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[Newbie] Question about interchangeability of DTI tips

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jmx66

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#1
Assuming the thread size matches, how interchangeable are tips? What are the advantages of ruby-tipped?
Here's the dumb question: what does length do to readings?. I would assume a tip that is twice as long would make the dial move half as much. I.e. if you want the dial to remain accurate, you must use the correct length tip?
Thanks all.
-John
 

JerryK

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#2
A longer tip will have more movement at the tip than
a shorter one to travel the same distance ( .001 )
Your indicator is geared , So .001 will always be .001
 

EmilioG

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#3
A longer than specified stylus will introduce cosine error and would have to be factored in to the reading.
If you want to avoid the math or other problems, just buy the stylus specified for your DTI. Ruby is used as a non marring
tip. Every DTI comes with a contact point designed for that model. Thread sizes, length, ball size, material. There is a ton of information
here: http://www.longislandindicator.com/p69.html
 

Billh50

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#4
I don't see how it will always be a .001 movement when the tip is out further from the axis point. If you have a tip that is 3 times longer then the original the tip will move 3 times the amount as the shorter one. The gears can not make up for that difference.
 

pstemari

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#5
Cosine error is introduced when the tip is at the wrong angle to the work. Usually indicators are calibrated for a 15° angle.

Longer tips require a correction factor if you care about the absolute numbers. However, usually a dti is simply used to adjust things for a null reading, in which case it doesn't matter. Zero difference is zero difference, no matter what you multiple it by.

It's possible to have gearing in the indicator you could use to compensate for tip length, but I've never seen or heard of such a thing.

Sent from my Pixel XL using Tapatalk
 

RJSakowski

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#7
Test indicators translate a linear movement into an angular movement. Assuming there is no cosine error, the DTI will only read correctly with a tip the same length as the original tip. Doubling the length will halve the angular travel for the same linear displacement.

The primary reason for using a longer tip is to reach locations which are not accessible with the shorter tip. Using one will decrease the sensitivity of the DTI by the inverse ration of the tip lengths. If you can live with the decreased sensitivity, it isn't a problem if you correct for it or are using the DTI to indicate zero as sptemari stated.
 

Billh50

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#8
When indicating a hole for center the length difference would probably not matter. But if you were using the indicator to check a step or length you would need to either calculate the difference when using a longer tip or use the right length tip.
 

jmx66

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#9
Thank you all for this informative (and surprisingly lively) discussion. :cool:
 

EmilioG

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#10
Cosine error is introduced when the tip is at the wrong angle to the work. Usually indicators are calibrated for a 15° angle.

Longer tips require a correction factor if you care about the absolute numbers. However, usually a dti is simply used to adjust things for a null reading, in which case it doesn't matter. Zero difference is zero difference, no matter what you multiple it by.

It's possible to have gearing in the indicator you could use to compensate for tip length, but I've never seen or heard of such a thing.

Sent from my Pixel XL using Tapatalk
Actually, Mitutoyo makes a custom stylus for it's DTI's that nullifies the cosine error. The stylus can be used at any angle.
 

Billh50

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#11
the angle of the tip really doesn't matter. the pivot point will still move the same distance. It is the tip length that changes the accuracy. As long as the tip is the same length as the original the indicator will read properly at any angle. If you don't believe me try it with size blocks and tip at different angles.
 

higgite

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#12
the angle of the tip really doesn't matter. the pivot point will still move the same distance. It is the tip length that changes the accuracy. As long as the tip is the same length as the original the indicator will read properly at any angle. If you don't believe me try it with size blocks and tip at different angles.
Just to clarify a little, the tip's angle to the DTI body at the pivot point doesn't matter, that's true. But, the tip's angle to the piece being measured does matter. A tip at 15 degree angle to the measured surface will read differently than a tip at a 45 degree angle to that same surface, due to cosine error.

Tom
 

EmilioG

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#13
Depends on the model:
From LIIS website:
Unlike other test indicators, Interapid indicators do not have serial numbers on them. Perhaps someday the manufacturer will remedy this situation but for now you'll have to assign your own unique serial number to each indicator if you intend to keep track of their calibration. It's easy to scratch a number into the painted body using a sharp tool. A permanent marker with a super fine point will also work. Do not use an electro-engraver because it creates sparks which have been known to damage the pinions on the gears.
cosine1a1.gif

Interapid series 312 test indicators are designed to permit a true reading when the angle between the stylus and the reference surface is 12° as shown in A and B (to the left).
In the rare cases when axial measurement is required with the stylus at 0° with respect to the reference surface as in C and D in the illustration, (for instance: small diameter bores) the readings must be multiplied by a factor of 1.022 (See further information on cosine error.)
It also follows that incremental calibration discrepancies can be corrected by adjusting the contact point angle.
 

rick9345

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#14
Cosine error is ove rated. If one knows enough to be concerned about or calculate it. Then it is not a hobby.
 

Tozguy

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#15
Actually learning the finer points of how things work and should be used is very much a part of the enjoyment i get from this hobby. I can be concerned about and dwell on any aspect of anything regardless of how trivial it might be to someone else. That is what defines a hobby to me.
 

pstemari

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#16
.... But if you were using the indicator to check a step or length ...
I'd hesitate to use a dti to measure a step, unless it was in conjunction with a stack of gage blocks on a surface plate. It's possible, but getting the angle just right is going to be trickier than using a drop indicator or a mic.


Cosine error is over rated. If one knows enough to be concerned about or calculate it. Then it is not a hobby.
Well, in that case you would probably be using the wrong tool for the job. I'm trying to think of some case where the numeric reading actually matters and another technique wouldn't be easier and more accurate. About all I can come up with is measuring the depth of a shallow groove or shoulder inside a hole that's too small for a telescoping gage or groove mic to get into.

Sent from my Pixel XL using Tapatalk
 
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