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View Full Version : Inidcator question- maybe a dumb one...



T.Hoffman
03-06-2012, 12:18 PM
I see different screw in points available for my Interapid indicator from McMaster. They have different tip diameters, but they also have different lengths.

Wouldn't the tip length difference slightly change your indicator deflection amount?
Or is it "close enough" because you are setting up the indicator arm to be close to parallel to the work?

Arthur.Marks
03-06-2012, 12:21 PM
For measuring, a different length is going to throw off your scale. I *think* you can account for it if you know the ratio/gearing/whatever, but the difference also changes along the arc of movement. Don't quote me on this :) I'd need to look this all up again... [EDIT:] Here you go> http://longislandindicator.com/p21.html

Simply put:
Use standard length only for measurement purposes.
Use whatever length is necessary to access your datum for comparison purposes.

TGTool
03-06-2012, 12:49 PM
I don't know which indicator you have, but I've got an Interapid that has a dial marked in half thousandths and both short and long points - I think something like 3/4" and 1-1/2". In any case, with the long points you just read every dial mark as one thousandth, essentially halving the dial movement. It's a useful feature to have. I don't use the long points often but they've sometimes been especially helpful.

Paul Alciatore
03-06-2012, 02:26 PM
You do not specify which type of "indicator" you are talking about. Using more precise terminology, there are "dial indicators" and there are "dial TEST indicators". Here, from McMaster's web pages, are examples of "dial indicators".

http://www.mcmaster.com/#standard-dial-indicators/=gjqixw

Dial indicators use a sliding shaft that moves in a strictly linear manner. They convert this linear motion to a reading using gearing (or other means if they are digital). This reading is accurate within the limits of the mechanism over the entire range of motion. If different tips are installed on this kind of indicator, they do not make any difference in the readings other than changing the starting point. Since they are relative reading devices, this does not change the accuracy of the readings.

Now, here are examples of "dial TEST indicators", again from McMaster's web pages.

http://www.mcmaster.com/#lever-style-dial-test-indicators/=gjqm3j

The difference here is that they use a rotating arm with a probe tip. Many of these have interchangeable arm/tips. In spite of the fact that these indicators have the same type of dial displays that are nicely marked off in thousands or sub thousandths of an inch or similar metric units, they are NOT all that precise in terms of measurement. Both types of indicators provide relative readings from some arbitrary starting point, but the dial test indicator does not provide the linear readings that a dial indicator does. This is due to the cosine error that is inherent in the rotating arm mechanism. As the arm rotates, it actually changes by a larger number of degrees per thousandth (or other unit of linear measure) as it moves away from (rotates) being perpendicular to the direction being measured. Hence, the scale is inherently inaccurate to begin with. When you change the arms/tips on this style of indicator, an arm with a different radius will provide different readings. This style of indicator is intended for work like centering or measuring relative run-out, not for accurate linear measurement. Hence, the manufacturers are less likely to be worried about the exact length of these interchangeable arms. And you should not be either.

If you have a dial TEST indicator, you should not trust it for any linear measure greater than a few thousandths of an inch and only for that if you have checked it against another means of measure AT THE SAME ANGLE OF CONTACT.

Frankly, I would not use a "dial TEST indicator" for an actual measurement: I would use a "dial indicator". There are "dial indicators" that have one inch and even two inch ranges and I use such indicators on my lathe for carriage movement measurement. They work quite well for this.

TGTool
03-06-2012, 03:16 PM
I think dial test indicators are rarely used for "measurement". They're almost always used as a null device to verify concentricity. You might use it on some occasions to check linear movement of a small amount, such as adjusting a tailstock to remove taper. In this instance if you knew it needed .002 of movement, you adjusted the tailstock but it still wasn't spot on, you'd be hard pressed to distinguish inaccuracy of the indicator from something else shifting as it was tightened up. If you're just using it to center a part or spindle it doesn't matter what the graduations are or whether they're exactly accurate. When the indicator reads zero-zero you've got the position.

Harvey Melvin Richards
03-06-2012, 03:43 PM
You do not specify which type of "indicator" you are talking about.


He did say "Interapid indicator from McMaster". I've never seen an Interapid dial indicator (plunger type), Long Island Indicators doesn't mention them, and I can't find any Interapid dial indicators on McMaster. Not trying to pick nits, but I knew immediately what he was talking about.

T.Hoffman
03-06-2012, 04:06 PM
You do not specify which type of "indicator" you are talking about. Using more precise terminology, there are "dial indicators" and there are "dial TEST indicators".

