Measuring Radii

Samples taken from box on shelf. Measured on Keyence IM-6020. Range of measures: 0.014 – 0.025. Target is .008 to .019. This is for the radius under the head of the part.

I ask employees how she measures radii on the comparator. I was taught to use the 3-point function on a comparator’s head.

Her (and others) method of measurement is:

1. Zero comparator X axis and Y axis so the cross-hairs were where the she could see the radius starting at the bottom part of the radius.

2. Next she moved the cross-hairs on the Y axis until on the point where upper parts of the radius started. The resulting number on the Y axis is the radius of the feature.

Is this correct? I have never seen this method before.

Assuming it is a 90Dgr angle then
Then seeing where the radius starts is up to interpretation IMHO

Peter

She is measuring the start and end point of the radius, not the actual radius itself. If the radius is perfectly formed that can work. If it's not then it won't be accurate. The same could be said for a 3-point check. High enough magnification makes it pretty easy to repeatably find the beginning and endpoints within reasonable limits. What magnification is being used?

A better solution might be to use a screen overlay with high and low limits and just make sure the part falls inside the lines. Unless you need an actual dimension?

If you have the option to use (up to) 99 points, I'd pick 5 or six. The DRO will tell you "best fit", and how far off the furthest from best fit lies.

I would either make a min and max overlay or buy a see through radius overlay that fits your screen. Just need to match it to your resolution (10 -20 -50 power) to measure with. Either option you mention have possible inaccuracies.

We do have overlays that we use for the head radius of parts. Good suggestion. thanks.

We are using 10X magnification, though we can go to 50X. Yes, we do need an actual measurement. Thanks for ideas!

An overlay with differing radius lines will be fastest. You can qualify the overlay yourself by generating easily calibrated external radii.

Is this method correct?

Like so many questions asked on this and other machining boards, that one is impossible to answer without more information.

The answer lies in the realization that a method that meets the needed accuracy and functionality of the part is a correct one and one that does not, is not.

A radius measurement implies the existence of a CIRCULAR arc. But a three point method is mentioned. Mathematically, three points can lie on any number of curves. They can even be on two straight lines. So, if three points are used, you know nothing about the actual shape of the curve. Or even if it is actually a curve and not two or more straight lines.

If the method used by that lady and company meets all their requirements, then it is a good one. And if it does not, then it is a bad one. But a lot more about what is going on is needed to determine the answer to that question.

An accurate measurement of a small radius on a part is a difficult one to make if a lot of precision is needed.

If accessible, hold test pin against. You may need to use pieces of music wire. The fitting size will not roll over but just sit. Balls work also, held by a magnet you can pass them along sighting for light like under a straight edge.

Electronic measuring equipment is for beginners.

Seems like you are using eyeball to determine it is a radius and coordinates to measure at some points.

Better might be to 10x your device and then overlay a plastic radius gauge. Plastic gauges so you don't mar your screen.
You will be able to gauge .008 .012 and .013 inches radius with the small metric gauge set at 10x
Set at 50x and using the 13mm gauge you will see a .0102 (.010) radius.
(REF: 13mm = .5118 inch / 50 = .0102 inch)
Assembly Tool Specialists | NO-MAR Gages - "NO-MAR" Radius Gage Set

Yes, you might overlay an Acrylic & Polycarbonate - Transparent Plastic on your screen and then use any (steel) radius gauge set.

Not uncommon to eyeball over a micrometer setting and measure a radius for a rough test, likey good for about .003 (with having young eyes.)...yes looking at/for the diameter

At Cutmore Tool our best guys at drawing would draw a fancy part on clear plastic and we would place them on /over the comparator screen. It was about the only way to get a bunch of angles/sizes correct..with grinding one facet/feature and then the next.