|Instead of solid polymer concrete bases, Hardinge reinforces key areas of cast iron bases with its Harcrete polymer concrete mixture to reduce machine weight.|
|A Maier polymer concrete base weighs about 8,500 lb and eliminates vibration.|
|Studer developed its Granitan polymer concrete mixture over 20 years ago and has since improved the material now designated as Granitan S103. |
Polymer concrete is dense, rigid, dimensionally and thermally stable, resistant to water and chemicals and will not twist or bend in reaction to stress.
In addition, the material displays superb vibration dampening qualities, and no machinist or engineer would deny that the biggest challenge in any precision machining process is eliminating vibration.
That is the reason that polymer concrete, which was once, long ago, derided as a cheap substitute for cast iron and welded machine tool bases, has become the preferred base to ensure rigidity in a machine tool.
Many machine tool builders are increasing their use of some form of polymer concrete in their machine bases.
James Kucharski, a product manager at Maier USA LLC (www.maierusa.com), said polymer concrete is the ideal choice for machine bases, especially Swiss-style turning machines, because of its efficiency.
Kucharski noted that the number of rapid movements in a machine are constantly increasing because of higher machine speeds and acceleration rates, and because positioning cycles and tool changes have become faster.
The machine base has been relied on to absorb that increased number of rapid movements, he said.
Maier has carefully measured absorption, and has found that polymer concrete absorbs vibrations eight to 10 times faster than gray cast iron. And, obviously, the more the vibrations are absorbed, the easier it is to take faster, deeper cuts while maintaining maximum accuracy and longer tool life.
Machine bases often are measured according to rigidity statistics, and polymer concrete can be classified as an isotropic, homogeneous material, Kucharski said. Because the material is so dense and its sections are so massive, machine bases weigh considerably more than typical cast frames.
A Maier polymer concrete base, for example, can weigh as much as 8,500 lb. The increased weight provides for a sturdier base that further improves vibration dampening.
But, is there such a thing as a machine base that’s too heavy?
Hardinge Inc. says “Yes.”
Hardinge (www.hardingeus.com) is well known for its rigid machine bases made from Harcrete, a polymer concrete mixture.
However, as machine sizes continue to grow, Hardinge finds that using Harcrete bases for bigger machines makes them too heavy, Don Thomason, an application engineer at Hardinge, said. So, for its larger size machines, Hardinge developed a base design that has reduced weight, yet with the same levels of rigidity and dampening qualities as a Harcrete base.
Hardinge still uses Harcrete for its big machines, but it is not the only component of the base. The company starts with cast iron, then uses Harcrete to fill and to reinforce critical areas that are prone to vibration. The company conducted extensive research to determine the key vibration areas in its bases.
Hardinge’s Harcrete is a licensed version of Granitan, a patented polymer concrete mixture that was pioneer by grinding machine builder Studer AG (www.grinding.com) over twenty years ago. Today, all of Studer’s grinding machines rest on a newer Granitan formulation designated as Granitan S103.
After several years of basing development on practical experience, Studer participated in a four-year research project with five European universities and four other industrial concerns.
Studer was the main industrial partner in the pioneering study.
The study was aimed at scientifically and systematically investigating and optimizing the use of polymer concrete. Chemical companies that formulate polymer concrete contributed to the study, and applications that were studied included the uses of polymer concrete for civil engineering projects.
Optimizing the use of the material depends on conditions during which the polymer is laid down and cures and other production variables, so those conditions were described using finite element analysis. The researchers also used computer simulations to represent the mechanical, thermal and chemical processes that occur in a machine bed while the polymer is cast and as it cures.
Once those conditions are simulated, the model of the hardened machine base can be subjected to simulated thermal and mechanical loads that would occur during the grinding process and that would affect the behavior of the grinding machine.
All of that research and years of observing machine beds have shown that Granitan is an extremely stable material. Additional tests have shown the material can even be used as an aggregate in the production of recycled polymer concrete.
After the research program, Studer switched its entire machine production to Granitan S103. That involved a new formulation for the mix and modifications in the company’s casting bay so that production would take place under clearly defined thermal conditions.
