[Metrology] Calibrating A Machinist Level

[MattM]

I scored a nice 18" Starrett level on CL yesterday ($50). It has adjusting screws. How do I know if it is in calibration? If not how do I calibrate it. Thanks.---Matt

 

[atunguyd]

Matt I guess it depends on what you need to do with it. For this like levelling a lathe, you need the precision that a Machinist level gives you, not necessarily the accuracy. Even if it reads 5 degrees out from horizontal it is still good enough to level a lathe as it has the precision to show you that your front and back are level with each other.

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[Tony Wells]

There is a difference between calibrate and qualify. To calibrate, you need to progressively elevate the end by precision increments to see if the lines are accurate representations of elevation, or "out of level". To qualify the level is simpler. Assuming you have a surface plate, place the level on it, and by rotating it, find a spot that is as close to "level" as you can find. Use a Sharpie or similar, and mark the position by tracing the outline. Then reverse the level and make sure you read "level" at that position as well. If you do, then it is fine to use for machine leveling. If not, you can carefully adjust the nut/screw arrangement on one end of the vial housing. Be very careful. Make sure your mating surfaces are extremely clean. If in doubt, send it to an accredited calibration lab.

 

[MattM]

Thank you. I'm just going to use it to level machinery.

 

[wrmiller]

Matt I guess it depends on what you need to do with it. For this like levelling a lathe, you need the precision that a Machinist level gives you, not necessarily the accuracy. Even if it reads 5 degrees out from horizontal it is still good enough to level a lathe as it has the precision to show you that your front and back are level with each other.

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Agreed. For absolute measurements referenced to gravity, the level needs to be calibrated. However, in the case of a lathe, the main requirement is that the ends of the lathe bed are at the same angle to prevent/remove 'twist' that will affect dimensional accuracy. For comparative measurements (one end of the lathe compared to the other), the surface plate process Tony describes will suffice to reduce gross error in the level.

I would also offer that a bubble level will only get you close. As each lathe is different and has it's own personality, my preference is to take the leveling process one step further by cutting and measuring the taper on a sufficiently (for your lathe) long unsuspended piece, i.e., no tailstock support. I then tweak the level of the tailstock end of the lathe to get a taper reading that suffices for my personal tolerances.

Mills and other machines are not as finicky and the spirit level should be sufficient.

 

[Dan_S]

I would also offer that a bubble level will only get you close. As each lathe is different and has it's own personality, my preference is to take the leveling process one step further by cutting and measuring the taper on a sufficiently (for your lathe) long unsuspended piece, i.e., no tailstock support. I then tweak the level of the tailstock end of the lathe to get a taper reading that suffices for my personal tolerances.

I use a slight derivation of this, instead of turning something, I chuck up a piece of hardheaded precision ground linear shafting and then indicate off the front and top sides to determine how aligned the lathe is. The shafting is cheap, rounder than most of us can measure, and straighter than most of us can turn.
http://www.amazon.com/Thomson-0-749...ting&refinements=p_85:2470955011,p_89:Thomson

 

[chevydyl]

Note that you don't have to have a surface plate, just something flat, and it doesn't have to be perfectly level, all your doing is a comparison when you rotate the level 180, like said above it must be in the same exact position when rotated, I use an edge like the mill table, or my lathe ways since I don't have a surface plate. Compare the bubble positions and make you adjustment until they are the same in both positions.

 

[wrmiller]

I use a slight derivation of this, instead of turning something, I chuck up a piece of hardheaded precision ground linear shafting and then indicate off the front and top sides to determine how aligned the lathe is. The shafting is cheap, rounder than most of us can measure, and straighter than most of us can turn.
http://www.amazon.com/Thomson-0-7490-0-7495-Diameter-12/dp/B00TSGMJTC/ref=sr_1_9?s=industrial&ie=UTF8&qid=1441469960&sr=1-9&keywords=linear+shafting&refinements=p_85:2470955011,p_89:Thomson

I must be one of the few then. I will try to generalize my responses a bit more, thanks.

 

[mikey]

I assume you're talking about a Starrett 98-18. If so, go here and download the powerpoint entitled "Calibrating and Maintaining Your Level" near the bottom of the page:

http://www.goodson.com/technical-library/

It pertains specifically to the Starrett 98 and is very clear.

 

[Dan_S]

I must be one of the few then. I will try to generalize my responses a bit more, thanks.

I don't want this to come off the wrong way, but if you can turn something to these tolerances I want to see it. Because the Thompson shafting has some seriously hardcore tolerances. Everything I've tested in my shop is well below the listed tolerances, or to small for me to accurately measure.

Surface Finish: 8 Ra Max
Roundness: .000080” Class L and S / .000050” Class N
Diameter: .0005 Class L and S / .0002" Class N
Straightness: .001” Per Foot Cumulative (.002” TIR)
Taper: .0001”