CNC machining centers have been constantly
improving over the past three decades. These machines are now so flexible in
design that with minimal reconfiguration they can be applied to anything
from very short-run tool making to long-run manufacturing. Aircraft
components that were once weldments or riveted assemblies are now being
carved from solid aluminum on high speed, high-horsepower machining centers.
Automobile engine production, once almost exclusively the province of
dedicated transfer lines, is moving toward the more agile concepts offered
by machining centers. An essential characteristic of these machining centers
is their ability to change tools automatically without any operator
The toolholder provides the standard
connection between various cutting tools and the machining center spindle.
It works in much the same way that an adjustable drill chuck allows the home
handyman to change bits in a portable drill motor. The socket in a machining
center spindle and the shank and flange areas of a toolholder are made to
standards that have been developed around the world over the last 25 years.
For the most part, these standards are well thought out, and, if both the
spindle and holder are in conformance to standard, a solid and concentric
connection between the spindle and toolholder will be the result.
Because of their standard configuration,
and because of the slow rate of innovation of tool holding systems relative
to the machines that they serve, the lingering perception among some
machining center users is that toolholders tend to be "commodity items."
Given the number of apparently successful toolholder manufacturers competing
in the marketplace today, the commodity conclusion is an easy one to draw.
But some end users do not accept the commodity theory. While it's true that
all toolholders of a specific type may look alike, definitely not all are
|Toolholders are a
critical link between the machine tool and the workpiece. This article
looks at four fundamental toolholder manufacturing parameters that
precision metalcutting shops need to consider when tooling for a job.
Differences In Toolholder Quality
What makes any one thing better than
another? In most metalworking applications, the difference between a good
part and scrap is often a tiny fraction of an inch on a critical dimension.
Likewise differentiating a high precision toolholder is a matter of
adherence to manufacturing tolerances.
The meaning of the first statement is
clear. The cutting tool must rotate exactly on the rotational axis of the
machine tool spindle. The means of accomplishing this near-perfect
concentricity are also clear, but complex.
To begin, the toolholder's tapered shank
must sit within the corresponding spindle taper very precisely each and
every time it is inserted. For this to happen, the mating surfaces must be
matched to very tight cone angle tolerances. These tolerances are specified
and published by national and international standards committees and are
generally available for review by anyone. Well-made toolholders are measured
for roundness and taper angle in gages that are calibrated by hard master
gages. The methods involved in using production gaging vary from hard
contact mechanical, hard contact/electronic analog to non-contact analog
techniques (such as air gages). All can be effective. The common denominator
of these methods is the hard master gage used for calibration.
There is a definite variation between the
master gages of the various toolholder manufacturers. This strong assertion
is based on the measurement of hundreds, if not thousands, of toolholders
made by many different manufacturers over many years. Simply put, they vary.
If it is assumed that all toolholders in the marketplace conform to their
corresponding manufacturer's gages, then it follows that the master gages
used by the various manufacturers are not the same. The problem is that this
situation results in variations of spindle fit from manufacturer to
manufacturer. The reason is easily understood. There is no "Grandmaster"
gage for standard tapers. The National Institute of Standards and Technology
(NIST), Gaithersburg, Maryland, and highly capable metrology laboratories
such as that found at Timken Corporation, Canton, Ohio, can measure tapers
with adequate precision on rotary tables under certain circumstances. But
there is no single reference hard gage that can readily authenticate other
hard gages of the same size and taper rate. Without a single source or
master gage from which to trace all gages, it is understandable that there
are variations in conformance to standard dimensions from part to part
across the marketplace and that these variations can affect the quality of
spindle fit. Let's go a little deeper into this.
To define toolholder quality, one must
first consider the function of a toolholder. A good toolholder can be
defined as follows:
A device that acts as an
interchangeable interface between a machine tool spindle and a cutting
tool such that the efficiency of either element is not diminished.
To hold with this definition, four
separate elements are essential:
The rotational axis of the machine
spindle and of the cutting tool must be maintained concentrically.
The cutting tool must be held
securely to withstand rotation within the toolholder.
The toolholder must be consistent.
The application of proper gages assures consistency from holder to
Toolholders must be balanced as
finely as the spindles in which they are installed.
