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[machinist] Use and limitations of dial test indicators and cosine error
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Use and limitations of dial test indicators and cosine error
- Date: Fri, 20 Jul 2018 01:59:11 -0400
Use and limitations of dial test indicators and cosine error
Forrest Addy
If we're talking about a dial test indcator the phenomenon is called "cosine error". Here's some info. http://www.cncexpo.com/IndCosine.aspx
You some time see high end DTI's with an odd bulb shape on the end of the stylus instead of a ball. This is an involute shape that gives you the correct reading regardless of the stylus angle with in reasonable limits. You never use a DTI for quantifiable data colletion without some form of calibration where a chart is generated to translate observed reading to corrected reading something like a compass card.
Cosine error can be a bitch to explain away at those "lessons learned" meetings that are supposed to be oh-so- civil-and-constructive but turn out to be back-stabbing sessions where turf and alibis are defended to the death.
Anyone usng a DTI for ither than dialing in a diamater on a lathe really should clamp a mike to an angle plate and compare mike spilndle face movement against the DTI reading at a number of different stylus angles. Cosine error can be a real eye opener. <><> AND <><> Lever type test indicators are best used as "null" indicators. When the needle never moves, your setup is correct. Anyone who wants to perform absolute distance measurement at this level is best advised to stick to plunger type indicators. Unless they understand cosine error in this application. <><> AND <><> I just checked the data sheet that came with my Compac DTIs. It says:
Repetition:
The repetition errors of this indicator shall not be judged by the values obtained in free travel but by those resulting from repeated drops of the contact point on a rigid surface. (has some nice pics)
What that means is the probe tip should be allowed to snap down to the surface being gaged and it should repeat. Sometimes I flick mine with a small screwdriver or scribe point.
Measuring:
the center axis of the contact point should be parallel to the surface of the workpiece when measurements are taken. It is only by observing this rule that accurate measuring results can be guaranteed. In case this requirement cannot be met, the angle a must be determined and the indicator reading multiplied by the correction factor.
Then it has a chart of correction factors for every 10 degrees of angle a and a nice little nomograph. Angle a is angle between the surface being gaged and the axis of the tip.
If you have a Starrett catalog it has a similar set rules for proper use of a DTI. Usually it is at the beginning of the section on dial indicators.
One thing that is very important even when using the best quality Interrapid and Compac DTIs that have the probe pivot mounted via tiny precision ball bearings is to keep the plane that the probe tip rotates in at right angles to the surface being gaged. If you don't you can get very inconsistent readings depending on which way you move the surface relative to the tip. It may read low in one direction of travel and high in the other. Best to stop every now and then and let the tip flick down to the surface. Starrett "Last Word" indicators are particularly prone to this problem.
Spend some "quality time" with your DTIs and other precision tools with some parallels and V-blocks and whatnot and get to learn their quirks. Even a Starrett "Last Word" will work fine if you know how to use it (and it isn't broken ) They also come with a little tear-dropped shaped tip that Forrest mentioned if you have the full kit.
-DU-
If we're talking about a dial test indcator the phenomenon is called "cosine error". Here's some info. http://www.cncexpo.com/IndCosine.aspx
You some time see high end DTI's with an odd bulb shape on the end of the stylus instead of a ball. This is an involute shape that gives you the correct reading regardless of the stylus angle with in reasonable limits. You never use a DTI for quantifiable data colletion without some form of calibration where a chart is generated to translate observed reading to corrected reading something like a compass card.
Cosine error can be a bitch to explain away at those "lessons learned" meetings that are supposed to be oh-so- civil-and-constructive but turn out to be back-stabbing sessions where turf and alibis are defended to the death.
Anyone usng a DTI for ither than dialing in a diamater on a lathe really should clamp a mike to an angle plate and compare mike spilndle face movement against the DTI reading at a number of different stylus angles. Cosine error can be a real eye opener. <><> AND <><> Lever type test indicators are best used as "null" indicators. When the needle never moves, your setup is correct. Anyone who wants to perform absolute distance measurement at this level is best advised to stick to plunger type indicators. Unless they understand cosine error in this application. <><> AND <><> I just checked the data sheet that came with my Compac DTIs. It says:
Repetition:
The repetition errors of this indicator shall not be judged by the values obtained in free travel but by those resulting from repeated drops of the contact point on a rigid surface. (has some nice pics)
What that means is the probe tip should be allowed to snap down to the surface being gaged and it should repeat. Sometimes I flick mine with a small screwdriver or scribe point.
Measuring:
the center axis of the contact point should be parallel to the surface of the workpiece when measurements are taken. It is only by observing this rule that accurate measuring results can be guaranteed. In case this requirement cannot be met, the angle a must be determined and the indicator reading multiplied by the correction factor.
Then it has a chart of correction factors for every 10 degrees of angle a and a nice little nomograph. Angle a is angle between the surface being gaged and the axis of the tip.
If you have a Starrett catalog it has a similar set rules for proper use of a DTI. Usually it is at the beginning of the section on dial indicators.
One thing that is very important even when using the best quality Interrapid and Compac DTIs that have the probe pivot mounted via tiny precision ball bearings is to keep the plane that the probe tip rotates in at right angles to the surface being gaged. If you don't you can get very inconsistent readings depending on which way you move the surface relative to the tip. It may read low in one direction of travel and high in the other. Best to stop every now and then and let the tip flick down to the surface. Starrett "Last Word" indicators are particularly prone to this problem.
Spend some "quality time" with your DTIs and other precision tools with some parallels and V-blocks and whatnot and get to learn their quirks. Even a Starrett "Last Word" will work fine if you know how to use it (and it isn't broken ) They also come with a little tear-dropped shaped tip that Forrest mentioned if you have the full kit.
-DU-
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- [machinist] Use and limitations of dial test indicators and cosine error, Lawrence London, 07/20/2018
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