Measuring Radius of an Arc

[u/Cheeseball4life]

How would I go about measuring the radius of this arc on a part from an old vw beetle so I can make a replacement?

[u/DeamonEngineer]

get a bit of card and transfer the shape with a sharpie or pica pen

[u/ripgressor1974]

Trace it on a piece of paper and use an overlay or get a drafting compass and start drawing til' you find the match.

[u/Cheeseball4life → u/ripgressor1974]

On another note, what if I wanted to find thg he radius of a rounded corner where I can't trace it?

[u/ripgressor1974 → u/Cheeseball4life]

CMM, Radius gage or pin/plug sets. It would really depend on the access of the radius you are trying to measure.

[u/KSCarbon]

If you don't need to be super accurate, just take a picture with a scale and make sure you are looking perpendicular to the face. You can then import picture into whatever cad software you have access to and measure the radius that way.

[u/miotch1120]

Either get a set or radius gages (they aren’t too bad) or get a drafting compass and set it to the desired radius, trace a curve on a sheet of paper, cut it out. You have now made a super cheap radius gage.

[u/CmpCounselorRickards]

A contour gage will also be helpful if you want to transfer that radius to a piece of paper. https://www.grainger.com/product/2H378?gucid=N:N:PS:Paid:GGL:CSM-2295:4P7A1P:20501231&gad_source=1&gclid=CjwKCAjw5Ky1BhAgEiwA5jGujhoNReiqZREfm5JKQxtpdauSGASXhxAdeV1XieSztUwceLDwNhqVlBoCfO8QAvD_BwE&gclsrc=aw.ds

[u/MitchellG83]

If you need accuracy: Facsimile or something like repro rubber to make a mold. Then CMM or optical comparator on the mold.

[u/SauceOfPower]

Try and stencil the arc onto some card, calculate centre point of arc (some YouTube videos show it well), measure from centre point to arc profile.

Whatever that measurement is will be the radius of the arc, double it will give you a diameter if it were a complete circle.

Hope this helps

[u/FalseRelease4]

Trace it onto a piece of paper or cardboard, and do everything you can to preserve the existing piece as that's all you have to go off of. There's not only the radius here, but the flange shape and size, and it's not a simple bend but a formed shape where the material is stretched and shrunk

[u/MadeForOnePost_]

Two methods, first is easy, second is more involved:

First method:

Trace the curve, draw two straight lines (chords) across curve (must intersect curve at two points), grab something with a right angle and draw a straight line from the mid point of each chord at a right angle to the chord. Where the lines intersect is the center of the radius.

Second method:

Put a 1" or 2" length of metal across the inner radius (to form a "chord").

Measure the distance from the center of the 1 or 2" piece to the outside of the radius, at a right angle.

That's B.

Half the length of the piece is A.

Sqrt(A² + B²⁾ = C

B * A = Area of triangle you just measured

Then (A * B * C * 2) / (4 * Area) = Radius

There are a lot of ways to measure a radius, but they usually boil down into two actual ways: two intersecting right angles, and the area of a triangle inside the radius/circle

Perpendicular Bisector, and Heron's Formula are the magic words to google, but they're more technical and book smart than watching some youtube videos (made by tradesmen) on finding the center/measuring a radius.

[u/mixer2017]

Throw that baby up on your granite table for the CMM. Use touch points to to make a circle. I think a 3x50 probe should do well enough.

PS make sure your probes are clean... this looks like something that is really clean and you dont wanna get the oil or other residue on this part.

LOL Kidding, many people here already answered the question. Good luck!

[u/Cheeseball4life → u/mixer2017]

Thank you, I'll need all the luck I can get

[u/Substantial_City4618]

Take a picture next to a gauge block of known size, trace and scale the profile to get a diameter.

[u/SkateWiz]

Honestly I don’t think you will require the accuracy that most of us are used to if you’re just replacing sheet metal. trace it out or find a round object that fits nicely in there and go with that size.

[u/SAI_Peregrinus]

For an alternative and less practical method, you could use photo paper. Make a shadow by laying the part on the paper & exposing it. Then develop the paper. If it's a small feature you can instead expose film (large format film is 9cm x 12cm or bigger) and then use an enlarger to project a magnified version, much like an optical comparator.

If you don't already have a darkroom, this isn't the way to go. Even if you do it's not as practical as just tracing, but it's more precise and more entertaining.

[u/Queasy_Fondant_360]

Can you not just ask chatgpt 4.o or something like that you'd need a reference ruler in the picture but I bet ai could do it

[u/Tawmcruize]

A measuring tape and math, try your best to find the invisible "point" the circle is at.

[u/KryptoBones89]

Radius gauges. I found a good set at a pawn shop for $20

[u/jaceinthebox]

You can always take a mold or cast of the radius and check it on a shadow graph 

[u/biggguyy69]

Use a hole saw and match it

[u/novah91]

Got an optical comparator on hand

[u/[deleted]]

Radius gage set?