|Home | ADCATS Info | Search | Site Map | Bulletin Board | Reports & Publications | Bibliography | Contact Us|
CHAPTER 5: TUTORIAL--STACK BLOCKS
|Home : Example Problems : Pro-E 2D - Modeler - Stack Blocks|
Because of the complexity of real problems, many assembly models will require more than one vector loop. Multi-loop modeling will be introduced using the three part Stack Blocks assembly.
CREATING THE ASSEMBLY DRAWING
The drawing with its dimensions and tolerances is given below in Figure 5-1. The assembly dimension of interest is the contact length A, between the Ground DRF and the Cylinder/Ground contact point.
Assembly datum points must exist wherever a DRF, feature datum, joint, or specification endpoint is to be created. The assembly datum points must not be created as a group. Each must be done individually. Assembly datum planes or feature surfaces must exist to define the sliding planes for all joints other than revolute. There must also be an assembly datum plane created to use as the 2-D reference plane. All assembly datum points must be created in this 2-D reference plane.
It is important to properly orient the parts within the assembly to prevent geometric error. All intersections should be true intersections or contact points can not be created correctly. Also, the drawing must be to scale or else errors will occur in reading the data. These errors will propagate throughout the analysis.
Figure 5-1. Dimensioned Stack Blocks.
For the remainder of this chapter, the following code will be used:
Select -- Select a menu option.
Pick -- Pick a point, plane, part, or modeling element on the drawing.
Enter -- Enter a value or name on the command line.
ENTERING THE TI/TOL 2D ENVIRONMENT
After an accurate assembly model of the Stack Blocks (complete with assembly datum planes and points) has either been created or called up in Pro/Engineer, the TI/TOL 2D application can be run.
SETTING THE DISPLAY
The first step in running the TI/TOL 2D application is to set the display. The Display option allows the user to define the plane in which the tolerance commands will be applied.
Note: If an appropriate assembly datum plane has not been created, Done/Return out of TI/TOL 2D and create one. Re-enter as outlined above.
A reference plane for the TOL-2D commands has now been defined. The size of the tolerance symbols can also be modified with the Display option. The default size is 2.00.
DEFINING DATUM REFERENCE FRAMES
The steps for the multi-loop problem will parallel those for creating the single loop problem. The only difference is that more loops are created (three in this example) and so the process is slightly longer.
Datum reference frames (DRF's) and feature datums must be created as shown in figure 5-2.
Figure 5-2. The Stack Blocks with Part DRF's and feature datums defined.
Creating the Ground DRF
Note: If an assembly datum point has not been created in the correct location, Done/Return out of TI/TOL 2D and create it. Assembly datum points and planes cannot be created inside of TI/TOL 2D, but if the user exits TI/TOL 2D by means of Done/Return, the work on the model up to that point will be saved in the database. The user can create the required point or plane, re-enter TI/TOL 2D, and resume modeling where they left off.
Creating the Block DRF
Note that the DRF's require an axis identification or a direction specification. Cylindrical DRF's automatically identify the longitudinal axis which is perpendicular to the cylinder's face. Rectangular DRF's, however, require the identification of a surface on the part which will establish the orientation of the DRF.
Creating the Cylinder DRF
The cylindrical datum reference frame will be created and the cylindrical DRF symbol will appear at the center point of the Cylinder.
DRF's have now been created for all three parts of the Stack Blocks assembly. If any DRF was created incorrectly, it must be deleted with the Delete command in the TOL-2D main menu.
Deleting a DRF
The incorrect DRF will be deleted and a new one can be created.
Modifying a DRF
A DRF may have its name and active degrees of freedom modified by the user. The name modification will be invisible in the modeler menus (for example when using Sel By Menu), but will be the node name seen when in the analyzer. Cylindrical DRF's may have their rotational degree of freedom turned off (or back on) in the modeler. We will change the Cylinder DRF name to Phi1.
DEFINING FEATURE DATUMS
Before joints can be created feature datums that define the paths from the joint location back to the DRF's of both parts associated with the joint must first be created. In the Stack Blocks problem three rectangular feature datums are required. Assembly points should already have been created in the Pro/Engineer modeler at the required locations (see Figure 5-2).
All of the feature datums for the Stack Blocks assembly have now been created. If a feature datum was created incorrectly, it can be deleted by following the same procedure outlined for deleting a DRF. Feature datums can also have their names and active degrees of freedom modified in the same manner as outlined for DRF's.
The next step in the TI/TOL 2D analysis is to locate the contact joints between each part. These joints represent the kinematic constraints between mating parts. The DRF's and feature datums created previously will be used to locate the joints by tracing a path back to the respective part DRF through controlled and kinematic dimensions. The paths back to the DRF's will vary depending on how the parts are dimensioned. For example, the path from the edge slider joint (joint 5) to the Ground DRF may appear either of the two ways below in Figure 5-3. For our model, the right-hand path will be used.
Figure 5-3 Two possible paths back to the Ground DRF.