Sorry, I was meaning DTI's.

Near the bottom of this page link, in the right column are the replacement points of different sizes and lengths listed with the Interapids:

http://www.mcmaster.com/#lever-style-dial-test-indicators/=gjqm3j

aboard_epsilon
03-06-2012, 04:21 PM
i think Paul's second type explains it all .

the longer tips are maybe for getting into deeper holes .

there Will be different readings on the dial according to what angle its presented to the work and the tip length ..and you would have to be a mathematical genius to work them out ...think the readings are only accurate with the standard tip in the vertical position.

i could be wrong ..i only use them for cantering stuff in the four jaw ......positioning vice parallel to mill table .........centering holes..checking run out of things ......but only using numbers as reference points.

all the best.markj

oldtiffie
03-06-2012, 05:37 PM
They are a nice indicator.

I wouldn't worry about different probe lengths or off-set angles as the difference in inch or mm won't differ much if used for "measurement".

Mine are used as comparators or "balancing/zero-ing" total indicated run-out (TIR) and they work very well.

Bob Fisher
03-06-2012, 07:21 PM
No dumb questions, that's how we learn. I do not believe I have ever used a DTI to actually measure something, they are mostly used to compare. Either to measure run out, or on a height gage to accurately locate your reference and part.Bob.

huntinguy
03-06-2012, 08:11 PM
your transmission has a gear in it that tells your speedometer how fast you are going - but only for the factory tire size - bigger tire, you are going faster than your speedometer. Smaller tire size, you are going slower than your speedometer.

There is a specific length of point that the indicator is geared to. Pretty simple.

Yes, there is a sine error but it has to do with the angle of the stem to the measuring surface not the diameter of the ball. Each indicator manufacture has built in sine error and you have to know what angle your indicator manufacture has compensated for.

IIRC Interapid is 15 degrees. The error in the few thousands range is so small from that corrected angle that breathing on the material would move it more.

gnm109
03-06-2012, 09:30 PM
I think dial test indicators are rarely used for "measurement". They're almost always used as a null device to verify concentricity. You might use it on some occasions to check linear movement of a small amount, such as adjusting a tailstock to remove taper. In this instance if you knew it needed .002 of movement, you adjusted the tailstock but it still wasn't spot on, you'd be hard pressed to distinguish inaccuracy of the indicator from something else shifting as it was tightened up. If you're just using it to center a part or spindle it doesn't matter what the graduations are or whether they're exactly accurate. When the indicator reads zero-zero you've got the position.

Yep, that's correct. In fact, most of them that I've seen have a span of only .030". That's ideal for centering parts which is what I use mine for.

adamc
03-10-2012, 12:00 PM
the longer tips are maybe for getting into deeper holes .

You can certainly use them for that. I think the advantage of the long needle is that the device essentially becomes more sensitive. With a normal needle, the range of the needle may be .015". With the long needle, the same full swing would read .0075". So you could swap needles and refine whatever you are doing.



There will be different readings on the dial according to what angle its presented to the work and the tip length ..and you would have to be a mathematical genius to work them out ...think the readings are only accurate with the standard tip in the vertical position.

You're right. What guys do is they use gage blocks to position the needle so that it is accurate, then leave the needle alone. I believe the Interapid angle is 15 degrees down. The Mitutoyo similar looking DTI is accurate at 0 degrees. I think the Interapid is a better design.

TGTool
03-10-2012, 01:19 PM
You can certainly use them for that. I think the advantage of the long needle is that the device essentially becomes more sensitive. With a normal needle, the range of the needle may be .015". With the long needle, the same full swing would read .0075". So you could swap needles and refine whatever you are doing.


Wrong direction, it becomes less sensitive. The longer point requires a greater movement to rotate the same amount on the dial. For greater sensitivity you'd theoretically need a shorter point. A more practical strategy would probably be just to keep a more sensitive indicator for those occasions. I like the Interapid with the two rotations of the dial each direction if something is way off. If the measurement is close and needs to be accurate to tenths I get out the tenths indicator.

adamc
03-10-2012, 05:29 PM
Wrong direction, it becomes less sensitive. The longer point requires a greater movement to rotate the same amount on the dial. For greater sensitivity you'd theoretically need a shorter point. A more practical strategy would probably be just to keep a more sensitive indicator for those occasions. I like the Interapid with the two rotations of the dial each direction if something is way off. If the measurement is close and needs to be accurate to tenths I get out the tenths indicator.
You're right, I got it backwards. Thanks.