Cutting
corners
Setup
time reduction can crack down on cost overruns and curtail delays in
the optimum working of CNC machines, says Gautam Doshi
CNC
machines have been accepted as a solution by most auto component vendors.
They manufacture large batches on their CNC machining centres. Very
reluctantly would they agree to do small batch production or develop
prototypes.
CNC machines were initially accepted in the aerospace industry, due
to their effectiveness in small batch and one time production. This
divergence is explained by the fact that most shops are not focused
enough in reducing setup change over time. If there is a reduction in
the overall time required for setup change, CNC machines can be most
effective in small batch production as well. Setup time reduction is
a prime ingredient of any successful lean manufacturing system.
PREPARATION
PROGRAM DEVELOPMENT
AND MAINTENANCE
There
are two approaches to CNC programme preparation. In case the programme
is simple, such as a 2 Axis lathe programme, it can be prepared directly
on the CNC machine. Most CNC controls have a Background Programming
feature. This allows a new program to be keyed in simultaneously while
machining is going on. On is very important to select a control, which
is 'programming’ friendly. Examples of such controls like Mazatrol or
that from Fadal, allow the programmer to programme without using G codes.
The programming software leads one through the programme steps in a
user-friendly manner, enabling someone who is a good machinist to develop
a good programme by following a few quick steps.
For instance, if one chooses the option to machine a tapped hole - based
on the tap dial and material one selects, it will automatically select
the centre drill and drill. It will also fix the tool travel of a roughing
tool based on the finishing tool. The CNC control through the use of
its material, tool and process parameter database complements the skills
of a programmer and in a short time the programme can be written without
making mistakes.
Usually
there is a tool path simulator, which will also dynamically simulate
the CNC operations so that programme verification can be completed and
cycle time estimated accurately. Sometimes CNC programme writing needs
calculations, especially where radii have to be blended or where tapers
meet. These ‘Shop Floor Programming Controls’ have special keys or features,
which allow blending, intersection or tangent generation to be done
automatically. The latest generation of PC based controls are now one
step ahead. They incorporate CAD/CAM on the CNC control itself. Thus
programmes can be automatically generated based on CAD data, using the
inbuilt CAM software.
An
example of such a control is that supplied by Fidia. It is recommended
to use similar CAM systems both in the workstation and at the CNC, so
that modifications can be conveniently carried out. In case the CNC
Machines are used in large shops or in a large production environment,
programming can be carried out offline in the office and only setup
on the machine. In this case CAD/CAM workstations can generate the CNC
programme in the office and through RS 232 interface, transfer the same
into the CNC. ********************
machine.
PC-based controls permit a network card (superior to RS 232 and DNC)
to be used as a LAN card. This allows the CNC machine to be connected
to a server and the workstation, which generates the CNC programme.
Data and programme backup can now be carried out in the server. With
a common programme and data pool, machining can be carried out at any
of the CNC machines available at that point of time. The machine control
need not have any sophisticated software for Shop Fl oor Programming
. Usually machines with ‘Shop Floor Programming Controls’ are costlier
than simple conks.
JOB
DOCUMENTATION
Job
documentation is of great importance in reducing setup change time during
repeat machining. With easy availability of PCs on shop floors, a ‘soft’
Job Docket can be prepared using all the Windows Office Tools. The docket
consists of a folder which will include a CNC programme, a pre and post
machined drawing, process sheet, operation layout, cutting tool setup
sheet, fixture setup sheet and a quality plan.
........CONTD
