Chemical Milling
Chemical milling is a non-traditional, subtractive manufacturing process in which material is removed by means of a strong, temperature-regulated chemical etchant. aPriori cost models support chemical milling of sheet metal parts that use the CTL/Shear/Chemical Mill routing, that is, parts that are first subject to the Cut-to-length, Shear, and (optionally) Roll Bending processes. Chemical Milling must be manually assigned to a part. See Routing a Part to Chemical Milling.
Note: Do not attempt to use Chemical Milling with any routing other than CTL/Shear/Chemical Mill. Chemical Milling with other primary routings will fail to cost.
Chemical milling involves submersing the cutting areas in a corrosive chemical known as an etchant, which reacts with the material in the area to be cut and causes the solid material to be dissolved; inert substances known as maskants are used to protect specific areas of the material.
Within aerospace applications, chemical milling is typically used on large sheet metal components to reduce the overall weight of the component. Common examples of this can be found on wing or fuselage skin panels where areas not being fastened to ribs or stringers are reduced in thickness.
The rate of chemical etching is slow in comparison with traditional milling machine removal rates. However, the advantages are that it can be applied to very large, complex shapes where a large amount of material is removed. Also, the etching time is a function of the pocket depths rather than the number of pockets being milled, therefore many features of the same depth can be etched simultaneously.
Chemical Milling GCDs
When a part is routed to Chemical Milling, Form GCDs that are of type STAMP are automatically assumed to be created by chemical milling, and are assigned to Chemical Milling operations. These GCDs are pocket-like regions whose floor and walls satisfy all the following:
• Floor consists of a region of thinner (lower gauge) material (compared to elsewhere on the blank).
• Floor is parallel to the opposite side of the part.
• Walls have a height that is less than or equal to the gauge reduction.
• Walls are perpendicular to the floor.
• Walls form a sharp angle with the surrounding blank.
Note that the walls can have a 90 degree fillet with the floor, or can consist entirely of a 90 degree fillet.
Note also that a pocket can be located within another, shallower pocket.
Users can also manually assign surface GCDs (Curved Surfaces, Curved Walls, and Planar Faces) to Chemical Milling—see Assigning Chemical Milling to a Surface GCD.
Chemical Milling Process Overview and Routing
Chemical Milling routings include the following processes in the following order. These processes (except Mask Spray) are part of the Machining process group:
• Bench Operation: This process has as children several operations (cleaning, identing, line sealing, spark testing, and final inspecting) that are performed at the same work area, though they are performed at disparate times during the execution of the routing. See Machine Selection for Bench Operation, Bench Operation Formulas, and Bench Operation Options.
Chemical Milling routings begin with the following Bench operations, which are performed on the Blank GCD:
o Cleaning: the entire part is cleaned in order to ensure that material will be removed uniformly from the surfaces to be etched. The cost model assumes the use of Diestone SR (PF-SR) Wipes. The unit cost of wipes is specified by a toolshop variable.
o Identing: a unique identifier is vibro-etched onto the part. By default, the time required is specified by a cost model variable; a setup option allows users to override the default.
• Mask Spray: a protective coating (maskant) is applied (by spraying) to all portions of the part surface. This maskant is made of a material that is chemically resistant to the etchant. The cost model assumes the use of AC-850-AERO-D-TAN. Maskant unit cost is specified by a toolshop variable. A setup option allows users to customize the number of maskant layers, as well as the programming cost. This process is part of the Surface Treatment process group—see Mask Spray Machine Selection, Mask Spray Formulas, and Mask Spray Inputs.
• Mask Cure: the maskant is developed (cured) either in a heated oven or in an ambient-temperature drying room--a setup option allows the user to specify which. By default, the time required is looked up in a lookup table; a setup option allows users to override the default time. See Machine Selection for Mask Cure, Mask Cure Formulas, and Mask Cure Options.
• Scribe: scribe lines are cut in the maskant along the perimeter of each GCD to be etched, in preparation for selective removal of maskant during etching. Scribing is performed on all GCDs, before any GCD is etched. The cost model assumes the use of a five-DOF robotic system with flexible fixtures and carbon dioxide laser. A setup option allows the user to customize the programming cost. See Machine Selection for Scribe, Scribe Formulas, and Scribe Option.
