Are test indicators only good until .015?

[Pcmaker]

I have 2 different test indicators I bought from Amazon. Both Chinese made cheapies.

The dial shows 0, 5, 10, and 15 on the 6 o'clock position, then goes back to 10, 5, 0.

I installed a quill DRO today on my PM-835S. I was checking it's accuracy because I had to do some modifications to get it to fit since it was made specifically for a Bridgeport. I used my 2 different test indicators, lowering the quill and touching off on the test indicator. I noticed that after .015, the accuracy on both test indicators are way off. I thought it was my iGaging quill DRO that was the issue, but I used my regular indicator and it was matching it.

After .015, I assumed that I would get .020, but I got something like .017 on the quill DRO.

I contemplated buying a Mitutoyo for well over a $100, but I'm thinking these test indicators just aren't meant to go over .015.

 

[markba633csi]

A good indicator should be accurate anywhere on the dial, although there is a slight angularity error depending on how far the finger is deflected. In most cases it's negligible I believe, at least for small measurements of a few thousandths
Not sure if that's what you are seeing
-Mark

 

[derfatdutchman]

The normal range for test indicators is .015, some of the higher reading ones only have a range of .004.

 

[Flyinfool]

Your set up can have a big influence on the reading. you want the arm to be perpendicular to the direction of deflection when the dial is at the position you expect to be taking the measurement.
If the arm is angled, then you will have cosine error to account for.

Yes that indicator is only designed for 0.015" in each direction. There are indicators that have longer travel, I have a couple with 1.000" of total travel.

 

[Alexander McGilton]

A good indicator will give you as labeled precision and accuracy over its own entire travel. Watch the link below 11min to 12min.
If you have a Compac Geneve test indicator or one of a few other brands they gave you an unnecessary high travel to be practical with the second dial, yet still held their accuracy. I have a .0001 indecator from them that has .020" of travel in both directions. like the one in the link, I wish that someone could tell all these people on ebay that the needle is expected to rest in the 7 to 10 o'clock position.

Redirect Notice

 

[RJSakowski]

Dial test indicators measure angle, not linear travel. The length of the tip multiplied by the angle traveled is equal to the length of the traveled arc which is approximately equal to the chord of the arc (straight line distance from the start point to the end point). For small angles, this approximation is close enough. The error doesn't become significant until the arc length exceeds .1 radian or 5.7º which would be .100" for a 1" tip length.

A more serious error arises if the tip isn't perpendicular to the direction of travel,as mentioned by Flyinfool. If a 1" tip started at 30º from the perpendicular and moved through a 5.7º arc, the actual distance traveled in the measured direction would be .0835" instead of the indicated .100".

 

[brino]

Right. In my shop I use a dial indicator for measuring linear movement up +1 inch.
I have one longer stem dial indicator that can measure up to +2 inch.

However, my dial test indicator only measures a range of +/- 15 thou. (Using the standard ~1/2 inch long tip.)

It's a choice; high range or high accuracy.
It depends on what I am measuring.

-brino

 

[MrWhoopee]

Dial test indicators are subject to (IIRC) sine error because the motion of the tip is not linear. They are generally used for checking relative differences rather than absolute measurements. A dial (drop) indicator would be better suited for what you are trying to do.