New Vista got the job: three machines to be supplied over a 2½-year interval. Now the hard part of designing and building the machine was at hand. As a custom machine builder, New Vista must evaluate the feasibility of different designs and materials in every area of each machine it builds, to make sure that its design best serves the application. One issue: what type of machine base to use?

Up to this point, New Vista had used cast iron or welded steel bases in all of its designs. These were standard approaches that yielded satisfactory results. Yet, New Vista engineers were familiar with, and intrigued by, the possibility of utilizing a cast polymer composite base for this application. The benefits of flexibility in casting design and greatly superior vibration damping would certainly be desirable here. But would the cost for the production of just a couple of bases outweigh these benefits? In a larger sense New Vista was on a path to answer the question: Would it ever make sense for a custom machine tool builder to order just two or three polymer composite bases?

Cast polymer composite bases for machine tools are not a new idea. Over the last 25 years, a small industry has developed around their manufacture in Europe (especially Germany) and they are increasingly developing a niche following in the United States. A number of manufacturers, most visibly Hardinge with its Harcrete-branded bases, now sell some machinery standard with a polymer composite base. In other words, enough time has passed that the technology has now been roundly studied and manufacturing techniques streamlined.

In the U.S., there are two primary sources for custom polymer castings, both in northeast Ohio: ITW Polymer Castings (Chardon, OH) (formed from the 2000 merger of ITW Philadelphia Resins & Precision Polymer Casting) and Anocast (Chagrin Falls, OH), a division of Rockwell Automation. New Vista contacted both companies, and selected ITW.

ITW recommended use of a wood mold, which is quicker and cheaper to build, but cannot hold tight tolerances. Life expectancy of a wood mold is just a handful of castings, which sufficed for New Vista. Other mold material options are fiberglass (high volume, low precision), sheet steel (high volume, medium precision), aluminum (high volume, medium precision, complex design), and steel (high volume, high precision). A steel mold can cost 3 to 4 times as much as a wood mold. For a steel mold ITW promises cast part flatness of .0025 in/ft and hole diameters of .001 in/ft, allowing for dowel holes to be cast directly into the composite. But threaded holes (and dowel holes when a wood mold is used) must reside in steel inserts. Inserts are pre-machined or machined after casting if better accuracy is required.

ITW influenced the direction of New Vista’s design from an early stage. Since cast composite is not as stiff as cast iron, much thicker cross sections are employed (but unlike cast iron, varying thickness won’t cause internal stresses). New Vista employed a 64” X 61” X 26” high solid block, with cut outs for forklift tines and adjustable mounting feet. Although composite has the same approximate density as aluminum, the base ended up being 6500 lbs., somewhat heavier than a cast iron base would have been.

The production process itself is easy compared to cast iron. Polymer composite, which is primarily crushed granite or quartz aggregate, combined with high-strength epoxy, a hardener, and coloring agent, is poured at room temperature and cures within 24 hours. No stress relieving or painting of the casting is required, although New Vista painted the sides of its base to match the customer’s factory colors.

The total cost of the composite base to New Vista was $13,500, including $4,500 for a wood mold, $1,600 for finish grinding the top surface flat to .0025, and $500 for secondary machining. Prior to quoting the job, New Vista had estimated spending $10,000 for a traditional base. Was the additional $3,500 for a composite base worth it? (New Vista suffered no additional internal cost designing or sourcing a composite base relative to a traditional base).

Yes, at least for the vibration damping benefits. Experts attribute to a composite base 5 to 15 times greater damping ability than cast iron. This has a number of benefits: better part surface finish, longer tool life, and decreased noise levels. Although part surface finish is not a critical objective for the customer, tool life is important. Six tool slides employ $220 worth of tooling inserts to perform a number of operations on the pipe fitting: trim end gate, machine grooves, turn “collars”, and apply inside & outside chamfers. With one ferrous fitting being finished every 11 seconds on a three-shift basis, even a 10% increase in tool life is significant.

Another widely-cited benefit of a composite base isn’t so clear-cut; the low thermal conductivity of a composite base can be a plus or a minus depending on the application. On the negative side, the base serves as a very poor heat sink, allowing unwanted heat to reservoir in work areas. Also, the use of dissimilar materials in composites causes unknown unevenness in thermal expansion across the base. On the positive side, although polymer composites have the same long-term expansion rate as steel or cast iron, the short-term speed of the response to heat is much slower. This means that a sudden temperature change caused by sunlight or a door opening does not have the same immediate tolerance-altering effect on a composite base. As the New Vista-designed machine tool doesn’t generate a lot of heat, and since there isn’t anything else tolerance-sensitive mounted to the composite beside the main slide and tool arch, the base’s thermal conductivity was not deemed a primary concern.

Now that ITW has developed the wood mold for this base, the second Automated Groove and Trim Machine, scheduled for delivery in early 2004, should make the proposition of a polymer composite base even more compelling.