Beyond the ROI calculation: Turning Performance

A rational approach to machine tool selection.

Since my first manufacturing job in 1976, which was in a screw machine shop, work for me has always been about the machines. Machines remained the focus as I progressed from summer help chamfering bar stock to novice graduate engineer to—many years later—a manager of manufacturing engineering. I’ve selected a lot of machine tools but realized there was no singular, formal defined method to select machinery. I looked around and found no book or selection methods with the possible exception of well-worn accounting principles.

Industrial and manufacturing engineers primarily perform machine selection and cost analysis by what is called “the engineering economy method.” These methods are strictly for capital investment analysis and comparison. They examine the return on investment (ROI) or rate of return based on enhancements to production capacity and capability. That is all well and good, but when I selected machinery, that was not where I started.

My method was an outgrowth of the methods I saw other engineers use. Those include spreadsheets, marketing data, discussions with plant management and similar “fishing trips” to determine real needs. There was no singular plan taught or used extensively to help the selector—typically the owner, engineer or manager—understand and quantify the considerations and alternatives. It was up to each designated selector to create a method to best pick the equipment. I’ve summarized my method in the graphic on the next page, the variety/volume matrix of machine tool categories.

Machine typeDescription and considerationsCosts (machine/per part)Operator knowledge1

Manual machines with DROs (toolroom equipment), a full range of basic, single-process machines are required. Accuracy is very dependent on operator skill.

Least/highest

Very good to excellent (toolmaker level)

2

Commodity/stand-alone CNC machines, a “no frills machine” with few manufacturer options to increase cost (e.g., through-spindle cooling).

Medium/medium

Basic operator, support by standard engineering

3

Stand-alone, “best in class” CNC machines with selected productivity features that increase part throughput and accuracy. An enhanced version of a commodity machine.

Medium to high/medium

Experienced with full operational support staff

4

Cellular/flexible manufacturing system arrangement. Built from machines that are capable of the concept and can have robotic handling of parts, if required.

Medium to high/medium to low

Experienced with full operational support staff

5

Hybrid machine tools, the new generation of multitasking machines and concept machines that do operations such as hard turning, with top-end tools from top-end makers.

High/medium to low

“Best and brightest” team is required

6

Custom version of No. 5, feature-rich, special design from builder with special workholding and special tools. This is a well-engineered machine with many part-specific innovations.

High/low to lowest

Same as No. 5

7

Transfer machine/multispindle bar machine. A “part family-specific” design and build, using standard modules available from the builder and a range of construction options.

High to highest/low to lowest

Same as No. 5, not for the”uninitiated”*

8

Custom version of No. 7, the highest end of the machine tool builder’s art with special features that can be proprietary to any number of suppliers. The best possible machine but at the highest initial cost.

Highest/lowest

Same as No. 5, not for the”uninitiated”*

* “Uninitiated” refers to the use of equipment by those without extensive previous experience using and supporting the equipment. The use of equipment in levels 7 and 8 will require a huge transition from level 6 equipment. The application will require a much greater level of support in terms of engineering, quality, maintenance, operation and supervision.

Machine tool selection must take into account diverse issues related to machine tool type, operator skill and total capital available. Furthermore, there are no “good” or “bad” machine tools—just ones that are better fits with specific sets of requirements. Awareness of machines and options enhance the ability to pick the “best” machine tool for a specific set of operational conditions, considerations and, of course, budgets. I’ve seen experienced hands achieve more with older, less-sophisticated equipment than those operating new CNC machines.

Machine tool selection is not just an exercise in finding the equipment with the right work cube size and voltage. It is a multifaceted process requiring skill and thought.

Companies have historically improved on manufacturing processes within their facilities. This is not news. In his book, “Fundamentals of Modern Manufacturing: Materials, Processes and Systems” (Prentice-Hall Inc., 1996), Mikell P. Groover stated, “The history of manufacturing can be separated into two subjects: (1) the discovery and invention of materials and processes to make things, and (2) the development of systems of manufacturing.” Machine tool selection addresses both areas.