Success in DIY Machining

John Staudinger

Keeping Your Precision-Machining Contract Shop Competitive

This 25-year-old contract shop's recipe for success includes CNC, coordinate measuring and computerized scheduling.

If you operate a precision-machining contract shop today, you know how tough the competition is. Fortunately, technology can help.

After starting 25 years ago with an $875 lathe in a basement, Arlington Machine had quickly grown to a 25-man shop with manual machines and one NC (numerical control) milling machine. However, business was drying up because prices for machining were being slashed by the competition. They were using computer numerical control (CNC).

This situation provided the impetus to plunge into CNC with a big investment in machining centers and CNC lathes. As more CNCs were installed, business increased accordingly. The CNCs allowed machining of workpieces ranging in size from the very small up to 80 by 30 by 25 inches and up to five-axis complexity.

By 1983, there were a total of eight CNCs. A coordinate measuring machine (CMM) was added to simplify and speed inspection. Today, there are eleven machining centers running two shifts, six CNC lathes, and it is clear that a second CMM is needed to keep up with steadily increasing business. In fact, the new CMM will require the added capability of CNC to preserve and continue the shop's growth.

Based on lessons learned through experience, three ingredients are necessary to succeed as a precision-machining contract shop. These are versatile CNC machining, a CMM for in-house coordinate-measuring capability, and computerized scheduling

CNC Machining

The first ingredient, computerized machining, requires more than one or two CNC machines. Only with multiple CNC machine tools will there be enough sales volume to support a methods department and a QC department (just as it takes more than one house in a community before the community can support a fire department and a police department.)

When a key CNC machine is down and switching to a manual machine is necessary to fulfill a delivery promise, the value of CNC stands out. For instance, it was discovered early that a gear housing with circular grooves could be machined for $360 each by CNC, but cost $1200 with manual machining.

The eleven machining centers equipped with pallets are important to Arlington Machine because a job can be set up once, the workpieces run, and the setup saved for running another lot later without repeating an extensive setup. This is particularly effective for those customers using just-in-time (JIT) production scheduling. Pallets also permit one job to be run while another is set up and multiple pallets can be loaded so the machining centers can run unattended.

Programming

A Schlumberger (Applicon) system provides programming for the CNC machines. It includes the Equinox computer graphics package. This combination allows work-pieces with difficult contours to be programmed easily, and is beneficial on jobs consisting of as few as one or two pieces. To machine an outside contour, for example, part boundry is described indicating whether it requires inside or outside contouring, and a G-code program is generated automatically.

Another advantage of this programming system is adaptability to machine tools and control units from various vendors. It produces a source program which, after post processing, runs any of the machining centers, regardless of brand.

Direct numerical control (DNC) is used where applicable, giving direct-line access to the CNC machines--punched tape can be run directly from the computer to the CNC machine.

Inspection with CMM

The answer to the question "You can make it. But can you measure it?" illustrates the second ingredient for success and demonstrates the importance of a coordinate measuring machine.

One example is a high-precision government job run on three CNCs at one time. Forty castings valued at $500 each were involved. Each had varied reference surfaces.

With manual inspection, four days were required to verify the first piece. The job could not proceed until verification.

At an average rate of $50/hour for CNC machining time and 10 hours of lost time daily for each machine, total cost of lost time on the three CNC machines was $6000. However, with the CMM, first-piece inspection takes 3 hours, for a total machining-time loss of only $450. With CNC CMM, first-piece inspection can be reduced to a half hour.

For another customer, 100 complex workpieces were received and machined with many compound angles having intersections located to [plus or minus] 0.001 inch. Thanks to inspection by the CMM, all 100 workpieces were shipped, and all 100 were accepted. With manual inspection, this job probably could not be completed successfully, with all pieces passing.

Over a two-year period, the company studied CMMs and analyzed various designs before purchasing a FJ1006 CMM from Mitutoyo. No problems have been experienced in the four years since installation.

A CMM can astonish workers only familiar with mechanical inspection methods. Yet, learning to use a CMM is easy.

Although a three-day training period is normal for an inexperienced CMM inspector, one inspector at Arlington took the manual home overnight and used the CMM the next day.

Generally, the output of one CMM is equivalent to the output of seven or eight inspectors checking manually. The machine has the ability to check to within 0.0002 inch. As for repeatability, the CMM gives an identical numerical readout when the same measurement is made six or seven times.

At present, the CMM does first-piece and final inspection.

The new CNC CMM will be dedicated to in-process and final inspection. On repeat jobs, the CNC CMM will contain the program for the previous job, and so a first piece can even be checked in less time than now needed. Later, the CNC CMM will be connected with the methods department, and the one program will be used for both making and measuring the part.

Computerized Scheduling

The third ingredient for success is computerized scheduling. For Arlington Machine, computerized scheduling is essential for locating open time for rush jobs and determining profitability of a job for a given number of workpieces.

A Profitkey International job-shop scheduling system is in use. It is capable of both forward and backward scheduling.

With forward scheduling, what-if modeling forecasts the match between jobs and capacities, and then establishes priorities.

Backward scheduling proceeds in a reverse time-line from desired completion date.

Before computerized scheduling, conventional cost accounting provided after-the-fact information, such as setup time and cycle time, only when it was too late to do anything about a particular job. Computerized scheduling provides before-the-fact information, which may indicate that reducing times by modifying fixtures or increasing speeds and feeds may be necessary.

Today, many customers are consolidating their vendor bases. To remain on their list, a shop must have a variety of CNC equipment; be able to supply CMM documentation; and use computerized scheduling.

Precision-machining contract shops which hope to compete today and survive tomorrow have to keep on top of technology. [Fig. 1 to 3 Omitted]