Creo Elements/Direct Sheet Metal provides a set of allowance tables plus allowance formulas for calculating the flat length of a sheet metal part that includes bends, hems and offsets.
The plane in which the length of a sheet metal part including bends remains unchanged is identified by the "neutral phase". The portions on either side of this neutral phase become longer or shorter, respectively.
The location of this neutral phase relative to the material thickness ("T") is described in terms of a parameter known as the "k-factor". ANSI (American National Standard Institute) defines the k-factor as the offset of the neutral phase relative to "T", while DIN (Deutsche Industrienorm) defines the k-factor as the offset of this neutral phase relative to "T/2". Both conventions are illustrated in the following figure.
Figure 6. ANSI vs. DIN Interpretation of K-Factor
Bend Allowance Parameters and Mathematics
The following diagram illustrates the geometric parameters and mathematical considerations included in the bend allowance calculation. Please note that Creo Elements/Direct Sheet Metal uses both the ANSI standard k-factor and the DIN k-factor.
Figure 7. Bend Allowance Calculation
Examples of allowance value tables are provided in the sha_demoshop.lsp file supplied with Creo Elements/Direct Sheet Metal. Allowance formula names and/or pointers to formulas are also specified in this file. The associated sample allowance formulas are included and fully documented in the sha_demoshop_func.lsp file.
Where an :ALLOWANCE_FORMULA is specified in a bend process table, Creo Elements/Direct Sheet Metal calculates the actual flat length by means of the applicable bend allowance formula or by a table interpolation method. The following predefined bend allowance formulas are provided in the sample sha_demoshop_func.lsp file:
If Creo Elements/Direct Sheet Metal cannot find a process dependent allowance value or if no allowance formula is specified for a given process, it looks for a shop-wide fall-back formula specified in the :ALLOWANCE_FORMULA entry in the shop definition file. In the sample sha_demoshop.lsp, this shop-wide fallback method is implemented through the following entry:
; fall back strategy for allowance values :ALLOWANCE_FORMULA sh_bend_allowance_din
If a tool is not available in the bend process table, the sh_bend_allowance_din formula checks whether the current bend process contains a K-factor. If a K-factor is found, the allowance formula for the roll bending process is used instead of the shop-wide fall-back formula.
The offset and hem tools use their own fallback formulas. These are specified in the :OFFSET_FORMULA and in the :HEM_FORMULA entries, respectively. Please note that the demoshop does not use the :HEM_FORMULA entry. For details see
Hem Allowance Calculation and
Offset Allowance Calculation.
Allowance Error Handling
The return properties of the allowance calculation formulas can be used to flag an error in case of DFM rule violations.
The setf return_values statement in each allowance calculation formula specifies the interpretation of error conditions. Each of these statements consists of:
• a headline string (which appears as the headline in the popup error box) explaining the nature of the error condition;
• an error type (taken from the *sha-severity* list shown above); advice about how the problem can be avoided.
• An example of such a setf return_values statement for the DIN allowance calculation formula is shown below.
:error-message "Relation radius/thickness should not be smaller than 0.65"))
The default strategies implemented for the Creo Elements/Direct Sheet Metal bend, hem and offset processes are discussed in the following sections.
Bend Allowance Calculation
Creo Elements/Direct Sheet Metal uses the following decision tree for its bend allowance calculation.
Figure 8. Decision Tree for the Bend Allowance Calculation
The following figure illustrates the default fallback strategy for the bend allowance based on this formula.
Figure 9. Fallback Strategy for the Bend Allowance Calculation
Bend Process/Tool Independent Allowance Calculation
This type of formula is used where bend process or bend tool data is not available or is not needed for the bend allowance calculation. The sha_demoshop.lsp file uses it as its fallback strategy as well as inside of the air_bend_allow formula. The input parameters and the associated return are specified below.
Input
sheet_thickness
Material thickness (mm).
bend_angle
Angle of bend (degrees). 0 implies "no bend".
bend_radius
Radius of bend (mm). 0 implies "no bend".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the sha_demoshop_func.lsp file for examples.
