# So, a Starrett 98 6" level walks into a bar...



## zondar (Mar 1, 2022)

I received a rather nice Starrett six inch level.

I had naively expected the glass to be rigidly mounted to the base at the factory after calibrating against a reference surface, but instead it's attached by bolts, i.e. for adjustment. The nuts and bolts were all loose when it arrived. So how do I trust that it's actually "level?" Given that it arrived loose, I don't see how I can.

Do I really need another level to "level" this one!? How is this supposed to work? Insert confused emoji here.

Thanks for any insight.
---
Edit to answer my own question: Oh, I see. One calibrates it manually by reversing its position end-to-end on a flat surface and observing the bubble, adjusting until it's in the same position regardless of which direction it's pointing. Nice! But is there a reason why it isn't just rigidly mounted from the factory?


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## ErichKeane (Mar 1, 2022)

>>Edit to answer my own question: Oh, I see. One calibrates it manually by reversing its position end-to-end on a flat surface and observing the bubble, adjusting until it's in the same position regardless of which direction it's pointing. Nice! But is there a reason why it isn't just rigidly mounted from the factory?

Exactly this!  Note that you want to find as level of a surface as you can to start, otherwise this becomes.... a pain.

The reason this is adjustable is 2 fold: 1- it allows you to calibrate it as things change with it over time.  2- It allows you to adjust it if you find yourself having to re-bed in the level glass.


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## WobblyHand (Mar 1, 2022)

A level is calibrated when it reads the same when flipped 180 degrees on a surface.  Decent levels need to be calibrated.  You need a flat surface, like a toolmakers flat, and probably another level.  But that second level is just to help you find a "relatively" level line.  Use your carpenters level to find a line on a flat surface such that the bubble is really centered.  If it is off by 1mm, then you won't be able to level your good level.

I used a toolmakers flat on top of a three legged table I made.  The legs were threaded and adjustable.  I leveled the flat as best I could using with the cheap level.  I then placed two 123 blocks on the flat so I could repeatedly place the good level in the same place, by nestling the level into the vee formed by the blocks.  It is important to rotate the level by 180 degrees, not 179, or 181, that's why the blocks are there.  After that, it is an iterative process to getting to calibration.  The line you have chosen as "level" on the flat, has to be within the total adjustment range of your good level, or you will never be able to calibrate.  It took me a while for this to sink into my head.


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## ConValSam (Mar 1, 2022)

Congratulations on the sweet new level and the fun of calibrating it spot on!

I have one too: love the old school castings and its' Made in USA pedigree.


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## zondar (Mar 1, 2022)

ConValSam said:


> Congratulations on the sweet new level and the fun of calibrating it spot on!
> 
> I have one too: love the old school castings and its' Made in USA pedigree.



Yes, I'm guilty of buying it instead of a less expensive import simply because it's so sweet.

I have one of those dirt-cheap B-grade surface plates, and I'll get to calibrating it pronto.  

Thanks!


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## zondar (Mar 1, 2022)

OK, I have another question about this level, out of curiosity: Why is the groove in the bottom involute rather than just triangular?

Possibly answering for myself again: Because that accommodates larger diameters?


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## Navy Chief (Mar 1, 2022)

zondar said:


> OK, I have another question about this level, out of curiosity: Why is the groove in the bottom involute rather than just triangular?
> 
> Possibly answering for myself again: Because that accommodates larger diameters?


To help it sit straight on round pieces.

Sent from my Pixel 5 using Tapatalk


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## zondar (Mar 1, 2022)

Navy Chief said:


> To help it sit straight on round pieces.
> 
> Sent from my Pixel 5 using Tapatalk


Well, yes, but why involute specifically rather than triangular? 

I'm guessing because if it's triangular, larger diameter rods would sit on the edges rather than the ground surfaces, whereas with an involute it remains sitting on the ground surface out to a larger diameter.


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## graham-xrf (Mar 2, 2022)

You are about to experience the joys of how a level is self-calibrating because you have that handy place at the centre of the Earth that supplies the best, most excellent, reference. You only need the recipe, and @WobblyHand shows the way.

My experience of this involved me "chasing my tail" going after the unique line across the surface that was through true level. If the level vial is far enough out of adjustment, use any old cheap level to get where it is, approximately.

