Feature Article
The Real Cost of Runout
A seemingly small amount of runout can still be too large for the
tool. Reduce this runout, and tool life or productivity may dramatically
increase.
By
Peter Zelinski
What constitutes “good runout” for
a cutting tool in a machining center?
|
| Concentric clamping of the tool allows
cutting forces and wear on the tool to be more evenly distributed.
The smaller the tool, the tighter this concentricity needs
to be. |
Many shops think about the answer to that question in terms of
a single number. When precision toolholder supplier Big Kaiser
(Elk Grove Village, Illinois) informally surveyed metalworking
facilities to ask what they considered acceptable runout, the consensus
seemed to be about 0.0005 inch.
Jack
Burley is Big Kaiser’s vice president of sales and
engineering. He says that consensus is correct—sometimes.
If the tool is 3/4 inch in diameter or bigger, then 0.0005 inch
may be good runout indeed. But applying this standard to all of
the tools in the shop can be a costly mistake, he says. For tools
that are very small, reducing runout to 0.0001 inch or less can
yield considerable gains in tool life and productivity.
A recent example
involved a shop that held runout to 0.0002 inch for a precision
drilling operation. This shop (like most other
shops) considered 0.0002-inch runout to be small. However, switching
to a toolholder designed for precision clamping allowed the shop
to reduce this runout to 0.00009 inch, or 90 microinches. Forces
and wear on the tiny drill became more evenly distributed. As
a result, the shop was able to cut faster with this tool, realizing
a cycle time savings of 20 percent. In addition, tool life increased
by 3 times.
 |
| Fig. 1—Here is a comparison
of the effect of runout on tool life for three different
3-mm drills
tested at four different runout values. The workpiece material
was 1055 steel. The HSS tools were run at 90 sfm and the
carbide tool was run at 250 sfm. Feed rate was 0.004 ipr. |
Runout Revealed
Figure 1 shows the results of an experiment conducted by Big Kaiser’s
parent manufacturer, Big Daishowa Seiki. In the experiment, three
drills were tested at four different runout values. Cutting conditions
were the same, and the tools were the same except for tool material
and length. The three drills included a carbide tool penetrating
to 3 times diameter, an HSS tool reaching the same depth and an
HSS tool with through-tool coolant penetrating to 5 times diameter.
Runout for all of the tools ranged from 0.0006 inch down to 80
microinches.
The findings illustrate various important points, Mr. Burley says.
One is that the right runout is relative not just to tool size,
but also to tool material. While the life of each tool improved
as runout got tighter, the carbide drill suffered the most when
runout was high.
 |
| Fig.
2—Slowly rotating a precision-machined
bar can measure a spindle’s static runout. The non-contact
measurement device below can evaluate runout at the spindle’s
operational speed. |
The
findings also show just how much significance a “small” amount
of runout can have on a 3-mm drill. Runout of 0.0006 inch is
close to the figure
that shops considered good runout in the informal survey. However,
reducing the runout to well below this value improved the life of
the carbide tool by almost 3 times. Even the least responsive tool
saw tool life improvement of 60 percent.
This effect does not just apply to drilling. When similar testing
was performed on a small-diameter, four-flute carbide end mill,
the difference between 80 microinches and 0.0006 inch of runout
was a tool life improvement of more than 30 percent.
Toolholder Features
To realize these savings, start with the spindle, Mr. Burley says.
Most shops are familiar with the common gage used to test spindle
runout error. The gage is essentially a bar machined to extremely
tight tolerances. This is slowly rotated in the spindle to measure
static runout. Less well known is the existence of dynamic runout.
At the spindle’s operational speeds, runout can change
as a result of heat, vibration and centrifugal force. Figure
2 shows both the common gage for static spindle runout evaluation
and a gage that is capable of measuring a spindle’s dynamic
runout. If evaluation of the spindle shows that it can maintain
an acceptably small runout on its own, then the key to imparting
that low runout to the tool is the toolholder.
Various design features of a precision toolholder allow it to
realize tight concentricity, Mr. Burley says. Those features include:
|
| Fig.
3—A toolholder with a smaller
collet angle can clamp the tool more precisely. Typical collet
angle is 16 degrees. The angle of the taper of this collet
is 8 degrees. |