Radius gauge, no maths (almost). 2 versions

This is a remix of Thyn's Radius Gauge for calipers. 
Initially I just added some nice fillets and made sure it fitted my plastic calipers and when I tried it I saw that a 38mm diameter cylinder yields a measurement of 7.9mm. Multiply by 2.4142 and you get 19.1mm. Close enough for what I do which is mainly making parts that fit to enclosures where I need to conform to the radii on the enclosures.

Not that there's anything wrong with having to multiply all the time, Fusion does it right on the model but I realised that if I measured in imperial units, I could read the radius value directly in mm, just multiply by 100.

It does require that the V angle be made slightly wider, 105.78°, more than 90° so I can still measure rounded corners on square structures but now I just read the radius in mm off the inch scale. Maximum radius it will measure without extending the arms is 75mm.
 

At last, a use for imperial units
 

Next thing was the possible error if the vernier isn't pushed all the way down. That could easily happen if I knock the attachment when I put it down. I have a nice tight fit but experience has shown me that it's better to anticipate a problem than have to do the rework - call it paranoia or maybe OCD but I decided to add a clamp (optional) to prevent this eventuality. Also known a poka yoking the design.

I might need to remove the attachment from the caliper so I used two small 8mm x 3mm self-tapping screws to hold the clamp on, it fits onto a feature on the plastic vernier that holds the depth probe down, M3 machine screws will also work. Yours may be different so this clamp may not work for you. If it doesn't work for you I suggest you use the step file and modify that part to suit your caliper. A simple flat piece and some CA glue might be a suitable solution for you if you want to dedicate the caliper to radius measurement.

I printed my clamp with the wide flat face on the bed. This means that the clamp is not particularly strong in clamping down because it relies on the layer adhesion. I decided this was OK for me because there isn't really much force pushing the tail of the caliper up.

I added my G-code files and you may notice that I printed my PLA at 230 degrees. This is not usual, I just bought some material that won't print properly at lower temperatures. Use a temperature that suits your material.

If it measures too big it is likely one of 2 things

• You weren't holding the V perpendicular to the curved surface face
• The flat surfaces that go against the item being measured may not be flat. Usually a bit of first layer compression, very lightly file the bulge flat, do it carefully and measure as you do it so that you don't go too far. If you do go too far, add a layer of tape to one face or paint it with some nail polish to restore some material.

Accuracy? I am amazed that I can measure to less than 1mm accuracy but it does depend on exactly how I hold the gauge. The big flat surfaces inside the V help a lot but it takes some time to get consistently accurate readings.

Some radii will not be consistent, the counter corner is one example where the curve is not a nice circular arc, you will get different readings. The template gauge shows the curve is not a consistent radius arc.

Update: 3/12/2022. Added radius to bottom edge of V so that the material spread on the bed is reduced thereby improving accuracy. Trimmed angle to exactly 105.78°. Added a recess for my metal vernier in the bottom of the Corner. Trimmed the tolerance on the clamp so that the fit isn't as tight.

Update: 3/12/2022 Added the larger version that uses a tyre depth gauge readily available online. It is designed so that this version just clips in, no clamp needed. It does use supports in 2 small areas that come off very easily. Measures from 2.5mm to 60mm radius. Measuring to less than 2.5mm radius is possible if you remove the re-enforcing flat section are in the corner of the V but this reduces the strength a bit