Machine Tool Alignment

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Mar 12, 2008
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Ok Krypto asked me to start a new thread about this since a bit more detail was wanted and I was in reality starting to throw his own thread OT. And since I can't know just how much anyone here already does or doesn't know, this is going to get a bit detailed and long.

As a reference and to avoid repeating it, some of the basic information and PDF links would be here, South Bend 10K Lathe T-Slot Cross Slide I'd strongly urge anyone interested to read that first and at least scroll through the linked PDF's. Studying what's in them would of course be even better.

Specifically Krypto wanted to know how some of the test checks could be done on lathes and mills with more average or common metrology equipment. While these certainly aren't the only way to do them, I know they work and the test numbers and methods can be trusted if your careful how your doing them. I've also proved I can sometimes adjust and tune a machine tool to produce much better than the manufacturer probably even designed for just with the information I've found online. But this would sure be a whole lot easier if I was set up to film and post Youtube videos and could just post a link instead. :)

All I really started to point out in Krypto's South Bend cross slide thread was that properly designed and manufactured lathes aren't set up to face dead flat. For some reason not many seem to know that. Why that misalignment is present is in the link I posted for that thread. The slight built in misalignment's the manufacturer's would use isn't very much, and the proper amount of deviation is measured over fairly long distances. So I'll try and stick close to round numbers to make the concepts a bit easier to visualize. And my apologies to any metric members, I think and work in imperial a lot better than I do in metric.

For the most part, lathe leveling for parallel turning is pretty well covered. But let's say you want to check the accuracy of and how well a lathe makes a facing cut across the work piece. If I recall the numbers correctly, Schlesinger mentions a tool room lathe should face concave by approximately .001"-.0015" or so over 12", or to an accuracy level of about .000083"- .000125" per inch. In post #15 on Kryptos thread, I made a mistake and said a couple of 10ths, that typo was supposed to read as a couple of thou, but I can't seem to edit that mistake out. How Schlesinger recommends doing it, the 12" distance measured over a facing cut would require a work piece diameter of roughly 24" So pretty tiny numbers over a facing distance most of our lathes couldn't even swing. However its a geometry problem if you think about it, and I finally figured out a simple way of multiplying those numbers for our smaller diameter test pieces as long as you have a face plate or can chuck up a short piece that's very close to your lathes maximum swing. While I don't recall seeing it mentioned elsewhere, there's zero doubt others have figured out the same.

For any that haven't done so, and the manufacturer's manuals never seem to mention it, any new face plate should be treated as only partially machined. They all need a finish facing cut after there mounted to the spindle there to be used on, and most seem to need a trim on the OD which helps the balance a bit. Any factory supplied and finished face plate I've refaced showed at least some warpage or axial wobble. I've also met people who had no idea face plates could even be re-machined to make them a lot better. For this and to avoid that lathe leveling, I'll have to assume anyone reading this does know how to and has already leveled there own lathe beds fairly accurately. And for a later obvious reason, that your gibs are well adjusted to limit any play in the slides.

I'll also need to use an analog clock face to be properly understood for the cutting and measuring locations and in which directions I mean. With a large facing cut on something like a face plate, I'd figure my maximum rpm for that face plates outside diameter and for the material its made from. So logically the facing cut would start from the 9 o'clock position and running into the face plates center hole. And just in case, yes the carriage lock is real important on these facing cuts. But whatever preference you have for facing in either direction will still work. You also have to ensure that facing cut fully cleans up across the whole surface and leaves no un-machined areas if it's warped. If you've got it, a slow power cross feed would be the best. If you don't? Then a careful and balanced hand over hand rotation of that cross slide hand wheel to keep any cross slide deflections to a minimum is important. Then with that cut completed, now you can start to measure your results with a dti. EXCEPT if you run the indicator in the same path from that 9 o'clock position into the center, it will tell you nothing. Your then just following the exact same path the cutting tool did, and with a lathe of decent quality and condition, the indicator will or should just show a dead flat surface no matter how correct or incorrect the head stock or cross slide alignment is set up. Instead you want to withdraw the cross slide back towards the operator, position the indicator and magnetic base with the indicator tip just barely PAST the face plates center hole. Zero the indicator, now using the cross slide, slowly run the indicator tip from just past that center hole out to the 3 o'clock position, or towards the rear of the lathes head stock.

What that will do is actually double the test numbers for the concavity your lathe is hopefully facing at. If it's not obvious yet, it would be a good idea to write any of these test numbers down as your known baseline. If your lathe is incorrectly facing convex, the indicator numbers will reduce as the indicator travels towards the rear of the lathe, If it faces concave, the indicator numbers will slowly increase. How much it does over half the diameter of the face plate and in which direction are the important parts. And yes a straight edge across a large faced part will also show with a light gap that the lathe is at least facing concave. A dti can measure by how much it does. And it's also a pretty good test for how the lathe performs under real world cutting conditions. Static no load indicator checks don't always give the exact results you would expect when compared to those more real world cutting loads because the machine tool isn't showing any part deflections that are present under those loads. Unfortunately and so far I haven't yet figured out a simple method of checking the vertical alignment on lathe head stocks without using a test bar. Anything held in a chuck or collet is of course subject to how true and square those are to the lathes spindle. Moving any lathes head stock is the very last thing I might do, and only after I was 100% certain that was in fact the problem.

