8

CE/TOL

Stacked Blocks Tutorial

Stacked Blocks Tutorial

For ME 477

Revised

3/22/2005


Purpose:

In this tutorial, you will learn how to analyze the Stacked Blocks model using CETol. You will analyze the model in both parametric and overlay modes.

1. Analysis Preparation

Start Up Programs:

  1. Start up Pro/E on any CAEDM PC (PC AppsàCAD ProgramsàPro ENGINEER).
  2. Start up CETol (PC AppsàCAD ProgramsàCETol 6 Sigma 7.1àCETol Modeler).
  3. Under the “Model” menu select “Synchronize with Pro/ENGINEER”.
  4. Select a connection version or create one if it does not exist. If creating a new one, you’ll want to change the settings to the following:
  1. Select “OK” & “OK” (it will take a few seconds to connect to Pro/E).

Open Stacked Blocks Assembly:

  1. Copy the following files to your own directory (e.g. create J:\CETol and then save files there) from http://www.et.byu.edu/~chasek
  2. àME 577àCE-TOLàStackblocks (NOTE: Truncate filenames to save as standard Pro/E files, that is, .prt and .asm)

·  base_park.prt

·  cyl.prt

·  mblock.prt

·  stack.asm

  1. In Pro/E, change your working directory (FileàSet working directory) to the directory where you just saved the files.
  2. Open stack.asm in Pro/E.
  3. Synchronize with Pro/E by pressing the icon in the top left of the CETOL window. (You’ll know that you’ve done everything right to this point by hovering over the icons in the bottom right of the CETol window and noting that you are “Synchronized with” and “Connected to Pro/ENGINEER”.)

2. Overlay Tolerance Analysis

Define Design Specification:

The first step in defining a tolerance model is to select what needs to be studied. In the stacked blocks model the gap between the base and the top of the cylinder is the dimension of interest.

  1. Select the Add Measurement button (the green button with the plus sign). This will switch focus to Pro/E.
  2. In Pro/E, select the under side of the overhang on the base part.
  3. Select the curved face of the cylinder part.

·  Note: the program will take a few seconds to identify the surfaces

  1. After CE/Tol has finished processing, go to Tolerances, for the Measurement, and change the precision to 6 then verify that the nominal value is 5.945699
  2. Ensure that the Zone Type is set to Symetric, and set limits at ± .15
  3. Under General, for the name type “GAP”
  4. Also change the Distribution to Normal Fit (This is done so that it will match results you would get by doing the analysis by hand, if you thought the distribution might have some skew you could select one of the other types.)

Define Kinematic Joints and Set Degrees of Freedom:

Now we must define how the parts fit together and which contacting surfaces control the position and orientation of the parts relative to one another.

Define the planar joint:
  1. Select the Add Joint button (the button with two stacked blocks with a plus sign)
  2. In Pro/E, select the slanted face of the base part.
  3. Select the bottom face of the block part

·  CETol will select the planar joint type

  1. Change the name to “Base_Block1”

5.  Under degrees of freedom, set all the joints as fixed except in the translational z-direction. This is done by clicking on the arrows showing each degree of freedom in the image. Highlighted means the model is free in that degree of freedom.

Define the Cylinder Slider Joint:

1.  Select the Add Joint button.

2.  In Pro/E, select the curved surface of the cylinder part

3.  Select the vertical inside face of base part.

4.  For the name enter “Base_Cyl”.

5.  Enter degrees of freedom according to Table 2.1

Table 2.1

Translational / Rotational
X / Kinematic / Fixed
Y / Fixed / Fixed
Z / Fixed / Kinematic
Define the Edge Slider Joint:

1.  Select the Add Joint button.

2.  In Pro/E, select the top left edge of the

block part (after the curved surface).

3.  Select the vertical inside face of the base part.

4.  In the name space, type “Base_Block2”.

5.  Enter the degrees of freedom according to Table 2.2

Table 2.2

Translational / Rotational
X / Kinematic / Fixed
Y / Fixed / Fixed
Z / Fixed / Kinematic
Define the Parallel Cylinders Joint:

1.  Select the Add Joint button.

2.  In Pro/E, select the curved surface of the block part.

3.  Select the curved surface of the cylinder part.

4.  In the name space, type “Block_Cyl”.

5.  Enter the degrees of freedom according to Table 2.3

Table 2.3

Translational / Rotational
X / Kinematic / Fixed
Y / Fixed / Fixed
Z / Fixed / Fixed

Note: When you have all of your joints constrained, your model should be over constrained by 6 degrees of freedom (it is a 2-D problem, this can be checked by pressing the “DOF Analysis” at the bottom of the window).

