Overlay Panels for a Car Project #2: Massaging the Numbers

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This is part 2 in a 2 part series: Overlay Panels for a Car Project

  1. Measuring the shape
  2. Massaging the Numbers

 With measurements in hand I now can enter them into a spreadsheet. 
 
Then I can create curves that run vertically on the doors.


And horizontally as well.


To get the curves to be this smooth I had to make a few corrections to the measured values. 
 
1.    Correct for misread numbers. Usually these were small corrections of 1/16” or less. A couple errors were more like ¼” where I likely misread the scale. 
2.    Correct for times when the frame was tilted. There a few sets of measurements where the error got larger at the top of the frame and got progressively smaller towards the bottom. Even though I clamped the frame to the beam each time sometime the beam would have a slightly different shape or I might bump it. The worst correction was less than 2 degrees. 
3.    Correct for times when the vertical and horizontal curves didn’t agree with each other. This was the last bit of fine tuning. 
 
It took about 6 ½ hours to for me to be happy with my final set of curves. I’m pretty sure I could have accepted larger mismatches but there will be plenty of ways to add in errors as I move forward. I might as well keep things as close as possible for now. 
 
After a couple of hours I have each of the 10 forms laid out in CAD.




The next step is to get the forms cut from a sheet of ½” plywood. My CNC is too small by about 3 inches so I am going to take this to friend who has a larger machine. It will take a while to match schedules up. Stay tuned! 
 
I have about 29 hours in the project so far. 
Sounds like a good use of a third order polynomial to do a statistical fit!
That and a lot of "adjusting"!

What software are you using your CNC?  Does it support tiling?  With tiling you can mill the area that will fit under your CNC and then slide it to do the other part(s).  As long as you move it precisely it works pretty well.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

I have Vetric software and I could tile with it. But I can just throw the whole sheet on my friends CNC and let it run. I could do it in two passes per part if I had to.
Steve, when I am designing curves like that, I like to test them by printing them on paper at full scale and then cut them out so that I can hold them up against the part.  Not sure that Vectric software has that ability but I know that OnShape CAD does.  I am sure that other tools could as well.  You can export your Vectric vectors to a DXF, import it into Onshape and then create a 2D drawing that you can save as a PDF.  Once you have a PDF, you can use Adobe Reader to print it at full scale across multiple pages and tape them together.  You can have Adobe print alignment crosshairs to make it easier to get it taped together.  For your purposes, I would probably tape them to a piece of cardboard before cutting them out to help hold it against the car to see how well you captured and transcribed the shape.   Might save some plywood. 

Let me know if you need some help with the OnShape side.  I could take exported DXFs and create the PDFs in OnShape for you if you want.   And at the end, you would have an OnShape file you could manipulate if you need to.  OnShape is free for personal use.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

Thanks Nathan! My CAD package lets me create a PDF. I might have the do the sanity check. Just a couple of test plots would let me know how I did.