Adjusting to get "the same reading" when rotated 180° can mess with your head, especially if you forget "which side" should the bubble be as you rotate it. Remember that the bubble has to reference to the rest of the planet, not where it was last seen on the level. Put a sharpie marker cross on the surface side the bubble is towards, or put your coffee mug somewhere that side. When you can reverse the level, and it delivers the identical (off level) reading, it is calibrated. All you have discovered is that the line on the surface is not quite the right one, and needs a little rotation.

Although you don't have to, you can use your "calibrated" level to search it out. Here is where it can get away from you. The bubble settles closer to the centre, but you discover no way can you make it "reversible". That lets you know calibration is not yet over, but you should be close! If you see it _is_ reversible, though maybe not in the centre, yay! The level is calibrated, but the line you placed it on is still not quite right. Use something heavy with a straight edge put against the side of the level to help you fix the line when you reverse the level. You get to the point where you realise you don't have to waste more time finding where across the surface the unique line happens to be. Your level will be just fine! Go use it to check your lathe ways - or whatever.

Adjust.. adjust.. seeking "convergence" Hmm.. With a very sensitive level, waiting on the bubble to settle, the whole scene can become a bit of a struggle, but if you have the recipe right, and understand fully what you are doing when you adjust, it's quite quick. I was surprised at myself how what seemed such a simple, obvious thing could get muddled! Then again, I am now not as sharp as I used to be 

The whole thing, getting into the weeds, was wrung out by HM folk here ..
--> https://www.hobby-machinist.com/threads/the-chinese-precision-level-teardown.97510/


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## Boswell (Mar 2, 2022)

Abom79 just posted two videos on calibrating a precision level and then leveling a lathe.
Here
and
Here


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## zondar (Mar 2, 2022)

Interesting videos.

I wonder about his technique, though. He fixed the level's position and repeatedly adjusted the level.

I'd imagine it could be more efficient, at least at the start, to adjust only the position of the level until swapping sides does not move the bubble, without actually adjusting the level. After that, you would in principle only need to adjust the level once, to put the bubble in the middle.

Both sides of mine arrived loose and almost entirely unscrewed. How tight should both sides be?


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## hudstr (Mar 3, 2022)

I think you have misunderstood what is going on when you adjust the level. A level is properly adjusted when readings are the same when it is flipped 180 degrees. This tells us that the bubble vial is parallel to the base it is mounted on. A level that reads x number of divisions off, then is flipped and again reads the same x number of divisions, is properly adjusted. You don't need to have the bubble centered when adjusting, people just do that because it is easier to keep track of where you want to adjust it to, instead of having to remember something like 4 1/2 divisions to the left.


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## zondar (Mar 3, 2022)

hudstr said:


> I think you have misunderstood what is going on when you adjust the level. A level is properly adjusted when readings are the same when it is flipped 180 degrees. This tells us that the bubble vial is parallel to the base it is mounted on. A level that reads x number of divisions off, then is flipped and again reads the same x number of divisions, is properly adjusted. You don't need to have the bubble centered when adjusting, people just do that because it is easier to keep track of where you want to adjust it to, instead of having to remember something like 4 1/2 divisions to the left.


Thanks, but I do understand that. I was commenting on the difference between changing the position of the level until it finds a genuinely level part of the surface and adjusting it once, vs. placing it anywhere and adjusting every time the ends are swapped. (Every flat surface, including those that are not parallel to the ground, is level to the earth along some orientation.)

So, technique 1 (as in the video): Place the level anywhere (non-level) and fix that position. Swap it end to end while adjusting the level each time its swapped until the readings are the same. You now have a calibrated level despite the fact that the bubble is not visibly centered. Then the level, if placed on a genuinely level surface, should have a centered bubble.

Technique 2: Place the level on any flat surface and swap it end to end. To the extent that the readings don't match (which it won't at first), change the orientation of the level along the surface. Turning it one way will make the readings converge, and the other way diverge. Converge on an orientation at which the readings are identical. You have now found a level position on the surface. Now just adjust the level to center the bubble. If you reverse it again the bubble should remain centered. The advantage of this technique is that you only adjust the level once, and you get the satisfaction of seeing the bubble centered.


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## ErichKeane (Mar 3, 2022)

Huh, I'd never thought of Technique #2 before!  That is interesting!  I never knew that "every flat surface is level to the earth in some orientation" bit, but I think it conceptually makes sense to me.


