In the range of the power train the machining of valve train components represents such a sequence of linked processes. In order to control uncertainty and guarantee process reliability in such a process chain a holistic approach including all necessary machining processes needs to be applied. Out of it a proper choice of machining strategies and tool concepts can be made.
Precision Machining of Bores – Applied Tool Concepts and Uncertainty Effects
The precision machining of bores is the quality defining process within a multi-staged process chain for the manufacturing of fit bores and precision bores with narrow tolerances. In this context, reaming is a frequently used processing procedure which also enables the machining of a pre-drilled hole to a designated final dimension. For this, tools with a guide pad, single-bladed tools or tools with several blades and chamfers are used. The advantage of tools with multiple blades is the increased productivity due to the increased feed per revolution. Single-bladed tools on the other hand can be adjusted to the respective machining task due to the distributions of their functions “guiding” and “cutting”. The basic research up until this point identified four uncertainty disturbance variables concerning the precision machining of bores. Firstly, an occurring concentricity error, an axis offset between the tool and the workpiece centre axis, an inclined scored area as well as an inclined pilot-hole. These disturbance variables cause a radial force which impacts the dynamic behaviour of the overall system. This results in an aggravated attainable bore quality regarding the cylindrical shape and the concentricity which leads to the fact that the required tolerances cannot be adhered to.
Already conducted tests illustrate that the occurring uncertainty during the precision machining of bores has a significant influence on the attainable bore quality. Especially the dynamic behaviour of the tool can deteriorate the bore quality significantly concerning the circular und cylindrical shape as well as the concentricity. There exists a knowledge gap concerning the occurring disturbance variables insofar that these variables overlap in the industrial practice and thus influence the process dynamics. Furthermore, the uncertainty review within a process chain, where several fine processing steps follow one another, was not jet subject of research.
Appearing Problems during the Production of Cylinder Head Components
The finishing of valve guides and valve seats in the cylinder head is a challenging task in the automotive serial production. Both components are subject the thermal and mechanical loads. These loads have increased significantly in the last couple of years due to the increase of the effective medium pressure following the guidelines for the reduction of the CO2 emissions and due to the down-sizing of combustion engines. Up until 2000 it was common practice to manufacture valve guides and valve seats from brass alloys, whereas now the components are manufactured almost exclusively via powder metallurgy for the automotive serial production. For this, the components are manufactured from high-alloy steels with a hardness over 30 HRC. This leads to a strong increase of the cutting force. Simultaneously, the specified tolerances have been constantly tightened so that a process-reliable machining without additional process steps cannot be ensured. The machining of components is a multi-staged process chain (see Image 1). The initial blank cast component is usually made from aluminium alloy. The first step is to drill the fit bores into this component to be able to mate the valve guide and the valve seat with the cylinder head. With the help of a force effect or by cooling the valve guides and valve seat inserts these parts are pressed into the prepared fit bores and finally finished.
Any occurring uncertainties – in the form of influencing disturbance variables on the succession process – are not being observed in the automotive serial production. The examination of the quality parameters takes place after the final finishing process due to business constraints. The mayor disturbance variables are the axis offset between the tool and the workpiece centre axis as well as an inclined scored area. Both variables cause deviations of the tool during the transient process phase which can cause the non-achievement of the required bore quality. Even the development of a robust tool geometry does not enable a process-reliable single-stage processing of the valve guide.
Objective and Approach
The primary objective of transfer project T4 is the development of a holistic approach to evaluate and select processing procedures within an uncertainty afflicted process chain like the processing of valve train components for the cylinder head of a combustion engine based on the developed methods during SFB 805. To control the uncertainty optimised processing tools and adapted processing strategies are used. The development of decision methodologies will enable a purposeful selection of robust tools and thus enable the selection the optimal processing sequences for each process chain. This leads to an increased processing quality as well as an increased productivity.
The methods for the realisation are based on the knowledge from the first two funding periods of SFB 805. When evaluating a complex process chain, it is necessary to expand the knowledge systematically. Therefore, it is necessary to consider all relevant processing procedures and processing variants for the computer-based design of the process chain. Therefore, it is necessary to expand existing models with the aspect of the reamer tools with guide pads. In addition is it necessary to consider the impacts of already conducted processes for the subsequent process steps to be able to control the uncertainty in the entire process chain. This will enable process-reliable processing sequences.
Development and construction of a test stand to illustrate the uncertainty afflicted linked processing procedures within a cylinder head
The so far unnoticed uncertainty within the processing procedures during the machining of valve guides and valve seats is identified. The focus lies on the possible fluctuations concerning the dimensions and the material properties of the unmachined cylinder head and the blank valve guides as well as blank valve seats, the pre-machining steps for the manufacturing of precision bores and the mating of sintered components. A model process is being derived in regard to the identified influencing variables. With the help of these influencing variables a test stand for already existing machining centres is being developed.
Implementation of a simulation model for tools with guide pads
In the industrial serial production, it is possible to use single-bladed reamers instead of multi-bladed reamers. To be able to design an uncertainty optimised process chain is it indispensable to consider this process chain. The during the first two funding periods already developed models can serve as the starting point. When using single-bladed reamers it important to deviate the tool to fulfil their function, which is not necessary when working with multi-bladed reamers.
Implementation of a simulation model for an uncertainty afflicted process chain
The impacts of tool wear on the behaviour of the dynamic overall system “tool” are up until now insufficiently studied. During the development of a holistic approach, to illustrate a process chain, these impacts are considered for all component models with machining processes.
Design of an uncertainty optimised process chain using Robust Design
Based on previous works and with their application, the objective is to develop an uncertainty optimised process chain, which is verified under laboratory conditions as well as tested in field test in the automotive serial production.
|Prof. Dr.-Ing. Eberhard Abele|
+49 6151 16-20478
|Dr.-Ing. Christian Bölling|
+49 6151 16-20841