Tooling tricks of the trade.
A few tips for extending die life.
By Steve Chamberlain and Rodney Hewitt
(reprinted with permission from The Fabricator, June 26, 2003)
 Just make it work!" they
scream. "I don't care how. Just make it work!"
As a tooling professional
(whether you are a tool- and diemaker, engineer, press operator,
or any other member of the team responsible for operating and maintaining
that die), you quickly find that "making it work" is subjective.You
may discover one or more tool scenarios:
- It was designed
poorly.
- It was made
"on the cheap."
- It has not
been maintained well, or even at all.
- The entire
tooling set would make a better fishing lure.
But you also know
you have to make it work, and typically, you have to make it work
right now.
When you find
yourself faced with a die with nasty problems, try using a few collected
tricks of the trade to help you get a problem die up and running
quickly.
Steel Brittleness and Hardness
Sometimes you
don't know what the die block material is, but determining the tool's
ancestry can be a chore in today's global marketplace. As offshore
dies come into the North American stamping marketplace, the documentation
becomes less and less accurate. Tools labeled as D-2 can turn out
to be lesser-quality materials such as 4140 and even cold-rolled
steel. Until an easy, very accurate way to test the steel type of
a tool is found, mistakes will continue.
Low-quality tool
steel can cause several problems in a production environment. It
may be too soft or too brittle, it can have cracks and surface flaws,
or it may be damaged by heat. Most of these conditions typically
are not apparent to the naked eye. Magnification of the area is
the only accurate way to pinpoint the problem.
Steel that is
too hard may become brittle and chip or crack. Brittleness on the
tool surface can be caused by electrical discharge machining (EDM)
or improper surface grinding.
To relieve surface
brittleness, remove the tool and polish or grind the damaged surface
until it's smooth. Then retemper it at 25 degrees F lower than the
previous tempering temperature. If you are in doubt about the original
heat-treating history, use the lowest recommended tempering temperature
for that steel.
If the cracking
is not just a surface condition, the block probably is too hard.
A new or existing tool can be heat-treated to achieve a lower hardness-a
process called underhardening. Hardening steel to 2 to 4 Rockwell
hardness (HRC) points less than the optimum hardness will make the
tool much more tough and durable. Underhardening is especially helpful
on trim steels that chip too often or in piercing and perforating
operations.
If the steel appears
to be smeared or crushed, then it has inadequate compressive force.
The first step is to measure the hardness. (Never assume that a
steel is fully hardened, because a lot of things can go wrong during
heat treating.) If the hardness is in the optimal range for that
steel, then you must either reduce the force produced or increase
the hardness. Increasing the hardness of a steel beyond its optimal
range may work for a short time, but it will lead to problems later.
D-2, for example,
becomes brittle past 60 HRC. Upgrading to a better tool steel can
solve this problem. Most tool steel suppliers sell brands of modified
D-2 that are durable up to 62 or 63 HRC. Another good general-purpose
steel is M-2. The M series, called high-speed steel (HSS), was designed
to be hard and very heat tolerant for use in drills and taps. M-2
can be hardened safely up to 62 to 64 HRC and still offer good toughness.
Coating
Two of the biggest
challenges facing metal formers today are the decreasing use of
petroleum-based lubricants and the increasing use of stainless steel.
Reducing friction
is one of the keys to successful forming, and lubricants make a
big impact on how much friction is generated in the forming process.
However, regulations have forced some fabricators to change lubricants,
and they've experienced production problems as a result.
Stainless steel
contains a lot of chrome, and so do most tool steels. The chrome
in the work material often attempts to weld itself to the chrome
in the tool, causing galling.
Tool coatings
can increase tool surface hardness and decrease friction, allowing
metal to flow more smoothly. They serve as an alternative to lubricants
and provide a permanent barrier between the tool and the work material,
reducing galling.
Coatings for trimming,
blanking, and piercing dies take many forms, from soft, low-friction
molybdenum-disulfide to high-friction thermal sprays. The key is
to match the coating type to the application.
- Titanium nitride
(TiN) by physical vapor deposition (PVD) doesn't work well under
pressure because of its low adhesion characteristics, but the
process's low temperature usually doesn't affect the steel.
- Titanium carbide
(TiC) and titanium carbonitride (TiCN), applied by chemical vapor
deposition (CVD), have higher adhesion but also higher application
temperatures, which can cause changes in the steel substrate.
