CAD
Spirograph Design for CSWP Contest
I’m going SolidWorks World next week and I’ve been told that the CSWP event on Monday night should be a good time. I remember seeing details about last year’s event where everyone got to play with the iCoaster. It looked cool. This year everyone gets to test drive an RC car. I’m not very good with RC cars, but there’s a design contest too. That’s something I can do.
The Problem: design a new wheel in SolidWorks for the SC10 RC car.
The Rules: Points will be awarded in these key areas: 1) Is the wheel designed and supplied in SolidWorks format? 2) Is the wheel rendered and/or animated in a cool way? 3) Is the wheel construction feasible? (decided by Team Associated engineers/designers)
The rules seem a bit awkward to me – the company obviously wants some free design work, but I want to have some fun. My best designs come when I ignore the Rules and just get creative (notice I said “ignore” and not “forget” – eventually you need to come back to the rules, but if you can ignore them for awhile, your creativity will not be limited.)
My wife still has her childhood Spirograph!
The first idea that comes to mind is an old toy – the Spirograph. What if my RC car wheel had spokes that looped around in a continuous weave that looked like one of those designs you can make with a Spirograph?
There must be a mathematical equation to describe the path made when a point on one circle is plotted as it rotates around another circle. If I can plot the path mathematically, maybe I can create a 3D path in SolidWorks to describe a sweep for the spokes of my wheel. After searching for “spirograph equation”, I discovered David Little’s page at Penn State University. As it turns out, the path of a Spirograph is called an epicycloid. It’s described by a pair of equations:
x(t)=(R+r)cos(t) + p*cos((R+r)t/r)
y(t)=(R+r)sin(t) + p*sin((R+r)t/r)
R and r are the radii of the 2 circles, p is the position along the radius of the first circle (the hole you put your pencil in on a Spirograph). Mr Little’s website has a cool Java applet that draws epicycloids based upon your input. If you experiment with this tool, you can discover all sorts of interesting geometry. After a few iterations, I found a combination that I thought might look like the spokes of a wheel using the values R=72, r=66 and p=67.
Where would engineers be without the spreadsheet? I remember seeing a documentary on PBS called Nerds where I learned that the first spreadsheet was called Visicalc and it was for accountants. Excel is the tool that I am most familiar with – version 2003 being most preferred. The Excel version of these formulas seems a bit more complicated. First, cells are made for all of the constants (R, r, p). I added a few extra for scaling – the final goal is an equation of a curve that will intersect the hub and rim of our wheel. Examination of the formula quickly reveals that t is an angular variable. I made a column for t in degrees because I know this is a cyclical function that will repeat every 360º (remember, it’s a circle rotating around a circle). I also made a column for radians (degrees times pi divided by 180) because Excel evaluates sine and cosine in radians. I probably could change this default somewhere, but that is something I would probably forget later and it’s easy to remember how to convert degrees to radians.
I know I want X and Y coordinates for the entire path and that they will eventually return to zero. I wasn’t sure how to know how many points to plot so I made some check formulas that subtract the current X and Y values from the original X and Y values – when the check columns both hit zero, I know I’ve returned home. The last step for X and Y is to plot a curve using Excel’s graph tool. The Excel graph matches the epicycloid plot so I know I’m on the right track. (Actually, I’m thoroughly appreciating the same feature that the original users of Visicalc appreciated, Excel’s power to iterate until I get it right.)
Some interesting observations: my epicycloid has 12 loops and the equation to describe it requires 3960 points at one degree increments. If I made another with 22 loops, I would expect it to take 7560 points to describe it (360 x 21). For some reason, my check equations did not exactly return to zero until I placed a round function into my formulas – I added a variable for the number of rounded decimal places as well.
What about Z? The equations for an epicycloid are only in 2 dimensions, but I want a 3D path for my wheel spokes. The options are limitless, but a sine function would make a nice smooth equation and it would behave in a similar way as the epicycloid equations. Some constants are necessary to vary how often the equation returns to zero and vary its scale. Graphing the function over the same range gives a clue how the path will vary in Z.
Excel’s worksheet functionality makes it easy to create formatted output. It turns out that the way the wheel was originally created, I need my X and Z data points to be swapped. By creating a new worksheet with only X, Y and Z values, I can swap the values very easily. The final result is a text file with 3960 points of X, Y and Z data.
Now to SolidWorks! The Insert Curve Through XYZ Points tool quickly creates a 3D path.
As a point of interest, I believe it was because of this tool that Walt Disney Imagineering chose SolidWorks as it’s 3D design tool – they had 3D points to describe the path of a rollercoaster and needed a tool to model the track. I created a second sketch on a principal plane perpendicular to the path. I started with just a sketch point and some dimensions tieing it down to the path.
Then I made the geometry for my sweep all tied to the sketch point. This way, if I want to change the profile from a circle to a square, I don’t lose my dimensions when I delete the circle. The Sweep tool turns all this hard work into an effortless expression of mathematical beauty. A few features to tie the spokes to the wheel and I have my entry.
Of course, I must return to the rules. I already know I will not win because, although my design is interesting, it cannot be molded which is the intent of rule 3. Rule 1 is easy – SolidWorks makes the whole thing possible. So Rule 2 is the only one left to satisfy.
I’m running out of time so a quick assembly with a cut-away view and a few minutes of rendering in PhotoView 360 and, at last, I have an entry.
Here are all of the files I used to create this design. -Amos
AEA Entry 1 of 2
AEA Entry 2 of 2
Followup: SolidWorks World was great. The CSWP event was very fun even though I am a very poor RC driver. 
I did not win the design contest (for obvious reasons). I did, however, win one of 40 SC-10 RC cars, which is VERY cool!