Studer said its nearly stress-free production process and other factors contribute to the significantly higher rigidity (modulus of elasticity), reduced thermal expansion and excellent long-term performance of Granitan S103. The mixture is made with minimal epoxy content which reduces the toxic additives for the compound and minimizes the use of hardener. The polymerization is completed through a special process control, and the result is a more ecologically friendly polymer base for machines.
Forming that special mix
Polymer concrete is comprised of several substances that make it an efficient, reliable and economical material. These substances include mineral fillers, quartz gravel, stone flour, a small percentage of epoxy-binder and various additives.
To create the optimal grain mixture, grain sizes can range from less than 0.003 in. to more than 0.1 in. for stone flour, and as much as 0.070 in. to 0.620 in. and up for gravel.
Maier applies epoxy resins as a bonding agent that provides high elasticity, low internal tensions, minimum cubic contraction and good longevity. Epoxy resins react to the hardener without splitting off a low-molecular connection to a three-dimensional network. In addition, other additives are included to improve process, adhesion and exhaust in the polymer concrete.
Maier measures, mixes, pours and consolidates all these components in molds that are made of wood, steel or plastic. The compounds cure in the mold.
To obtain reproducible material properties, all the individual components of the bonding agent — the polymer resin and the hardener — and the filler mixture must be proportioned correctly.
Maier uses a series of metering screws and dosing pumps to ensure the weight for the fillers falls between 90 percent and 93 percent, and that the bonding agents are between 7 percent and 10 percent of the finished compound.
The mixing process takes place in two phases and is continuous. First, Maier mixes the portions of the various grain sizes and the bonding agent components to meet the finished grading curve. Second, the company creates the optimal connection of the polymer concrete material by a complete wetting of the fillers with the epoxy-resin bonding agent.
When casting a machine base, the mixture is poured directly from a machine or a foundry ladle into the mold. Processing efficiency is critical because the amount of time that the mixture can be processed is limited. With epoxy resin-based polymer concrete systems, the processing time — known as pot life — is five to six hours.
During pouring, the mold vibrates to compress and remove air bubbles from the mixture. A frequency of 70 cycles per second and an acceleration of up to 2.5 G create a solid polymer concrete machine base.
Heat development — the curing process is an exothermic reaction — to 113-degrees F., during the hardening process signals that the epoxy resin and the bonding agent are reacting, and the base begins to solidify. Heats in excess of 113-degrees F. during this process have adverse effects on the machine base because it later contributes to stresses from cooling and shrinkage, Kucharski said. Blank machine bases require 12 hours to be proved from the mold so, typically, one base can be produced per day.
Builder benefits from polymer concrete
Sunnen Products Co. lists several reasons that it built its new SV-1000 vertical CNC modular honing machine on a polymer concrete base.
Such a base lowers production costs after the initial cost of the mold is paid, provides excellent product consistency because the casting process duplicates the features machined into the mold, and the base closely matches the thermal expansion of steel and the density of aluminum. In addition, the base delivers better vibration dampening than steel.
Sunnen (www.sunnen.com) also said using polymer concrete allows it to shorten machine build and delivery cycles because the base does not need machining after it is cast. Inserts and finished surfaces are cast into the one-piece base.
Modularity allows Sunnen to scale up the SV-1000 series of machines from single-spindle models to fully automated multi-spindle systems for ultraprecise bore sizing and finishing.
The company designed the basic single-spindle model from the base up for accepting future automation. The machine’s side enclosure panels are removable for flow-though part processing.
Bases that fit
Drake Manufacturing Services Co. (www.drakemfg.com) has been using cast polymer concrete bases for its machines since the early 1990s and has had very good success, James Vosmik, president of the company, said.
“While the vibration dampening and thermal stability of cast polymer are very attractive properties, the ability for us to tailor our machines to fit the customer’s part-making requirements, as opposed to forcing a solution on them that happens to ‘fit’ onto a pre-designed, pre-cast and pre-aged cast iron machine base, is the benefit for us,” Vosmik explained.