As I also said in that link to my post in Krypto's thread that properly aligned lathes should if the manufacturer did there job, come with the tail stock pointing slightly uphill and towards the operator by roughly .001" over that same 12". Pointing dead straight at the head stock while not ideal, might still be ok. Pointing down and/or away from you is pretty bad. Highly worn lathes will almost always point down because of the front wear on the tail stocks bottom way surfaces. After the initial bed leveling, one of the very first checks I run on any new or new to me lathe is to extend the tail stock quill almost out to it's full travel, lock the tail stock down to the lathe bed, and set the quill lock. Then use a magnetic base and indicator on the cross slide. Run the carriage back towards the tail stocks position, and set the indicator tip at the rear and top of that tail stock quill as close to the tail stock casting as you can get. Zero the indicator on the quills high point by running the cross slide in and out. Then use the carriage to move the indicator and check along the whole top of the quill. Any indicator deviation will show the direction the quill is pointing, either up or down. Do the same test against the front side of the quill. Those will give you some general numbers if there's any real deviations away from where the tail stocks quill should be pointing.

A far better test is if you can afford what are known as Morse Taper test bars. There's some off shore one's being sold now for much more reasonable prices than what I bought quite awhile ago. How good they are I can't say, but at the minimum I'd still test whatever I bought with a good dti and spinning the test bar in a couple of V blocks and checking for any runout. My test bars are guaranteed to 50 millionths or 1/2 a 10th. And mine do check out under that number. If it passes, then the test bar can check something a dti can't, how straight and well aligned that female morse taper really is to the tail stock quills OD. Yes it should be, but is it? Using those MT test bars is also shown in that Schlesinger PDF. But the initial rough check without an actual test bar will certainly show any major deviations that simply shouldn't be there. My old Sieg C6 lathe as one example showed a .009" upward tilt above the head stock spindle center line on its tail stock quill over just 2 inches. Add to that the length of a drill chuck and center drill, and that lathe from the factory broke the tip off any center drill I tried to use when it was in that condition. And that lathe is part of the reason I had to really go looking for information about proper machine tool alignment because it wasn't just the tail stock that was out.

And I'm out of room for post length, I'll add a part 2
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A far better test is if you can afford what are known as Morse Taper test bars. There's some off shore one's being sold now for much more reasonable prices than what I bought quite awhile ago. How good they are I can't say, but at the minimum I'd still test whatever I bought with a good dti and spinning the test bar in a couple of V blocks and checking for any runout.
I purchased a 12-14" length? cylindrical/MT3 test bar from Ebay seller a few years back. They are typically from India seller although seems to be a centerless grinder shop because most of their offerings were along those lines, various tapers & lengths. Maybe there are other sellers now, I haven't checked recently. I was not setting my expectations very high for the price. Amazingly the finish was beautiful & diameter within a tenth down the entire bar, or at least to what I felt my tenths reading Mitutoyo micrometer could faithfully display. And this was also measuring in different cross plane orientations.

I used the bar to do some checking on my lathe headstock & tailstock, my rotary table socket.... Anyway it got stored in its original box laying flat in foam. A few years later I pulled it out again to loan to a friend & inserted to my spindle with my prior notes in hand. After re-insertion & much fiddling to confirm, I'm pretty sure it bowed a couple thou over time. Maybe stress relief? The diameter was still consistent but axis appeared to have gone slight banana which is about as useful as a curved ruler. I didn't have a granite surface plate at the time but I do now. I've been meaning to put it in Vee blocks to see what I can see. From what I recall the 'real' ones are very expensive.

Looking forward to your next instalment. This is a favorite subject of mine, recognizing I still have much to learn.
My sincere apology's for going missing. Personal issues (sigh)

That really sucks about your test bar warping Petertha. Possibly I'm overly cautious, but I store mine coated in oil, wrapped in cloth and stored vertical inside PVC plumbing pipe with a glued on cap and the other end a threaded cap. It's basically air tight. Mine are U.S. made by a one man shop that I honestly can't remember the name of right now. I've probably had them at least 20 years now and so far mine are still straight.

The real issue about machine tool alignments are I think it takes a few sources that aren't that well known unless you've gone looking. Even then it takes a lot of thought to finally put all the concepts together well enough to make sense. So there's a lot of well meant I suppose but less than well understood ideas and methods being posted. I've read a few threads where people mention using those test bars to check a lathes head stock alignment before leveling and verifying the bed is in fact true with between centers test cuts. It can't be done because without that straight and square bed, any indicating on the test bars parallel section and using the carriage is following what's almost for sure a twisted bed.

There's threads showing test cuts being made using chucks or collets. Again any inaccuracy in either directly affects your results that may or more likely isn't a head stock alignment issue. I level as best I can, then use that to indicate and center the tail stocks Morse Taper bore side to side. With that correct, then I do the longitudinal test cuts. Every lathe I've leveled no matter how perfectly with a high end Mitutoyo machinist's level has still required at least some minor tweaking of the bed adjustment bolts to get the last 1/2 thou or so correct and turning parallel. I then indicate the top and front side of that bar and write any deviations from zero down. You may or probably will see a few 10ths deviation due to static loads verses dynamic cutting loads. That's simply the inevitable lathe deflections under those cutting loads. Any time I off set or want to check my lathe bed, or tail stock alignment and especially after off setting it, that bar can be used to bring everything back in to the same numbers that I know are correct. Obviously this doesn't work well on a highly worn machine, so a bit of judgement about what's possible is required.
People also forget it's not just about short parts and a lathe turning parallel at least well enough to work on those SC. Any twist in a lathe bed will seriously affect drilling, reaming and boring. As the tools get longer, any misalignment effects become a lot larger. On top of that, the carriage and bed wear unevenly. A lot of what I've had to learn was because of one particular C6 Seig lathe that never should have left the factory as poorly machined and ground as it was.

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