CMM Inspection
CMM (coordinate measuring machine) inspection is the process of physically measuring a part in order to obtain precise, relative positions of various part features. aPriori's CMM Inspection process models the times and costs associated with CMM inspection, assuming the use of a touch-trigger probe. The model is described in the following sections:
Machine Selection for CMM Inspection
aPriori selects the default machine if it is feasible; otherwise, aPriori selects the feasible machine with the lowest overhead (specified by the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate). The machine feasibility rules require that both the following hold:
• Part dimensions can be accommodated by machine bed, as specified by the machine properties Bed Length, Bed Width, and Bed Height.
• Part mass (the product of part volume and material density) does not exceed the machine property Max Allowable Mass.
Formulas for CMM Inspection
The CMM Inspection cost model relies on the formulas below.
Labor Cost = Labor Time * Labor Rate / Final Yield
Labor cost depends on the following:
• Labor time (see formula)
• Labor rate (specified by the machine property Labor Rate)
• Final yield (see the section on Yields in the chapter for the primary process group)
Labor Time = Cycle Time * Number of Operators * Labor Time Standard
Labor time is the product of the following:
• Cycle time (see formula)
• Number of operators: specified by the machine property Number of Operators.
• Labor time standard: specified as the machine property Labor Time Standard. This multiplier is used to account for otherwise unaccounted for factors that affect labor time, such as operator fatigue or time spent by the operator for cleaning or maintenance.
Cycle Time = Process Time * Cycle Time Adjustment Factor
Cycle time is the product of the following
• Process time: see formula.
• Cycle time adjustment factor: specified by the cost model variable cycleTimeAdjustmentFactor (1 in starting point VPEs). Administrators can customize this value in order to globally adjust cycle times for Assembly Molding.
Process Time = (Number of Probe Touches Per Part *
Default CMM Probe Time * Total Sample Size) / Part Batch Size
Process time is the number of probe touches performed per batch, amortized over the number of parts in a batch.
• Number of probe touches per part: this the number of probe touches for a single part, which is calculated based on the number of features of various types found on the part. See the formula below.
• Default CMM probe time: this is the time for a single probe touch. It is specified by the cost model variable defaultCMMProbeTime (2 seconds in starting point VPEs).
• Total sample size: this is the number of parts to be inspected per batch. By default, this is looked up in the lookup table CMMInspection, based on the following:
o Batch size (specified in the Production Scenario screen of the Cost Guide)
o Inspection type: this is Standard Sample Sizes (AQL General Inspection) by default in starting point VPEs. Administrators can customize the default with the cost model variable defaultCMMInspection. Users can override the default inspection type on a part-by-part basis with the setup option Type of CMM Inspection (Per ANSI Standards)--see Setup Options for CMM Inspection for more information on inspection types.
The lookup table CMMInspection contains information provided by the ANSI/ASQ Z1.4-2008 standard.
Users can override the default sample size with the setup option Inspected Sample Size Per Batch—see Setup Options for CMM Inspection.
• Part batch size (specified in the Production Scenario screen of the Cost Guide)
Number of Probe Touches Per Part =
Simple Hole Total Touches +
Planar Face Total Touches +
Curved Surface Total Touches +
Curved Wall Total Touches +
Pocket Total Touches
For each of five GCD types (SimpleHole, PlanarFace, CurvedSurface, CurvedWall, Pocket), the following formula is used:
<GCD Type> Total Touches = Number of <GCD Type> Features *
Number of Touches per <GCD Type> Feature * Percent of Part to be Tested
A part's total number of touches for its GCDs of a given type is the product of the following:
• Number of <GCD type> features: this is the number of features of type <GCD type> on a single part. For example, if <GCD type> is SimpleHole, this is the number of SimpleHoles on a single part.
• Number of touches per <GCD type> feature: this is the number of probe touches performed on each feature of the type <GCD type>. For example, if <GCD type> is SimpleHole, this is the number of probe touches performed on each tested SimpleHole of the part. It is looked up by GCD type in the lookup table CMMInspectionProbeTouches.
• Percent of part to be tested: this is the percentage of a part's features of type <GCD type> that are tested. By default in starting point VPEs, this is 100%. VPE administrators can customize the default with the cost model variable defaultPercentToBeTested. Users can override the default on a per-part basis with the setup option Percent of Part to be Tested--see Setup Options for CMM Inspection. So, for example, if the percent of the part to be tested is 50%, then half the part's SimpleHoles are tested, half the part's CurvedWalls are tested, and so forth.
Amortized Batch Setup =
(Setup Time * (Labor Rate + Direct Overhead Rate)) / Batch Size
Batch setup cost per part depends on the following:
• Setup time (specified as the machine property Setup Time)
• Labor rate (specified by the machine property Labor Rate)
• Direct overhead rate (see Direct and Indirect Overhead)
• Batch size (specified in the Production Scenario screen of the Cost Guide)
Setup Options for CMM Inspection
The CMM Inspection process provides the following setup options:
• Type of CMM Inspection (Per ANSI Standards): this specifies the type of inspection to be performed, which affects the number of parts to be inspected per batch (see Process Time in Formulas for CMM Inspection). There are two broad inspection levels, as described in ISO 2859-1 and ANSI/ASQ Z1.4:
o Standard Sample Sizes (AQL General Inspection): includes inspection levels I, II, and III.
o Smaller Sample Sizes (AQL Special Inspection): includes inspection levels S1-S4. With these levels, a smaller percentage of the total batch is inspected, compared to General Inspection levels.
By default in starting point VPEs, the inspection level is Standard Sample Sizes (AQL General Inspection). VPE administrators can customize the default with the cost model variable defaultCMMInspection. Users can override the default on a part-by-part basis with this setup option.
• Inspected Sample Size Per Batch: this specifies the number of parts to be inspected per batch. By default, this is looked up in the lookup table CMMInspection, based on inspection type and batch size (see Process Time in Formulas for CMM Inspection). With this setup option, users can override the default, and specify the sample size explicitly.
• Percent of Part to be tested %: this specifies the percentage of features of various types to be tested on each part in the tested sample. For example, if this value is 50%, then half the part's SimpleHoles are tested, half the part's CurvedWalls are tested, and so forth. The following feature types are included:
o SimpleHole
o PlanarFace
o CurvedSurface
o CurvedWall
o Pocket
See also Process Time in Formulas for CMM Inspection.
By default in starting point VPEs, this is 100%. VPE administrators can customize the default with the cost model variable defaultPercentToBeTested. Users can override the default on a per-part basis with this setup option.