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Gravity vs Measurements

PeterT

Ultra Member
Premium Member
#1
On another forum someone posted this Richard King video. I was quite intrigued by this. I've held my DTI in the little stub arbor & the bar included in the kit & observed no such deviation. But its a different matter when you do teh same thing with a typical indicator mag stand holder assembly. The theme is gravity acts on the assembly and gives erroneous readings upright vs inverted. A classic example of this (and which I am personally guilty) is putting a DTI assembly in the lathe chuck & rotating about a fixed arbor in the tailstock to determine if its coaxial to the spindle. I suppose the lateral in/out position of the TS is more believable because the assembly is in the same vertical plane, but now I'm even suspicious of that. Comparing the readings of DTI facing down vs up, is more suspect. How much more? Surprisingly a lot.

I crudely mimicked this with my equipment & got the following on a mini mag holder vs typical mag holder. Your mileage may vary.
Noga Mini + Mitutoyo 0.0001" DTI: Level reference = 0.0000", 90-deg = 0.0005 - 0.0008", 180-deg (inverted) = .0011
Noga 12"? + Mitutoyo 0.001" DTI: Level reference= 0.000", 90-deg = .0042", 180-deg = .0055" (suspect a smidge more because it didn't return to zero & there might be some flex in my bar).

Conclusions
- I'm confident the DTI by itself probably has negligible gravity effect. A short rigid holder like the dovetail stem & probably the bar is probably close to nil
- but the contribution of the typical mag base DTI holder assembly sure can yield incorrect numbers
- if you have to use a holder, best to find something more rigid. I don't know what to say on that front. Maybe those classic post & clamp style are better? If you have one do a test & let us know.
- gravity acting on extended frame is the enemy here. So I suspect a dial indicator would be worse yet because it weighs more than DTI & is cantilevered out to the extremity
- for this reason, don't use a long mag base arm when a mini will do
- just to be clear, all should be well rotating an indicator assembly as long as you are maintaining the same positional plane of the dial. The problems happen when teh assembly is rotating & gravity tends to pull it different in one orientation vs another


Here is the RK video

 

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John Conroy

Well-Known Member
Premium Member
#2
Makes perfect sense when you think about it and good to keep in mind when ever we use an indicator for comparison measurements. Thanks for posting that Peter.