Machine Selection for Machining
Machine selection is described in the following sections:
Machine Selection for Milling and Turning
In addition to actual machines, each starting point VPE provides a series of virtual machines so that you can easily compare the costs of machines of comparable capabilities. For example, you can compare the cost of machining a part on a 3-axis mill as opposed to a 5-axis mill with comparable spindle power and spindle speed.
The use of virtual machines reduces sensitivity of the aPriori results to specific machines, and helps support Design Engineering use cases such as early design cost estimation or design alternative evaluation. In practice, machining cycle times and costs are highly sensitive to the precise speed and power characteristics of the machine actually used for manufacturing.
These virtual machines:
• Provide consistent initial assumptions for machine capabilities between process routings for the purposes of initial/baseline costing.
• Minimize confounding factors to correct routing cost and cycle time directionality.
• Provide an underlying mechanism to allow for the initial preference of a particular subset of machines within a routing, in this case what we refer to as "Virtual" or generic machines.
Virtual machines are chosen by default. You can change this behavior by setting the cost model variable preferVirtualMachines to false.
The virtual machines and their specifications are shown in the following chart:
Machine selection has three distinct modes, in the following order of preference:
1 Select the smallest virtual machine that will fit the part if:
• Virtual machine selection is turned on in the VPE (default)
• There is a virtual machine large enough for the part
2 Select the default machine specified for the process in the VPE manager if:
• A default machine is specified
• The machine is large enough for the part
3 Select a machine as described in Non-virtual, Non-default Machine Selection for Milling and Turning.
This translates into the following default behaviors:
• Virtual Machines will always be selected for the in-scope processes.
• If the VPE is configured to not allow virtual machine selection (by setting the cost model variable to false), the system will select the default machine -- if specified and feasible -- ahead of all other machines.
• If any machine is tagged true in the Is Virtual machine data field, it is considered within scope of virtual machine selection if turned on.
• There is intuitive and consistent cost and cycle time directionality between routings for a given part:
o A simple, good 3-axis candidate part will naturally cost less on the 3 axis machine than the 5 axis machine.
o Directionality remains consistent and intuitive across a range of part sizes
Non-virtual, Non-default Machine Selection for Milling and Turning
In non-virtual, non-default machine selection mode (see Machine Selection for Milling and Turning), machine selection for milling and turning picks the least expensive machine (see Determining Machine Expense) that meets all the following criteria:
• Machine is big enough to accommodate the part’s dimensions (or the biggest machine, if none is big enough to accommodate the part)
• Spindle speed (the machine attribute spindleSpeed) is about half way between the following values:
o Minimum spindle speed: spindle speed of the machine with the lowest spindle speed of all machines big enough to accommodate the current part’s dimensions (or the spindle speed of the biggest machine, if none is big enough to accommodate the part)
o Maximum spindle speed: spindle speed of the machine with the greatest spindle speed of all machines big enough to accommodate the current part’s dimensions (or the spindle speed of the biggest machine, if none is big enough to accommodate the part)
• Spindle power (the machine attribute power or spindePower) is about half way between the following values:
o Minimum spindle power: spindle power of the machine with the lowest spindle power of all machines big enough to accommodate the current part’s dimensions (or the spindle power of the biggest machine, if none is big enough to accommodate the part)
o Maximum spindle power: spindle power of the machine with the greatest spindle power of all machines big enough to accommodate the current part’s dimensions (or the spindle power of the biggest machine, if none is big enough to accommodate the part)
The selected machine’s spindle speed and power are approximately as close as possible (given the available machines) to halfway between the minimum and maximum values, without falling under the halfway mark.
You can adjust how close to the maximum (relative to the minimum) the selected machine’s spindle speed and power are, by specifying a value between 0 and 1 for the following cost model variables:
• spindleSpeedBias
• spindlePowerBias
These variables specify a fraction of the distance to the maximum (from the minimum). For example, 0.75 specifies three quarters of the distance from the minimum to the maximum. These variables both default to 0.5.
If routings for the parts at a plant generally have a lot of contouring, increasing spindleSpeedBias and spindlePowerBias will typically lower cycle times.
Machine Selection for Drilling
For the Deep Bore Machine process, aPriori selects the least expensive machine (see Determining Machine Expense) that meets both the following requirements
• Machine’s Max Hole Length can accommodate the part’s length (select Component > Base Properties > Component in the Geometric Cost Drivers pane, and see the property length)
• Machine’s Max Hole Diameter can accommodate the part’s width (in the Geometric Cost Drivers pane, select Component > Base Properties > Component, and see the property width).
