# Indicator Sag: How Do You Deal With This?



## EmilioG (Jun 8, 2015)

Does this happen to all indicators and all mag bases?  How much does gravity play a part?
Would this happen with an Indicol?


----------



## Tony Wells (Jun 9, 2015)

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.


----------



## EmilioG (Jun 9, 2015)

There has to be a way to indicate w/o sag even on a lathe tailstock.
Talking with highly skilled professional machinists, the experiment Richard King shows is disingenuous.
Not scientific.


----------



## tertiaryjim (Jun 10, 2015)

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.


----------



## Tony Wells (Jun 10, 2015)

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.


----------



## 4GSR (Jun 10, 2015)

That's me in the red shirt at about 1:00 into the video.


----------



## EmilioG (Jun 10, 2015)

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.


----------



## 4GSR (Jun 10, 2015)

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.


----------



## EmilioG (Jun 10, 2015)

Not something to dwell on, but some one thought enough of it to make a You Tube video.


----------



## hermetic (Jun 10, 2015)

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


----------



## EmilioG (Jun 10, 2015)

Disingenuous was a bad choice of word.
I only meant to say that the experiment didn't show the mag base in another configuration.
It would have been nice to see a demo on how it should be done to minimize error.
He also mentioned the Blake co ax, especially when using the long stem.  
I would also like to know which DTI's are best.  I guess I'll just find out for myself through
trial and error.


----------



## Tony Wells (Jun 10, 2015)

Interapid is the best, IMO. Pricey, but none better.


----------



## 4GSR (Jun 10, 2015)

I'm too cheap to buy Interapid's.  I use Brown & Sharpe's for measuring .0001" and .00005".  And Starrett for  the rest.  The B & S's that I have pretty much stay on the surface plates or in their boxes when not being used.  Oh, I also have a couple Federals that I use around the machines when needed.

I got off subject a little, Sorry...


----------



## Holescreek (Jun 10, 2015)

OK, I watched the video and didn't find anything new: gravity is a *****.  First off, the only ones indicating a hole in a circular motion on the horizontal plain are on horizontal boring mills. Having run one for 6+ years  my first reaction was "who in their right mind would use a side dial indicator like that?" I suppose if I were stuck on a desert island with a machine shop and only one indicator and a mag base I would.  The majority of people in a shop are going to be indicating vertically  like in a Bridgeport spindle where gravity is equal all around the hole.  Side dial indicators are for working in the horizontal, not traming holes IMO.  It's not news that using too long of a tip in an indicator not designed for the extra length exponentially changes the needle movement.  While he was at it he should have mentioned the proper tip angle for an accurate reading.

Obviously he didn't intend to teach indicator etiquette, someone just recorded the speech and dropped it n the net where it was taken out of context.


----------



## Wreck™Wreck (Jun 12, 2015)

EmilioG said:


> Not something to dwell on, but some one thought enough of it to make a You Tube video.


There is a youtube video for EVERY subject.


----------



## T Bredehoft (Jun 12, 2015)

I've been wondering if this would turn up here.   

Use an indicator in one position relative to gravity. You can rotate it around a vertical shaft, but not a horizontal one. 

In the last century, in the automotive industry, I had a job re-aligning two opposing boring heads. The two heads were mounted on bearing blocks 8" square and 30 inches long, hydraulically moved fore and aft.  The part, a brake disk, was mounded in a collet for machining the bearing diameters and seats. Each boring head machined one side. (Inside and outside) Tolerance was  _Mount the disc between bearings, the disc shouldn't run out more than .0030_.  Engineering had determined that one of the bearing bores was misaligned, it was my job (over Christmas vacation, at triple time). I wound up building an indicator holder out of 6 inch bar stock, two feet long, to reach from one head to the other.  (The collet was removed to allow more easy access to the two boring heads).  I screwed an indicator to the end of the 2 foot bar, and had no difficulty aligning the two heads in X, horizontal position.   Part of the problem was that  to check alignment, it meant tearing down my set up, putting the machine back together and cycling one part. Of course the part had to be carefully marked so I'd know what I was accomplishing. I finally determined that the 6" dia. two foot long bar sagged .002 at the end. I had to put it  out of alignment in Y (Vertically) by those .002 to get two bores concentric. 
Subsequent to this I made a solidly mounted indicator on a steel bar, not magnetic attachment, and by turning it upside down could illustrate as much as .007 'sag.' I could not couple it in any way to totally eliminate 'sag.'


----------



## JimDawson (Jun 12, 2015)

Sounds like using a laser to align non-vertical equipment is the way to go.  At least the light beam is much less effected by gravity.  Either that or hang a lathe vertical to align the spindle with the tail stock for instance.


----------



## Holescreek (Jun 12, 2015)

Those things didn't exist in the 80's and 90's when I was on a DeVlieg! 

I occasionally had a job that required boring a 16" diameter hole trough two sides of a cast iron press bed that required all 20" of my spindle travel with a 68 pound boring head on it's end. I can't recall anymore how much I had to raise the Z to finish the rear hole to be in line with the front.  The only saving grace was that back then there wasn't even a good way for anyone to inspect it.


----------



## stupoty (Jun 12, 2015)

JimDawson said:


> Sounds like using a laser to align non-vertical equipment is the way to go.  At least he light beam is much less effected by gravity.  Either that or hang a lathe vertical to align the spindle with the tail stock for instance.



Yeah you have to be carful with light too, it gets bent by gravity, to be fair though not a lot 

Stuart


----------



## astjp2 (Jun 14, 2015)

A tapered hollow inside hole may have eliminated some of the flex due to gravity on your indicator holder, the diameter and cross section needs to get thinner so it is lighter at the end.  They talk about this and test bars in the Connelly  book....Tim


----------



## EmilioG (Jun 14, 2015)

astjp2 said:


> A tapered hollow inside hole may have eliminated some of the flex due to gravity on your indicator holder, the diameter and cross section needs to get thinner so it is lighter at the end.  They talk about this and test bars in the Connelly  book....Tim



A tapered hole?  Please elaborate.


----------



## astjp2 (Jun 15, 2015)

Its on figure 16.4 in the Connelly book, basically you make the test bar hollow, but the inside in not straight/parallel its tapered as it gets towards the ends.  The diameter retains the stiffness where a  solid shaft will sag....Tim


----------

