Introduction

In this post I will setup a simple parametric Catia model of a Pinewood Derby Car (PWD-C). All files are available for download at the end of the post including a PDF of the drawing template. The motivation to create this model are:

  1. Introduce basic Catia workflow
  2. Develop parametric CAD models with dynamically linked assemblies and drawings
  3. Quickly design with drawings automatically updated to trace with a band saw
  4. Introduce engineering tools to my son (*never too early to brainwash my minion*)

In this post I’ll be going over: parts, assembly and drawing creation. Then shaping of the body will be performed using splines and style curves where you will see several designs created by Pete, my 7 year old son.

(*No childhood memories were harmed in the making of this post. Aside from setting up the Catia model and running the band saw my 7 year old son Pete did all real the work.*)

Sections

  1. Basic part design
  2. Assembly
  3. Drawing
  4. Shaping car
  5. Updated assembly
  6. Updated drawing
  7. Finished real car!

Basic Part Design

In Catia there are many modeling strategies that can be used. Here I employed a simple approach having the wheel and axle in a single CATPart and created a separate CATPart for the car body. In future posts I will go into some of the modeling strategies such as: top down, bottom up along with their merits and hurdles.

Wheel and axle

The wheel and axle are standard parts with no modification allowed, in our league. Two sketches were created on the XZ plane. Each started with an axis aligned with the Z axis and their length was dimensioned corresponding to the overall length of the part. I find that if you have the first sketch entity to scale then your as sketched geometry is usually quite close to the actual dimensions requiring little manipulation upon dimensioning. If you click the little down arrows on icons in toolbars there are additional commands. In the case of the circle command there are multiple ways to define a circle or arc segment. From here lines and arc segments defined the shape and dimensions were placed to fully define the sketches.

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Then the trusty calipers were brought out to fully define the sketches. Each sketch was revolved into their own part body.

Wheel

Block with axle holes

A basic standard block shape was created. Axle holes were parametrically dimensioned referencing the distance from the edges of the block. The block was extruded half of the width and then mirrored to complete the block. This assures symmetry during the stylizing. Wheels were drawn in concentric to the axles and output as sketch features which were extruded as surfaces for visualization purposes when stylizing the block in later steps.

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Assembly

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The car body was placed in the assembly and fixed in position. The wheel/axle parts were added to the assembly. A coincident constraint was added between the axle and axle holes in the car body. Another coincident constraint was added to the inner edge of the wheel and the side of the block. This procedure was repeated for the other wheels.Image 724

Drawing Creation

View creation wizard was used to create orthogonal view of the assembly. Unlike most engineering drawings this one’s only purpose was to create a 1:1 scale drawing of the top and side views which will be glued to the block and cut with a band saw. Because of this no dimensions or annotations were provided it’s also the reason for the odd and cramped arrangement so that it fits on a standard A4 (8.5×11″) paper. This scale drawing is also provided in PDF form for downloading for your own PWD-C for your kid, troop or even you to use (I know you want to make one too).

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Shaping Car

As is often the case in engineering the manufacturing method can dictate the acceptable design space and this case is no different. The desire is to use a band saw to cut the side profile and then to cut the top profile. The model was created to accommodate this by using 2 extruded cuts. Sketches created on the top and side planes were created with splines. My son Pete then manipulated these splines by dragging their handles or adjusting the tangency angles.

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He was very happy with his results but I could see how the manipulation methods were constraining him from creating his desired geometry smoothly. I replaced the splines with Imagine & Shape sketch curves. Sketch curves allow you to repeatedly draw the desired shape and the curves evolve much like drawing on paper with a pencil and eraser. Below you can see his final design along with the sketch curves in orange.Image 720

Using this same model I added in a wood grain and modeled up my younger daughter Meg’s (daddy’s) car by simply redrawing the sketch curves.

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Updated Assembly

The assembly updated with the new design. Pete wanted it dark blue with fillets in black.

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Updated Drawing

The drawing also updated with the new design. It was printed, cutout and glued onto the block of wood.

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2015-05-01

Finished Real Cars!

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Here is a great video on the science of Pinewood derby cars by Mark Rober ex-NASA engineer.

Thanks!

Rob Stupplebeen

Rob@OptimalDevice.com

To access the files created for this post including the PDF scale drawing click here