Precisely measuring a 2D object - What's the best method?

[photekq]

I have a Korean custom here with 2 PCBs but just 1 plate. I would like to get another plate made for the second PCB.

The files aren't available (don't ask why.. blah blah exclusivity blah blah rarity blah blah), so I figure I'll try my best at measuring the plate and transferring it to dxf format.

I initially planned to just measure it will calipers, but then I discovered my calipers are a tad inaccurate. That got me thinking.. is there an easier/more precise way to go about this, or is taking some calipers to it the best way?

It should be noted that the key spacing is a little different from usual I think, so I must assume that all measurements are completely unknown.

The only measurement I've been able to obtain is the measurement for 1u, since I was able to take measurements for different multiples of u and subtract the difference, eliminating the inaccuracy from my calipers. I was thinking I could maybe scan the plate in and use the value for u as a base measurement, and from there deduce the rest of the measurements with pixel counts.

Anyone got any thoughts?

Here's a picture of the plate, to give an idea of what I'm working with :

http://i.imgur.com/tCiEBFa.jpg

[derzemel]

You are still a student, see if you can get access to a touch probe or a laser scanner (for school projects )

I believe you could also "freehand" it using a 0.5mm or smaller pen (you can find them in architects supply shops) and carefully draw the contour on a thick paper and then scan the result.

Or, as you said, you already have the 1u measurement, stick it in scanner and see what comes out

Oh, wait, you already have a picture of the plate... Get you hands on a better camera with a better lens (to be sure you won't have lens distortion) take a picture of the plate and use that + your measurement to create the cad file.

Or, find 3 more callipers and take each measurement 3 times with each caliper and calculate the average.

[chzel]

I think a flatbed scanner is your best bet (assuming limited budget!).
Most camera lenses have enough distortion to mess up the measurement.
Scan the plate and use it either to measure or as a template to trace it in CAD.

[derzemel]

Scanning it on a flatbed scanner could work.

Here is an image of an Alu plate that I scanned a minute ago with a low quality A4 scanner, from a multifunctional printer:

(click on it for the full size, as it is 3825x2154)


The image looks crooked because the scanner is an A4 sized one and the plate is larger than the flatbed. Also, the scanning head does not scan on 90°and those shadows show up

So, if you can find a higher quality A3 flat scanner, you may have good results.

[Findecanor]

I think a flatbed scanner is your best bet (assuming limited budget!).
Most camera lenses have enough distortion to mess up the measurement.

With a regular camera the biggest problem is distortion from just having perspective projection. In my other hobby I replicate objects from photographs and perspective effects are a big annoyance when you try to get measurements out of them.

I would definitely use a flatbed scanner. If you use a office scanner/printer/copier combo that is controlled from the machine itself, make sure that you set it to give you the highest resolution possible because defaults are often set to reduce file size.

A scanner should give you correct dimensions and embed the resolution (DPI) in the image file. Otherwise, would need a reference measurement: in pixels and in the real world. Make sure that this is as large as possible.
When measuring something that is tiled (such as a keyboard part), remember to always measure from left edge to left edge or similar, and not from a left edge to a right edge.. I know it sounds simple, but it is a common mistake.

I am a Linux user, so I often use The Gimp (free) not only for editing but also for measurement. In the Scale dialog you can do two things that are useful: You can resize an image in percent, and you can set the resolution (without modifying any pixels) in either DPI or pixels/mm.
At the bottom of the image window there is a popup-menu for changing the unit used by the measurement tool between pixels, mm and inches. If the resolution is set correct, you could then measure millimetres and inches directly with the measurement tool.
I think Inkscape (free) allows you to load a bitmapped image as background which you could draw vectors on top of which could then be exported to DXF, but I really prefer QCad over Inkscape for editing.

[photekq]

You are still a student, see if you can get access to a touch probe or a laser scanner (for school projects )

You're right.. but I'm nowhere near anywhere with those resources! Perhaps I can get access to that kind of thing (and CNC mills hopefully ) next year at university.

I don't have a flatbed scanner or even an A3 scanner, but I have an A4 one. I'll try and get some high DPI scans with that and see what I can come up with.

[photekq]

Looking good..



I'll just go a bit further down the scan.. Oh, nevermind..



I've managed to find some decently accurate calipers, so I'll try just measuring it..

[photekq]

Getting there..

[derzemel]

Uuuuuu... naiisssss man, ya doin a great job.

How did you do it in the end? Did you scan it piece by piece with the A4 scanner?
Used better callipers?
Paid an industrial spy to steal the plans?

[photekq]

The scan ended up being a bit warped, and in a way that I couldn't fix with photoshop. Take a look at the pictures above..

I ended up just using calipers. It turns out that they're fairly accurate when measuring external edges (the zero is about 0.05mm off), just not internal ones.

[bpiphany]

If you use a scanner, place a good ruler (or two perpendicular) in there as well. Then you can calibrate the image by calculating the actual pixel size. After that the measure tool in your image editor should be very useful.