An example of such a function is provided in the sha_demoshop_func.lsp file. The name of this function is Sh_bend_allowance_din. This function does not use the sheet_material parameter. For details see:
This method is used where bend process and/or bend tool data is available and can be used for the bend allowance calculation. Creo Elements/Direct Sheet Metal uses this method where an :ALLOWANCE_TABLE and/or where an :ALLOWANCE_FORMULA is specified in a bend process table. Examples included in the sha_demoshop.lsp file are the functions linear_interp and air_bend_allow (although the latter does not make use of the extra tool information).
If a formula is called, it has the following parameters:
Input
sheet_thickness
Material thickness (mm).
bend_angle
Angle of bend (degrees). 0 implies "no bend".
bend_radius
Radius of bend (mm). 0 implies "no bend".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the
sha_demoshop_func.lsp file for examples.
An example of such a function is provided in the sha_demoshop_func.lsp file. The name of this function is air_bend_allow. Instead of using the tool information, this function calls the DIN based allowance formula directly. For details see
Allowance Calculation Formulas.
Bend Allowance Tables
Where an :ALLOWANCE_TABLE is specified in a bend process table in the shop definition file (see the sha_demoshop.lsp file for an example), Creo Elements/Direct Sheet Metal uses the specified table to compute the actual flat length of each bend created by the associated bend process.
For the bend forming process (which uses a separate tool for each bend angle), you can add a specific allowance value to the bend_form_allow table for each unique material/tool combination in your shop definition file(s). For the other processes (which use arbitrary bend angles), the allowances are defined through calculation formulas.
For example, the sha_demoshop.lsp file includes the bend_form_allow table for the bend forming process. The default contents of this table is shown below.
You can edit this allowance table by modifying existing values or by adding empirical allowance measurements taken from your own test specimens. The following text block is an example of a custom bend allowance table section for 2 mm aluminum.
This example implies the following allowances: When the bend angle is 45 degrees and the bend radius is 0.5 mm, the bend allowance is -2.56 mm. When the bend angle is 90 degrees and the bend radius is 3.2 mm, the bend allowance is -4.21 mm.
Which Formula Is Used for Each Bend Process?
By default, the sample sha_demoshop.lsp file specifies the following pointers to allowance formulas (defined in the sha_demoshop_func.lsp file) for the five predefined bend processes:
Bend Process
Formula Call
Bend Forming
None -- uses table access to allowance values.
Air Bending
air_bend_allow — formula in sha_demoshop_func.lsp
Bend Folding
linear_interp -- formula in sha_demoshop_func.lsp
3 Pnt Bending
air_bend_allow -- additionally points to an allowance table interpolated by the formula
Roll Bending
roll_bending_allow_din — formula based on a K-factor to specify the allowance value
Cone Bending
cone_bending_allow_din — formula to specify the allowance for conical bends
The air_bend_allow statement calls the actual bend allowance formula. In the sha_demoshop_func.lsp file, air_bend_allow calls the DIN based allowance formula (Sh_bend_allowance_din). Two of the possible alternatives (function calls to Sh_bend_allowance_K_fact and linear_interp) are commented out by preceding semicolons.
The linear_interp function uses a linear interpolation formula. This formula accesses an allowance table to compute values through linear interpolation. The name of this allowance table is derived from the :ALLOWANCE_TABLE entry for the given bending tool. The tool ID of the bend tool is one of the parameters passed to the formula. This implies that a bend process which uses the linear_interp formula should have both the :ALLOWANCE_FORMULA and the :ALLOWANCE_TABLE entry.
In the sha_demoshop_func.lsp file, roll_bending_allow_din calls the general DIN based allowance formula. This means that the K-factor should follow the DIN standard, that is, its value should be between 0 and 2.
Example: Changing the Formula Used in the air_bend_allow Function
If you want the air_bend_allow formula to use one of the other two alternatives (commented out by default) instead of the DIN based allowance formula, proceed as follows:
1. sha_demoshop_func.lsp file or in your working copy of this file, place semicolons in front of the function call you want to disable. For example, if you want to disable the function call to the DIN based allowance formula, comment it out by adding semicolons as shown below.
