biggest problem for such companies is the little experience they have in milling these materials. First cutting tests nearly ever result in very bad surface qualities and then companies often stop their efforts. The focus of the cornet project HPM was to overcome these hindrances for SMEs. The biggest problem therefore is the delamination of upper and lower layer during the cutting process. The Institute for Production Engineering developed a method to reach optimised surface qualities according to the application for different types of fibre reinforced materials like Aramid and Dyneema. Authors DI (FH) Dominik Berger; ao. Univ. Prof. DI Dr. Friedrich Bleicher; DI(FH) Christoph Dorn; DI Falko Puschitz, Vienna University of Technology, Institute for Production Engineering, Landstrasser Hauptstrasse 152, 1030 Vienna, Austria Key words: aramid, dyneema, fibre reinforced material, milling

Authors´ data: Dr. Berger, D[ominik]*; Prof. Dr. Bleicher, F[riedrich]**; DI (FH) Dorn, C[hristoph]***; DI Puschitz, F[alko]***, *Institute of Production Engineering / Vienna University of Technology, Institut für Fertigungstechnik, 1030, Wien, AT, **Vienna University of Technology, Fünkhgasse 4/26, 1140, Wien, AT, ***Vienna University of Technology - Institute of Production Engineering, Landstr. Hauptstr. 152, 1030, Vienna, AT, TU Wien, berger@ift.at, bleicher@ift.at, dorn@ift.at, puschitz@ift.at This Publication has to be referred as: Berger, D[ominik]; Bleicher, F[riedrich]; Dorn, C[hristoph] & Puschitz, F[alko] (2008). Optimised Machining of Fibre Reinforced Material, Chapter ?? in DAAAM International Scientific Book 2008, B. Katalinic (Ed.), Published by DAAAM International, ISBN 3-901509-69-0, ISSN 1726-9687, Vienna, Austria DOI: 10.2507/daaam.scibook.2008.01

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DAAAM INTERNATIONAL SCIENTIFIC BOOK 2008 pp. - CHAPTER ??

Fig. 2. Delamination of Aramid strands (Microscope view)

The forces during the chipping process are too high to be absorbed by the matrix, normally being a type of epoxy resin. As a consequence the fibres or singular fibre strands (Fig. 2) come loose from the laminate. Another reason is that Aramid and Dyneema fibres are not as stiff as e.g. glass fibres. This makes them quite difficult to cut without any support of the delaminating matrix. In order to prevent delamination, either the matrix, especially the upper and lower layer, or the fibres themselves, have to be made stiffer and therefore easier to cut. Both ways have been analysed and are detailed below. 3. Material

Before describing the different tests, there is given a short overview of the various fibre materials that are available on the market and which of those have already been tested (Flemming et. al., 1995; Flemming et. al., 1996; Ehrenstein, 2006):

• Glass fibres: They are yarned from melted glass with a steady round diameter of 3.5 to 24 μm. The covalent linkage between oxygen and silicon is responsible for their strength. Different types of glass, E-glass (electrical glass – good isolator), R/S-glass (resistant/strength – high-strength glass), C-glass (high chemical resistance), ECR-glass (a mixture of strength and chemical resistant E-glass) and AR-glass (alkali-containing glass), are used to make glass fibres.

• Carbon fibres: Because of their graphite structure they are very strong and stiff. Mostly they are made of Polyacrylnitril (PAN) rather than of pitch. There are different types of carbon fibres available: HT (high tensile), HS (high strength), HM (high modulus), UHM (ultra high modulus).

• Aramid fibres: Other common names are for example Kevlar® and Twaron®. Aramid fibres are linear organic polymers with high tensile load and stiffness with oriented covalent linkages along fibre axis. The fibres are made of Polyphenylenterephthalamid (PPTA) in a wet spinning process. During the manufacturing process the E modulus, both tensile and breaking

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DAAAM INTERNATIONAL SCIENTIFIC BOOK 2008 pp. - CHAPTER ??

The tested Aramid fibre reinforced material (Fig. 5) was a plate of 16 layers Aramid fibres with a standard epoxy resin and plain weave (Fig. 4) interlace. It has a fibre ratio of 50.3 Vol. % and a thickness of 3.5 mm. The Aramid fibre used is Style 160 UD (company Swiss-composite) with a grammage of 163 g/m². The shoot has a yarn count of 23 tex and the warp 166 tex. Each layer has an offset of 45° to the former layer.

The tested Dyneema fibre reinforced material (Fig. 5) was a plate of ten layers Dyneema fibres with a standard epoxy resin and twill weave (Fig. 4) interlace. It has a fibre ratio of 47 Vol. % and a thickness of 3.5 mm. The Dyneema fibre used is Dyneema SK60 (company C.Cramer & Co) with a grammage of 180 g/m². Both, the shoot and the warp have a yarn count of 132 tex. Each layer has an offset of 45° to the former layer. 4. State of the art in cutting Aramid and Dyneema

Considering that there is hardly any descriptive literature on cutting processes for Aramid and Dyneema in general, and for milling in particular, we analysed literature for the milling of glass and carbon fibre reinforced materials. Most manuscripts deal with lifetime behaviour of milling cutters but not with getting optimised surface qualities. In order to increase the lifetime of milling cutters when cutting glass fibres, special PCD (polycrystalline diamond) milling cutters are used. However, the problem with PCD milling cutters is that you can not get a very sharp cutting edge (big chip and clearance angle) which is necessary to get a sharp cutting line on the material. Therefore solid carbide tools are used in order to reach these geometry requirements (Neitzel & Schlimbach 2003). The biggest cutting problems with Aramid and Dyneema are (Wiendl, 1987):

• Distinctive delamination • Insufficient surface quality • Low durability In order to reduce delamination it’s recommended to cut with high cutting speed

and low feed rate. However, also the ratio and orientation of fibres are very important for optimised results. 5. Tests at the Institute of Production Engineering

The first step was to test different milling cutters by milling linear grooves with different cutting parameters. We varied the cutting speed from 100 up to 1,000 m/min and the feed rate from 0.1 up to 15 m/min. To be able to test different milling cutters we contacted several international tool resellers. In total we tested 13 different milling cutters, some of them explicitly recommended for the milling of fibre reinforced materials with different geometries and numbers of cutting edges. All tests ended with more or less the same result: very bad surface (more than 1 mm delamination) quality due to delamination of the upper fibre layer. Especially tests with micro toothed milling cutters for glass and carbon fibre reinforced materials resulted in low quality surfaces.

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