First published in the August 1997 issue of Cutting Tool
Engineering magazine.
Watch Out For Convenience Features!
Early NC machines (before computers were incorporated) were quite difficult
with which to work. Machine tool builders and control manufacturers have worked
very hard to make their machines and controls easier and easier to work with.
Truly, current model CNC machine tools, while still quite sophisticated, are
much easier to program, setup, and operate than their predecessors.
We call any feature developed to make CNC machines easier to work with a
convenience feature. You must be aware that almost every convenience feature
can make you pay some kind of penalty. In some cases, the penalty paid is
acceptable, based upon the company’s needs. In others, the penalty will be
very severe, and alternatives must be found.
One classic example is related to the machining center feature tool length
compensation. While this is an extremely helpful feature (one that we recommend
every CNC user should use), you must be careful in your use of this feature to
minimize the setup-time-related penalty it may be forcing you to pay. If for
example, your setup people must measure tool lengths during setup right on the
machining center, the machining center is being used as a very expensive height
gage during these measurements. There are ways to use tool length compensation
that allow you to measure tool lengths for the next setup off line, while the
machine is in the current production run. Of course, there are situations when
even this severe setup-time-related penalty is acceptable. Perhaps lot sizes
are small, cycle times are short, and/or only one person (the CNC operator) is
used to perform all CNC tasks. In this case, tool lengths may have to be
measured on line during setup, and doing so right on the machine may be
acceptable.
This is but one example of how convenience features can force you to pay a
penalty. Others include:
Probing systems - While very convenient to use, be sure to exhaust
all other possibilities before deciding to use a probing system. Those used to
help your people during setup (like spindle probes, tool length measuring
probes, and turning center tool touch-off probes) force you to pay the penalty
in setup time. Those that must be executed in every cycle (like spindle probes)
force you to pay the penalty in cycle time. While there are many excellent
applications for probing systems, be sure you are applying them wisely.
Parametric programming - While this feature can facilitate your CNC
usage in many ways, it does require quite a bit of processing power from the
CNC control. In some cases, the calculations and logic done with parametric
programming will increase cycle time.
Canned cycles - Like parametric programming, some canned cycles
require enough processing to cause noticeable pauses during program execution
(increasing cycle time). Additionally, there are many control parameters that
affect how your canned cycles behave. In deep hole peck drilling, for example,
a parameter controls the distance the tool will be kept from the current hole
bottom between pecks. This distance, of course, has a great deal to do with how
long it will take to drill the hole.
Many more - Again, any feature that makes it easier to work with a
CNC machine tool will force you to pay some kind of penalty. As CNC users, you
will have to identify the penalty, consider your alternatives, and determine
whether the penalty is acceptable based upon your own applications.
