Dial Test Indicator probe position

[Archer60x]

I picked up this used Mitutoyo DTI. I have two questions.

I have heard DTI are only accurate when pushing on the probe from certain angles. If you get too far away from the optimum angle you loose accuracy.

Question one what orientation should this DTI be at for the most accurate reading? Should the body be straight up and down like this picture?

Question two can i adjust the angle of the probe?

 

[francist]

It's called "cosine error" and it's more about the angle of the stylus to the surface than the angle of the body to stylus. There are lots of descriptions on net about it -- this one from University of Florida was fairly straightforward....

Indicators

All of my indicators have either a ratchet click that allows the stylus angle to be changed or a friction clutch which does the same thing.

 

[Cadillac]

Ive heard the same. Indicator arm should be straight for the best accuracy. I swing my arm perpendicular to the body in both directions sometimes. I’m not as accurate in my machining for any of it to matter in my eye. They do sell vertical and horizontal mounted test indicators. Or you can change the stem position on some.

 

[ChazzC]

I picked up this used Mitutoyo DTI. I have two questions.

I have heard DTI are only accurate when pushing on the probe from certain angles. If you get too far away from the optimum angle you loose accuracy.

Question one what orientation should this DTI be at for the most accurate reading? Should the body be straight up and down like this picture?

Question two can i adjust the angle of the probe?

View attachment 472275

That is a good angle if the body is held as shown and you are checking the stop surface. If you want the body to be horizontal (or if you are checking a vertical feature, then you do need to adjust the point to get a similar angle (guidelines are usually around 15°) to the measured surface.

To change the angle go the point on most DTI's you simply push the point in the direction you want it to go while holding the body with your hand. However, there are some Mitutoyo DTI's (and maybe others) and Starrett Last Word indicators that only read in one direction: these indicator will have a lever on the side that needs to be switched to change the direction of measurement – while you still push the point to adjust the angle, if you push into the wrong direction with these indicators you will damage them. Your DTI looks like the type that will read in both directions.

Quinn (Blondiehacks) has a good video that discusses both Dial Indicators and DTI's (DTI detailed portion starts here).

 

[RJSakowski]

The angle of the stylus to body doesn't matter. The cosine error for an 5º angle to the surface is about .4% so for a 1" stylus, a ,001" difference in height would read as .000995" on the dial. For a 10º angle the cosine error is 1.6%. so that .001" difference would read as .000986". For 15½, to would be ,00097" and for 20º, it becomes .000940.

 

[Archer60x]

Thanks everyone! I am reading up on the cosine error now.

I was able to just push the stem and it moved to a new location. I was afraid to break it.

Thanks for the help and confidence to push the stem. I will keep reading!

Chad

 

[gard]

To change the angle go the point on most DTI's you simply push the point in the direction you want it to go while holding the body with your hand. However, there are some Mitutoyo DTI's (and maybe others) and Starrett Last Word indicators that only read in one direction: these indicator will have a lever on the side that needs to be switched to change the direction of measurement – while you still push the point to adjust the angle, if you push into the wrong direction with these indicators you will damage them. Your DTI looks like the type that will read in both directions.

I have an old starrett that has the lever to change direction, how do you know what is the right or wrong direction rotate the needle angle?

 

[ChazzC]

Unfortunately Starrett doesn’t have downloadable instructions, and I wasn’t able to find applicable videos on YouTube. However, if you search YouTube for “Starrett 711 Last Word” you will find numerous videos that have bits & pieces of information. Basically, if you gently press on the point it will only move in one direction: if that direction is opposite that which you want, then you need to change the position of the lever. Please note these instructions for one of my Last Words:

 

[Bone Head]

With test indicators (any indicator actually) one of the things you should notice is any flat spot worn in on the tip. Think cumulative error. Replace tip/probe as required.