# Coaxial Indicator Problem



## BRIAN

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.



The information with the indicator defiantly says :0005" per div offset.



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??




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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## Bob Korves

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?


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## Bob Korves

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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## buffdan

check out this chart from Blake: http://www.blakemanufacturing.com/pages/coaxvalues.html
And an interesting video on coaxial indicators: 




They are relative reading, not absolute.

Dan


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## BRIAN

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.


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## Bob Korves

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?


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## BRIAN

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


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## Bob Korves

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^)


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## wyldboar

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 ?


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## John Hasler

wyldboar said:


> 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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## BRIAN

WY  thanks for your question.
We do not need to use the indicator to see if it reads as advertised just push the feeler the correct amount and see if the dial agrees.

 John. Yes the coax  is indicating the null. The point is how wide is the null. On my indicator when the short feeler is used the dial is pushed by a Class One leaver of about 5 to1 ratio. so even when the in it's best mode. at zer0 diameter with the effort at 90° the accuracy is degraded by that amount.The longer the feeler the worse the result is
Brian


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## wyldboar

I am still not sure I understand,  if a 5 to 1 ratio than .0005 would equal .0025 of linear movement, is that correct ?

Sent from my SM-P600 using Tapatalk


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## BRIAN

Yes correct. sorry to cut you loose  but it's one in the morning and I am off to bed. see you later
 Brian.


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## wyldboar

Good night

Sent from my SM-P600 using Tapatalk


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## Mark_f

I kind of hate to step in this , but my understanding of these coax indicators is, they are not for measuring, they are for centering only. That meaning they are strictly for centering on a ID or OD of a part or centering on  a hole, in which case the numbers are for reference only. I know I have a Best Test indicator with a long and short arm. With the short arm , the numbers are accurate, but if I use the long arm the numbers are off by a factor of 2. They tell you this in the literature and the long arm is not for measuring but more for lining up on a hole or OD where it is for convenience. I understood, and I could be wrong, but a coaxial indicator is not for measuring ( that is why I don't own one now). That is my opinion.


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## wyldboar

As I understand it  BRIAN thinks he has an error with his co-ax indicator  and was using a linear  measurement  to verify  if there was an error  or not , I was trying  to understand the process he was using.

Sent from my SM-P600 using Tapatalk


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## BRIAN

Hi Mark . thank you for joining the conversation, all contributions are welcome.
WY  you have it right I was trying to establish if the resolution of the indicator was as stated  under the best conditions.  Short feeler .  Zero diameter.  effort at 90°. I do not use the indicator to "measure".
But the thread took off down another path .
The point is my indicator has a resolution 5 times worse than is stated in the instructions and on the dial.
I am interested to see if other  (Quality)  brands have the same difference. so is any one prepared to do this simple test that takes all of 10 minutes,  Or do we just say Bad resolution is not a problem.
Brian.


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## wyldboar

Well I set things up the same as your pictures, my dro and dial have been checked with a known accurate  test indicator and I ran the same test as described  about 5 times and the dro averaged  .0072 . I didn't use an indicator  to measure movement at the end of the arm like you did, not sure it matters? My co-ax is by SPI , if that helps, I have used it for other set-ups and it was always accurate  for my needs. You may need more accuracy  than I do.

Sent from my SM-P600 using Tapatalk


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## BRIAN

Hi WY
You are correct the second test is not relevant, thank you for running the test
 So using the short feeler  moving the Coax one division takes .0072 movement of the cross slide
Thank you for your time and interest.
Any others willing .
Brian


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## Bill Gruby

Small comment here. I see someone put the Blake video up. Do not try to compare the Blake  with any other Coax. Most are not anywhere near the accuracy and quality of the Blake. If you really want to test them you need both a Blake and an off brand. Yes I am a tad biased on this, I have used many and the Blake is the winner hands down in my opinion.

I can understand the tests and reasons for them. In the end, the indicator with the finest resolution will be the winner It's hard to use something when it is not to spec.

I wish you good luck in resolving this issue Brian.

"Billy G"


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## BRIAN

Hi Billy G,
Thank you for your input.
I am doing this not only for my benefit but because it affects all owners of Coax indicators,
 Most owners just say "OH" I am happy with mine,  But have never tested it.  So I do not find there answers very useful.
Are you happy with what the manufacturer tells you ??  Think VW  ??
The test is simple.
Short feeler.
Zero radius.
Ensure the feeler travel is horizontal.
Load the end of the feeler against the tool post a little with the cross slide.
Zero all .
Move the cross slide untill the indicator moves 10 divisions.
Take the cross slide reading and divide by 10.
Compare with what the manufacturer says.

