Thread: Metrology for Amateurs

Originally Posted by Stustoys

But the same goes for a micrometer? The "marks" on a DTI are no good to you if you dont know what they mean, Even for zeroing you need some idea of what the marks mean.

How would you measure runout with a micrometer without a fixed reference point?


Stuart

I was not going to add to this thread anymore (at least this part of the discussion) as I seemed to be causing further confusion - perhaps due to my poor explanations, or maybe because it relies too much on definition. However...

I was trying to explain that because a DTI is a relative measuring device and relies on repeatability, the marks are all you need. Even if you had a blank face on a DTI, brought it down on a surface, put a mark on the face and then took it to your reference (whether that be a heightmaster, slip etc) and brought the DTI back to the same mark, it has done exactly what it needed to - behaved in a repeatable fashion so you could transfer your measurement. The marks and graduations on the face are convenient but not necessary. If I wanted to check the run out of a shaft, I could take my blank faced DTI and put marks on the face at minimum and maximum deflections, head back to the reference and by comparison determine the run out because the DTI is behaving in a repeatable way. I don't need accuracy (although it is nice) but I do need repeatability.

(As an aside, we use bore gauges at work and then take the measurement using a micrometer to get the same deflection of the dial. The dials are calibrated but the discussion point is do they need to be? They are not used to measure, only transfer a measurement)

On the other hand, with a measuring device that provides an absolute reading (be it heightmaster, micrometer, caliper) you must have accuracy because that item is acting as a measurement standard. Without graduations or numbers it can't do what it is meant to do. Contained within the accuracy of the device is a requirement for repeatability, but the primary concern is accuracy.

While DTIs and DIs have graduations and numbers on them, they are always making relative readings. They do not relate to a primary standard - that is, you can not look at a DI and say something is 3mm tall without having established a reference frame work to compare to - whether that be a flat surface, a high spot on a shaft or what ever. They show a reading on the dial that relates to another value. Provided the DI behaves repeatably I can use it regardless of how accurate or not the graduations may be (although I admit it would be a right royal pain in the ...)
On the other hand, I can pick up a micrometer and measure the 3mm tall thing and say that it is 3mm because of the accuracy in the device, linked back to a primary standard.

Michael

Run out is a relative measurement, so if you only had a micrometer you would have to measure a maximum position and a minimum position relative to a fixed reference (say a straight edge) and then subtract the two. Easier to use an indicator