13-05-2013, 04:06 PM
Modern machining of die and mold tools
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Abstract
Modern production of die and mold tools is quite different in comparison with conventional machining. The basic theory of cutting
process and cutting geometry is similar, but the techniques and technology is quite different. High-speed cutting (HSC) principles are not
applicable with conventional machine-tools. Difference between conventional and high-speed cutting velocity is analysed on chip formation.
The comparison between EDM and HSM has been made and shows great HSC benefits. Usage of modern software (CAM) optimises tool
production and helps us to save unnecessary additional machining time and costs.
Introduction
High-speed machining (thereinafter HSM) is a relatively
new production technology that allows a higher productivity,
an excellent surface finish and a good dimensional accuracy
in the manufacturing process. Slovenian die tools production
mainly contains: plastic molds, die casting molds, vacuum
molds and forging dies. High-speed milling is one of the most
important of all high-speed cutting (HSC) methods [1–3].
Thanks to the advances in machine-tool performance as a result
of improvements to the main spindle, feed drives, etc.,
high-speed milling has become a cost-effective manufacturing
process that produces products with a high surface quality,
low variations in the machined surface and dimensional
accuracy. High-speed milling was first used successfully in
the aircraft and automotive industries for machining complex
machine parts made of aluminium and its alloys. Recently,
with the advances in cutting-tool materials and technologies,
high-speed milling has also been used in the machining of alloy
steels in their hardened state (above 30 HRC up to 60–65
HRC) [4,5].
HSM in tool-making industry
High-speed machining assures two times more efficient
productivity, which is achieved at first with the cutting speed
and secondly with feed-rate. Conventional feed-rate is increased
with software solutions and modern machine-tool
design up to 1600 mm/min.
Fig. 1 shows water bottle mold machined with HSC by
using modern CAM programs. By deep cavity the machined
surface quality is very uncertain, especially the second part
where polishing is used as a finish operation.
More exacting machining of 3D (three-dimensional)
workpiece surfaces is successful only with five-axis machinetools,
which enables milling of any surface. Five-axis NC
programming is similar to the one applied with three-axis parallel
machining. Five-axis machining enables additional cutter
side deviations and onward–backwards deviations from
the normal, which orientates the surface. Among many
CAD–CAMpossibilities, five-axis machining is the most universal
machining procedure, which enables side or tip milling
of any shapes.
Finish machining of heat-treated tool steels
Modern cutting materials, tools shaped as a ‘pencil-tools’,
enable small metal removal rate but also much faster machining.
It is very important to use right technological parameters
for successful machining (Fig. 3).
Fundamental parameter of heat-treated tool steels highspeed
finish machining is small cutting depth.
The cutting depth should not exceed 0.2/0.2mm (ap/ae)
value. In this way we prevent tool deflexion/deviation and
preserve high level of accuracy (tolerance and geometry).
Cutting tools for finish machining of heat-treated materials
should be heat resistant, thus coated (e.g. TiAlN). Relating
to tool-wear, diffusion is one of the main reasons for
tool-life reduction. Temperatures exceeding 800 ◦C are very
detrimental to all tools, which are not coated with TiAlN and
TiCN, or multi-layer coated surfaces (Fig. 4).
Some theoretical backgrounds
There are several criteria used for defining high-speed machining,
i.e. the criteria for determining the boundary between
conventional and high-speed machining.
These include [7], the magnitude of the cutting speed,
the revolutions of the spindle or the rotating tool (the spindle
speed), the DN number (DN is the spindle diameter in
mm multiplied by the spindle speed in rev/min), the dynamic
behaviour, and the workpiece material. The most appropriate
definition of high-speed machining is based on theworkpiece
material grade (or type) being machined [7], Fig. 5. For example,
the cutting-speed values from 500 to 700 m/min is the
high-speed region for machining alloy steels.
CAD–CAM system
Today’s PC-based CAD–CAM software grows more sophisticated.
The prospective CAD–CAM users are presented
with dozens of options through trade shows. It becomes difficult
for a shop to decide exactly what they need. Due to a
rapid development of information technologies, CAD–CAM
packages can perform NC programming tasks that would
have been impossible a few years ago without an expensive
workstation-based system. In general, mould shops should
not purchase a system for their far future plans, because at
that time the software will already have been out-of-date.
Eksperimental results
Software programs for optimization can improve part
quality. That is especially true, when using a lowperformance
CNC machine tool that does not have much of optimization
algorithms built in CNC control unit. Using of off-line optimizing
algorithms lead us to improve of the geometrical
accuracy and surface quality of machined parts. These kinds
of software are usually easy to use and are inexpensive regards
to high performance CNC controller. One such software
package named G-optim program has been described
in this paper.
Conclusions
Entrance to the European/Western market of technology
and manufacturing requires introductions into modern methods
of cutting technologies.
HSM assures competitive position in the field of mass
production in automotive manufacturing. Mass production
in quality tool-making, which demands short time for
(die/mold/forging) tool preparation, represents key element
of profit.
Furthermore we can list the following HSC benefits:
• shortening of the whole machining process,
• surface quality improvements–less additional machining,
• lower temperature in the cutting zone and longer tool
life,
• the reduction of EDM usage,
• machining of accurate and thin-wall electrodes,
• possibilities of five-axis machining.