Metrology Monday: Precision Machinist’s Level

What Is A Precision Level For?

This is one of those tools that you don’t need until you do. As evidenced previously, I have managed to turn out some parts that are less perfect that one would hoped. While the list of possible sources of error is long, a fundamental part of setting up machine tools is levelling them. Even a solid casting like a lathe bed or a milling machine base can acquire detectable twist in it when you bolt it down. To take out the twist, you put shims under the low corners or some other adjustment until the measured surface is level. To detect twist, you use a precision level.

I have avoided going through what is almost certainly a fiddly process with either of my machine tools but I decided the time has some so I acquired the Starrett level depicted. The certificate in the box asserts that each division on the tube represents .005″ difference in elevation over 0ne foot. This ought to do it.

Initial readings on the mill Y-axis ways (which are the ones that are part of the base casting suggested that my previous attempts with carpenter’s level were less than perfect. At least I didn’t buy this thing for nothing.

Fortunately, the stand has leveling feet so off I went with a wrench and a lot of bobbing up and down and cranking. It turns out that one of those .005″ marks represents about a half turn on a foot. Presuming that the other foot is on the floor… I need to develop a method that avoids trying to level an inadvertent tripod.

After a lot of faffing about, I got to about one mark of level and at least the back and front agreeing on which way that out of level goes. I called it done for now because I was chasing that mark around the corners. I need to do some more reading up but I suspect that this is where the shims come in. The top of the milling vise show just about the same reading so nothing inherently funny is going on.

Alternatives

There are methods of detecting twist in a lathe that don’t involve a level. Twist in a lathe bed manifests as a taper in what should be a parallel turned surface. You can turn two ends of a round bar and measure the result with a micrometer. You then shim whichever tailstock corner is low. Repeat until both ends come out matched.

You can also buy a pre-turned bar, mount it between centers and use a dial test indicator to compare ends. At least I think that’s how it works.

Presumably, you can carry out the same sort of process with a mill but given the way the mill table takes up most of the space, it would be a pain to say the least. You can’t trust the spindle or column at this stage so I think you would have to remove the table and sweep the ways with an indicator mounted on the ways. Suddenly, a fancy level doesn’t seem such a bad idea!

Options

This is one of those purchases that will get seldom used which is why I avoided it as long as I did. My “budget” model Starrett level was about $180CDN. You pay for greater length so my 6″ model is the shortest I could get away with. The 18″ one is about $1200.

There are digital machinist levels that cost more than the analog ones. I don’t know if they work as well. They claim similar resolution and I even found one online that has an app that relays the readings. That would be handy if you wanted to avoid jumping up and down to check as you adjusted things. It would be a huge time saver if you were levelling a CNC machining center, for instance.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s