SolidWorks World Presentation
Last week I taught a course at SolidWorks World called Nonwelded Weldments – Using SolidWorks Weldment Tools and 3DContentCentral to Create 80/20 Structures. I always enjoy teaching and this event was especially gratifying when a couple of the attendees told me that they had been designing 80/20 frames for years but had never thought to use the Weldment tools to do it. It will save them a bunch of time. I plan to show the steps here in future posts, but I thought I would post the PowerPoint and support files for those who are interested. -Amos
80/20 Weldment Profile Library Features
SolidWorks does an amazing job of making the design of frame structures very easy. I’ve given a few presentations lately about how to do this using 80/20 products as the example. As promised, here are all of the Weldment Profile Library Features I created and uploaded to 3DContentCentral. I will give step-by-step instructions in a future post. -Amos
CSWP Advanced Weldments Specialist
I discovered yesterday from reading Mike Pucket’s blog that the SolidWorks certification team had released another advanced exam, CSWP Weldments. Since I am scheduled to teach a hands-on session at SolidWorks World next month titled, “Non-Welded Weldments – Using SolidWorks Weldment Tools and 3DContentCentral to Create Cool 80/20 Structures”, I thought it would be a good idea to get certified.
The first thing I discovered is that SolidWorks is just like any other company – their website is not up-to-date. The certification page in the Customer Portal lists a coupon code for subscription customers to take an advanced exam for free (one of the perks of being on subscription). The code didn’t work. It had expired 5 days ago. The Weldments exam wasn’t even listed. A quick email to certification@solidworks.com brought a new code from Av (Avelino Rochino, Certification Specialist) that worked just fine.
On to the Sample Exam. The nice thing about the sample exam which can be said about all of their sample exams, is that it is, in some ways, harder than the actual test. I know Weldments fairly well after creating over 40 weldment profile library features of 80/20 extrusions and uploading them to 3DContentCentral. It turns out that my weakness is 3D Sketches. I really had to work to get my answer to match the key on the last page of the sample. [Note to self, work on 3D sketch techniques.] Next the sample exam showed me something I’d never done before, a 3 member miter.
It’s possible to have 3 members come to a corner and miter all 3 together. It was new to me and took a few minutes to master – a few minutes well spent. So the sample exam was a challenge, but I did all right, so on to the real thing.
120 minutes, 27 questions. I jumped into the first few with fervor and made a few profiles. Something to consider if you work mostly in inches, make yourself a part template that is in metric as most every CSWP exam I’ve taken has started most excercises in metric units. Another suggestion that I teach my students is to add the Weldment Profiles directory to your Design Library task pane.
It makes it easier to create weldment profile library features (drag the sketch from the tree to the pane and drop it) and you get a nice preview of the profiles that you don’t get inside the weldment tools.
So I felt pretty confident after the first few consecutive questions and then I remembered my failure in the cave of 3D sketching. I took my own advice from the Sheet Metal exam – stop and read every problem. I’ve recently made lots of drawings with weldment cut list tables, so I jumped on those questions next. Then to the advanced questions which focused on proper profile alignment and finally the big 3D sketch problem. I was glad I saved this part for last because at that point I figured I had already passed the test. I’d also wasted 15 minutes of test time on a quick bio break and an urgent discussion with a customer about their customer who wanted us to fudge our paperwork because they made a mistake on their paperwork. There’s nothing like having someone ask you to throw away hours of work you did over 2 years to fix a paperwork glitch to distract you during a CSWP exam.
I tried hard, it looked perfect, but my 3D sketch answer didn’t match any of the choices. I triple checked, redrew some of it and still no match. Finally I chose the closest answer and the test ended. Wow, 120 minutes are gone and I passed. Not a perfect score, but I can proudly say that I’m the first person in Minnesota that chooses to be listed in the CSWP database to pass the Weldments exam. Hmmm. Well, my kids weren’t that impressed either. The good thing is that I can teach my SolidWorks World class with official credentials. And, I can now add CSWP-AWS to CSWP, CSWP-ASMS and of course, BSME. Woohoo!
The exam was pretty fair. There was one question series that didn’t specify a material in the questions but it was already chosen for you in the downloaded file you must start with. The other exams are notorious for throwing in material changes to catch you up, so I’m hyper sensitive to this – it should have been in the question a least for verification. Also, on my system, everything seemed to hang for an undetermined period of time after downloading a file (several times during the exam). This is a bit unnerving for fear the exam is locked up and all of your time will be wasted. Overall, a good test – glad I passed. I’m not sure where you stand on divine intervention, but I can honestly say that I thanked God more than once during the test for showing me something in question 12 that corrected a slight mistake I made in question 8. It’s a good thing you can go back and improve your answers. Say a little prayer before you begin, it will help.
-Amos
R2-Table2
I’m headed to SolidWorks World and I’m getting in for free! I submitted a proposal to teach a 90 minute hands-on session called, “Non-Welded Weldments – Using SolidWorks Weldment tools and 3DContentCentral to create 80/20 structures”. My plan is to demonstrate how to design a mobile cart in SolidWorks while my assistant actually builds the same (or similar) cart from 80/20 parts. My goal is to demonstrate how easy it is to both design and build stuff using SolidWorks and 80/20. I was having a hard time deciding what to build for the event. The proposal I submitted referenced a “a cool collapsing mobile workbench” but I wanted to build something like a Battle Droid. My friend Jamie suggested R2-D2 and the idea for R2-Table2 was born. It’s all 80/20 components. It’s modular and collapsible and mobile and very cool.
-Amos
Added 2/25/10 – Disclaimer – 80/20, Inc. and John Henry Foster provided the kit of 80/20 parts for my assembly of the R2-Table2 at a discounted price (it was not, however, free).
Swell Ideas
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