That is very common. When we dial in a barrel for threading, chambering, crowning , or like work, we start with a .001 indicator to rough it in, switching to a .0005 to get what looks like dead on. Once it looks dead on with the .0005 indicator we go to a .0001 indicator to get it right.
Even with it not moving the .0005 indicator, it will still swing a good bit with the .0001 indicator.
A good .0001 indicator is just more sensitive.
I have exactly the same experience with my .0005 and .0001 indicators.
Neither are top quality but the .0005 one seems to be more adapted to my needs.
It makes me wonder if a better quality indicator or a different stylus on the .0001 indicator would be any different.
I see .0001" Indicators used in grinding shops who are always dealing with Tenths and most Machine shops who's tolerances are not as tight use .0005" and sometimes .001" 90% of the time I use a .0005" and eyeball the tenths. If I am holding .0002 or more my Best Test or Inter-rapid .0005" works just fine. If I am scraping a Jig Bore or super precision Hardinge lathe or indication a spindle that I am rebuilding I use a .0001".
A funny story: A few years back I had a MD who had a Rung-Fo mill in his garage and was so upset because he was indication the table with his Inter-Rapid .0001" . He said it bounces so much I can't read it and my machine table makes my indicator run out of travel. I told him to use a plunger type or Last Word .001" indicator. He said he wanted the best indicator he could by. I suggested he buy a Mill that was the best he could buy. He and I rebuilt his Rung-Fo and used the .001 first and used a .0005" to finish.
If you look closely at both videos, they each have glitches in their movement. The expanded range of the .0001indicator just makes the glitch more obvious.
Given the choice between higher and lower sensitivity, I will take higher sensitivity every time. The above videos are supporting evidence. The .0005" indicators movement is more subtile and could easily be missed. If I were concerned with surface finish, I might just say "good enough". With the .0001" indicator, it would be hard to ignore the glitch.
I personally have no trouble with seeing a movement of .0004" and saying that is is good enough for the task at hand. On the other hand not being able to see movement gives a false sense of security.
The only down side with the higher resolution indicators that I can see is the limited travel.
This is a brand new mill so you would think the spindle would be perfectly smooth, no? Maybe not I guess there could be grind marks.
I originally used the .0005" indicator and got what looked like .0004" which is double the spec so I made an issue about it. The quill was sent in and tested. I was never told what they saw other than the spindle was within spec. Something else must be going on. I paid good money for the mill so continued to ask questions. I was told the manufacturer would not listen to any complaint unless a tenths was used so i used a tenths. I just got it back from a repair guy. I think it is too jerky and he is going to look over it once again.
I'm amazed at the difference though one is definitely not correct. Do you guys agree or am i not seeing something? .0004 on the one and ~.00025 on the other.
I agree, you are seeing around 0.0002 and maybe a little more with the 0.0001" test indicator. Also, a test indicator is more useful for relative movement as opposed to absolute measurement due to the angle of the tip to the work and the change in accuracy with swing. You also have some inherent +/- tolerance for the rated indicator.
I also agree that you can easily be deceived measuring anything when you are measuring outside the rated spec for the indicator. I had a similar issue trying to setup the stylus tip on edge finders which required to be within 0.0001" concentric. I tried with a Interapid 0.0005" indicator and kept on getting errors with the edge finders (both mechanical and electronic). I broke down and purchased a Compac 215 GA and it solved the problem because the accuracy improvement. What I like about the Compac 215GA is a large easy to read dial, and a range of 0.024", so much more forgiving. If I need to do direct measurement I use my Starrett 25-511 and 25-611 dial indicators, these do a great job of measuring down to 0.0001" with a 0.200" indicator range. So one dial indicator pretty much does it all for me.