Posted by Nicholas Farr on 17/03/2022 15:09:20:
Hi Dave, as JasonB says, the surface doesn't need to be dead horizontal, just as reasonable flat as possible for the type of level you wish to adjust. From JasonB's first link and under the heading of useful information > useful articles, you can find How to calibrate engineers level
Regards Nick.
I've been misunderstood again! I'm describing how to meet the requirement for 'as reasonably flat as possible for the type of level you wish to adjust'. Nick's 'How to calibrate engineers' link is in line with what I'm trying to say : 'In order to calibrate an engineer’s level, an adjustable flat surface that is mechanically stable is required. It does not need to be perfectly level to start with, but must be within the range of the level you are calibrating.'
Fitting a new sensitive vial is rather different from tweaking the adjusters on a decent level where the reference flat is horizontal enough such that reversing keeps the bubble near centre. As a new vial could be a long way off when first inserted, not 'decent', I suggest it's much easier to initially align a sensitive bubble when the level's body is known to be horizontally accurate. Which can be achieved by finding the horizontal line on a plate that's not itself horizontal. However the plate is tilted, the horizontal line will be close to perfect.
Using a sensitive machinist's level is frustrating because the bubble takes an age to settle, the slightest movement shifts it, and a tiny tilt will move the bubble off the scale. Setting a vial up from scratch is worse than using the level, and trying to set a vial on an inadequately horizontal or bendy surface makes the job harder than it need be. It's to the fitter's advantage to put a tight tolerance on his 'as reasonably flat as possible' surface. And whatever he perceives 'reasonably flat' to be, his horizontal must be within the sensitivity of the vial.
Ordinary spirit-levels aren't sensitive enough to need a careful horizontal but the bubble on my Engineering Level moves 2.5mm on a slope of only 0.02mm per metre. My Wixey and builder levels all quickly agree the surfaces in my workshop are horizontal. My Engineer's Level shows they're all dreadful optimists, and also that my 'stiff' worktops bend surprisingly easily! Not much, but deflections of a fraction of a millimetre aren't difficult to detect with a sensitive level.
Not a fan of sensitive Engineering Levels unless absolutely essential because using them is so slow, finicky, and frustrating.
Dave