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USDA offers help for evaluating CNC woodworking equipment.

Say that you manage a furniture or cabinet manufacturing plant. As manager, you are responsible for purchasing a CNC machine that not only performs a specific task now, but also has the flexibility to handle future tasks. How do you determine which machine meets your needs?

Answering this question can be difficult, because much of the needed information is not available in the sales literature. Also, the information available is not in a form that allows direct comparison between machines. So you might make the right decision and make substantial profits, or you might make the wrong decision and lose a great deal of money.

Not only buyers but also the producers and sellers of CNC woodworking machines are concerned about this problem. If you buy brand "A" and it does not meet your needs, you're going to be upset and probably tell others about your dissatisfaction with the machine. The negative advertising produced by this situation may cause potential buyers to not even consider brand "A" when that machine may be exactly what their production situation requires. Thus, both the seller and potential buyers of the brand "A" machine lose, because one poorly informed manager purchased the wrong machine for a particular milling situation.

To alleviate this situation, we need evaluation procedures that help both the seller and the buyer of CNC woodworking equipment determine the best machine for a given production situation.

The Advanced Hardwood Processing and Technical Resource Center of the USDA Forest Service at Princeton, W. Va., can provide that help. The Center has three primary functions:

* Conduct research to increase the value and utility of the hardwood resource through the development of new or improved manufacturing processes.

* Provide information to the industry about woodworking machines, complete and current research and international economic and marketing data. Several on-line information databases have been developed that can be easily accessed to answer specific questions in these areas.

* Conduct evaluations of computer-controlled machines for efficiency, safety, accuracy, computer control functions and economic criteria.

As part of the third function, the Center is developing and testing evaluation procedures for CNC woodworking equipment. Methods found in standards provided by organizations such as the American National Standards Institute and the American Society of Mechanical Engineers will be used when they meet evaluation needs. As draft procedures for a particular type of machine are finished, a committee of engineers from the equipment manufacturers and sellers and the equipment users will critique them and provide direction in their final development. Thus, the Center and industry will cooperate in the development of these procedures.

Preliminary evaluation procedures

Because CNC routers are some of the more complex pieces of equipment, we started with preliminary evaluation procedures for them. With these procedures, we have attempted to provide the information on spindle characteristics, table characteristics, spindle vs. table relationships, machining accuracy/repeatability and health/safety features that managers need to evaluate routers for specific machining tasks.

* Spindle characteristics: Several procedures have been developed to evaluate router spindles. First, we determine the power available, in kilowatts, for the spindle motor to machine the raw material. This available power is used to determine the maximum tool cutter diameters that can be used to process medium density fiber-board, plywood and solid wood of different species.

Next, we analyze the vibration in the spindle due to bearings, balance and tool centering ability of the collet assembly. With this information, the maximum out-of-balance allowable for different tool diameters and revolutions per minute can be determined. We also determine tool run out, trueness of tool rotation and tool deflection due to side load during cutting.

* Table characteritics: There are various procedures for evaluating the table. First, we determine the flatness of the table. Then the table travel is evaluated for parallelism, pitch and yaw. These table travel parameters reflect the straightness and alignment of the way shaves that guide the table. We also determine movement in the table due to the side pressures that occur during cutting. This measures side-to-side movement in the table due to slack in the pillow blocks that attach the table to the way shaves. It also measures slack in the ball screw assembly that drives the table.

* Spindle vs. table relationships: There are a number of procedures for evaluating the ability of the router to perform as directed by the machine's computer controller. In other words, we evaluate the relative travel between the spindle and the table top by determining: accuracy of cutting feed speeds, deviations in parallelism of the spindle travel to the table top, positioning accuracy of the spindle in all of its axes, straightness of spindle travel in all of its axes and squareness of spindle travel in each of the two axis groups.

* Machining accuracy/repeatability: Another group of procedures evaluates the actual machining capabilities of CNC routers. First, we evaluate the machining capability of the router under light cutting and normal cutting loads. Under light cutting loads, figures (circles, squares and diamonds) are cut in 1/4-in. sheets of cast acrylic. This allows an evaluation of the machining capability of the router without the effects of large side pressures that can occur during the cutting of MDF, plywood and solid wood. Then, under normal cutting loads, the same figures are cut in MDF board, plywood and solid wood to determine the machining accuracy of the router on these materials.

* Health/safety features: Finally, there are procedures which evaluate the efficiency of the machine's dust removal system and the noise level that can be expected during the machining of different types of material.

Committee review of procedures

We are in the process of organizing committee of engineers to help us develop the final CNC equipment evaluation procedures. The members will advise us not only in finalizing the preliminary router procedures, but also in the development of evaluation procedures for other types of CNC woodworking equipment.

The committee will be composed of engineers regularly involved with CNC equipment. These will represent a cross section of the industry and include both the procedures/sellers and the users of the equipment. We are also seeking representatives from large and small companies.

Once a set of preliminary evaluation procedures for a particular type of CNC equipment has been developed, the committee will meet to discuss modifications needed. This may result in the elimination or addition of procedures. Every attempt will be made to limit time demands on the committee members.

After the initial meeting, the Center staff will make the modifications recommended by the committee and test them on the equipment. The new procedures then will be mailed to the committee members for technical review. The review process will continue until the procedures are accepted by a two-thirds majority of the committee.

These accepted equipment evaluation procedures should help solve the communication gap that exists between the sellers and buyers of woodworking machines. Using the procedures, both sides will be talking the same language when trying to match machines to jobs. This should significantly reduce the possibilities of a buyer getting a machine that does not satisfactorily do the job.

The evaluation procedures will not provide specific values for the parameters being evaluated or measured. In other words, a procedure would not say that spindle positioning accuracy must be within 0.005 of an inch. This value must be specified by the machine seller or buyer based on the job to be done. What the evaluation procedure will do is define how that accuracy is to be measured.

Sellers and buyers probably will not use all evaluation procedures on every machine. They will consider only those procedures necessary to provide the information needed to assure satisfactory machine performance for a particular job.

For example, a manager may need a CNC router to cut fancy mirror frames and table tops out of oak. Both of these parts are to have ogee edges routed on them. In this situation, the manager wants to be sure that: the spindle motor has enough power to make the desired cuts in oak, the parallelism of the spindle to table travel is close enough to provide a satisfactory ogee edge on the parts, and the Z-axis positioning accuracy/repeatability will consistently provide the proper depth to the ogee edges. The manager would specify tolerance values for the evaluation procedures that are related to these factors. Since a fairly wide variation in the flat dimensions of both the mirror frames and table tops is acceptable, evaluation procedures related to the flat dimensions may not be used.

In the final analysis, the success of the evaluation procedures will depend on their acceptance by the industry. Thus, it is essential that representatives from industry be involved in their development.

Edward L. Adams is a forest products technologist for the USDA Froest Service, Forestry Science Laboratory, Rt. 2, Box 562-B, Princeton, WV 24740.
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Title Annotation:US Department of Agriculture's Forest Service; computer numerical control
Author:Adams, Edward L.
Publication:Wood & Wood Products
Date:Jan 1, 1992
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