The following steps will outline how to create the five kinematic joints required for the Stack Blocks assembly.
Creating the Cylindrical Slider Joint Between Ground and Cylinder (Joint 1)
Note: If Sel By Menu is used to select the part DRF's, the Done Sel option on that same menu must be used after each DRF selection to complete the sequence.
Creating the Cylindrical Slider Joint Between the Cylinder and Block (Joint 2)
Creating the First Edge Slider Joint Between the Ground and Block (Joint 3)
Creating the Second Edge Slider Joint Between the Ground and Block (Joint 4)
Creating the Third Edge Slider Joint Between the Ground and Block (Joint 5)
All of the contact joints necessary to create loops have been added to the drawing (see Figure 5-4 below). If a joint was created incorrectly, the Delete command in the TOL-2D main menu can be used to delete the incorrect joint. The procedure is identical to deleting a DRF. The joint can then be created correctly before continuing with the tolerance modeling.
Figure 5-4 Kinematic joints of the Stack Blocks.
A joint may have its name and active degrees of freedom modified by the user. The name modification will be invisible in the modeler menus (for example when using Sel By Menu), but will be the node name seen when in the analyzer. For this example, no joint degrees of freedom will be modified, but the names of the edge slider joints will be changed to Phi2, Phi3 and Phi4.
The next step in the modeling process is to create the kinematic loops which relate all assembly parts and contact joints to the resultant assembly dimensions. Although the Specification command in the TOL-2D main menu precedes the Loops command, closed loops must be created before closed loop specifications (dependent lengths and angles) can be applied. However, open loop specifications should be created before loops are generated. The Stack Blocks assembly requires three closed loops. The first will be generated manually to demonstrate that process. The remaining two will be generated using Autoloop. All three can be created using AutoLoop, but the loops will be slightly different than those shown. The assembly variations calculated will be the same for both sets of loops, though.
The first loop will be created manually.
Loop 1 will now be created and appear as shown in Figure 5-5. The remaining two loops will be created using the Autoloop option.
Figure 5-5. Stack Blocks Loop 1.
Loop 2 and Loop 3 will appear as in Figure 5-6 below.
Figure 5-6. Stack Blocks Loop 2 and Loop 3.
Loops can be deleted by following the same procedure as for deleting part DRF's. If there are one or more closed loop specifications applied to a closed, those specifications must be deleted before the modeler will allow that closed loop to be deleted. Open loops have no such restrictions.
Loop vectors can be modified in three ways. The user can change the vector names and vector tolerances. They can also equivalence vectors for cases when the variations of two vectors are not independent of each other. For example, if the radius of a cylinder is over-sized at one point, it is likely to be over-sized at all other points.
Modifying Vector Names
The user can apply new names to the loop vectors. These new names will remain invisible until the user enters the Analyzer.
Note: The order of the vector naming scheme outlined above will only be valid if the DRF's, feature datums, and joints were created in the order outlined in the first part of this chapter. If they were created in a different order, the order that the vector are highlighted will probably be different.
Modifying Vector Tolerances
Each vector that corresponds to a manufactured dimension must have a tolerance associated with it. Vectors that represent kinematic assembly dimension (closure lengths) should not be assigned a tolerance.
Equivalencing Two Vectors
The Stack Blocks model requires two sets of vectors be equivalenced. The variations of the radii of the Cylinder are not independent of each other. They will be equivalenced in order to link their variances in the tolerance model. There are also two vectors on the Block that lie on top of each other (the two that were labeled I). They are actually the same dimension, so they also must be equivalenced.
DEFINING DESIGN SPECIFICATIONS
A Dependent length specification will be applied between the Cylinder and Ground. This specification relates the point of contact between two parts.
Creating a Dependent Length Specification
The dependent length specification will be applied and the specification symbol shown in Figure 5.7 will appear along the specified vector. An incorrect design specification can be deleted by using the Delete command in the TOL-2D main menu.
Figure 5-7. Dependent Length Specification for the Stack Blocks Problem.
DEFINING GEOMETRIC TOLERANCES
TI/TOL 2D includes geometric tolerancing options. Geometric tolerances allow an engineer to account for machined surface variations such as flatness, circularity and perpendicularity. These surface variations can accumulate and propagate kinematically through the model the same as dimensional variations.
Applying a Flatness Tolerance to the Ground
Applying a Circularity Tolerance to the Cylinder
The specified circular tolerances will be created and the circular tolerance symbols will appear at the joints. The new geometric tolerance symbols will be placed on top of the symbols that already exist at the specified joints (see Joint 1 and Joint 2).
Applying a Flatness Tolerance to the Block
The model should now appear similar to Figure 5-8 below.
Figure 5-8. Geometric Tolerances for the Stack Blocks.
Blocks | Remote
Modeler: Crank Slider
|The ADCATS site: Home | ADCATS Info | Search | Site Map | Bulletin Board | Reports & Publications | Bibliography | Contact Us|