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Indicator Sag: How Do You Deal With This?
- Thread starter EmilioG
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- Jan 22, 2011
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To a greater or lesser degree, yes. The heavier and less rigid the mag base, the more pronounced this will be. Conversely, the lighter and more rigid, the effect is less.
Most test indicator use is unidirectional and the setup is not flipped upside down. The most common application where this could present is when using a DTI in checking the centering of a tailstock quill on a lathe. Naturally, when you swing the indicator to the lower position, the sag would be there, and give a false reading.
Can't say about the Indicol. Never tried it.
Most test indicator use is unidirectional and the setup is not flipped upside down. The most common application where this could present is when using a DTI in checking the centering of a tailstock quill on a lathe. Naturally, when you swing the indicator to the lower position, the sag would be there, and give a false reading.
Can't say about the Indicol. Never tried it.
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- Mar 3, 2014
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Use a test bar. Mount the indicator to it as you will later on do on the lathe.
You may have to machine a solid clamp to represent the lathe but if your using an extension on the lathe then also use it on the test bar.
Support the test bar just outside the clamp or magnetic base and the indicator contact. Leave room to rotate the indicator setup 360deg.
Zero the indicator to the test bar while in the top position.
Rotate the test bar checking your reading every 90deg.
If the indicator sags at any position you will have a reading.
If the cause is in the indicator I don't know how to make a correction.
If its due to sag in the mount you must consider (probably) it is sagging at the top as well as the bottom position.
A better mount should be made but if that isn't reasonable and accuracy isn't paramount then just use the same sag reading on the lathe and add or subtract your reading to get the difference for adjustment.
You may have to machine a solid clamp to represent the lathe but if your using an extension on the lathe then also use it on the test bar.
Support the test bar just outside the clamp or magnetic base and the indicator contact. Leave room to rotate the indicator setup 360deg.
Zero the indicator to the test bar while in the top position.
Rotate the test bar checking your reading every 90deg.
If the indicator sags at any position you will have a reading.
If the cause is in the indicator I don't know how to make a correction.
If its due to sag in the mount you must consider (probably) it is sagging at the top as well as the bottom position.
A better mount should be made but if that isn't reasonable and accuracy isn't paramount then just use the same sag reading on the lathe and add or subtract your reading to get the difference for adjustment.
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- Jan 22, 2011
- Messages
- 8,031
Emilio, I don't know who you are talking about when you refer to "highly skilled professional machinists", nor do I care to know. But the laws of mechanics and physics are generally reliable. There is spring tension from the indicator, as well as mass and gravity at play here. And, there is really no such thing as a perfectly rigid support. Years ago at a World's Fair, I can't recall which one, GM put a extremely sensitive indicator on a steel plate several inches thick and invited people to press down on the plate. Most were surprised that even hand pressure was enough to deflect the bar. All these factors combined add up to what Richard is demonstrating. I would grant that the exact setup that Richard used could have been chosen for its susceptibility to this phenomenon, but the facts are that what he showed is true. It was only a demonstration. Proper choice of materials, etc, can mitigate the effects shown, but not totally eliminate them. Whether it is acceptable or not depends on the accuracy need in any particular situation.
And BTW, we have a number of highly skilled professional machinists here as well.
And BTW, we have a number of highly skilled professional machinists here as well.
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4GSR
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That's me in the red shirt at about 1:00 into the video.
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- Jul 20, 2014
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Yes, that is why I posted here, because I know there are many highly skilled, smart, people of all backgrounds.
I just had to question the validity of the test and ask as many knowledgeable people as I could.
The test wasn't very scientific.
That's just me, I question everything and try to use critical thinking on any problem.
I just happen to have a good friend and trusted source that I go to AND here as well, to get as many views
and opinions as possible. I thought that was the point of this forum? Thank you.
I just had to question the validity of the test and ask as many knowledgeable people as I could.
The test wasn't very scientific.
That's just me, I question everything and try to use critical thinking on any problem.
I just happen to have a good friend and trusted source that I go to AND here as well, to get as many views
and opinions as possible. I thought that was the point of this forum? Thank you.
4
4GSR
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I think Tony pretty much sum it up with his explanation.
Yes, you do get some amount of deflection. It's all in the setup you use. In what Richard was demonstration, the amount of movement was a couple of tenths.
And yes, you must pay attention to measurements with a test indicator when you start moving it in an angular rotation when in the horizontal plane or any plane off of vertical.
Something to think about. But only if you are one of them that like working in measurements less than .001".
Most of us in general day to day operations don't even think about things like this and or have no desire to do so.
Yes, you do get some amount of deflection. It's all in the setup you use. In what Richard was demonstration, the amount of movement was a couple of tenths.
And yes, you must pay attention to measurements with a test indicator when you start moving it in an angular rotation when in the horizontal plane or any plane off of vertical.
Something to think about. But only if you are one of them that like working in measurements less than .001".
Most of us in general day to day operations don't even think about things like this and or have no desire to do so.
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- May 2, 2011
- Messages
- 811
The test shown in the video is indicating what NOT to do, and to that end, it shows the worst possible set up for a mag base and DTI, to illustrate the maximum sag. It is not showing that the equipment has drawbacks, it is showing that the equipment has drawbacks if used incorrectly! You have to think about what you are trying to measure, and provide the DTI with the most rigid base possible to make the measurement. It is not rocket science, but it is not 100% accurate, and it doesn't need to be. There is very little point in making measurements of less than .0005 unless for alignment or centering as at that resolution expansion and contraction will be greater than the measurement you are trying to make. Sensible use of DTI and mag base should get you comfortably to within .001. Engineers work to tolerances, not dead sizes.
Phil
Phil