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Software cures tolerance stack up.

If you have ever watched your parts passing through machining operation after machining operation only to find that deviation from engineering drawing dimensions had gone beyond acceptable limits, there's a good chance manufacturing process planning caused the problem.

In the manufacturing engineering world, these deviations in machining linear dimensions are referred to as tolerance conflicts or "stack ups," explains Lee Wagstaff, plant manager of Equitable Engineering Co, Troy, MI.

Because the effect of missing tolerances on a series of machining operations is additive, at the very least, you may produce a part that doesn't meet blueprint specs. The worst-case scenario is that you may run out of stock before the part is finish machined.

Mr Wagstaff explains that process planners, the people who manufacture parts on paper, have been using tolerance charting for years to prevent tolerance stack-up problems caused by interrelated blueprint dimensions and their associated tolerances. The charts provide protection against exceeding the allowable variations (tolerances) assigned to the linear dimensions of a part by the part designer and do not apply to diameters or other non-linear characteristics.

The problem with tolerance charting, at least up until now, is that it is a time-consuming manual process that can be flawed because of the possibility of arithmetic errors. In addition, misjudging tolerance interaction can lead to catastrophic results in final part production, says Mr Wagstaff.

Equitable Engineering, a gear manufacturer that specializes in flight critical" aircraft components, has developed a software solution--the EEC Tolerance Charter--that computerizes and simplifies tolerance charting.

The software enables the manufacturing engineer to develop a part manufacturing plan, define the step-by-step process by which the part will be made, enter the data into the chart, and review and edit the process--without ever cutting metal.

The first step is marking up an engineering drawing or tracing (see Fig 1). Each linear surface is defined with a final dimension and its related tolerance. The surfaces are numbered consecutively, in left-to-right fashion, doing both internal and external surfaces that appear parallel to the starting surface, or, from another perspective, at right angles to the left-to-right scan.

Once this is done, the planner is ready to build a computerized chart. Selecting the Build function from the program's main menu, the user enters data according to the prompt screens.

The user is able to see each operation on the Review/Edit screen (Fig 2), including the "from" and "to" planes involved in each cut for that operation, the dimensions and tolerances that will ultimately be used in the process plan when it is released to the shop floor, and the anticipated stock removal for each cut.

An error-message column indicates whether the cumulative tolerances used during the machine cuts exceed final engineering drawing tolerances and if proposed stock removal exceeds previously defined maximum stock removals.

While at this screen, the user can make final adjustments to the machine-cut dimensions and tolerances and perform a recalculation to fine-tune the chart.

Viewing the Balance Summary screen (Fig 3) shows the software's tracking capability. Each blueprint drawing dimension and tolerance is shown along with its balance tolerance, which is the result of all machine cuts having a direct impact on final blueprint requirements.

"The system will report when tolerances can be relaxed, when they should be tightened, and when they must be tightened," says Mr Wagstaff.

Equitable is marketing the software, which is priced in the "higher" range, to other companies, says Mr Wagstaff. The EEC Tolerance Charter is currently available on tape for use on Hewlett Packard 260 and 9000 series computers. An IBM-compatible DOS format is being developed.

For more information on tolerance charting, circle 374.
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Title Annotation:EEC Tolerance Charter; deviations in machining linear dimensions
Publication:Tooling & Production
Date:Jul 1, 1992
Previous Article:Turning to CNC modeling.
Next Article:CNC software speeds run times.

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