# Co-axial Indicator Measurements



## BGHansen (May 7, 2016)

There have been a few threads regarding co-axial indicators and what the numbers on the dials equate to in actual inches.  Frankly, I had no clue so did a couple of simple set ups on the lathe and collected data.  Basically mounted the co-ax in a collet (overkill), locked the spindle with the tilting plane of the indicator flat to the world.  Loaded a couple (1.7” and 6”) of indicator rods in the co-ax.  Set the lathe compound to parallel with the cross feed.  Adjusted the cross feed/compound until the indicator was against the QCTP and swept side to side to check tram of the QCTP to the Z-axis.  Once that was verified square it was time for data collection.

I backed off the compound to its limit of travel, turned in a few tenths and zero’d the micrometer dial.  Advanced the cross feed until the needle on the co-ax moved, then zero’d out the co-ax.  I then started advancing the compound and recorded the number on the co-ax dial face.  Data was plugged into Excel and the attached plots made.

I saw on the face a note for ‘axis offset 0.0005” ’ and had no idea what that was.  Changed my set up by turning the compound to parallel with the lathe spindle and rotated the co-ax so the QCTP would directly push against the machined bottom surface of the co-ax.  Zero’d out the compound micrometer dial and took more data.

The envelope says . . .  the travel with an indicator rod is not linear through the full travel of the co-ax so you can’t just throw a multiplying factor at the number on the co-ax dial and generate an actual offset in inches.  For example, the first 0.050” of travel with a 1.7” indicating rod showed a dial offset of 19.65 units on the co-ax.  The last 0.050” of travel with the same indicating rod showed a dial offset of 24.65 units.

With a 6” indicator rod, the first 0.050” gave a co-ax offset of 6.9 units.  Last 0.050” of travel was 9.325 units.

However, as expected the direct push on the housing of the co-ax is linear.  Each unit on the co-ax dial indicates the housing has moved up/down 0.0005”.  Makes sense since at this point the co-ax is just a dial indicator.

What I can take away from the data is if you use a 1.7” long indicator rod and set a deflection of 1 on the co-ax, you are within about 0.002”.  Using the 6” long rod with a deflection of 1 on the co-ax means you are within about 0.009”.  So, the shorter the indicator rod, the more accurate you’ll be.  Hope this helps answer some questions!  I could only attach photos of the plots, send me your email address if you'd like the Excel file (can't attach it to this post).

Bruce


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## John Hasler (May 7, 2016)

I think that the important feature of the coax indicator is that the tip of the probe always moves in a perfect circle centered on the axis of rotation even if the spindle has some runout and misalignment as long as the reference surface of the instrument is flat and square to the axis of the instrument.


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## Bob Korves (May 7, 2016)

The coax indicator is intended to be a centering indicator, not a measuring indicator.  Ignore the numbers on the dial and simply try to minimize the needle oscillation, minimizing the offset from concentric location.  If you want to measure the hole ID or cylinder OD, use another tool.


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## COPE 19 (May 9, 2016)

I agree with Bob. I am not a machinist by any means, just a beginner but have been using my mill and lathe for a good while only for my needs in the small engine world.  I only use my coax ind when setting a small engine block on my Lagun Mill for cylinder boring or installing over sized valve seats, works great every time. I never pay any attention to the numbers, other than making sure it is the same all the way around. However I am learning a whole lot about a lot of things and a lot of tools with the help of this forum. Thanks to all the pros that post here, it really helps.


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## John Hasler (May 9, 2016)

I


COPE 19 said:


> I never pay any attention to the numbers, other than making sure it is the same all the way around.


It's useful to know what the numbers mean, though,  so that you can judge the significance of your results and place an upper limit on the error.


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## BGHansen (May 9, 2016)

Looks like lots of views of this thread, so many are interested in the proper use of a co-axial indicator.  No doubt they should only be used to verify you've centered your spindle on the appropriate feature on your work.  I read a number of other posts where members asked how to interpret the numbers on the co-ax dial.  Quick version is don't worry about what the numbers are, the needle moving is just an indication of variation, but not how much variation.  Nothing is mentioned in the manual of my Shars co-ax, but the shorter the indicating rod, the more accuartely centered you are. 

It'd be really simple at the time you were using the co-ax to set the rocking plane to in line with your x or y table movement and move the axis 0.001" and see how much the needle moves.  That'd give you some indication of how much you're off but it's not the intended use of a co-ax indicator or the best way to measure your error.

Bruce


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## Bob Korves (May 9, 2016)

Make sure you have a good idea what the wiggling needle means.  It might be a nice round hole, and the indicator is showing you the offset from the axis of the hole.  That is the intended, desired, and best case condition.  The wiggling might also be from an out of round hole, or from surface defects bouncing the needle.  In those cases you might not be achieving what you expect.  It is precisely for those reasons that I have not been looking for a co-ax indicator.  Swinging a dial test indicator around slowly shows me what is really going on there and can be done reasonably quickly.  If I was doing production work my choice might be different...
http://www.noga.com/nogaProducts.php?prdID=NF1018 
If you set it up with the dial facing upward and the stylus moved to a nearly 90 degree angle downward, it is easy to watch the dial all the way around the circle and without a mirror or a neck like a stork.


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## John Hasler (May 9, 2016)

Bob Korves said:


> It might be a nice round hole, and the indicator is showing you the offset from the axis of the hole. That is the intended, desired, and best case condition. The wiggling might also be from an out of round hole, or from surface defects bouncing the needle.


The jiggling due to poor finish is obvious.  If it exceeds your  tolerance for the hole you've got work to do.  You adjust your dials until the dial wiggle is minimized.  If it is zero the hole is perfectly round and centered to with the accuracy of the instrument. If it isn't zero you are as close to the center of whatever shape it is as it is possible to get.  The amount of residual wiggle places an upper limit on the runout.  If you are off-center the needle moves each way once per revolution.  If you are on center and the hole is elliptical and the needle moves each way twice per revolution.  Runout and angular error in the spindle do not effect accuracy.


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## Tony Wells (May 9, 2016)

I think the coaxial indicator is a useful tool but it's largely a matter of personal preference. I don't own one, nor have any strong desire to get one. If a good trade deal came up I'm not saying I'd pass, but it would have to be a good trade deal. I have used one, on loan from a coworker years ago, so I have used one a good bit. I'll give it a nod for speeding things up or making some things easier to do. But to me, they are not a necessity. 

When I have used one, I note the same thing as the OP, and I expected it. It's all about levers and fulcrum points. Common sense if you study the mechanism a bit. It's clever, and it works as advertised. If you can live with the minor(IMO) shortcomings it has and it fills a need, then by all means get one. But don't think that it is on everyone's TopTen Must Have (TTMH) list, by any means. But then, the people I know that have them, by and large, like them and won't part with them, so that says something too.


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