Heat Effects on Spirit Levels

graham-xrf said:
Yup - this happens! My Dad (builder) had a traditional lead plumb bob set into a "teardrop shaped" cutout in a hardwood plank, and with a fine V-line cut into it. The whole thing, which stood as high as a man, had a cover box that could be removed sideways, with viewing aperture, all to stop air disturbances. He told me the same thing about Table Mountain (Cape Town). The middle of that peninsula mountain is made of iron, or something dense, because all the buildings stand slightly off true geographic vertical, and plumb bobs hang slightly pulled toward the mountain.

Not a problem for building purposes, because the forces in the buildings are still properly catered for by regular spirit levels. I expect any satellite tracking dishes in the region would have to correct for clinometers being somewhat untruthful. The messed up way levels worked in Cape Town has been well known since back in the 19th Century because of the importance of the Observatory in Cape town, and the huge efforts to establish the Southern Arc of the Meridian, and prove the Southern half of the Earth oblate spheroid was the same shape as the Northern half.

The deviations from vertical were known as "La Caille’s discordances". I am not sure it is true that at the base of the mountain, coffee in a cup will look slightly skewed off level. I never noticed anything like that, and I lived for a while in a University residence on the slopes of Table Mountain.

View attachment 386582
Great history and pictures here -> Arc of the Meridian Survey
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The effects of any nearby mass would be negligible for all practical purposes. In any event, a plumb bob and spirit level would experience the same forces and should agree. The effect of the gravitational pull of the sun and moon would be greater as evidenced by ocean tides. Any edifice constructed near a very large mass such as a mountain would be plumb and level to the eye but in actuality, it would be slightly askew. There would be no simple way to check this as any reference would see the same effect. One possible way would be by observing the position of astronomical features although this would require very precise time measurements. An interesting thought.

There has been some locations where whole mountains have been leveled due to mining operations. Any edifice would now be leaning slightly away from the missing mountain. I suspect that the change would be insignificant. Certainly, we wouldn't be able to observe the effect as our internal plumb bob that allows us to observe the vertical would see the same forces.

An interesting experiment would be to level a stable surface with a sensitive level and obser4ve any changes over time. These changes, if any, could be compared with the position of the moon and the sun over the course of a month to see if there was a correlation.
 
@RJSakowski
I did think the deflection would be negligible. It seems not negligible enough for the surveyors doing the Cape Flats baseline, nor for the baseline on the Grand Parade Square. You are quite right that it does not affect our use of spirit levels at all! Who cares if the downwards pull is not truly at the Zenith, so long as it is the same over the whole thing the level is on.

I am always surprised at how non-intuitive is my appreciation of the effects of gravitational attraction. The way warships parked next to each other will supposedly "pull together" over a couple of days (do they?).

I still have a couple of old videos taken in a university basement, of a qualitative gravity experiment using what looks like boule game steel balls, also called la pétanque. The time lapse was triggered remotely with nobody in there, and all sorts of care taken to eliminate air disturbance, but the actual kit was just hung under a stepladder. I am not sure what was used for the suspension, maybe fishing line or fine thread of some kind.
Check the timeline. It took hours!

View attachment Gravity1.mp4

View attachment Gravity2.mp4

For our purposes, deflected or not, we only need levels to provide a handy (intangible) reference so we can go measuring along surfaces, and discovering the actual dimensions and wear on our machines. We choose horizontal, but in theory, if we decided to always use the level some constant amount tilted, we can still do the measuring job, only having burdened ourselves with extra arithmetic, and that would be a truly nut-ball thing to do.

Temperature effects?
Lacking a really good straight edge, I am grateful that I can work my way along the ups and downs of a worn surface with a thirty quid Chinese level claiming 0.02mm/metre, or about 4 arc-seconds angle sensitivity. The levels do not seem to care about being near freezing sometimes.
BUT..
The situation might be different for the ways of a knee mill if it has been in a below freezing shop all night, and then experiences a significant temperature gradient. You get the heat on, and start milling madly. The motor heats up, etc. Maybe the bed will start to curl. I guess the way to see it would be to try for a video with it illuminated by monochromatic light, taken through a diffraction grating, or some such hologram-type tricks.
 
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I think it is worth stating the following for newbies who are wondering why any of this is of interest to hobby machinists.

My understanding about the topic of “leveling” a lathe has always been NOT to level the machine in the purest sense of the word, but rather to remove TWIST.

If leveling were the key to a successful use of the lathe then there would be no such thing as a vertical lathe, or for sake of discussion, vertical boring machines or milling machines.

Twist is our enemy not level. That is why mills need to be trammed.

With that said it makes perfect sense why we have to be mindful of temperature effects on the spirit level when removing twist from the lathe bed.


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This has been very interesting reading so far. All good information. Metrology is a very interesting subject
 
vtcnc said:
I think it is worth stating the following for newbies who are wondering why any of this is of interest to hobby machinists.

My understanding about the topic of “leveling” a lathe has always been NOT to level the machine in the purest sense of the word, but rather to remove TWIST.

If leveling were the key to a successful use of the lathe then there would be no such thing as a vertical lathe, or for sake of discussion, vertical boring machines or milling machines.

Twist is our enemy not level. That is why mills need to be trammed.

With that said it makes perfect sense why we have to be mindful of temperature effects on the spirit level when removing twist from the lathe bed.
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You are right in that there is nothing absolute about mounting a lathe "level", and I did say that the value in "leveling" was only to have easy access to an external reference to measure from. Lathes can, in principle, be mounted in any attitude, so long as where it is does not eventually introduce any self-weight distortions or twists.

It's an incredibly convenient intangible reference, and we get there with a spirit level. Lathes mounted on ships can hardly be ever be "level", but when they are securely attached to the ship, they use the ship's line reference for the day. It may not be truly horizontal at the time, and keeps changing during construction. Again, the handy transfer gadget is a spirit level, albeit sometime deliberately offset to some other reference.

A lathe in operation might not be much affected by whether it is "level", but discovering where it's bits are in relation to others, or the ups and downs of wear along it's ways, or whether there is twist, is most handily done with a spirit level. It could be done by other means, but with a whole lot more trouble. Of course twist is the enemy, and I see the level as the opposite, the friendly accurate tool to reveal it, and quantify it, and help us fix it!

The convenient starting point is just to level the ways, and it is here we discover that the whole thing is a bit bendy, and will twist if it's design load is not shared correctly between the supports. Over time, a support can relax. The sag may be imperceptible, but the force it delivers can disappear!
 
Any ex Navy machinists out there? It would be interesting to see what the approved practice for setting up a lathe was.
 
was there more than 30 seconds of video?
I don't do any social media, so I am a guest viewing. It ended after 30 seconds?
 
woodchucker said:
was there more than 30 seconds of video?
I don't do any social media, so I am a guest viewing. It ended after 30 seconds?
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There were two videos, 24 and 28 seconds, respectively. You can view them without restrictions.
 
so what it says is the spirit size is different, meaning the reading of out of level is off. But level is not affected. the change is not one side or the other, only a non level item reads off. In other words if its .0005 it might be off by .001
 
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