Getting precise measurement of a complex rod

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I'm trying to build a headrest for my Sayl office chair. For that, I'm designing a 3d-printed part that's going to fit on one of the existing rods of the chair.

Check out this picture:

How would you go about in getting the exact measurements of that white rod? I tried a caliper, and I'm able to get the width and depth, and I can just assume that the rod's profile is a perfect ellipse, which is probably a close estimate. But say that I want to get a more precise measurement. Is there any technique to do that?

Ram Rachum

Posted 2018-05-20T15:36:15.710

Reputation: 243

You willing to disassemble the chair and use scanning techniques such as a 3-D laser scanner? More to the point: what is the intended operation of your add-on part? Does it need to fit to a high precision or can it go with an oversize hole? – Carl Witthoft – 2018-05-21T14:15:45.923

you can make a mold with modeling clay and then slice up the mold to take "samples" at a regular interval with a pair of calipers. – dandavis – 2018-05-22T05:55:05.727

Answers

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In an earlier comment you stated that you cannot take it apart. So without taking it apart, you could try to determine the profile the old-fashion way with a piece of cardboard and a short pencil, just cut the rough shape of the rod and place it onto the rod, then take the short pencil and draw the profile onto the cardboard with the pencil parallel to the rod. Measuring the distance from the pencil center to the pencil radius will give you the profile of the rod with that off-set. This technique, or the technique used to create notches in logs for log cabins may be used to find the profile at various sections which have to be entered in a 3D CAD model program and splined/lofted to get the surface of the rod.

Alternatively you could use a profile shape tool carpenters use: enter image description here

0scar

Posted 2018-05-20T15:36:15.710

Reputation: 25 570

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You need to disasemble the part and measure it with special equipment, a caliper can help but only as reference since the part has an angled shape.

I recommend to use an optical comparator (base shadows) with this you can have X and Y data to calculate the angle and curves. If you want more precise measurements you can try an Optical Measurement Device (based camera), this also can give you Z but X and Y will help you a lot. Both equipments use a system called Quadra Chek many industries has at least one of this to assure his quality due meets all requirements for Ford GDT guides (geometric dimensional tolerances). no matter the manufacturer brand.

I´ve tried to measure with the phone or table application but is hardly to calibrate on each required dimension. I had to buy an optical comparator.

Fernando Baltazar

Posted 2018-05-20T15:36:15.710

Reputation: 1 280

Depends on the accuracy required to meet the needs of the add-on part. – Carl Witthoft – 2018-05-21T18:53:57.117

In an earlier statement the OP said Nah, can't pull it apart. – 0scar – 2018-05-22T05:27:58.907

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You could pull it apart and have it 3d scanned if you want to know the exact dimensions. There are companies that can do that for you at a certain price. Our company has used such services in scanning various parts before we obtained our own laser scanning device.

The question is whether you want exactly the same (dimension wise) part (maybe you do for ecstatic reasons) while a part that is a little more beefier would work also.

Edit: Although tagged with 3D scanning, the OP did not mention whether he would be able to disassemble the part. 3D scanning is an option when taken apart. Another solution has been posted since.

0scar

Posted 2018-05-20T15:36:15.710

Reputation: 25 570

1Given that the question is already tagged "[3d] scanning", I don't think this really helps much. How do you get something 3D scanned? This is the equivalent of answering "How do I take a portrait photo" with "Using a camera.". – Tom van der Zanden – 2018-05-20T20:45:03.507

Nah, can't pull it apart. Thanks for the information though. – Ram Rachum – 2018-05-21T06:57:50.047

Well, if 3d scanning is out of the question if it is not taken apart, the whole answer can be deleted. Either way, the tag should be deleted as well. – 0scar – 2018-05-21T13:09:38.873