• Bench Operation: see Bench Operation, above. The following operations are performed after scribing and prior to etching:
o Cleaning: performed on the Blank GCD. The part is cleaned to remove maskant dust created by the scribing operation.
o Line Sealing: scribe lines are manually resealed. One execution of the Line Sealing operation, performed on the Blank GCD, uses a roller to apply line sealing solution to the surface of the entire part. Then additional executions of the Line Sealing operation use a brush to apply line sealing solution to scribed pocket edges. There is one such additional execution for each scribed pocket.
o Spark testing: performed to detect pinholes in the maskant. A setup option allows customization of the time required.
• Etch Cell: This process performs one or more Etch Cell Cycle operations, followed by desmut and desmut rinse procedures. Pockets are grouped by depth (pockets in the same group have the same depth), and there is one Etch Cell Cycle for each group. During each cycle, the following operations are performed:
o Pocket Peeling: at the beginning of each cycle, the maskant is peeled away from the pockets in the cycle’s associated group.
o Pocket Etching: following pocket peeling, the part is immersed in an etchant that chemically attacks those portions of the part surface that are not masked. When the desired amount of material has been removed, the part is withdrawn from the etchant. A setup option allows customization of the etching rate.
o Rinsing: following etching, the part is rinsed to stop the etching process.
o Drying: following rinsing, the part is dried.
o Depth Inspection: at the end of each cycle, the depths of the pockets are checked for depth tolerance.
The deepest pockets are etched in the first cycle, as well as in every subsequent cycle; the shallowest pockets are etched only in the final cycle. All pockets are etched to their final depth in the final cycle.
• Remove Mask: any remaining maskant is removed from the component, and then the pocket edges are manually dressed. This is performed by the following operations:
o Removing Residual Maskant: performed on the Blank GCD after all etching is complete.
o Dressing Edges: performed on each etched pocket.
• Bench Operation: See Bench Operation, above.
o Final Inspecting: performed on the Blank GCD.
• 5-Axis Mill: the part is routed to the 5-Axis Mill process in order to trim the blank addendum (which is added for ease of transportation and protection from the etchant), as well as to create the part’s holes (if there are any).
Routing a Part to Chemical Milling
When you route a part to Chemical Milling, Form GCDs that are of type STAMP are automatically assumed to be created by chemical milling, and are assigned to Chemical Milling operations (see Chemical Milling GCDs).To route a part to Chemical Milling, follow these steps:
1 In the Production Scenario tab of the Cost Guide, click the ellipsis, …, next to aPriori-computed routing in the Process Routing section:
The Routing Selection dialog appears.
2 In the Routing Selection dialog, scroll down if necessary, and click the + to expand the Machining node next to the CTL/Shear/Chemical Mill node:
The node expands to reveal multiple CTL/Shear/Chemical Mill routings:
3 Select the routing with the Chemical Milling node:
4 Click OK.
Note: If you expand the Machining node next to nodes other than CTL/Shear/Chemical Mill, a routing with a Chemical Milling node is among the revealed routings. Do not attempt to use any routing with a Chemical Milling node other than the CTL/Shear/Chemical Mill routing. Chemical Milling with other routings will fail to cost.
Assigning Chemical Milling to a Surface GCD
When a part is routed to Chemical Milling, Form GCDs that are of type STAMP are automatically assumed to be created by chemical milling, and are assigned to Chemical Milling operations (see Chemical Milling GCDs). You can also manually assign surface GCDs (Curved Surfaces, Curved Walls, and Planar Faces) to Chemical Milling. When you do this, you can use a setup option to specify the depth to which you want the surface etched (that is, the depth of material to be removed by etching). If you don’t enter an etch depth, the cost model uses the depth of the shallowest form of type STAMP, if there is one; otherwise, the cost model uses 50% of the blank thickness.
Follow these steps to assign Chemical Milling and enter an etch depth:
1 Ensure that the part has been routed to Chemical Milling (see Routing a Part to Chemical Milling), and costed at least once.
Note: If your part has no Form GCDs of type STAMP, you must disable process-level feasibility checks, cost the part, and then follow the steps in this section. Be sure to disable feasibility checks on both the CTL/Shear/Chemical Mill process node and the Chemical Milling process node: in the Routing Selection dialog, right-click each node, select Rules/Status, and then uncheck the checkbox in the dialog that appears.
2 In the Viewer, select the surface that you want to assign to Etching (the surface must be orthogonal to the depth direction). Then right click in the Viewer, and select Edit Operation….
3 In the Operation Sequence Selection dialog, click Chemical Milling to select that operation sequence. Then right click Chemical Milling and select Process Setup Options.
4 Enter the depth of the pocket that you want to be created, that is, the depth of material to be removed by etching. Click OK.
