Gurbok Tibadwi

[email protected]

 

 

Introduction:

 

Traditional machine peen forming of aircraft wing skins is based on trial and error to determine Almen intensities required to form flat sections to the desired radii. This works very well when material is flat. However, virgin wing skins, as a rule, are out-of-flat and vary wildly from severely concave to severely convex.  This being the case, forming a wing skin to conform to specifications relies heavily on the experience and skill of the machine operator as well as extensive manual local correction of over-formed/under-formed sections.

 

In order to overcome this difficulty, software for machine shot peening of wing skins was developed. Extensive tests using data from over 200 wing skins formed manually indicate that deviations from required curvatures can be limited to .001 in. (6-in arc length) which may occur in only 20% of the surface. Consequently, undesirable spherical, waving and fanning deformations may be minimized.

As well, minimal local correction requirements could result in dramatic reductions in production time

Software Description:

The software was developed for machines with fixed nozzles or impellers driven by variable speed motors. It calculates Almen intensities required to form wing skins to desired curvatures. These may be converted to any working unit such as psi or rpm.

In a typical application, wing skins are either suspended vertically from hangers or set up on a table. Forming is essentially a single-axis operation where the hangers/table move through the nozzle/impeller housing or the nozzle/impeller housing moves over the wing skin. Peening intensity is adjusted either manually or by a programmable controller.

Wing skins are mapped into a suitable number of sections selected according to factors including:

1. Material thickness
2. Change in thickness
3. Masking requirements
4. Curvature
5. Steps
6. Ribs
7. Ports

Forming parameters for each section are determined and recorded in a parameter table. These parameters define the conditions that influence material deflection and are “constants” for the same part

Section flatness is checked and values entered directly into a scaled drawing displayed on the user’s screen. In multi-level iterative calculations, these values and the forming parameters, yield required intensities.

These may be:

1. Printed out for machine operators
2. Saved as a file for future runs
3. Exported in a format suitable for use in a programmable controller

Input Section:

Drawing:

 

Figure 1: Wing Skin Map

Scaled AutoCad .dxf files are required. Import capabilities include:

1. Text
2. Lines
3. Polylines
4. Circles
5. Ellipses
6. Arc
7. Spline
8. Square
9. Rectangle
10.  Other shapes converted to polylines

Figure 1 shows a skin mapped into 6 segments (A-F) by 7 sections (1-7) for a total of 42 cells. Segment and section dimensions may be expressed in any unit of measure.

Parameter Table:

A parameter table is created for each section to calculate the required intensities. These are converted to psi/RPM as required. Parameters include:

1. Active cell: Only active cells are processed

2. Vertical adjustment: The influence a cell may have on adjacent vertical cells. Over-spray of a higher intensity from adjacent vertical cells, if significant, modify deflection of cells above and below it

3. Horizontal adjustment: The influence a cell may have on adjacent horizontal cells. The illustrated wing skin is progressively formed from left to right (the software is capable of mirror image forming; left/right hand, flipped). Preceding sections therefore pre-form those that follow. In most cases this is desirable as it helps to smoothly blend changes in curvatures. Where instantaneous change is required, as in a dihedral break, the extent of pre-forming must be determined and entered as a % of the dihedral intensity.

4. Curvature: Cell radius or arc height required.  For UWP4M, curvatures are expressed in terms of arc height for an arc length of 6 units.

5. Model: Cell deflection in response to changes in shot intensity prior to vertical/horizontal adjustments. Two sub-models are available if needed:

a)     -mod: deflection if cell is concave

b)     +mod: deflection if cell is convex  

Any of the following models may be selected for the -mod and/or +mod:

1.      Linear Regression

2.      Polynomial

3.      Exponential

4.      Power Law

5.      Yield Density

6.      Growth

7.      User defined

 

 Figure 2a: Parameter Table: Model, Curvature, Active, Vertical Adjustment

Default number of segments is 6. This number may be increased as required. The table is for Section 1 with a length of 8 units.  Analysis of the parameter table shows:

Cell F1:           Active             No       (FF = 0)

Cell E1           Active             Yes      (EE=100)

                        -mod               1          (linear response to increase in intensity)

                        +mod              1          (linear response)

                        curvature        10        (curve=10x1/1000, 6 units arc length)

                        Vertical Adjustment

                             F1             0          Not active

                             E1              100     Active 100% response

                             D1              5          Active 5% response

                             C1-A1        0          Not adjacent

 Cell D1:          Analysis is similar to E1

 Cell C1:          Analysis is similar to E1 except:

                        -mod               1          (linear response to increase in intensity)

                        +mod              2          (polynomial response)

 Cell B1&A1    Analysis is similar to E1                  

 

 Figure 2b: Parameter Table: Horizontal Adjustment

Default number of sections is 10. The scroll bar was moved to the right to show sections 1-9 (UWP4M has only 7 sections, sections 8-10 are ignored)

For segments A-F:

Section 1:       0          In progressive forming, a certain amount of pre-forming is always present. For cells in Section 1, it is indeterminate.

Section 2-7:   0          Changes in curvature are gradual and “blend in/fade out” is desirable

Section 8-10:             Not used. By default, set to 0

 Parameter tables for the remaining sections are similar.

 Sample Run:

 

Figure 3:         Section Flatness/Intensities

Section flatness may be entered anywhere in any active cell. Values are displayed on the right and calculated intensities on the left.

Note that Almen intensities required to form cells D5-D7 are many times higher than those of adjacent cells. In typical applications, cells C5, E5-E7 as well as Section 4 may have to be masked to prevent over-forming. In this case, vertical adjustments in the parameter tables are set to zero.