Now who will be the first person with a Blake or any other  "Quality" indicator to set the standard.
Brian.


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## rgray

Mine is an MHC. Don't know much about it the label is mostly unreadable, but the mhc is formed into the plastic case. No name on the indicator or on the face. Says .0005 per division.
10 divisions is .025 on the cross slide. I don't have a fancy dro just an igaging but both it and the cross slide dial agree at .025


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## BRIAN

Thanks to Wyldboar for mentioning the other half  it made me think , this is the total movement of the feeler and will allow us to compare  resolution , but to compare the axis offset we must divide the answer by 2.  this still leaves my indicator having to move .00135"  for one division instead of the quoted .0005"


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## BRIAN

Thank you rgray divide the reading by 10 gives .0025"resolution to compare the axis offset divide by 2 = .00125"
Can any one find anything I have missed in this test.


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## wyldboar

Wouldn't  you need to account  for the 5 to 1 ratio of the lever arm?

Sent from my SM-P600 using Tapatalk


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## chips&more

OK, I hope I have all the answers for you. I have a Blake CO-AX. I just ran a test like Brian did and to my surprise, I got about the same results. I have had/used a Blake for decades and never noticed the discrepancy in feeler travel verses dial readings. I have only used the CO-AX for centering not for measuring anything. Cosine error, different feeler lengths, feeler angle and whatever else can effect quantitative measurement. "The manufacture also points this out". But why such a gross error? I can see the copy indicators having the same error BECAUSE they copied the design of the original Blake indicator. The error was built into the copy from the original, huh. OK, so now where is this stated dial reading/graduations coming from??? I found out that the space from the bottom of the CO-AX case to one of the feet on the spinning feeler holder gives the correct readings on the dial, imagine that! Kinda meaningless, but that’s the deal. Just use the CO-AX as a centering tool and not for measuring and you will be good to go…Dave.


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## Bob Korves

Do the dials on the various coax indicators show the value?  Does it say something like .001"?  Or are they just undefined numbers on a scale?


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## Mark_f

I have a question. I just looked at several different brands of co-ax indicators and NONE of them say what each division is. They all say on the dial "AXIS OFFSET  .0005"
What does this mean?


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## chips&more

mark_f said:


> I have a question. I just looked at several different brands of co-ax indicators and NONE of them say what each division is. They all say on the dial "AXIS OFFSET  .0005"
> What does this mean?


My Blake manual says: “Dial Graduation is divided such that each division represents .0005 inch of AXIS OFFSET (deviation from true location) when feeler is tracing a diameter of two inches. (varies slightly with feeler angle).”


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## Mark_f

chips&more said:


> My Blake manual says: “Dial Graduation is divided such that each division represents .0005 inch of AXIS OFFSET (deviation from true location) when feeler is tracing a diameter of two inches. (varies slightly with feeler angle).”


That is what I am talking about. It does not say each division equals .0005" of movement. it says it is axis offset, what ever that means. offset from what and where.
I give up. I'm not going to get one of these.


I know for a fact, that the dial indicators are accurate if you use the arm straight out. when you bend it ( or I should say angle it) the accuracy changes. if you use the auxiliary long arm that comes with it in the case, the accuracy goes out the window. the same is true with the Starrett indicators. they are accurate alone, but if you start using the fancy attachments that comes with it in the case, the accuracy is shot and by a lot depending on the length of said accessory. I think we are chasing our tail here and to get back to the original question, I think there is no clear answer as they don't say that .0005" is readable movement....it is offset .... and for me that makes the waters muddy. That is what I think.


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## JimDawson

mark_f said:


> That is what I am talking about. It does not say each division equals .0005" of movement. it says it is axis offset, what ever that means. offset from what and where.




Mark, that is the offset from the center of the hole (or pin) to the spindle centerline.  Normally in use you don't worry about actual dimensions, just adjust X-Y position for minimum needle movement when centering on a part.


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## Mark_f

JimDawson said:


> Mark, that is the offset from the center of the hole (or pin) to the spindle centerline.  Normally in use you don't worry about actual dimensions, just adjust X-Y position for minimum needle movement when centering on a part.


an expensive center finder......


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## JimDawson

mark_f said:


> an expensive center finder......




