Machine tool alignment Part 2

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Mar 12, 2008
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Ok lets say your tail stock does pass as pointing in the correct direction and within the allowable limits, or at worst, even straight in both axii during those initial rough checks. With any lathe that has an adjustable tail stock and for my usual order of operations, I'd now center the tail stocks female taper to the lathes head stock center line. For any that haven't yet experienced it, gravity and indicators in the horizontal orientation do not play nice with each other. And gravity always wins. You can attach a magnetic base and dti indicator to a spindle mounted lathe chuck or face plate and rotate it by hand with the indicator tip adjusted to sweep the tail stocks female morse taper. For any that don't already know it, what you CANNOT do with that method is accurately check the vertical center line alignment to the head stocks center line. And for most dti types, you'd also need to read the indicator as it's sweeping either the bottom or top of the MT with a mirror and depending on which direction its orientated in. Any extension out from that magnetic base and no matter how light you think your dti is, the assembly WILL deflect an unknown amount during that top to bottom rotation. The very first time I got caught by that gravitational effect, I spent a couple of hrs before I finally realized what was really happening. But more about a cheap to do check for that tail stocks vertical alignment in a Because gravity works as a constant force and directly down, you can still use that magnetic base and indicator to check the tail stock bore for it's side to side horizontal alignment to the head stocks center line because gravity isn't affecting the magnetic base and dti assembly side to side repeatability. A Co-Axial indicator might be easier, and its what I now use because if you use a short probe with those, it can check both the horizontal and vertical alignment and still view the dial the whole time.

At least for my Blake, it has a stiff internal spring that's well above the effects of gravity. But a dti will still work for that probably much more important horizontal side to side zeroing. So how do you check the quills vertical is correct without a Co-Axial indicator? Measure your tail stocks outer quill diameter very accurately, write that down, now grab any short scrap of metal that's a bit larger in diameter than your quill diameter. Chuck it up, accurately face and then turn maybe a 1/2 inch long section of it's outside diameter to exactly replicate the diameter of your tail stock quill. Within a 10th or two difference if you can manage it. Now move the carriage out of the way towards the head stock, slide the tail stock up, lock it down and extend the quill until it just barely kisses the face of the part in the chuck. Remember to set the quill lock. Now use a micrometer to measure across both OD's. The micrometer anvils should span across both the machined scrap and the tail stock quills O.D. Any size difference due to misalignment will show up as a number larger than your quill diameter. If or which way the tail stock quill is either high or low can be felt easily just with your fingers. Or you can also use a more direct method. That same magnetic base and dti on the cross slide. Run the indicator tip across the top of either the now machined scrap or quill until you find the high point, then do the same for whichever part the indicator wasn't zeroed on. Any deviation high or low of where the tail stocks quill elevation is to the head stocks calculated center line will show up as a direct number. And yes dti's are known to not be all that accurate when making linear measurements, but as long as it's only a few thou of height variation, it will still be a pretty good indication of where you are. If really you don't trust it, then use the micrometer.

A tail stocks vertical alignment as long as its within reason, is mostly helpful for accurate drilling and reaming without bell mouthing the hole entrance. Visualized correctly for how direction to the head stocks spindle center line will create a tapered shaft by that much even if everything else is perfect. There's also a one further issue worth at least knowing about, but it still may be something impossible to fully solve. Even Monarch Lathes 10EE's that were well known as one of the worlds best tool room lathes show unavoidable deviations in the exact side to side center line location as the tail stock is moved to various positions on the lathe bed and then locked down. For whatever reason and no one I've ever seen yet has explained exactly why, it happens. It wasn't much on the EE's, about a maximum of .001" allowable variation for the full bed length on the EE test certificates I've read. So just because you get the tail stock perfect in one bed location, that may not be true at other positions on the bed.parallel turning happens, a vertical misalignment will have very little part size effect or create a taper along a shaft held between centers unless it's seriously excessive. A horizontally misaligned tail stock is completely different, every thou its out in either direction will produce a taper in a shaft large or small at that end depending on which direction the off set is.

Besides that lathe spindle alignment, there's some checks to run on the spindle nose and even chucks. Pretty simple really, all your testing is the important working faces. The spindles morse taper run out, any chuck register faces or OD's. I've seen people on Youtube indicating the outside diameter of lathe chucks as if that would mean anything. It doesn't because no part will touch or reference off that surface as its being machined. So unless the lathe spindle or chuck mounting surfaces were so incorrectly machined that severe vibration were already evident, checking a chucks OD is just about pointless. What surface is important would be the chuck and face plate faces. Those are what a lot of work will reference against. So any axial run out showing up on those faces is important. And that's worth remembering to check whenever you machine a new chuck back plate and bolt a chuck to it. Much tougher to check properly are the chuck jaw faces and ledges. Ideally they need to be preloaded in there working position. How chucks are preloaded and then ground would give the information about how its supposed to be done. To be honest, I'm not fully convinced those are worth checking unless you have a tool post grinder to make any corrections if needed.

Just so there's no confusion, there's many methods out there that get mentioned all the time. For example the Rollies Dad's Method. (RDM) or what's called a Two Collar Test, (TCT) Either of those don't and aren't meant to check any of what I'm trying to detail, that RDM or TCT are used to either check for lathe bed twist or help make lathe bed adjustments until there is no bed twist. There a couple of different methods to hopefully get a lathe bed square and parallel without the expense of a high end machinist's level. So for any method, you first need to fully understand what it's testing and what it can or can't check. And the less you understand about machine tools, the more careful you need to be about what your trying to test and how you do so. And yeah all this sounds like a whole lot of time consuming work. It really doesn't take that long and after there done, it will be many years before they need doing again. With known to be correct alignments, then the usual lathe leveling checks once in awhile are all that's needed. What isn't nice is finding out any machine tool isn't quite as good as you assumed. When I knew a lot less, I thought my machines were always a lot better than they were.

I've also not mentioned what to do, how to fix or with luck, possibly adjust out any serious misalignment's that might be found. That would take knowing what the exact problem is, which part of the lathe, and maybe even the options it has. Taper turning attachments for example have a set of alignment tests just for them. And making hand scraping corrections are completely outside what I want to get into here. I also know zero and can't help about anything cnc or even electrical related. For now this is already way too long, so I'll do a separate thread for how to check a Bridgeport type knee mills alignments because there's a whole lot more to it than just tramming the head in on any mill. Most of that can still be directly used on any round or dovetail column mill. I also can't stress enough about actually studying that Schlesinger PDF until you do understand the reasons behind and why he's recommending the test methods he does. I had to work at it until it finally started to click into place. It's also not only about lathes, it shows the accepted test methods for most machine tools any of us might have.

Fwiw, I'm sure not needing any attaboys, and I have real thick skin. So I can post here or not. But if people here don't want threads of this length and depth, or I hope might be getting at least something helpful out of what I've tried to explain, then whatever negative or positive comments you have are just fine with me.
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