As a result of this change, any bend process using the air_bend_allow function specified in the :ALLOWANCE_FORMULA statement will now use the fixed k-factor formula instead of the DIN based allowance formula.
Example: Specifying a Formula Directly
If you want a given bend process to use one of the predefined bend allowance formulas included in the sha_demoshop_func.lsp file, you can specify a direct call to that formula. For example, if you want the bend forming process to use the fixed k-factor formula, change the :ALLOWANCE_FORMULA statement for that process as shown below.
:ALLOWANCE_FORMULA Sh_bend_allowance_K_fact
Alternatively, if you want the bend forming process to use the linear interpolation formula, change the :ALLOWANCE_FORMULA statement for that process and specify the associated table for the linear interpolation as shown below.
This works only because the process-independent functions (Sh_bend_allowance_K_factor and linear_interp) accept the longer process-dependent parameter list due to the use of the &allow-other-keys statement. This special LISP statement simply skips the tool ID parameter included in the parameter list of the function definition.
Hem Allowance Calculation
Creo Elements/Direct Sheet Metal accounts for the fact that allowance values for hems differ from regular bend allowance values due to the two-step mechanism in which hems are produced.
The hem allowance is defined as the difference between the length of the two lips and the flat length of the sheet metal part. This relationship is illustrated in the following figure.
Figure 10. Hem Allowance Calculation
Creo Elements/Direct Sheet Metal uses the following decision tree for its hem allowance calculation.
Figure 11. Decision Tree for the Hem Allowance Calculation
Creo Elements/Direct Sheet Metal uses the following fallback strategy for its hem allowance calculation.
Figure 12. Fallback Strategy for the Hem Allowance Calculation
Hem Process/Tool Independent Allowance Calculation
This type of formula is used where hem process or hem tool data is not available or is not needed for the hem allowance calculation. The input parameters and the associated return are specified below.
Input
sheet_thickness
Material thickness (mm).
hem_dist
Distance between the lips of the hem (mm). 0 implies "touching lips".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the sha_demoshop_func.lsp file for examples.
Hem Process/Tool Dependent Allowance Calculation
This method is used where hem process and/or hem tool data is available and can be used for the hem allowance calculation. Creo Elements/Direct Sheet Metal uses this method where an :ALLOWANCE_TABLE and/or where an :ALLOWANCE_FORMULA is specified in a hem process table. The hem allowance formula uses the following paramters:
Input
sheet_thickness
Material thickness (mm).
hem_dist
Distance between the lips of the hem (mm). 0 implies "touching lips".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the sha_demoshop_func.lsp file for examples.
Hem Allowance Tables
Because of its dependency on tool and shop process characteristics, the hem allowance is based on empirical data. The following example shows the relevant section of a predefined allowance table for hems included in the sha_demoshop.lsp file.
For 1.25 mm Zincor, for example, this table implies that a 1.25 mm hem tool is associated with an allowance value of -0.20 mm.
You can edit this table by modifying existing values or by adding custom values to these allowance tables based on empirical allowance measurements taken from your own test specimens.
Offset Allowance Calculation
Creo Elements/Direct Sheet Metal accounts for the fact that allowance values for offsets differ from regular bend allowance values due to the mechanism in which offsets are produced. Creo Elements/Direct Sheet Metal represents the offset by two connected bending zones. Where the offset height is equal to or smaller than the sheet thickness, special radius calculations are used.
The offset allowance is defined as the difference between the length of the original sheet metal flat and the produced sheet metal part containing the offset. This relationship and the geometrical representation used in Creo Elements/Direct Modeling are illustrated in the following figure.
Figure 13. Offset Allowance Calculation
Although visually similar to two adjacent but opposite bends, the offset has to be produced with a special tool in a single step because regular bending tools require a non-zero distance between two bends. This special offset forming tool generates specific stretching phenomena in and between the two bending zones.
Creo Elements/Direct Sheet Metal uses the following decision tree for its offset allowance calculation.