Yup   but with a certain cool factor


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## chips&more

Yes, we are all on the same page in understanding the authors question. And my Blake is doing the same thing? That means using a 2” feeler in about a 2” hole should give 0.0005” offset readings/div (manufactures recommendation), but it does not? It’s more like 0.0025” actual/div. What am I missing? Instead, the dial reading is the space from the bottom of the CO-AX case to a finger on the spinning feeler holder...Dave.


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## chips&more

JimDawson said:


> Mark, that is the offset from the center of the hole (or pin) to the spindle centerline.  Normally in use you don't worry about actual dimensions, just adjust X-Y position for minimum needle movement when centering on a part.


Yes, but that means my Blake that advertises 0.0005”/div or you could say its sensitivity or resolution is not as advertised. It’s actually not that sensitive now that I did findings on it today. It’s more like 0.0025”/div. It would be hard to true position anything with a Blake down to 0.0005”!!! Because you could not see that kind of needle movement on the dial! Or, am I missing something?


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## Bill Gruby

If you move the cross slide 2 divisions, how many divisions does the Blake move? This is no time to have a broken ankle. Dang it all

"Billy G"


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## buffdan

As an aside, here is an individuals plans to build a coaxial indicator.. just a FYI: https://drive.google.com/file/d/0BwXDcXKx2KzlaUVrZ0l6QU4yaTA/view

Also, statement from Long Island indicator repair, regarding Blake Coaxial indicators:

"Additionally, we have the following statement from the manufacturer :
The Blake Co-Ax indicator is a *centering device*. It requires a reading of both sides of the bore (or boss), in each axis, to arrive at center. *Its sole function is to find center. It is never to be used as a measuring instrument.*

You can not certify the Co-Ax on a surface plate. It must be put in the spindle of the machine in which it would normally be used and indicate a precision ring or plug. Then remove it and use a .0001" indicator to check that you are on center.

The Co-Ax indicator is made to be used under power. When checking the Co-Ax position with a test indicator the spindle should be rotated manually for both indicators due to the fact that spindle torque under power on some machines will give a different reading for center." _- manufacturer's instructions _


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## Bill Gruby

This is correct. The resolution only allows you to see how close to center you actually are and the needle tells you which direction to move. It is all position, nothing else.

 "Billy G"


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## chips&more

Bill Gruby said:


> If you move the cross slide 2 divisions, how many divisions does the Blake move? This is no time to have a broken ankle. Dang it all
> 
> "Billy G"


If you move the cross slide 0.0025” the Blake will move 0.0005”. The Blake is fitted with a 2” feeler and 1” offset from datum centerline.


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## Bill Gruby

Then the Blake is correct in resolution. Resolution on a Coax indicator merely tells you how fine a movement (adjustment) you have. If the resolution was say .001 the movement would be coarse, .005 fine movement (adjustment).  All you are seeking is absolute"0".

"Billy G"


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## BRIAN

My apologies I have been down at the boat for the morning thank you for continuing this thread.
It appears that in my ignorance my assumption that if I was down to one division on the indicator the job was within.ooo5" is totally wrong and that figure is only obtainable when the indicator reads zero movement,  So if I cannot get to zero movement I must look for some other reason why .
Brian.


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## Bill Gruby

Good one Brian. You are absolutely correct. Now, lets see what other possibilities there are.

 "Billy G"


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## Bill Gruby

A Co-Ax Indicator in itself measures nothing. It is for centering the work to the spindle, that's all. It matters not where the reading is as long as the needle is in the same place and not moving for 360 degrees. It's probably one of the easiest ways to center a spindle and work.

 "Billy G"


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## chips&more

Yes, you are correct. The CO-AX is only for centering, BUT, the question now is how well can it center? The dial on my Blake says 0.0005” OFFSET. The manufacture claims this is offset deviation from true position/div. So, at first glance, you are thinking this tool has a very good sensitivity or resolution and can find center within a few tenths. NOT TURE, with a 2” feeler you would be hard pressed to see tenths. And with a longer feeler in place, you would be hard pressed to see deviations of a thousandth on the dial. Or am I MISSING SOMETHING? Somebody please chime in. I can’t believe the Blake manufacture has been advertising this for all these years and nobody corrected them. What am I missing?...Dave


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## Bill Gruby

I have no clue what you are missing. I see your Blake the same as I see the two I have. I am having no trouble centering with either of them. The Graduation values that Blake give you are spot on. I have centered stock up to 8 inches with no trouble.

 "Billy G"


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## buffdan

*Blake Specifications*:
Feeler ball diameter: 1/8 inch

Measuring range: 1/4 inch axis offset

Capacity: 5/32 to 4.25 inches internal; 0 to 4.25" external. Optional feelers extend these capacities to 12 inches. See accessories list below.