If there is no such machine, the machine with largest Max Table Length is used.
For the Gun Drill process, aPriori selects the least expensive machine (see Determining Machine Expense) whose Max Drill Length can accommodate the part’s box length (in the Geometric Cost Drivers pane, select Component > Base Properties > Component, and see the property length). If there is no such machine, the machine with largest Max Drill Length is used.
For the Drill Press process, aPriori selects a machine as described in Machine Selection for Milling and Turning.
Machine Selection for Broaching
Broaching selects the machine with the lowest overhead (specified by the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate).
Machine Selection for Jig Bore and Jig Grind
aPriori selects only from preferred machines, if there is a feasible preferred machine and the cost model variable usePreferredMachines is set to true (the default in starting point VPEs). If there is no feasible preferred machine, or if the cost model variable usePreferredMachines is set to false, aPriori selects from all machines. A machine is preferred if the machine property isPreferred (typically displayed as Is Preferred) is true.
CNC vs. manual machines: When aPriori selects from preferred machines, it selects only machines whose Machine Style property matches the value of the cost model variable jigBoreGrindPreferredMachineStyle (CNC in starting point VPEs).
A machine is feasible if it can accommodate the part dimensions. The largest machine is always considered feasible—the cost model assumes that the part is supported by rests in that case.
Machine Selection for Bench Operation
aPriori selects the feasible machine with the lowest overhead rate, where overhead rate is the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate. A machine is feasible if both the following hold:
• Part length is no greater than the length of the work area (specified by the machine property Length of machine).
• Part height is no greater than the width of the work area (specified by the machine property Width of machine). Note that the cost model assumes that the part is on its side during this process, with the part’s height oriented along the width of the work area.
Machine Selection for Mask Spray
See Mask Spray Machine Selection in the Surface Treatment chapter of this Guide.
Machine Selection for Mask Cure
aPriori selects the feasible machine with the lowest overhead rate, where overhead rate is the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate. A machine is feasible if both the following hold:
• Machine dimensions can accommodate the part dimensions.
• If the setup option Heated cure cycle has been set to false, the Machine property Heated Cure is false (see Mask Cure Options).
The cost model assumes that the part is on its side during this process, with the part width oriented along the height of the oven. Feasibility requires that the part fit in one of the following two orientations:
• Square fit: Part length oriented along the oven length (and the part height oriented along oven width)
• Diagonal fit: Part length oriented along the oven length/width diagonal
The oven dimensions are given by the machine properties Usable Height, Usable Width, and Usable Height.
Machine Selection for Scribe
aPriori selects the feasible machine with the lowest overhead rate, where overhead rate is the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate. A machine is feasible if the bed dimensions can accommodate the part dimensions, that is, if all the following hold:
• Part length does not exceed the machine property Bed Length.
• Part width does not exceed the machine property Bed Width.
• Part height does not exceed the machine property Bed Height (the machine’s Z travel)
Machine Selection for DeMask
aPriori selects the feasible machine with the lowest overhead rate, where overhead rate is the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate. A machine is feasible if both the following hold:
• Part length is no greater than the length of the work area (specified by the machine property Length of machine).
• Part height is no greater than the width of the work area (specified by the machine property Width of machine). Note that the cost model assumes that the part is on its side during this process, with the part’s height oriented along the width of the work area.
Machine Selection for Etch Cell
aPriori selects the feasible machine with the lowest overhead rate, where overhead rate is the sum of the machine properties Direct Overhead Rate and Indirect Overhead Rate. A machine is feasible if the machine’s tank dimensions can accommodate the part dimensions.
The cost model assumes that the part is on its side during this process, with the part width oriented along the depth of the tank. Feasibility requires that the part fit in one of the following two orientations:
• Square fit: Part length oriented along the tank length (and the part height oriented along tank width)
• Diagonal fit: Part length oriented along the tank length/width diagonal
The tank dimensions are given by the machine properties Tank Depth, Tank Length, and Tank Width. Note that these are different from the machine dimensions, which specify the footprint of the etch cell. The etch cell includes six tanks: two for rinsing and one each for etching, drying, desmut, and desmut rinse.
Determining Machine Expense
aPriori selects the least expensive machine by combining the following machine attributes:
• Labor Rate
• Labor Time Standard
• Overhead Multiplier
• Overhead Rate
These factors are combined as follows:
LaborRate * LaborTimeStandard * (1 + OverheadMultiplier) + OverheadRate