Figure 14. Decision Tree for the Offset Allowance Calculation
Creo Elements/Direct Sheet Metal uses the following fallback strategy for its offset allowance calculation.
Figure 15. Fallback Strategy for the Offset Allowance Calculation
This type of formula is used where offset process or offset tool data is not available or is not needed for the offset allowance calculation. The input parameters and the associated return are specified below.
Input
sheet_thickness
Material thickness (mm).
offset_height
Height of the offste (mm). 0 implies "no offset".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the sha_demoshop_func.lsp file for examples.
An example of such a function is provided in the sha_demoshop_func.lsp file. The name of this function is Sh_offset_allowance. The associated formula is derived from the empirical offset allowance values shown in the "offset_allow" table in the sha_demoshop.lsp file. This formula does not distinguish between material types (e.g. steel vs; aluminum). The formula describes a cubic curve plotting allowance values vs. offset height until the offset height is equal to twice the thickness of the sheet material. For larger offset height values, the allowance is equal to the sum of the offset height and a special constant. The error yielded by this approach can be taken to remain below 0.1 mm for the conditions specified in the sha_demoshop.lsp file.
This method is used where offset process and/or offset tool data is available and can be used for the offset allowance calculation. Creo Elements/Direct Sheet Metal uses this method where an :ALLOWANCE TABLE and/or where an :ALLOWANCE_FORMULA is specified in an offset process table. The offset allowance formula uses the following paramters:
Input
sheet_thickness
Material thickness (mm).
offset_height
Height of the offste (mm). 0 implies "no offset".
sheet_material
Material property list. The following syntax applies:
The optional return properties can be used to flag an error which may have been detected during the allowance calculation. The error feedback to the user consists of:
The error type should be one of the types specified in the *sha-severity* list in the sha_demoshop_func.lsp file or your working copy of this file; for instance:
• a headline string which is displayed as the headline in the error popup box,
• an error type defining the error severity level and thus the color of the error popup box,
• a more detailed string describing the error condition.
:DFM_rule_violation
Each defined error type is mapped to one of the three severity level colors:
• :low = yellow
• :medium = orange
• :high = red
You can customize these severity level mappings as required by your organization. See the sha_demoshop_func.lsp file for examples.
Offset Allowance Tables
Because of its dependency on tool and shop process characteristics, the offset allowance is based on empirical data. The following example shows the relevant section of a predefined allowance table for offsets included in the sha_demoshop.lsp file.
By default, Creo Elements/Direct Sheet Metal uses a DIN based formula as a general fall-back method for bends, hems and offsets as well as inside of the air_bend_allow function and thus implicitly in the air bending and 3 point bending processes. The name of this DIN based formula is:
Sh_bend_allowance_din
This formula is used for the correct flattening of parts which were generated without the use of Creo Elements/Direct Sheet Metal, or without an explicitly defined bend, hem or offset process.
It is included and fully documented in the sha_demoshop_func.lsp file supplied with Creo Elements/Direct Sheet Metal. This formula calculates the location of the neutral phase (K_factor) as as function of the sheet thickness (gage) and the bend radius according to the following DIN stipulation:
K_factor = 0.65 + 1/2 * lg radius/thickness
Customizing the DIN Based Allowance Formula
To customize the k-factor used in your DIN based allowance calculation formula, you can choose one of the following methods:
• Substitute the coefficients 0.65 and 1/2 by your empirical coefficients.
• Create and apply your own calculation formula. See Creating and Applying Custom Allowance Formulas for details.
Fixed K-Factor Formula
This method is widely used in the U.S. It is implemented in Creo Elements/Direct Sheet Metal through the following formula:
Sh_bend_allowance_K_fact
This formula derives the location of the neutral phase (K_factor) by scanning the K_FACTOR column in the material specification tables in your shop definition file. If no such column is found, the formula looks for a K_FACTOR column in the shop specification. If no such column is found, the formula uses its default k-factor. In the sample sha_demoshop_func.lsp file supplied with Creo Elements/Direct Sheet Metal, this factor is set to 0.4.