Remember that we are still dealing with
the issue of keeping the tool on the center line of the spindle. Even if the
spindle fit confusion issue were resolved any time soon, the problem moves
forward to the business end of the toolholder. At this point, the end user
has even more reason to lose confidence because now there is no national or
international standard for reference. Spindle and toolholder taper fit and
tool changing function have been specified by standards organizations such
as ISO, ANSI, JIS or DIN, but at the other end, where the cutter fits the
toolholder, the toolholder manufacturers are on their own. They alone set
tolerances for inside diameters of end mill holders, cone angles on collet
chucks, and face squareness on shell mill adapters. They alone set the
tolerance of the concentricity of these holding features relative to the
tapered shank that enjoins the machine spindle. Only the toolholder
manufacturers can fixture their machines so that the mating surfaces are
machined with the tapered shank, with spindle interface as the surface of
Before the discussion of concentricity is
ended, there is another, more general, variable to be considered. That
variable is the innate capability of the manufacturer to make not only good,
but consistently good, products. Within any industry, there are variations
between manufacturers in the ability to make high quality products day in
and day out. In your own business circles, you can name good and not so good
competitors and suppliers. Usually there is a paragon company in every
market and the good manufacturers are striving for that status.
Therefore, on the first and most crucial
attribute of a good toolholder, concentricity, there is good reason to
believe that there are likely variations in toolholder quality from
manufacturer to manufacturer in terms of gaging, product standards,
tolerances and manufacturing capability.
|Collets and collet
chucks should enhance, not defeat, the inherent accuracy and
repeatability that today's machining centers are built to deliver.
Holding Strength—Second Criterion
Here is where the "commodity theory"
believers score some big points. In case nobody has noticed, most
toolholders made to a given standard look very much alike. Be they CAT, BT
or HSK, they are made up as side lock end mill holders that use a setscrew
against the flat on the tool to prevent the tool from coming loose. They may
be made to accept standard collets that allow the holder to be more
adaptable to a variety of tool diameters. They may be specialized to hold
threading tools or Morse taper drills. And, there are many others. The fact
is that virtually every manufacturer of CNC toolholders offers a solution to
facilitate the use of most cutting tools in most machine spindles.
The principal source of variability in
holding force, the ability to keep the cutting tool from spinning in the
holder, occurs when collets are employed. Single angle collets, such the ER
and TG series are usually preferred. Single angle collets exert more holding
power and tend to offer superior concentricity. But collets, just like
toolholders, are subject to considerable variability from manufacturer to
manufacturer. To obtain the best possible concentricity from a toolholder,
collet, and cutting tool assembly, a means of measuring concentricity,
either with a specialized gage or a high quality tool presetter, is
essential. When the best combination of holding power and concentricity is
the goal, either hydraulic or heat-shrink toolholders should be employed to
eliminate the collet entirely.
Some manufacturers do indeed offer heavy
duty versions of collet chucks and end mill holders for extreme
applications. But with the possible exception of hydraulic and heat-shrink
technology, most makers have done an effective job of copying each other's
product lines so that there is an incredible sameness of appearance of
toolholders in the marketplace. The differences are in proper execution,
which brings us back to the conclusions reached above—there are differences
in gaging, internal standards, tolerances and, most of all, manufacturing
capability. Simply put, toolholders are not all the same.
like those shown, have a guaranteed concentricity of 0.0002 inch TIR.
For high precision applications, concentricity is critical for accuracy
and tool life.
An Illustration Regarding Gaging—Third
Here's a case in point that illustrates
gaging variability. Six years ago when our company began producing holders
to the emerging HSK standard, the first step was to purchase the required
gages. As access was obtained, a rather broad variety of HSK products
produced by competing manufacturers (in those days, mostly European) was
measured relative to the master gages we obtained from Germany. The results
were appalling. None of the parts measured were within tolerance. With what
can only be described as a healthy case of paranoia, a new master gage was
purchased from a different German gage maker. Amazingly, both gages agreed
exactly. The difference was in manufacturing. While making holders in
conformance to the HSK standard is certainly no walk in the park, a better
result should surely have been the case.