Establishing Dimensioning Schemes:

In order for CETOL to be able to correctly analyze the tolerances of the assembly it must have information about how each part is dimensioned. All the features that are referenced to other parts are already included in the part diagram, these must be tied together. If there are any critical intermediate features between these features they must be included as well. For instance suppose we have some part that has 3 faces all in the same orientation and for some reason the manufacturer dimensions plane B from A and C from B. Well if A and C are ones that are used for constraints to other parts then B must be included because it is an intermediate feature.

To assist in working through the relationships it is recommended that the user rename the features in CETOL to more indicative names. For consistency change the name of the features shown below to the names listed. If the feature is not yet in CETOL when/if it is added use the name shown. Bottom and left should also be used for the obvious sides. The cylinders feature should be named curve (you may notice there are 2 features of the cylinder, select them both and merge them to one).

Establishing the Dimensioning scheme for the Base Part:
Creating Constraints
  1. Add the bottom surface of the base as a feature, this is done using the “add feature” button, the orange plane with a plus sign.
  2. Rename the plane appropriately and drag it to the top of the features listed under the base. Order is very important in the listing in the left pane.
  3. Select the inside top feature and go to the “Constraints” tab.
  4. Under constraints click add and select the bottom plane out of the ProE model. (If we had not already added the bottom as a feature this would automatically, but it would have been lower on the list than the inside top.)
  5. Toggle the state of the two angles to invariant by double-clicking on “variant”.
  6. Select the Offset angle in the very left pane and change its name to “f”. You can also change other factors such as limits etc for the variable you have just added.

You will want to repeat the steps above for the entire assembly until it appears as the following.

You can change the diameter measurements to radius by selecting them in the listing and changing there type. You will want to do this to get the sensitivities to match what is given the example. You will notice also that the radius R is repetitive with the offset of the edge, currently we have been unable to relate these two dimensions to one and the output sensitivities will need to be summed.

Analysis – Calculate Sensitivities and Review Results

The purpose of this exercise is to calculate the sensitivities of each dimension on the Gap dimension. CETol will calculate these sensitivities, and knowing them will allow us to know how much effect each dimension has on the Gap.

  1. Select the Analyzer Window button (the distribution) on the tool bar.
  2. This will open a new window. In this window select new, under file or on the toolbar.
  3. This will bring up a warning message about overconstraint, this is expected because we are forcing it to 2d.
  4. You can now review the sensitivity of the variables as well as their contribution. Generate a report by the command under “Analysis”. Compare the chart in the report to the following. This is identical to the one in the Chpt 13 handout except that Offset and R must be summed.

4. Discussion of Results

When CETol solves a tolerance problem, it has built in tolerance models. In the Stacked Blocks model, it makes different assumptions in Parametric Mode then it does in Overlay Mode. Because of the different assumptions, the program solves two similar, but different problems. In this section of the tutorial you will learn some assumptions CETol makes and the different problems it was solving.

Background:

There are two ways to call out a tolerance on an angle. You can apply a tolerance on the angle itself, or, using GD&T you can call out an angularity tolerance zone.

When you call out a tolerance on the angle itself, you simply give a range of angles the actual angle must fall within. This creates a wedge-shaped tolerance band that radiates from the axis of rotation (see Figure 4.1).

When you use an angularity tolerance call out, you define a set of parallel lines falling on either side of the desired line. The actual line must fall somewhere in that band. This allows the line to rotate around an axis at the center of the line (see Figure 4.2).

CETol Assumptions:

In the analysis of the Stacked Blocks model in Parametric Mode, CETol placed the axis of rotation of the angle at the corner. For this reason, the analysis in Parametric Mode modeled the angular variation as a wedge shaped tolerance zone.

In Overlay Mode, CETol will always place the axis of rotation of the angle at the midpoint of the angled side. The analysis in Overlay Mode models angular variation as a rectangular tolerance zone, similar to a GD&T angularity call out.