DTIs - 5 vs 1 Tenth?

To me, tenths indicators are for inspecting high-precision parts on a surface plate, particularly after surface grinding. 50-millionths indicators are for inspecting surface plates :)

My standard high-precision test indicator has a half-thou scale, and that’s good enough for most inspection, aligning machines, and indicating in vises and toolposts, etc. My South Bend lathe has a static spindle deflection test with a range of 7-12 tenths, and that’s easy to target with a half-thou indicator. For indicating parts in a four-jaw, even that spins the dial frustratingly, and a 0.001 indicator is more practical. I’d rather have a 0.001 dial indicator with a 3” dial than a half-thou test indicator with a 1.5” dial for most things.

Rick “has a tenths indicator but never uses it” Denney
 
I have a Mitutoyo 20-020/0.001”, several SHARS 15-0-15/0.0005” (vertical, horizontal, large dial, yellow/white dial) and a Links 4-0-4/0.0001” I bought from Enco in the 80’s. I usually use one of the yellow/black SHARS for setups in the mill. I use a 0.250/0.0005” dial (drop) indicator for centering in the lathe.

The only times in recent memory I’ve used the 0.0001” DTI was checking TIR of tooling - after I had checked with one of the 0.0005” ones.
 
RJSakowski said:
A DTI isn't an absolute measurement instrument. You observe changes in readings as when checking runout, tramming a mill head, or sweeping a vise jaw. With careful observation, you can see a .0001" change with a .0005" DTI. This is more than accurate enough for most operations than one would encounter in a hobby machining environment.

For super-critical operations, the .0001" DTI has some value but it isn't necessary for most day to day work. If the choice was one or the other, I would opt for the .0005" DTI.
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The only place I ever saw a .0001 indicator in practical use(Practical, not general, ie necessary) was in Demag-Delavals steam turbine shop.

Toured that place twice during my apprenticeship and was in awe of their processes both times.

And for steam turbine work where a 6’ turbine rotor that spins at 20,000rpm that indicator was used as a setup tool for then going to the more sensitive .00001 indicator.

What we consider fine measurements were in this place used to keep the actual fine instruments from crashing or bottoming out in use.


That place had everything from a 20’ Edelstaal lathe down to watchmakers lathes for instrument repair.
 
A general rule in metrology is that a calibration tool should be 10x more precise than the object you are calibrating. To that degree, I would use a .0001" DTI for checking surface plates or straight edges, for calibrating a precision square, or for leveling a lathe. For most routine work, I use my .0005" DTI.
 
Canus said:
As an aside, can a DTI be calibrated for linearity or only repeatability?
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Dial test indicators are subject to linearity error and will never be really linear, because the feeler moves in an arc. I seem to recall they are usually calibrated to minimize this error when the tip approaches the surface being measured at 12 degrees and centered in their range. They are not calibrated at all for absolute measurements, because they are used to measure deviation, not dimension.

Dial indicators don't suffer from linearity error, and they are also available in tenth-reading models, if one needs absolute deviation. But they are subject to cosine error if the plunger doesn't approach the measured surface exactly a right angle. Test indicators can fit in places a dial indicator may not fit, so they are more convenient for some measurements.

I don't know that any of these can be easily calibrated by the user, other than by zeroing the dial. The units I've had apart on the watchmaker bench use a rack and pinion to move the indicator works.

Rick "more likely to use a tenth-reading dial indicator than a tenth-reading DTI" Denney
 
Canus said:
As an aside, can a DTI be calibrated for linearity or only repeatability?
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A DTI is by nature an imprecise instrument in that it is measuring rotational angle rather than linear movement. Changing the angle of contact will change the calibration. Changing the probe length also changes the calibration. I view it as best used checking for zero movement. When measuring TIR with a DTI, I am generally looking at a few thou movement and whether the TIR measured is 3.5 or 4 thou isn't that important
 
RJSakowski said:
A DTI is by nature an imprecise instrument in that it is measuring rotational angle rather than linear movement. Changing the angle of contact will change the calibration. Changing the probe length also changes the calibration. I view it as best used checking for zero movement. When measuring TIR with a DTI, I am generally looking at a few thou movement and whether the TIR measured is 3.5 or 4 thou isn't that important
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Excactly, It's more of a movement clock.
 
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