Intro: Pinewood Derby Car on Shopbot
Where my son competes in Pinewood Derby, they have pretty strict rules on what work the boys must do on their own cars. The boys divisions have strict rules about what can and can't be done.
But THEN there is a DAD's division! The rules for the Dad's division are as follows:
1) Weighs less than 5 ounces
2) Fits within 1-3/4" by 6-15/16"
3) Uses the wheels in the kit
4) Bring it!
Now, I am a member at TechShop (the one in RDU) - http://techshoprdu.com
I decided that I would use the opportunity to learn some new skills, and I would use the Shopbot!
Follow along for an adventure to learn what to do (and what not to do....)
Step 1: Computer Work
I had never done any 3D CAD before so I started with Google Sketchup. I decided it would be interesting to learn, and it had the added benefit of being FREE and running on my mac. (If Autodesk 123D had run on my mac that would have been a better choice.)
It turns out that Sketchup was NOT designed for this type of work. I wasn't going to pay $495 for the pro version (which had solid tools.) I should have just gone to Techshop and used Autodesk Inventor there. (But I didn't know how to use that either, and I decided that I would just go ahead.)
Anyway, I browsed the Google 3D Warehouse and downloaded a model of a Camaro. Then I scaled it from the full size down to the width I needed my wheelbase. (1.5") I deleted side view mirrors and other details (like the inside of the car).
I downloaded a free plugin that let me export STL from Sketchup. Then I pulled it into Partworks3D (on the Techshop computers) which let me do the CAM work. (That is the photo above.)
Step 2: Preparing the Wood
I was going to try to do fenders that went over the wheels, so I glued 2 blocks together to get the additional width.
Step 3: Run Shopbot
I start the shopbot, do an "air cut" (where you run your program several inches above your material so you can make sure everything looks good.). A friend of mine stops by to watch. I start the program and the block of wood goes spinning off the wood as soon as the router bit hits it. I slam the emergency stop button.
Obviously not enough hot glue to hold it. I then cover the bottom in hot glue. I put everything back, tell the shopbot to go home and the bit comes flying across and breaks in half when it hits the block. I hadn't recalibrated the Z azis after my escapade.
I go buy another bit from the Techshop retail store, and continue.
The bottom is cutting fine. Then I put in the ballnose bit, and because the outer part hadn't been fully removed yet the collet runs into the wood and causes some burning. (from the friction.)
I hit the e-stop again. And decide that I have found enough problems with the cutting from the bottom, that I'll cut from the top and find the OTHER problems in my CAD/CAM work.
I go to remove the hot glue, and it won't come up. In the process of getting it to come up, I slice my hand with a chisel. Luckily it was a sharp chisel so the cut was small. (sorry, no picture of the blood....)
Finally I get it to come up, and then glue it to the otherside of the board.
Step 4: Pass 1 - Top
So I run the rough cut, followed by the ballnose pass. And wow, it looks really good.
Unfortunately, I look and the fenders aren't even close to the size they need to be. I am puzzling over this when a friend mentions to me, "Of course you realize that the wheels in pinewood derby aren't scale to a real car." As soon as he says this it is obvious he is right.
I try to increase the size in Sketchup, but am failing miserably. (Have I mentioned how much Sketchup is not made for this task yet?)
I realize that I only have time for 1 more Shopbot session before the race, so I need to get it right. So I decide to remove as much error from the process as possible.
1) Make it so I can do it with only a single cutting pass from the top. This means reducing some detail.
2) Make it with no fenders, out of just a single pinewood derby block. This will make it more the dimensions of a "normal" pinewood derby car.
I finally find a Sketchup extension where you can select multiple vertices and set their Z to the minimum of all of them. This lets me drop the sides of the car so there won't be wheelwells.
Step 5: Pass 2
So I put the block on a piece of wood (plenty of hot glue this time). Set the block on the table, and start the shopbot.
I realized looking at it with just the rough cut, that the car wasn't centered on my wood. Turns out the table wasn't perfectly zeroed. Luckily I had one more piece of wood and some time left in my shopbot slot.
Step 6: 3rd Time Is the Charm...
I cut it out of the wood and my only problem was that my wood was a 1/16" inch shorter than I had modeled it in CAD/CAM so it didn't have the little spoiler on the back. I didn't have another block, so I decided that was fine.
Step 7: Sand, Paint, and Race
A little bit of sanding (not nearly the amount I should have.) Then a spray paint coat of primer, a spray paint coat of Red, and a little bit of black Testor's paint for the windows. Wheels put on with an hour before the race.....