- Vanadium carbide
(VC) applied by thermal diffusion (TD) is a hot process that works
best under high forming pressure. The coating is metallurgically
bonded to the substrate, which creates a strong adhesive bond.
The high-heat process can change the shape, size, and hardness
of the tool, so it's important to know how the tool was heat-treated
before having it coated.
Polishing
 Metal's microsurface is surprisingly rough, even when
it appears smooth to the naked eye (see Figure 1).
Melting, heat treating, cutting, and grinding can leave hills, pits,
craters, holes, scratches, and cracks in the steel substrate. All
these imperfections increase metal flow resistance during part forming,
which causes friction, heat buildup, galling, and tool wear. The
better the finish, the fewer problems you will have.
Most tooling users
polish a tool only when it has a problem, such as to remove gall,
and then put it back in service. Instead, polish a tool to achieve
a mirror finish and then try to keep it in that condition as a part
of regular maintenance (see Figure 2). It's not
easy or fun, but it can be a real productivity enhancement.
For proper polishing,
start with the proper equipment:
- An abrasive
wheel
- Silicon carbide
polishing stones
- Mineral spirits
or degreaser
- Variable-speed grinder with felt bob
- 3- to 7-micron
diamond paste
If the block is
galled or severely scratched, start with the abrasive wheel-and
go easy! Use the polishing stones, rubbing by hand in the direction
of the material flow, starting with a 200 to 300 grit and working
progressively finer to a 900-grit stone. Continually use the mineral
spirits to keep the stones clean and moist, and use a grinding wheel
to shape the stone to fit the contour of the workpiece.
Use an electric
or air grinder with a soft felt bob and a very small dab of diamond
paste to finish the tool to a mirror finish. You won't need a mirror
finish everywhere, but it makes a huge difference on radius sections.
Once the radius
finish is complete, transition that mirror finish to a B and then
to a C finish evenly across the rest of the working area of the
tool.
Die Blocks
Sometimes metal
forming operations create enough severe stress to overwhelm the
hardened tool steel. This is common if the metal is thinned or coined
and is typical with the inside corners of a rectangular draw. It
also can occur if the die is bottomed to set against springback.
Instead of fighting
this problem every week or month, just eliminate it. The use of
inserts is a quick, simple, and economical solution, and with today's
EDMs, it's also easy.
First, choose
a proper replacement steel, such as M-2, M-4, or modified D-2, and
determine where you need the extra hardness. Inserts should be about
half the original wall thickness and deep enough to cover the entire
depth of the draw.If you are going to coat the insert, then leave
some grind stock material on the areas with critical sizes. When
the block is coated, you can go back and grind the nonworking areas
exactly to fit the insert pocket.
Make sure you
polish the insert while it is in the block. Polishing each part
separately could result in taped edges, creating a mark on the part
or an area for galling to begin.
Plastic on Blanks
Usually you would
use plastic only as a last-resort lubricator, but with tighter and
tighter tolerances and less effective lubricants, it's possible
you may need it for use in a production pinch. Plastic garbage bags
have been used by toolmakers for years. The intention is to introduce
a slippery membrane between the tooling and work material to reduce
friction and encourage metal flow (see Figure 3).
Figure 3
Usually you would use plastic only as a last-resort lubricator,
but with tighter and tighter tolerances and less effective
lubricants, it's possible you may need it for use in a production
pinch. The intention is to introduce a slippery membrane between
the tooling and work material to reduce friction and encourage
metal flow. |
You can use plastic to make more metal flow in specific corners,
or on one side as a way to determine the height and location of
draw beads or pad pressure in tryout. Some parts require that extra
advantage to even run production, and several companies have rigged
a simple stand and electric motor to feed a roll of plastic into
the die with the material.
Better Lubricant Application
Lubricant application
is a technical issue, and every situation will have specific demands
and need individual assessment. Just looking at your die is not
always enough to know if you are applying lubrication as effectively
as possible. For some operations, spraying lubricant from outside
the die might be a good choice, while others might require application
of lubricant directly to the coil or blank. An application specialist
can help you decide which method is right for you.
The next time
they scream "Just make it work!" remember these tricks of the trade
and apply them to some of your stickier situations, preferably with
a new replacement tool at your disposal. All of these tips have
actually worked and might for you as well. But like any good solution,
the keys are to determine the correct application and experience.
Steve Chamberlain
is business development manager and Rodney Hewitt is a sales engineer
with the TD Center, 2020 15th St.,
Columbus, IN 47201, 877-832-3687 or 812-378-1675, fax 812-378-1591.
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