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## hudstr (Mar 3, 2022)

zondar said:


> Technique 2: Place the level on any flat surface and swap it end to end. To the extent that the readings don't match (which it won't at first), change the orientation of the level along the surface. Turning it one way will make the readings converge, and the other way diverge. Converge on an orientation at which the readings are identical. You have now found a level position on the surface. *STOP HERE *Now just adjust the level to center the bubble. If you reverse it again the bubble should remain centered. The advantage of this technique is that you only adjust the level once, and you get the satisfaction of seeing the bubble centered.


That is correct until the 5th sentence, no adjustment is necessary. You can't move the bubble to the center by adjusting if the previous sentences are true, if flipping a level end for end shows the same reading. What this proves is that the the bubble vial is parallel to the base it is on, and that the surface the base is on isn't level (in that orientation). In this situation, you would just be able to keep rotating the orientation until you actually find a spot where the bubble shows the surface is level (with the bubble in the center), and to prove the bubble vial is parallel to the bottom of the level(by having the same reading 180 degrees apart).



Seems I misinterpreted your use of the word identical. What you are saying is both readings have the same value, but one is "positive" and the other is "negative". Then adjusting the bubble half of the discrepancy, aka to the center of the lines, and then you are done. I thought you mean identical as in the exact same.

tom lipton has a video that explains it better than I can.


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## Parlo (Mar 3, 2022)

zondar said:


> Thanks, but I do understand that. I was commenting on the difference between changing the position of the level until it finds a genuinely level part of the surface and adjusting it once, vs. placing it anywhere and adjusting every time the ends are swapped. (Every flat surface, including those that are not parallel to the ground, is level to the earth along some orientation.)
> 
> So, technique 1 (as in the video): Place the level anywhere (non-level) and fix that position. Swap it end to end while adjusting the level each time its swapped until the readings are the same. You now have a calibrated level despite the fact that the bubble is not visibly centered. Then the level, if placed on a genuinely level surface, should have a centered bubble.
> 
> Technique 2: Place the level on any flat surface and swap it end to end. To the extent that the readings don't match (which it won't at first), change the orientation of the level along the surface. Turning it one way will make the readings converge, and the other way diverge. Converge on an orientation at which the readings are identical. You have now found a level position on the surface. Now just adjust the level to center the bubble. If you reverse it again the bubble should remain centered. The advantage of this technique is that you only adjust the level once, and you get the satisfaction of seeing the bubble centered.


Technique 2 makes perfect sense and is what I will use from now on. I do prefer to have the level on two supports though to rule out any flatness issues.


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## zondar (Mar 3, 2022)

Tom Lipton is performing a "technique 1" calibration, at least at first. He started with what turned out to be an arbitrary position and keeps that position while adjusting the level over and over. He does move it once later, but just to find a second position that is closer to level, and then he proceeds with "technique 1" again. 

I do stand by my assertion that the "technique 2" is correct and is a better way to do it. Perhaps I could have been more clear about what I meant by "identical" readings, maybe? I meant the same as what is done in the "technique 1" version. (Edit: Yes, "identical" except for the sign, so +X and -X, where X is identical)

I especially like that the "technique 2" version does end up with a centered bubble, because you found a level orientation, and which can be reversed again to prove itself. You don't get that with the "1" version, and just have to trust that you would have a centered bubble if it was on a level surface.

I also note that Lipton is making adjustments by turning the slotted screw rather than the pair of nuts. I've wondered about that, as a fine screw thread should be a more sensitive adjustment than a pair of clumsy nuts. Which does one actually use?


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## WobblyHand (Mar 3, 2022)

zondar said:


> Tom Lipton is performing a "technique 1" calibration, at least at first. He started with what turned out to be an arbitrary position and keeps that position while adjusting the level over and over. He does move it once later, but just to find a second position that is closer to level, and then he proceeds with "technique 1" again.
> 
> I do stand by my assertion that the "technique 2" is correct and is a better way to do it. (Perhaps I could have been more clear about what I meant by "identical" readings, maybe? I meant the same as what is done in the "technique 1" version.)
> 
> ...


Finding that level line on the plane isn't that easy to do with an uncalibrated level.  However, you can bound that approximate line with a poorer level.  What makes it tricky is if your level (to be calibrated) is very sensitive.  Then you have to find a levelish line on the plane whose slope is within the total number of divisions of that sensitive level.  If that line exceeds that slope, then your level will always peg, no matter what you do.  A pegged level won't calibrate.

