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August 1996/Volume
48/Number 5
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By Richard McCarthy
By setting up
tools outside the machine, a shop can increase its production without
purchasing another machining center.
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Operator Robert Finegan prepares to preset
a roughing head with the KPT Diaset contact presetter on the
workbench at Quality Machining Co., Columbus, OH. |
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All too often, shops invest hundreds
of thousands of dollars in a new machining center to increase production,
when they could accomplish the same thing with existing machinery
at a fraction of the cost. Many machines cut metal less than 50%
of the time during working hours—even less in a flexible manufacturing
environment where part runs are generally short. For the rest of
the time, these machines sit idle while their operators are setting
up tools in the spindle. Instead of purchasing an additional machining
center, a shop can invest in equipment that allows it to set up
tools outside the machine, so the spindle can make chips while the
next tool is being adjusted and prepared.
Some shops, unaware of the capabilities
and cost efficiency of today’s tool-presetting equipment, have dismissed
offline adjustment as an expensive extravagance. However, setting
up tools on a machine tool essentially means using the machine tool
spindle as a “presetter”—a very expensive presetter. For example,
consider a shop running four CNC mills for two shifts per day at
a shop rate of $40 per hour. If each operator takes only 1 hour
per shift on each machine for tool setup, the shop is losing about
$320 per day, or $80,000 per year, to spindle downtime. And if this
setup time isn’t sufficient to ensure optimal cutting conditions,
short tool life and scrapped parts will quickly erode profits.
If this shop used tool-presetting
equipment, then the operators wouldn’t have to waste valuable spindle
time touching off tools or making trial cuts. Instead, they could
preset tools for the next operation or reset tools after changing
inserts while the machine continues to run. As a bonus, the operators
could preset cutters with adjustable pockets for a balanced chipload,
rather than setting these indexable tools on the machining center,
which is very time-consuming.
Many shops have found that routine
use of offline tool- presetting equipment significantly reduces
spindle downtime and keeps their machine tools producing quality
components. Offline adjustment of boring-bar length and diameter
can reduce tool-changeover time at the machine spindle from 15 minutes
to less than 1 minute. On CNC lathes, similar reductions in setup
times are routinely achieved. With tools that require a length-only
setup, shops can realize a reduction in setup time from 5 minutes
to less than 1 minute per tool.
Presetter Selection
Tool-presetting equipment can cost hundreds of thousands of dollars;
even the most basic system can cost thousands of dollars. Therefore,
it is very important to select a model that will maximize setup
efficiency. To choose the presetter that best suits your requirements,
you first must determine the maximum tool length (z-axis) and diameter
(x-axis) that will be measured. This will determine the presetter
travel capacity that will be necessary to measure your full range
of tools. If you anticipate using larger tools in the future, you
may want to get a system that is slightly larger than your current
requirements.
Next, consider the tolerances that
must be held in your machining operations. How accurate and repeatable
does the presetter need to be? Precision to within 0.0001" is available,
but it makes sense to buy no more than what your operation requires.
A presetter with a 0.0001" accuracy, for example, will typically
result in cuts with a 0.0001" accuracy.
Finally, consider the different
tool shanks used in your operations. The presetter should be fixtured
for the shanks you run, whether they be standard V-flange, DIN-standard,
BT-flange, NMTB, or lathe tool shanks. Adapters are available for
a variety of tool shanks and toolholding systems.
Contact vs. Noncontact
With information on lengths, diameters, precision, and shanks, a
quality tool-presetter vendor can help you select the most cost-effective
measuring system for your operation. Two classes of presetters are
available: contact and noncontact.
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Figure 1: The Diaset™ contact presetter
has a large digital readout and a data memory with capacity
for 15 separate zero points. |
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Contact presetters range in cost
from about $5000 to $10,000. They use a probe that touches the tool
to make measurements. Contact presetters may achieve overall accuracy
of less than 0.0005" and check runout to less than 0.0003" TIR (Figure
1).
The magnetic-scale technology associated
with some earlier models of contact presetters is outmoded. Glass
scales and encoders now offer many advantages and much finer resolutions
than their magnetic-scale predecessors. Magnetic scales output a
digital pulse and have a fixed resolution (generally 0.000500");
glass scales output an analog signal that allows the readout electronics
to be configured in such a way that enables the manufacturer to
select the resolution. Most presetter vendors resolve the analog
pulse to 0.000040", but a few very high-precision systems offer
resolution as low as 0.000019".
Because they are more sophisticated,
noncontact (optical) units generally cost more than contact presetters.
However, noncontact presetters are now more cost-efficient than
ever before. In the past, a basic projector-type tool presetter
cost more than $30,000; today, a benchtop projector-based system
can be purchased for less than $15,000.
By using shadow graphs, noncontact
presetters provide a view of the tool profile for both inspection
and presetting. Users of these systems can measure tool length and
diameter, calculate nose radii and angles, and detect damaged or
unusable cutting edges. Noncontact presetters minimize the possibility
of chipping the tool’s cutting edge when setting the tool-offset
distance. They also allow the user to monitor and statistically
document tool runout in the holder—even in hollow mills, trepanning
tools, and short tools that cannot be monitored by using contact
presetters.
