CAD EXERCISE 1.1 MODELING A SPIRAL STAIRCASE

CAD EXERCISE 1.1 MODELING A SPIRAL STAIRCASE Figure 1: Spiral staircase and its model tree.

Learning Targets In this exercise you will learn: Grouping features Using dimensional pattern Using relations Notes about the Exercise Some common combination of letters: RMB is right mouse button LMB is left mouse button MMB is middle mouse button (press the wheel) In Creo Parametric, MMB is mostly used to rotate the view (holding MMB) and accepting the tools (press ones MMB). By holding RMB, you can open an additional menu, which depended on the selected items. The used program in this exercise is Creo Parametric 3.0 M050. 2 / 13

About the Case A spiral staircase (Figure 1) has (possibly varying number of) repeating features, so modeling one properly definitely involves patterning. However, most CAD packages do not have sufficient tools for such both radial and linear patterns. The conventional dimension pattern of Creo (been there from the times of Pro Engineer) does the job, once appropriate feature to be patterned is available. Creo patterns only individual features, so creating a feature group that has right dimensions to be patterned is a must. To get started, start Creo Prametric 3.0 and create a New ( spiral_staircase (Figure 2). ) solid part and name it as Figure 2: Creating a new solid part. 3 / 13

First Features Base pillar Using Extrude ( ), select TOP datum plane and create a Ø200 mm Circle (, Sketching group). Accept the sketch ( ). Extrude it 4000 mm (Figure 3) and accept it ( ). Rename it as PILLAR. Datum plane Figure 3: First feature. Then create a new Plane ( offset (Figure 4). ) using the default offset method, TOP plane as reference and a 100 mm Figure 4: Accepting a new datum plane with a 100 offset. 4 / 13

Creating a Stair Extruding first stair Use Extrude ( ) and sketch a closed profile on the new plane (DTM1) as shown in Figure 6. You can use Center and Ends (, Figure 5) arc and two Line (, Sketching group) features. To place an ancle dimension, use Normal ( them to place a dimension there., Dimension group) and select two lines and then press MMB between Figure 5: Choosing Center and Ends under Arc menu. You should be able to freely rotate the sketch by changing the value of the lower angular dimension, try! Exit sketcher ( ) when ready and extrude a 200 mm thick stair. Rename the feature as STAIR. Figure 6: Sketch containing one arc, two lines and three dimensions. 5 / 13

Creating a group Now comes the most important step: Select the extrusion (STAIR) and the new datum plane (DTM1) in the model tree. This is done by holding CTRL and clicking the corresponding features. When selected, click Group in the RMB popup menu (Figure 7). The resulting group feature has all the needed dimensions for the dimension pattern. Figure 7: Selecting Group from the RMB menu. Making a dimensional pattern Select the previously created group, click RMB and select Pattern ( ). Holding CTRL, select two dimensions (A and B in Figure 8) that define the stair location as references in Direction 1. Open the Dimensions slide-down panel and enter the proper increments (200 mm and 20º). Start with 15 instances, for example. Finish the pattern (MMB). 6 / 13

Figure 8: Ready to accept dimensional pattern. Make sure that you understand what happened. Dimension pattern simply takes given amount of copies of the selected feature, adding given increments to the selected dimension(s). If second direction is used, the whole set of first direction copies is similarly copied into given the number of instances, again incrementing selected dimension(s). Very simple and effective, yet traditionally one of the most tricky things about Creo to learn! Save your model (CTRL+S). Introduction to Parametric Modeling Firstly, what we want to do here is to make sure that the pillar starts from the bottom of the first stair and ends to the top of last stair. If the number of the stairs is constant, this job would be easy. We could do it just calculating the height or even better, locking the height of the pillar to the top surface of last stair. This method however doesn t work if we have a possibility to change the number of the stairs because the extrusion locks to the N:th member of the pattern, regardless of the number of the pattern. 7 / 13

Now what we do is, we add relation between the height of the pole and number of the stairs, and later on, we want to include the elevation of the stairs to be taken into account. The first step is easy. We want the pillar to begin from the bottom of the first step. In this case, it is obviously easier to lower the first step to the TOP plane. Expand the pattern (Pattern 1 of LOCAL_GROUP) from the model tree and expand the first Group. Then select the DTM1 and click RMB and select Edit ( seen in Figure 9. Then change the offset from 100 to 0. ) as Figure 9: Editing the location of the first datum plane. 8 / 13

Relations Next, we add a relation to the pillar s height. Select Relations ( ) from Model Intent group (Figure 10) and Relations window opens. Figure 10: Selecting Relations from Model Intent group. In to the text field, we can add relations and functions to the dimensions. All of the dimensions are coded to the model as a variable, for example d2 or p4. D stands for dimension and p for pattern. Click PILLAR form the model or from the model tree and you can see the dimensions of the feature (Figure 11). Figure 11: The dimensions of the feature PILLAR showed. You may have different dimension names! 9 / 13

Now type: d0 = 15*200 to the relations text box, click OK and click Regenerate (, Operations group) (or press CTRL+G). NOTICE: Use the correct dimension, it might not be d0! And also notice that if you click the dimension from the model, Creo adds the dimension to the relations box. Now the height of the pole should be suitable for 15 steps. Of course, now if we change the number of steps, the pole height doesn t change. Open the Relations ( ) box again and click one of the steps from the model or from the model tree to get the dimensions of the pattern. Now find the p-dimension (p#) which defines the number of the pattern s instances. Then replace 15 from the relation with p#. Next, find out the dimension name (d#) of the height of the step (d#) and replace 200 with it. Then, again, click OK and regenerate. If your number of steps is 15, nothing should happen, so now change the number of instances form the pattern. (From the model tree: RMB on pattern and select Edit ( ).) 10 / 13

Creating a parameter Figure 12: The amount of pattern instances changed to 10 and model is regenerated. There is also an easier way to change the number of steps. We can create a parameter that defines the number of the steps. First select Parameters ( ) from Model Intent group and the parameters window opens. Now click the 13. -button to create a new parameter. Then fill the new line as in Figure 11 / 13

Figure 13: Parameter STEPS created as an Integer. Close the parameters window (OK). Now we only have to add one more relation to the relations box. Open the Relations window ( ). Notice that you can access the parameters also from the bottom of the relations window (Expand Local Parameters). Add p# = steps above the existing relation. And again, check the right number (#) for p#-dimension. Now change the STEPS-parameter value to 20, for instance, and close the window and regenerate the model (CTRL+G). Now the pillar s height should follow the number of steps. 12 / 13

Last thinks to do without guidance 1) Find out why the p# = STEPS were added above the existed line instead of below? 2) Change the relations and parameters so, that you can define the height of the pillar (using parameter named HEIGHT), and the program calculates the needed number of steps. 3) Create also parameter called STEP_HEIGHT that defines the height of a step. Ensure that this parameter works together with HEIGHT parameter. Notice, that you need also to adjust the increment in the pattern (while in Relations, select the pattern feature to see its dimensions). 4) Correct the error of the height of the pillar (for ex. when height is 2100) so, that the program To Return The model reduces the height of the pillar according to the amount of the steps (without changing step height). Hint: with floor() function in Relations you can get rid of regeneration warning (yellow traffic light instead of green one). The.prt file (Creo native file format) itself is returned to the auto-assessment tool. Link can be found in MyCourses. If there are some mistakes in your model, read the auto-assessment tool s feedback (Figure 14). NB! Your model should have parameters called HEIGHT and STEP_HEIGHT, otherwise the auto-assessment doesn t work! Figure 14: Feedback from the auto-assessment system. Upper one passed, lower one failed. 13 / 13