Using the Fixed K-Factor Formula as the Shop-Wide Fallback Formula
If you want to apply this formula instead of the default Sh_bend_allowance_din formula as the shop-wide fallback method, replace the following block in the shop definition file (sha_demoshop.lsp or your working copy of this file):
; fall back strategy for allowance values :ALLOWANCE_FORMULA sh_bend_allowance_din by:
; fall back strategy for allowance values :ALLOWANCE_FORMULA sh_bend_allowance_k_fact
Using the Fixed K-Factor Formula for a Specific Process
Directly:
If you want to apply this formula to a given process, set the :ALLOWANCE_FORMULA function call of that process to the sh_bend_allowance_k_fact formula as shown below.
:ALLOWANCE_FORMULA sh_bend_allowance_k_fact
Indirectly:
If the :ALLOWANCE_FORMULA of a given process points to a process-specific allowance method defined in the sha_demoshop_func.lsp file, you need to enable the call to the sh_bend_allowance_k_fact formula in this file. For instance, in the sha_demoshop.lsp file, both the air bending and the 3 point bending process point to the following default allowance method:
:ALLOWANCE_FORMULA air_bend_allow
In the sha_demoshop_func.lsp file, air_bend_allow defaults to its first allowance alternative: Sh_bend_allowance_din. Proceed as follows if you want air_bend_allow to use the fixed k-factor formula:
1. sha_demoshop_func.lsp file or in your working copy of this file, place semicolons in front of the function call to the DIN based allowance formula. The resulting text block is shown below.
As a result of this change, any bend process which calls the air_bend_allow function in its :ALLOWANCE_FORMULA statement will now use the fixed k-factor formula instead of the DIN based allowance formula.
Customizing the Fixed K-Factor Formula
To customize this formula, you can choose one of the following methods:
If you know from your design experience that different k-factors apply to your various materials, you can apply these k-factors and the fixed k-factor formula by performing the following steps:
• Add a :K_FACTOR column to the material table in your shop definition file.
• Specify the fixed k-factor formula in the statement in the appropriate bend process table.
To add a k-factor column:
Proceed as follows to add a K_FACTOR column to the material table in your shop definition file(s):
1. Declare a :K_FACTOR column in the :COLUMNS section of the material table.
2. Specify K-FACTOR for the :K_FACTOR column in the :COLUMN_NAMES section.
3. In the :UNITS section, specify nil (because no unit is associated with the k-factor).
4. Add a :K_FACTOR column to the :CONTENTS section of the material table.
The following example shows the "Sheet Metals" material table containing a newly added :K_FACTOR column.
(sha-define-shop-table "sheet_metals" :TABLE-TITLE "Sheet Metals" ;; :MAT_ID Ident. number/string of the material e.g. "7144-0503" ;; :MATERIAL material description e.g. "UST 1405" ;; :THICK material thickness e.g. 0.88 ;; :MIN_BEND_RAD minimum bending radius e.g. 0.4 ;; :K_FACTOR fixed k-factor for allowance calc e.g. 0.41 :COLUMNS (:MAT_ID :MATERIAL :THICK :MIN_BEND_RAD :K_FACTOR) :COLUMN-NAMES ("Mat Id" "Material" "Thick" "Min Bend Rad" "K-Factor") :UNITS (nil nil :mm :mm :nil) :CONTENTS ( ("8888-0009" "AA 5052" 1.00 0.4 0.38) ("8888-0003" "AA 5052" 1.60 0.4 0.39) ("9999-0477" "UST 1203" 1.00 0.4 0.39) ("9999-0344" "UST 1203" 1.25 0.4 0.41) ("9999-0345" "UST 1203" 1.50 0.4 0.41) ) :KEY (:MATERIAL :THICK) ; The material key MUST not be changed ; A FLAT-TEXT info is not used :HELP "sha_sheet_metals" )
To specify the fixed k-factor formula:
Proceed as follows to apply the fixed k-factor formula to a given bend process:
1. Below the :CONTENTS section of the bend process table, specify the :ALLOWANCE_FORMULA as follows:
:ALLOWANCE_FORMULA Sh_bend_allowance_K_fact
2. Delete any other allowance formula and/or allowance table specifications from the table.
Changing the K-Factor in the Formula
If you know from your design experience that a fixed k-factor other than 0.4 is more appropriate for your specific shop environment, you can substitute 0.4 by the applicable value. For example, if you want to apply a fixed k-factor of 0.42, edit the K_factor argument as shown below.