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Figure 2: The Speroni Model STP-56 high-precision
noncontact presetter can position accurately within 0.000040". |
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The technological innovation of
vision-based tool measurement, combined with faster computers, has
taken the art of highly precise tool-inspection to new levels. They
can position accurately within 0.000040", even when measuring tool
diameter. Only the most accurate systems will have a resolution
this fine. Because presetters measure radially from the centerline
of the spindle out and double the value for diameter, a 0.000020"
resolution on the radius is required to provide a 0.000040" on the
diameter. These highly precise noncontact systems also will have
a maximum TIR at the full travel length of the presetter of 0.000078"
or less (Figure 2).
Noncontact presetters are recommended
for most computer-integrated-manufacturing applications, including
flexible manufacturing systems and manufacturing cells. While it
is possible to interface contact presetters to computer systems,
contact units are more commonly used in cell-based applications
where tools are set up only when inserts wear out.
Use of a noncontact presetting unit
is also preferred for operations using toolroom-management systems
to reduce tool-management time. Optical presetting becomes essential
for high-speed machining and for operations where it is important
not to touch the tool or where it is necessary to view and evaluate
the integrity of the cutting surface.
Design Criteria
After you’ve made the choice between a contact and noncontact instrument,
you should consider two design criteria in selecting a tool presetter
for precision applications. First, to measure a tool accurately,
you must hold the tool accurately. Look for a presetter spindle
with a high level of rigidity and minimal runout. High-force retention-knob
clamping is essential for the presetter to consistently duplicate
the way the tool seats in the spindle. The highest degree of error
in accuracy and repeatability that you could have with a tool presetter
is the sum of the linear positioning error and spindle rotational
error. Be sure that the positioning error is stated in the manufacturer’s
specifications in a diameter measurement mode, not radially. A radial
measurement must be multiplied by 2 to calculate the diameter, thereby
doubling any stated error.
Second, to measure a tool as accurately
in a presetter as you would in a machining center, find a tool presetter
constructed to the standards established for machine tools. The
ideal presetter will be manufactured from aged cast iron, just as
your machine tool is. Also consider the damage your shop-floor environment
might do to your presetter: An enclosed design provides dust protection
for sensitive parts and makes the unit easy to clean. Such a sealed
design may be essential, considering the coolant mists, welding
or iron contaminants, and temperature changes that can interfere
with the instrument’s ability to provide repeatable and accurate
measurement.
Temperature changes can cause significant
problems. A steel workpiece 39" long expands 0.000433" during a
short-term temperature change of 2.2°F. Aluminum expands substantially
more, cast iron somewhat less; granite remains almost the same size.
The wide variances in these coefficients of expansion suggest that,
in the absence of an extremely well-controlled environment, it is
not advantageous to have steel permanently fastened to granite or
to have any dissimilar materials fixed or attached.
A tool presetter constructed solely
of one material such as aged cast iron is generally more costly
to manufacture and, therefore, may carry a higher price tag. However,
this initial investment will be less expensive than the cost of
creating a temperature-regulated environment in the shop or allowing
significant measurement errors in the manufacturing process. A temperature-stable
structure will prevent changes that could cause distortion in the
linear axis or in the positioning of the guideways, thereby assuring
that long-term repeatability and accuracy will not be compromised.
Presetter Features
Once you’ve found a presetter construction that suits your shop-floor
environment, evaluate the features that your system should include.
Consider the following list of functions that are important to entry-level
and cell-based users:
- Although it’s always prudent
to verify the linear gage references when tolerances are tight,
machine-axis home position saves you the trouble of having to
calibrate the system every time you turn on the power. This is
an added bonus of the more sophisticated readouts that are generally
available only on presetters equipped with glass-scale and glass-encoder
technology.
- In addition to larger numbers
and buttons, digital readouts are now available with data memory
for many different zero points. Rather than actually performing
the calibration for a tool-shank changeover, you can simply select
a different zero point from the memory.
- Many presetters have the ability
to switch back and forth from inch to millimeter, from radius
to diameter, and from incremental to absolute measurements from
a known gage point.
- You may require a presetter that
allows fine adjustment of any distance along the z- and x-axis
within the unit’s full range of travel. You may also need to be
able to make geometrical calculations and store tool data in the
presetter’s memory or in your own database.
- A built-in label printer or computer
connection allows you to print labels that may be affixed to tools,
keep preset data handy for use at the machine, or download tool-offset
data through a direct-numerical-control interface system or directly
to machine tools.
One presetter may be enough for
a shop with a lot of machines if operations don’t change often.
But a shop with only a few machines that handle short part runs
may have operators waiting in line to preset their tools.
Whatever the specifications of your
operation, a carefully selected tool-presetting system can increase
shop efficiency and enhance production of quality components. It
will help your machine tools do what you purchased them to do—make
chips.
About the Author
Richard McCarthy is national product manager, tool-presetting
and tool-measuring systems, at KPT Kaiser Precision Tooling Inc.,
Elk Grove Village, IL.