Friday, December 7, 2012

Precision Level Calibration

I bumped into a problem the other day that is something other meccanica might be interested in. I loaned my precision level out to a friend of mine who had it for a few months under unknown conditions. I started to level the new lathe in the shop and like a good mechanic I got the lathe close and then started swapping the level end for end as I worked closer. After a few rounds of this I was getting some odd readings so I decided to go ahead and calibrate the level to make the fine leveling easier.

Levels are one of those cool tools that are self proving. As long as nothing is loose or broken you can bring even the most sensitive levels into calibration with a simple sequence. Its easier if you have a precision leveled surface to use but its definitely not necessary. It starts out with the premise that every plane has a line in contact with the surface that is perfectly level even if the plane itself is not level. Another way of saying this is if you have a table or surface and you place a level on it you can rotate the level in contact with the surface and find a line that reads level.  So how does that help us calibrate a level? Look at the sequence below.
  In this three picture sequence I rotated the level counterclockwise until I found a line on the surface plate that the instrument read exactly level. If the level is in calibration I should be able to swap the level end for end and get the exact same reading if I don't change the position. In the case of this level it appeared it needed a minor adjustment. Typically on the better levels one end of the bubble vial has some kind of adjustment screw. In the case of this old South Bend lathe level it has a pair of opposing set screws that bear on two steel balls the index in a hole in the vial holder. As one screw is advanced the other needs to be slackened. When levels come from the factory the screws are sometimes potted with plaster or putty to keep pesky tinkerers away from the adjustment screws.

This level has a bit of a story attached to it. My old mentor Doug had a son that worked with us for a few years in the sheetmetal shop. He approached me one day and said his roommate had some machinist tools that his dad or grandfather had left him that he didn't want and was I interested. I'll give you a guess what I said. Later that week he brings in a box of stuff and there are a few things of interest in it. The only two I can remember are the Southbend level, and a very cool mechanical tachometer which I used in my  blog article on the high speed Bridgeport speeder. Its made by the Herman Sticht company.  I had the old tachometer repaired by the factory for a fraction of the price of a new instrument. Its has a transmission and four speed ranges from 20-12,000 rpm which I thought was pretty neat and worth repairing.

So I see the level in its wood box but it seems to be missing the wood slide cover. I ask about the box cover and get a I think its lost answer. He says he will look around. Needless to say I did buy the deal. Can't remember what I gave him but it wasn't too much. So I have a nice precision level and a cool mechanical tachometer that's broken pretty good deal right?. I never thought I would use the tach that much but it has come in handy many times over the years. I guess if you have something handy you end up using it.

So quite a few months later Doug's son comes up to me and hands me the lid to the level box. Its dirty and one end looks like it was in a fire for a while. So the story goes, him and his roommate rolled the barbeque out when the weather changed and he saw the level box lid was being used to stir the briquettes in the barbeque. His roommate was bummed out when he took his stirrer to give it to me. This explains the burnt end. Every time I use the level I think about that barbeque and how lucky I was to get the lid.
When swapping the level end for end its helpful to block in the level so it goes back into the exact same position at each swap. The level is then carefully and slowly adjusted until the reading is as exactly identical on each swap. Its important to let the level settle for a few minutes at each swap when your getting close. This old SouthBend level is like that and needs some extra time to reach equilibrium.
The screws should be torqued equally when you reach the final calibration position.
This is the final result of the offset from the starting position that the level was blocked in at. Remember the level was out of calibration so I expected the position of the bubble to show an out of level condition at that position when it was properly calibrated. When the level is swapped end for end it reads the same which is what the goal is. So from this we can see the level was about half a division off from the starting condition.

So now after all that I can go back to what I was working on when this came up. Leveling a lathe. The level is just the first step in the proper setup of an precision lathe. More fun to come.

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