Every manufactured item has tolerances
which are basically the measure of manufacturing variability—less
variability, higher tolerances. Therefore, the above anecdote does not deny
that all of the holders looked at were functional on some level or in some
spindles. But the toolholder manufacturer has no way of controlling the end
use of a toolholder. It is not within the province of a toolholder
manufacturer to deviate materially from a standard if the product is offered
for sale under that standard.
provide a simple yet highly accuract and repeatable grip on the cutting
tool. The tool socket is slightly undersized for the cutting tool. When
heated, the bore expands to allow the tool shank to enter the bore. When
cool, the bore contracts around the tool with a balanced and strong
More and more of the machining centers
sold today come equipped with spindles designed to run at maximum spindle
speeds of 10,000 rpm or higher. Not only must the tools for these machines
be concentric and strong, but they must be balanced as finely as the
spindles in which they are installed. If they are not, the result will be
unwanted vibrations that create chatter and will ultimately diminish surface
finish and cutting tool life. In extreme cases, when imbalances are large,
spindle damage can result. Much has been written about the physics of
toolholder balance in recent years and need not be repeated here. An
important point, however, is that the force generated by an unbalanced
condition within a tool/toolholder assembly is proportional to the square of
the spindle speed. Negligible forces generated at 1,000 rpm are one hundred
times greater at 10,000 rpm and four hundred times greater at 20,000 rpm.
The need for excellent concentricity is also more important at elevated
spindle speeds because if the tool is not rotating on the spindle center
line, it becomes a prime source of additional imbalance.
Sources of imbalance in toolholders are
often functional, such as locking screws in end mill holders or unequal
depth drive slots in CAT holders. Often, however, they are random due to any
number of reasons. In any case, it is required that material be removed to
offset the imbalance to effect correction. Other solutions to the balance
problem entail the use of balanceable holders that allow the end user to
offset toolholder imbalances by manipulation of radially located setscrews
or balancing rings. In these cases, correcting imbalances in toolholders
requires that the imbalance be located and quantified which, in turn,
requires the use of a sophisticated balancer. In some cases a shop is better
off using pre-balanced toolholders obtained from a reliable source and used
in combination with high quality cutting tools as a cost effective
alternative to the purchase and operation of a balancing machine.
Don't Overlook Toolholders
There are a number of examples where
improvements in spindle fit, concentricity and balance have facilitated
difficult machining operations. At Gem Tool, a Minneapolis mold maker, a
simple change in collet systems from a double-angle design to a single-angle
DR20 series improved concentricity and repeatability on tool changes,
increasing tool life and accuracy.
|Consistent balance is
achieved with high speed, two-plane toolholder balancers like this from
Hoffman. These are permanently calibrated systems with
microprocessor-controlled hard bearing systems.
At Boeing (Everett, Washington), a bank of
30,000 rpm routers was successfully tooled for the machining of composite
panels. For these collet chucks, special nosepieces were designed to further
enhance balance and concentricity because the tools were very small and had
almost no tolerance for imbalance.
At another Minnesota manufacturer, a
single point boring operation produced several hundred thousand holes with a
single diamond insert. The tolerance on the hole was three ten thousandths
(0.0003) inch. Precision of the spindle fit was of absolute importance to
accomplish this feat over that many automatic tool changes.
Some of these examples are extreme and
some, like the collet series switch, are not. The point is this: In the
extreme cases, production would not have been possible were it not for
precise spindle fit and concentricity coupled with balance. While these
concepts virtually enable extreme applications, the benefits that they
generate carry over into everyday shop operation. Exact spindle fit results
in part-to-part repeatability and makes the holding of tolerances over a
production run easier, and thus less costly to achieve. Concentricity
balances the chip load per tooth on cutting tools and makes them last
longer. This reduces tool cost and down time. Balance assures that the full
capability of the machine tool can be used to make chips and in doing so
efficiently, reduce cycle time.
Precision manufacturers looking to reduce
process variability should consider an incremental cost differential for a
quality toolholder. A better toolholder can reduce cutting tool expenditures
and machine downtime while increasing hourly production.
About the author: Dan
Chartier is a consulting engineer for Command Tooling Systems, Ramsey,