Every machine shop
has a tool measurement device, says Richard McCarthy. In some shops,
the tool measurement device is the machining center itself. That can
be an expensive way to go.
Mr. McCarthy
is a national sales manager for tool measurement systems with Big
Kaiser Precision Tooling (Elk Grove Village, Illinois). The tool
measurement devices he helps shops to implement are commonly known
as “presetters.” The term may be somewhat misleading,
because shops using these devices enjoy a range of benefits even
if they don’t literally “preset” their tools.
Typically, the most significant of these benefits is saving time
at the machining center. As an alternative to using feeler gages
and test cuts to determine tool offsets at the machine, using a
presetter to perform independent tool measurement away from the
machine can free up considerable productive time.
Who
Needs A Presetter
Mr. McCarthy says job shops often assume they aren’t good
candidates for presetters because their production quantities are
small. In fact, job shops often represent the very best applications
for presetters. A process that might not need a presetter is one
that runs the same part all day long, day after day. A process like
this requires tools to be changed out only because of wear. By contrast,
job shops change tooling not only because of wear, but also because
of new jobs. The more frequently a shop has to load a fresh tool
in a machining center, the more savings off-line tool measurement
can deliver.
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However, even
facilities with long stretches of time between tool replacements
might benefit from off-line measurement. An additional advantage
relates to the impact of runout on tool life and productivity. By
using a presetter to set up its tools, a shop can detect runout
problems and hold each tool to a defined runout limit. Without this
kind of control, machining with too much runout accelerates tool
wear by forcing one edge to perform the brunt of the cutting. A
shop that doesn’t recognize this problem may run slowly or
take shallow cuts just to preserve the life of the tool. By identifying
and curing the problem, the shop can realize more aggressive material
removal.
Controlling runout
is particularly important for small-diameter tools because the acceptable
runout for any tool is proportional to its size. According to Mr.
McCarthy, this is the reason why one high-volume manufacturer implemented
presetting. While time lost at the machining center was not a concern,
the shop believed tool runout might be a significant problem for
its small-diameter tools. After discovering runout error resulting
from the toolholders, the shop switched to holders that permitted
faster production. Without using the presetter to perform this diagnosis,
the shop may never have considered this fix.
Prerequisites
Shops implementing presetters often learn lessons like this because
the presetter forces a more disciplined approach to measuring tools.
In fact, it also tends to force a more disciplined approach to tool
management. Tooling stored in toolboxes and bins throughout the
shop needs to be gathered together in a central location close to
the presetter. Getting all the tooling in one place may save the
shop considerable time in its own right because operators no longer
have to roam the shop in search of tools.
This centralization
is just one of the requirements for successful off-line tool measurement,
Mr. McCarthy says. Another requirement is an investment in having
a sufficient number of tools and holders on-hand. If the shop is
no longer doing tool measurement at the machining center, then it
should have enough extra tooling to run production while the presetter
measures the tools that will be needed next.
Which
Presetter Is Right
Various presetters are available across a range of performance levels.
Big Kaiser offers contact and noncontact models from Diaset and
Speroni, respectively. The contact measurement involves an indicator,
while the noncontact measurement is optical and might be performed
either manually or through CNC.
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| One
factor that affects measurement accuracy on a presetter is
the location accuracy of the adapters that accommodate different
toolholder types. The presetter shown here holds and locates
adapters using a locking mechanism that provides for this
accuracy. The operator is inserting an adapter for holding
40-taper toolholders. |
The contact presetters
are the least expensive. The noncontact models tend to be both more
accurate and more efficient. Mr. McCarthy points out that optical
models are upgradeable, so it’s usually not necessary to begin
with an aggressive menu of capabilities. “About the only thing
we can’t expand is the iron,” he says—meaning
the presetter’s size should be carefully chosen to match the
largest tool the shop is likely to need to measure in the future.
The other fundamental
consideration is accuracy. If the shop accepts the rule that gages
should have 10 times better accuracy than the part tolerance, then
this rule can suggest the required accuracy of the presetter. For
example, if parts are to be machined to accuracies as loose as ±0.005
inch, then tools can be inspected to ±0.0005-inch accuracy
using a contact presetter, Mr. McCarthy says. At ±0.002-inch
part tolerances or tighter, an optical system is necessary to achieve
the corresponding accuracy of tool measurement.
Getting
The Data To The Machine
No matter how precise the measurement, there is still the challenge
of getting the tool measurement data to the machine—a step
that offers plenty of opportunity for error. Some presetters offer
a label printer to avoid human error in writing numbers down. The
correct measurements are printed out and affixed to the toolholder.
Even then, there is the “fat finger” problem, which
occurs when data from the label is miskeyed into the control, Mr.
McCarthy says. To avoid this error, some presetters compose NC programs
for assigning correct tool offsets. The machine tool’s CNC
runs this program before running the machining cycle. The operator
simply has to load the tools into the correct pockets.
To avoid the
potential for error even in this tool loading, a still more aggressive
approach is the use of tool ID tags capable of storing electronic
data at each toolholder. With a system such as this, the presetter
can write tool offsets to each tool-and-toolholder assembly. The
CNC using a reader for these tags can identify the tool automatically,
read its offsets, and cycle the tool magazine around to the right
pocket for loading.
Qualified
Tools
One final way some shops try to avoid errors in communicating tool
data is by keeping the tool offsets always fixed. They use “qualified”
tools. That is, they use tools that literally are “preset,”
because the user employs the tool measurement system to help him
adjust the tool’s length within the toolholder until it matches
a certain predetermined value. (Only in a case such as this is the
measurement device truly used as a “presetter.”)
A few types of
machines demand these qualified tools because they have no freedom
to apply offsets. These machines include transfer lines, twin-spindle
machines that use identical tools in parallel processes, and five-axis
machines with CNCs that lack the capability to adapt complex tool
paths for tool offset changes. Boring tools also have to be preset
to specific dimensions. Apart from these applications, most users
of presetters employ the device to measure the tool as it is.
However, the
potential to take this opposite approach—determining the offset
first and setting the tool to match—illustrates the increased
range of options the shop has available with a presetter. The need
to transfer data to the CNC can be rendered unnecessary as the shop
applies the presetter to standardize the tool dimensions and achieve
even greater control over the management of its tools.