(setf K_factor 0.42)
Linear Interpolation Method
The third allowance calculation method documented in the sha_demoshop_func.lsp file is the linear interpolation method. Any bend process using the linear interpolation method requires the following:
• An :ALLOWANCE_FORMULA column specifying the interpolation formula to be called. The linear_interp allowance formula defined in the sha_demoshop_func.lsp file is a useful example.
• An :ALLOWANCE_TABLE column specifying a separate allowance table containing existing allowance values from which any additional values are interpolated. This allowance table must contain the material key columns, the tool key column and (unless included in the tool key) an :ANGLE column plus the :ALLOWANCE TABLE column. Please note that the :ALLOWANCE TABLE column has to be the last column in the allowance table. The Bend_fold_allow table is a useful example.
If no such table is found by the linear_interp formula, it uses the "general_allowances" table provided in the sha_demoshop.lsp file as a fallback table.
The sample linear_interp formula included in the sha_demoshop_func.lsp file does not extrapolate outside of the range of defined table values. Therefore, the allowance value table should normally start with a 0 bend angle (no bend) and a 0 allowance value. It should end with the maximum permissible bend angle for the given tool.
The interpolation formula starts by seeking a valid combination of sheet material and associated bending tool. The formula uses this information to interpolate between the given angles. The following functions are used for this purpose:
• Special Creo Elements/Direct Sheet Metal functions:
◦ sha-get-shop-tab-entry
◦ sha-rowkey-plist-to-generals-string
◦ sha-table-entry-eq
◦ sha-separate-plist
• Functions included in the Creo Elements/Direct Modeling Integration Kit:
◦ sd-get-logical-table-number-of-rows
◦ sd-read-logical-table-row
You can find an example of the linear interpolation method in the "Bend Folding" table in the sha_demoshop.lsp file. The relevant section of this table is shown below.
These entries tell you that the allowance formula for this process is named linear_interp. The existing values used for interpolation are taken from the bend_fold_allow table.
Using the Linear Interpolation Method as the Shop-Wide Fallback Method
If you want to apply this method instead of the default Sh_bend_allowance_din formula as the shop-wide fallback method, replace the following block in the shop definition file (sha_demoshop.lsp or your working copy of this file):
; fall back strategy for allowance values :ALLOWANCE_FORMULA sh_bend_allowance_din
by:
; fall back strategy for allowance values :ALLOWANCE_FORMULA linear_interp :ALLOWANCE_TABLE "Bend_fold_allow"
Directly:
If you want to apply linear interpolation to a given process, set the :ALLOWANCE_FORMULA function call of that process to the linear_interp formula and specify the bend_fold_allow table as shown below.
If the :ALLOWANCE_FORMULA of a given process specifies a process-specific allowance method defined in the sha_demoshop_func.lsp file, you need to enable and/or specify the call to the linear_interp formula and its associated bend_fold_allow table in this file. For instance, in the sha_demoshop.lsp file, both the air bending and the 3 point bending process point to the following default allowance method:
:ALLOWANCE_FORMULA air_bend_allow
In the sha_demoshop_func.lsp file, air_bend_allow defaults to its first allowance alternative: Sh_bend_allowance_din. Proceed as follows if you want air_bend_allow to use the linear interpolation method:
1. In your sha_demoshop_func.lsp file or in your working copy of this file, place semicolons in front of the function call to the DIN based allowance formula. The resulting text block is shown below.