Of course, if you already have a flat truly level surface, then you are golden.   Many of us don't have a truly flat plane in their place.  All of us may have that level line, but it isn't simple find it within the precision of a sensitive level.  5 degrees rotation from the level line, and you are well out of range, at least with a 0.02mm/m level.  Less sensitive levels are easier to calibrate.

Have fun calibrating.


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## zondar (Mar 3, 2022)

WobblyHand said:


> Finding that level line on the plane isn't that easy to do with an uncalibrated level.
> 
> Have fun calibrating.


You find the level line on a generally non-level surface with your uncalibrated level by rotating it until the bubble is +/- an equal amount from center when flipped. No need to know where that level line is before hand. 

Of course, you do have to start with a flat surface that is at least somewhat level, or else your bubble will be out of range in most orientations. But with the 6" 98, this is not very hard (more sensitive levels are harder). It only takes a minute or so to get very close (I've tried it). After that, it's just a matter of how obsessive-compulsive the tool's owner is.

Also, checking the Starrett 98, the slotted screw thread cannot be used for adjustment. It just creates a pivot point. It probably shouldn't be screwed down too tight, either, as while that end of the level rests on a ball-shaped pivot, the bottom of the screw's head is flat. I'm still a little puzzled about this configuration. But on the 98, you apparently do have to use the nuts.


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## zondar (Mar 3, 2022)

Here's a link to a video that shows the "technique 2" approach in action (he does do a bit of a mix, though):

Steven Lang calibrating a level.


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## WobblyHand (Mar 3, 2022)

zondar said:


> You find the level line on a generally non-level surface with your uncalibrated level by rotating it until the bubble is +/- an equal amount from center when flipped. No need to know where that level line is before hand.
> 
> Of course, you do have to start with a flat surface that is at least somewhat level, or else your bubble will be out of range in most orientations. But with the 6" 98, this is not very hard (more sensitive levels are harder). It only takes a minute or so to get very close (I've tried it). After that, it's just a matter of how obsessive-compulsive the tool's owner is.
> 
> Also, checking the Starrett 98, the slotted screw thread cannot be used for adjustment. It just creates a pivot point. It probably shouldn't be screwed down too tight, either, as while that end of the level rests on a ball-shaped pivot, the bottom of the screw's head is flat. I'm still a little puzzled about this configuration. But on the 98, you apparently do have to use the nuts.


I did level a level that is 10x more sensitive that a 98-6.  Finding the level line within the range of the level is a lot harder!  If you go in 5 degree increments, you might find it.  Using 15 degree increments, you won't find that line, or at least that is my experience.  The more level the plane, the easier the level line is to find  

At least you have the sense to get a level that isn't too sensitive.  Alas, I didn't, and it took me quite a while to get it to converge.  My level could tell if someone entered the room, 10 or 12 feet away.  If I stood on one side of the level or 90 degrees to the level, the readings were different.


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## zondar (Mar 3, 2022)

WobblyHand said:


> My level could tell if someone entered the room, 10 or 12 feet away.  If I stood on one side of the level or 90 degrees to the level, the readings were different.


Wow! 

Of course, either technique would suffer from the same frustrations caused by hyper-sensitivity. But should I really pat myself on the back for getting such a horribly low-sensitivity level? Lol.


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## WobblyHand (Mar 3, 2022)

zondar said:


> Wow!
> 
> Of course, either technique would suffer from the same frustrations caused by hyper-sensitivity. But should I really pat myself on the back for getting such a horribly low-sensitivity level? Lol.


In retrospect, yes.  You did good.  It will be fine.  Too sensitive of a level makes you chase phantoms.  I can see the effect of the carriage position on the lathe.  When better weather arrives, I will make a better lathe stand.  Don't want to deal with welding outdoors, a bit too cold for me.  

For now, I have to tell myself, I would barely have been able to detect this, (and would have been happy), had I just bought a level like yours.


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## zondar (Mar 3, 2022)

Being a total newbie, my thought process went like this:

1. I need a level for a project that exists only in my mind.
2. I like Starrett products, and their's looks very nice.
3. Um, let's see... I guess a 6" one sounds about right.
4. Hit buy and wait impatiently at the window for the truck to come by.

So I guess I lucked out. Lol.

Go down the rabbit hole far enough, and all of metrology is about chasing shadows. Nothing at all is certain or absolute.


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