Coaxial Indicator Problem

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BRIAN

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Just lately I have had cause to question the accuracy of my coaxial indicator. so as a test I held the stem in the lathe 3 jaw. and placed the end of the short feeler ( the most accurate ) against the tool post zeroed the indicator -- the cross slide dial and the digital read out. fed in the cros slide to give one division on the Co ax -- said to be .0005" offset,
And the dial and digital read out read about .0025" this did not look good. so to get better accuracy I went to 10 divisions and divided the result by 10 and came up with .0027"per div the long feeler gave a figure of.0064" per div.
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The information with the indicator defiantly says :0005" per div offset.

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So confused and just a bit annoyed I placed a dial indicator against the body of the co ax and lo and behold one division on the co ax is .0005" on the indicator. IT's the dial resolution not the indicator.
How is that for a useless piece of information??
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The manufacturers have forgotten about the ratio of the feeler arm. Test yours you may be surprised.

I am waiting for a reply from the suppliers
Brian is not a happy boy.
 
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The value of each increment on the dial changes with the length of the feeler probe. The actual value of the off center indication does not need to be even looked at to center a part. Just move the handles until you get the least movement on the dial. Pay no attention to the numbers.

What were you hoping for beyond that?
 
A co-ax indicator is not even on the list of tools I want to acquire. A test indicator stuck in a collet, or for bigger work a test indicator on a Noga arm in a collet does the centering job just fine. I suppose if I found a co-ax indicator for dirt cheap I would buy it, but it would mostly stay buried in my tool box while I reached for other quicker and/or more accurate solutions.

Edit: Another issue is that the blurry needle caused by the high rotation speed does not let you see if the range of needle motion is because of offset or because of surface imperfections. By turning an indicator around slowly you can see if there is a nick in the surface or a burr sticking up causing the needle deflection, which can be ignored while sweeping the rest of the diameter. Using the coax, when you reduce the overall needle movement you are centering on the highest point and the lowest point of the surface, often not what you are looking for...
 
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Yes I do know how to use a co ax. And do understand geometry.
Looking at the Blake chart it says that at zero radius with the small feeler (as I tested it) one division is .0005" it is not, it is .0027"on this indicator. and this is at it's most accurate. With the long feeler if I get the wiggle down to one division I can still be .0064" out.
Obviously the Blake is in a different ballpark as far as accuracy
You only find this out when the parts you have spent days making do not line up as you think they should.
Brian.
 
OK, you were using the numbers and directions on the co-ax dial for meeting tolerance. I certainly understand that and, yes, the tool should meet the specs given in the product literature. Unfortunately, in the real world tools do not always meet the specifications or our expectations. Again, that is why I use a trusted DTI, set up correctly, to indicate holes and lots of other surfaces. I can also see the idea of using a co-ax indicator for doing production or serial work where it would be a lot quicker than a DTI. Thanks for the product report. Do you have a brand and model name we can tie it to, so we don't buy one like yours?
 
Hi bob I am still waiting for the answer from the supplier before I give any names I feel I must give the man a chance to answer. but he has had my query for 10 hrs so I am not holding my breath.
Brian
 
Good on you, Brian! If more of us would call them and let them know that they have a problem, and that we will tell many other about it, maybe they will change their ways.

Also my apologies, I did not catch at first that you were a global moderator and probably had a clue... 8^)
 
I don't know a lot and read these forums to try to pick up some information, so I have a question . It looks like your test only checks for half of the indicated sweep and without the other half you would not be able to find the accuracy of the indicator. What am I missing ?
 
I don't know a lot and read these forums to try to pick up some information, so I have a question . It looks like your test only checks for half of the indicated sweep and without the other half you would not be able to find the accuracy of the indicator. What am I missing ?
I think you're right. I thought that a coax indicator was primarily a nulling device anyway, though.
 
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