PROBING
ACCURACY
The
Machinist analyses the critical usage of high accuracy probes in bettering
the mould making process of the US-based Western Industrial Tooling
Washington-based
Western Industrial Tooling (WIT), Redmond,Washington, an ISOcertified
tool maker has turned electrode machining into a production operation,
for competitive moulds with plug-and-play 1.3+ cpk capability. It's
ironic that one of the most productive manufacturing processes – plastic
injection moulding – has often started out with one of the most laborious,
low-production processes: mould making. WIT uses high-accuracy Renishaw
MP700 machine tool probes to 'spark productivity' in machining its Electric
Discharge Machines (EDMs) electrodes.
Employing
solid-state technology to collect accurate data regardless of axis approach,WIT
uses the MP700 to probe all its CNC-machined graphite electrodes including
very small, detailed electrodes using ultra-thin styli down to 0.3 mm
in diameter.WIT churns out electrodes to keep pace with its CNC EDMs,
while maintaining accuracies of +0.000 to -0.013 mm. It is this initial
step in the mould making process that demands the tightest tolerances.
ELECTRODE
MACHINING
Each
new mould begins with a customersupplied 3-D database for a new part.
WIT programmers use Surfcam to write both the machining programme and
the probing routine from that database, ensuring that potential mistakes
in the machining program are not repeated in the probing routine. The
machining program is downloaded from a central computer to the machine
control of one of the four vertical milling centres. Each vertical centre
is equipped with way scrapers and vacuum systems.
MP700
FOR MACHINE SET-UP
Before
production begins on a new batch of electrodes,WIT uses the MP700 for
machine set-up.With electrode blanks fixtured, an operator launches
a WIT-written macro that directs the MP700 into a ring gauge mounted
in the middle of the table. If the probe determines that the ring gauge
has moved from its previous location, all machine coordinate positions
are reset in relation to the gauge’s new position.
After
all the milling work has been completed and graphite dust cleaned from
the electrode, the MP700 is again loaded for post-process inspection.The
probe inspects the electrode and the data is compared with tolerances
defined by the customer (WIT works with 25 per cent of the customer’s
tolerance). Since the electrode remains mounted on the machine, no refixturing
errors are introduced.
With
inspection completed and the electrode removed, the MP700 again inspects
the ring gauge and makes compensation for any position changes before
the next electrode is machined.After machining and probing, the electrodes
are delivered to the EDM department where mould burn-in begins.After
burning, first article and final checks of succeeding moulds are made
on the CMM.
MP700
FOR INSPECTION
On-machine inspection of an EDM electrode demands that probes are extremely
sensitive so as not to break off a delicate part feature or deflect
the graphite, which would generate erroneous data.However, the probe
must also be ...
.........CONTD