Speed: spindle rotation up to 800 rpm

*Locating accuracy: .0002" of true center*

Initial positioning: within 7/16 inch diameter of true location

Graduations: .0005" or 0,01mm

The entire gage will just fit into a 2.2" diameter cylindrical space

Overall length of gage, without the feeler is roughly 5.2", with the shortest (standard) feeler installed, the overall length is about 6-3/4 inches. When the upper shaft is inserted into a chuck, the overall length is about 5-3/4 inches.

The spindle diameter is 3/8" on the inch model, 10 mm on the metric model.


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## Bill Gruby

This is the Blake Manufacturing Site. At the top is a contact box. If you feel that there is a genuine problem just ask them. You will get a direct answer from them with no BS.

http://www.blakemanufacturing.com/pages/aboutus.html

 "Billy G"


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## chips&more

Bill Gruby said:


> I have no clue what you are missing. I see your Blake the same as I see the two I have. I am having no trouble centering with either of them. The Graduation values that Blake give you are spot on. I have centered stock up to 8 inches with no trouble.
> 
> "Billy G"


Just to clarify please. Did you actually set-up your Blake like Brian did in a lathe? And did you find the graduation values accurate on your Blake? Did you put in a 2” feeler and move it say 0.001” and then get a 0.001” reading on the dial? Or what reading?


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## Bill Gruby

I have to rely on what you guys are doing. I have a broken ankle and cannot navigate the stairs to the shop. We tested the Blake years ago when GM bought them for the Bearing Plant  here in Bristol. We found them to be the most accurate available. Ours were Lab tested. I was not on the test crew so I would guess that anything I had to say now would be meaningless to you. You guys have fun.

"Billy G"


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## Mark_f

This may ruffle some feathers .... but ..oh well..... the way I see it , the co-ax indicator is correct. it will not measure accurate movement and wasn't intended to. it was intended to find a center point. the divisions vary widely in the distance they represent depending on the length of the stylus. I'm sure there is a stylus length that may work out to .0005" , but I bet it is really short. It is merely an expensive center finder using a dial gage to read center. MY opinion....I could be wrong....


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## Bill Gruby

I have a question on your test Brian. You are testing a Co-Axial Indicator. How many positions are you checking it at. IMHO you would need to check it at a minimum of 4 locations and take the average of all 4. More positions would prove more accurate. The killer is that even a Blake was never meant to be accurate. As Mark and others have said, it is an expensive center finder. Blake claims .0002 center accuracy. That is closer than most will ever need. Now I bid you all good bye. I can add no more to this thread.

"Billy G"


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## Mark_f

And as my final thought on this. Bill said a key word " .0002 CENTER accuracy" is the claim.


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## chips&more

I just solved my CO-AX discrepancy, actually two of them. First, and this is I believe for Brian also. Initially I set-up the CO-AX as Brian did on the lathe and used the cross slide as a calibrated translator. Big mistake, if you use the cross slide, the dial on the cross slide does not read 1 to 1. That dial is made to read for diameters. My second mistake was assuming the small feeler I had was 2” long, it’s more like 1 1/2”. After I undid my mistakes the Blake does read accurately…Dave


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## BRIAN

Well we appear to have thrashed this one to death,  and the answer seems to be, Just use it and don't question it.
 Thanks to everyone that has taken part your input is much appreciated.
And Mark I don't think any feathers have been ruffled this is possibly the only site that a discussion like this can take place in a gentlemanly manner.
Brian.


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## kd4gij

The paper says ..0005 dia. and the op is measuring in radius.


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## Bill Gruby

The indicator is a center finder. What difference does it make. It indicates minus to plus with the half way mark being center ( -.0001 ---- 0.0 ---- +.0001) that is a .0002 margin of center  error just as it says. That's close enough for me.

 "Billy G"


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## rick9345

BRIAN said:


> 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.
> View attachment 126830
> 
> 
> The information with the indicator defiantly says :0005" per div offset.
> 
> View attachment 126831
> 
> 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??
> View attachment 126832
> 
> 
> 
> 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.


Amateurs tend to beat the wrong end of a dead horse


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## Uglydog

Used my recently acquired Blake for the first time today.
Wow!!
The Zero-it goes to the back of the drawer.


Daryl
MN


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## rick9345

coaxial DTI are only meant to i_ndicate/show_ an _offset exists_. Not meant to measure a defined offset. they will help to locate a spinning spindle to the center of a hole. The designed purpose.Some may find other uses,but defined off set should not be one.


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