Using & trusting a brand-new Starrett 98-6 machinist's level

[astjp2]

I should have you check my level, its pretty old but I think its in good shape. Tim

 

[cazclocker]

My day job is as a metrologist, so I have calibrated many precision levels, like the Starrett #199. When I calibrate these levels at work we need to make sure that the temperature hasn't changed more than 2 degrees in the past 8 hours & that the level has been in the same environment for that 8 hours. We try for 68F (20C). The first check is for flatness. This is the hardest part of the calibration & requires some skill & practice. But for you guys at home long as it doesn't rock move when you push on any one corner, & then you are probably flat enough. Yes the levels are made slightly concave & you shouldn't use them on anything that is shorter than the level.

Clean the level & the surface plate, checking the level for any burrs or other damage. Level the surface plate (using the level you are calibrating) & let the level sit for a few minutes, re-check level & readjust as needed. Then rotate the level end for end putting back up against your reference point. I use two 2x6 granite parallel bars with 1 perpendicular to the other, & held in place to the surface plate with modeling clay, to maintain a reference point. A few minutes later after the level settles it should indicate the same level reading. If not double check your first reading, rechecking to ensure that everything is clean before adjusting the level. Screws & jam nuts type adjustments are very touchy, & it may take some time to get it spot on, the only to find that when you tighten down the jam nuts you throw the level out of whack again. A light touch is needed & always allow time to let it settle. Sometime I'm not even be sure I moved it at all when adjusting it, that how light of a touch you need, unless it's way out already, probably from moving the adjustment to much in the first place.

At work we also check the level sensitivity to ensure that each division on the level equals a certain amount of angle. With the Starrett 199 one division equals 1/2 thousandth (0.0005) of an inch per foot. When I check the 199 it's typically about 8 arc seconds per division. So figure 4 arc seconds if you are only off half a division when you use it. To calibrate the sensitivity we use a Brunson 470 small angle generator. It's basically a granite sine plate with a big micrometer head to adjust the angle down to 0.1 arc seconds per division. I move the bubble exactly 1 division then record the reading, move it another record, etc... then at the end average the readings. The factory usually specs levels as a certain amount per distance, such as .0005" per 12".

It usually comes down to the person eyeballing the bubble to determine the sensitivity. The biggest error we encounter is the person looking at the level isn't looking at the bubble dead on the same way, using the same eye, each time they make a measurement.

Hope this was helpful.

illillill, thanks for your input. It's been extremely enlightening to me to learn that the bottom surfaces of these fine levels are just slightly concave. I have a Starrett 98-6 which is longer than the width of my milling machine table, so I see that my 98-6 can be applied to the table lengthwise, but not widthwise. Luckily, I just acquired an extremely sensitive Chinese-made level that was made for Chinese army's artillery - and I got it for just $12.00. It's only 4" long so I use it for leveling my table's width. Also, I can understand just why it would be ideal to calibrate a level under steady temperatures - I wish it was possible in my home shop, but unfortunately it just isn't! I live in northern Arizona, so we have some pretty wild temperature swings. I guess we homeshop type guys just have to do the best we can do with what we have.

 

[illillill]

I don't think that the temperature differential is that big of a deal for the kind of work that most people will do in the shop & probably could be ignored for the all but the most exacting work. When you calibrate your level take your level & a surface plate in the house, & leave the heat set at the same temperature all night. The next day everything should be at the same temperature, & you will be fine. Even the concavity of the base of the level might not matter if you don't need that kind of accuracy. The concavity on the Starrett 199 is something like < .00005" iirc, so it's not much. So using a precision level on a part shorter than the level will still be much more accurate than eyeballing it or using a framing level. You could probably calculate a worst case using some trigonometry, & it would still be only a few arc seconds difference. Always make sure to look straight down at your bubble using your good eye, to minimize any parallax & line up the bubble on the lines the same every time. All of the precision levels I have used have a surface that is not perfectly flat, but even a brand new framing level is awful in comparison. But make sure the level & what your putting it on is clean & free of burrs that could scratch the surface.

I only mentioned the temperature thing because it's part of every calibration we do at work. We wear cotton gloves to prevent the heat from our hands to cause the material to expand possibly affecting our measurements. One thing I teach the guys at work is heat causes stuff to grow, & cold causes it to shrink. Think about your anatomy, to remember this. A cold swimming pool causes shrinkage & a hot girl can cause a swelling in your pants. You'll never forget it if you think of it like that.

To get a little deeper into it you can find out the coefficient of linear expansion, for the material you have & see that a few degree temperature change affects it a little, but not that much. It's probably not enough to measure, unless it's a big temperature change from 68ºF (20ºC), or you have some really expensive & accurate equipment. The Coefficient of Linear Expansion for a given type of steel is something like .000012" per degree C away from nominal of 68ºF (20ºC). So say it's freezing in you shop, @ 32ºF (0ºC) that's only .00024" that the item has shrunk (20ºC temperature change x .000012 =.00024". It's unlikely that anyone would try to perform any precision work at that temperature. Also if your measuring instruments have also been in that freezing environment they would also have contracted or shrunk a bit depending on the material, so the true difference in size would be even harder to measure.

Not to get too off topic, but in case all this talk about temperature has caused some confusion let me state that a temperature change can cause a size change that will be noticeable to most people. Like when machining a part to a specific size, because the temperature difference is much greater when that part you are machining gets hot. Say the part got hot during machining & is now 212ºF (100ºC). That's 144ºF (80ºC) difference from the nominal of 68ºF (20ºC). That much of a temperature swing will cause a noticeable affect on you measurements. Again using steel for our example; .000012" x 80ºC = .00096" Almost .001" is a measurable difference in size & could cause a part to not fit. This would be more measurable than the previous example because your micrometer or other measuring instrument would not be at the same temperature as the part you are measuring.

That explanation is over simplified of course, the part size, & material type/quality will affect it as will other feature such as a hole, & probably a bunch other things.

I'm not an engineer, but I think my simplified math is correct.

 

[cazclocker]

illillill, thank you for the elucidation. You're my kind of engineer!
...Doug

 

[coolidge]

I used a machinist level to level my new lathe last week, turns out I had leveled it with my Home Depot level to within .002 of the machinist level end to end

 

[tripletap3]

I just recently purchased a 98-6 level myself, for my new PM 1236 lathe, and PM 962 Mill.


I Called Starrett, and Scott the technician educated me a bit on levels. Machinist level bottoms are slightly bowed, so you have to have a surface longer than the level to place them on, as only the ends touch the surface you are leveling. That was good to know !

RH

Thank you! Thank you! Thank you x100! I have a pristine older Starrett 98-8 that I bought off Ebay a few years ago. This thing drove me bonkers while trying to level my lathe until I figured out it had a bow in the middle. I thought it was warped and just threw it in the tool box and have been meaning to mill the bottom flat to try and save it I just haven't got around to it yet.

 

[cazclocker]

Re: Using &amp; trusting a brand-new Starrett 98-6 machinist's level

I used a machinist level to level my new lathe last week, turns out I had leveled it with my Home Depot level to within .002 of the machinist level end to end

Dang! Yer good! Way ta go...!!! :tiphat:

- - - Updated - - -

Thank you! Thank you! Thank you x100! I have a pristine older Starrett 98-8 that I bought off Ebay a few years ago. This thing drove me bonkers while trying to level my lathe until I figured out it had a bow in the middle. I thought it was warped and just threw it in the tool box and have been meaning to mill the bottom flat to try and save it I just haven't got around to it yet.

Yeah, I know what you mean...that was a revelation to me too. I wouldn't have figured they were supposed to have a bow in them.