Thomas Schade, Vice President, International Mold Steel, Inc.
In the past 30 years there have been
tremendous improvements in machine and cutting tool technologies.
High-speed machining can accomplish cutting speeds and accuracies that
toolmakers in the past could only dream of.
Advances in cutting tool metallurgy and
coatings technology have dramatically improved tool life. However,
toolmakers still limit their ability to take full advantage of these
marvelous technological advances by using a mold steel that was
developed in the 1950s: P-20.
cheap and familiar steel, why switch? Because there is a better way to
make molds. In the past few years mold steels have been developed that
are easier to machine and weld. One of these new materials is a
precipitation hardening steel of uniform microstructure and hardness
that was developed by Professor Asada and Dr. Watanabe of Daido
Industrial College in Nagoya, Japan. This material was introduced to
the plastics industry as
Although NAK 80 is considerably harder
than P-20 (40 Rockwell C compared with 32 Rockwell C for P-20), it
machines 15-20 percent faster than P-20 and can be polished to a Class
1 optic-quality finish. To further improve machinability, a small
amount of sulfur was added to NAK 80 to produce a new grade of steel
NAK 55. Some NAK 55 users report as much as 50 percent reduction
in machining time compared to P-20. There are several reasons for the
increased metal removal rates for NAK 55 over P-20. NAK 55 is a vacuum
arc remelted, age-hardened steel. As such, it is exceptionally clean
with very uniform grain structure and hardness from surface to core.
means to the mold builder is:
- You will never hit a hard spot
and break an expensive finishing cutter.
- You never need to do an
intermediate stress relief. The dimensional stability of NAK 55
is such that if there is any movement during machining it will
be measured by .0001 of an inch.
- Increased cutting tool life.
The optimum cutting conditions for
NAK 55 vary by machine tools. Cutting tools incorporating the
recommended geometries will produce superb finish machined
surfaces, often completely eliminating grinding. NAK 55 does not
Milling: High-speed steel
cutters give excellent results and very smooth machined surfaces.
The best result will be obtained with cutter geometry
incorporating a positive rake angle of 15 degrees–20 degrees and a
relief angle of less than 10 degrees.
NAK 55 Grain Structure
NAK 55 Hardness Curve
Carbide cutting tools (P-40 grade)
yield excellent results if the positive rake angle of the inserts is
approximately 8 degrees and the relief angle is less than 10 degrees.
When a negative rake carbide is used, insert grades with greater
toughness than P-40 will give better results. In general, a positive
rake configuration is superior to negative rake for milling NAK 55.
Small, single flute carbide cutters will give excellent results.
Grinding: NAK 55 grinds easily.
It is recommended that it be ground wet.
Drilling: NAK 55 drills easily.
The cutting speed should be lowered as drill diameter increases. A
smaller than standard twist angle and shorter length will reduce the
danger of broken tools.
Tapping: NAK 55 is a 40 HRc
steel; therefore the following is recommended to facilitate tapping:
- Use a sharp, premium grade tap, tin
coated and spiral pointed.
- Use a tapping oil or highly
chlorinated sulfurized oil. If neither is available, a mixture of 50
percent kerosene and 50 percent cutting oil also works well.
EDM: Copper or graphite
electrodes are suitable, or the steel may be used as an electrode when
burning mating halves together to achieve a matched fit. The recast
layer from EDM for NAK 55 is soft (approximately 32 HRc). Other low
alloy grades, such as P-20, or more highly alloyed steels such as S7
and H13, have extremely hard recast layers equivalent to 55–60
Rockwell C. Because the EDM white layer must be removed, the
subsequent stoning or grinding of NAK 55 is much easier than with
The success of
NAK 80 and
NAK 55 is only partially attributable to excellent machinability.
Usually a mistake in the mold or part design must be corrected by
welding. It is very difficult to weld, polish and retexture the
surface of P-20 without the welded area being visible in the steel of
the molded part. If the mistake occurs in a highly polished textured
area, the entire cavity may need to be scrapped. The mold made from
NAK, when the proper procedures and NAK-W weld rods are used, will
have uniform hardness across the weld. This allows the toolmaker to
purposefully match the polish or etch.
A tooling engineer from
Apple Computer (Cupertino, CA), explains: “In today’s business
environment, to shorten time-to-market, we are usually forced to build
molds while the parts are being designed. This means we make a lot of
changes to our molds, both during the construction and after the first
tryout. Our ability to make these changes with NAK, sometimes in an
already textured area, has saved us countless times. We have also
found that NAK etches much better than other mold steels.” Design
changes can be made during the life of a part as well.
In 1989, the Daido team of engineers
surveyed the mold building industry to identify what mold builders
considered their worst reoccurring problem. The resounding response
was “welding”. Perfect welds are obtainable with the NAK series of
mold steels. But, proper pre- and post-heating is required. According
to the mold builders surveyed, a pre-hardening steel that doesn’t
crack when welded at room temperature, and could be repolished or
retextured without post, would be ideal.
In 1993, Daido introduced a new
pre-hardened mold steel called PX5 that combines the weldability
and machinability of NAK with added toughness. PX5 has the same
hardness as P-20 (32 Rockwell C) but is more consistent through
the entire workpiece. The steel’s tensile and yield strengths are
similar to NAK and P-20 (see chart).
A quench and temper heat-treating
process is used to manufacture PX5, but the steel is quite stable.
It can be machined to size without the need to relieve stress on
the block before the finish cut. The sacrifice in hardness for NAK
(40 Rockwell C) is compensated for by extreme toughness. PX5 is 60
percent tougher than P-20. This improved toughness allows
increased design creativity and flexibility. But it’s the
weldability that really makes PX5 unique.
Welding this steel requires neither
pre-heating nor post-heating, which greatly reduces welding time
and cost. According to Ernie Beutel, vice president of Technical
Services for International Mold Steel, “the increase in hardness
in the area heated during welding is very low and distortion is
minimal, resulting in low overall repair time and cost. As with
NAK steels, the weld can be textured or polished with no
observable difference in the surface. As with NAK 55, the use of
positive rake cutters will always give superior results when
cutting PX5. Consistent hardness and the elimination of hard spots
will improve tool life.”
Ron Field, process development
engineer for the Makino Die/Mold Division, decided to put PX5 to
the test. Field compared the machinability of PX5 to P-20 by
machining Makino waves on a Makino V55 vertical machining center.
Roughing the P-20 took 102 minutes. Roughing the PX5 took 56
minutes. The superior machined finish obtainable with PX5
practically looked polished. A major mold base manufacturer
obtained similar results.
Improvements in machining efficiency of 20 to
30 percent can
be anticipated. PX5's
consistent hardness and microstructure will
allow dependable, unattended machining.
When Roland Krevitt, program manager
for Apple Computer specified the mold steel for the 20th anniversary
Macintosh, he chose PX5. “I used PX5 and was amazed by its
performance. Machining time was reduced by 30 percent, stress
relieving was never required and welds in critical areas textured
perfectly,” said Krevitt.
20th anniversary Mac was a high-profile launch—it had to be
perfect. I knew we would have to weld extensively. The design was in a
constant state of flux. PX5 was the only choice. I still laugh when I
think of a comment made by one of the CNC operators involved in the
project: ‘After machining PX5, I realized that putting P-20 on my
Makino is like putting kerosene in my Corvette.’”
Competitive Mold Maker,
Volume 4, Number 2
Copyright © 1999 Makino Inc.
All rights reserved.