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

[zondar]

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?

 

[ErichKeane]

>>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.

 

[WobblyHand]

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.

 

[ConValSam]

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.

 

[zondar]

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!

 

[zondar]

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?

 

[Navy Chief]

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

 

[zondar]

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.

 

[graham-xrf]

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/

 

[Boswell]

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