As a result of this change, any bend process which calls the air_bend_allow formula in its :ALLOWANCE_FORMULA statement will now use the fixed k-factor formula instead of the DIN based allowance
Roll Bending Allowance Calculation Formula
Creo Elements/Direct Sheet Metal uses the roll_bending_allow_din function to define the allowance for bends with angles that are greater than 180 degrees. In the sha_demoshop.lsp file, the roll bending process points to the following allowance method:
:ALLOWANCE_FORMULA roll_bending_allow_din
The roll_bending_allow_din function retrieves the K-factor from the tool table and uses the Sh_calculate_bend_allowance_DIN formula. This formula handles bends that have angles greater than 180 degrees.
If the tool is not available in the bend process table, the following sequence of steps applies:
1. If there is only one tool in the bend process table for the given material the K-factor of this tool is used.
2. If the given radius value is between two radius values in the bend process table, the K-factor is interpolated.
3. If the material is not defined in the bend process table or the given radius is not between the smallest and largest radius values in the bend process table, the K-factor is retrieved as follows:
a. From the material table
b. The global K-factor which is defined in the shop
c. If everything else fails, 0.8 is used as the K-factor.
Cone Bending Allowance Calculation Formula
Creo Elements/Direct Sheet Metal uses the cone_bending_allow_din function to define the allowance for conical bends. In the sha_demoshop.lsp file, the cone bending process points to the following allowance method:
:ALLOWANCE_FORMULA cone_bending_allow_din
Additionally the following formula is defined as the fallback formula for cone bending:
:CONE_BEND_FORMULA cone_bending_allow_din
The cone_bending_allow_din function returns a K-factor instead of an allowance value. The function retrieves the K-factor from the tool table.
If the tool is not available in the bend process table, the following sequence of steps applies:
1. If there is only one tool in the bend process table for the given material the K-factor of this tool is used.
2. If the given cone angle value is between two cone angle values in the bend process table, the K-factor is interpolated.
3. If the material is not defined in the bend process table or the given cone angle is not between the smallest and largest cone angle values in the bend process table, the K-factor is retrieved as follows:
a. From the material table, if the material is defined.
b. The global K-factor which is defined in the shop.
c. If everything else fails, 0.8 is used as the K-factor.
The K-factor defined in the process table and as the fallback is defined using the DIN standard. Accordingly, the shop entry :K-FACTOR-NORM :DIN should be set to :DIN. Internally, Creo Elements/Direct Sheet Metal uses the ANSI standard and converts the K-factor values of the shop according to the above setting.
Using Customized Allowance Tables and Formulas
The location of the neutral phase relative to the sheet thickness is governed by a number of criteria such as:
• bending process
• material thickness (gage)
• rolling direction ("grain") of the sheet metal stock
• material surface characteristics
• material grade
• damping method (jigs and fixtures)
• bending force
• temperature
The most accurate way to deal with this complexity in your own fabrication environment is to make test specimens and to include the resulting measurements in an allowance table and/or an allowance formula. Sample allowance tables are provided in the sha_demoshop.lsp file supplied with Creo Elements/Direct Sheet Metal. Sample formulas for calculating the resulting length are included and fully documented in the sha_demoshop_func.lsp file. To meet the specific needs of your organization, your TDB adimistrator can modify these tables and formulas and/or define and implement custom tables and formulas.
When creating your own working copies of these files for customization, be sure to specify these new files in your customization file (your working copy of the default sha_customize file). For details see
Structure of the Technology Database.
Examples of customized allowance tables are provided earlier in this chapter. The following section provides general instructions for creating and applying your own (custom) allowance formulas.
Creating and Applying Custom Allowance Formulas
Optionally, you can create and apply your own allowance formulas based on empirical data gathered in your fabrication environment. The general procedure comprises the following steps:
1. Create a working copy of the sample sha_demoshop_func.lsp file supplied with Creo Elements/Direct Sheet Metal.
2. Review and understand the predefined allowance calculation formulas documented in this file.
3. Create your own allowance formula in your working copy of the sha_demoshop_func.lsp file.
4. Specify the name of this formula in the :ALLOWANCE_FORMULA statement in your shop definition file. Refer to the sha_demoshop.lsp file for examples.
