Cast vs. Fabricated Machine Tool Structure Design Impact on Supply Chain Design

Spending on new metal working equipment in the US is projected to increase by $2.3 Billion in 2015 according to Gardner Research (1). How does the manufacturer of machine tools in the United States take advantage of this growth? This is an important question for machine tool design engineers and purchasing managers. The end user of a machine tool has many options that can meet her needs. In today’s competitive environment the end user considers the degree to which the machine tool meets her specific needs, the price and the lead time. To take advantage of the growth in capital spending machine tool builders must use machine design and supply change design to beat the competition. This article will explore the alternatives of cast versus fabricated structures in machine tool design and supply chain design.

Gardner Research’s 2015 Capital Spending Survey Machine Tools Executive Summary (1) reports that projected 2015 capital spending for US metalworking facilities will grow 37% compared to the latest estimate for 2014. Several industries are projected to have an increase from 2014 to 2015 of over 100%. These include metal cutting job shops, non-automotive forming and fabricating, industrial motors/hydraulic/mechanical components, custom processors, aerospace and plastic rubber products. If you are a builder of machine tools this is an opportunity you want to take advantage of.

The end user of machine tools wants performance, low price and short lead time. How does machine tool design impact these customer requirements? Machine tool design considerations for performance include damping, natural frequencies, inertial forces of moving members (Bamburg (2)), load bearing strength of components, stress when loaded, (AFS (3)), vibration control and heat control (Micromanufacturing (4)), and stiffness (Tobias and Koenigsberger (5)) to name a few. Design decisions will also impact:

  1. The degree of machine customization that can be offered
  2. The lead time to first production
  3. The lead time for production machines
  4. The fixed cost and variable cost structure of manufacturing the machine
  5. The cost to change design features at startup
  6. The cost to change design features over the life of the machine in the manufacturer’s portfolio

Performance of the machine tool is the primary consideration. Cast iron structures are able to handle high loads and high machining forces. Cast iron structures will transfer heat less effectively than fabricated structures, making them more stable under conditions of changing temperature. Cast iron dampens vibration better than steel. (Walker Machinery (6)). Picture #1 is a large CNC Vertical Gantry mill used for precision heavy machining of large components. The bed and column are cast iron. This is a large milling machine designed for heavy stock removal rates and machining precision.

Cast vs. Fabricated Machine Tool Structure Design Impact on Supply Chain Design, Picture 1 – Large Machine Tool Structure – Cast Iron

Picture 1 – Large Machine Tool Structure – Cast Iron

In many cases a well-designed steel fabrication can meet the performance characteristics of a cast iron structure. The design engineer has to determine if a cast base is required or if a fabricated structure can meet the performance requirements. Picture # 2 is a fabricated steel structure for a metal cutting machine. This machine is designed for lower metal removal rates, very low vibration at the tool tip, and high accuracy.

Cast vs. Fabricated Machine Tool Structure Design Impact on Supply Chain Design, Picture 2 - Fabricated Machine Structure

Picture 2 – Fabricated Machine Structure

How important is flexibility in machine design? Will offering customers the ability to customize each machine give the manufacturer a competitive advantage? Will the ability to incorporate new features and technology quickly help the manufacturer increase sales?

Cast iron components require a casting mold or pattern. Design, build and qualification of a casting mold or pattern takes time. If a change in design to the structure is required the mold or pattern must be modified or replaced. The requirement for a mold or pattern limits the flexibility for design changes. A structure that is fabricated from steel plate and steel tube requires little if any fixed tooling. Each structure that is designed can be unique.

Cost is an important consideration in the design of a machine tool structure. The casting mold or pattern is a fixed cost that must be incurred with cast structures. This fixed cost is not incurred with fabricated structures. The designer of a cast structure is able to put material in the structure only where it is needed. He has a great deal of design flexibility. He may be able to reduce the amount of material needed for the structure through casting design. (AFS (3)) There will be welding cost in a fabricated structure that is not present in a cast structure. The variable cost of the fabricated structure may be higher than the cast structure. It will depend on the design of each.

Lead times are an important consideration in design. The manufacturer that can deliver quickly often gets the order. The lead time to produce a cast structure is longer than the lead time to produce a fabricated structure. The casting has the lead time for the mold or pattern as well as the lead time to cast and machine the structure. Even after the mold or pattern is built, the lead time to pour a large casting may be longer that the lead time to obtain standard sizes of steel plate and tube and fabricate. The fabricated structure has the lead time advantage.

There are at least two types of design flexibility. One is the flexibility to modify a standard machine configuration for a specific customer. The other is to completely redesign a standard machine to incorporate the latest technology or machine features. Every machine can be different with a fabricated structure. The base design is modified to add or delete features. There is no or little fixed costs that will become obsolete if an entire standard machine design is discarded and a new design machine takes its place. Because a fabricated structure does not need a mold or pattern, the fabricated structure has the advantage in terms of design flexibility.

The structure design issues have been addressed above. Let’s take a look at design of the supply chain for a machine structure. There are three main processes in the manufacture of a cast structure. They are casting, machining and painting. There are four main processes in the manufacture of a fabricated structure. They are fabrication, stress relieving, machining, and painting. As the structure gets bigger the number of foundries that are able to cast the structure gets smaller. The capital investment in the facility to fabricate a large structure is smaller than the capital investment in the facility needed to cast a large structure. There are more companies that can fabricate a large structure than there are companies that can cast a large structure. The larger number of potential suppliers gives the machine tool company purchasing manager more options in looking for the right combination of price and delivery.

Transportation of the machine tool structure can be expensive. Large structures may require oversize load permits and their added cost. The ideal supply chain will have all of the processes performed by a single supplier. This will reduce transportation cost. If they cannot be completed by the same supplier having all the processes performed at facilities close to each other will minimize the transportation distance and cost between processes.

An indirect but real benefit of having one supplier responsible for all processes is the reduced supplier management costs incurred by the machine tool builder. The purchasing professional at the machine tool builder will have to manage only one supplier rather than several. There is also an increased accountability. With one supplier the machine tool builder and the supplier know who is responsible if there is a quality or delivery problem. With multiple suppliers there may be disagreement and time and money wasted finding the root cause of a problem and determining who is accountable for the costs to correct the problem.

Machine performance requirements may drive the machine tool builder to select a cast structure design. There are many advantages to a single source for fabricated steel machine tool structures. If the machine tool design permits a fabricated steel structure cost and lead time can be reduced. A single source for the complete fabricated structure can provide a high performing supply chain that is accountable for quality and delivery.

References:

  1. 2015 Capital Spending Survey Machine Tools Executive Summary
  2. Rapid Machine Design, Bamberg, Slocum,, Massachusetts Institute of Technology, Cambridge MA.
  3. Steel Fabrications vs. Castings, American Foundry Society (AFS), Schaumburg IL.
  4. Good vibration (control): Small vs. large machines, Kennedy, Micro manufacturing.
  5. Advances in Machine Tool Design and Research 1967, The University of Manchester Institute of Science and Technology, September 1967
  6. Walker Machinery

June 23, 2015

 


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