Interpreting these bearing blue patterns

Following Pete Barker's helpful article "Scraping ML7 headstock bearings" in the April 20 MEW, I decided to check my own ML7.

The 'shake test' he described showed 3 thou vertical slop. Hence a bearing bluing session was needed to investigate further.

This is my first foray into blue/checking fit of bearings. I followed Pete's instructions as closley as possible. Below are a series of photos (this post has taken ages to prepare!) Please answer list of questions - simple lanaguage please!

1) Why is there scoring at all?

2) Was the blue applied correctly? I finger on, then off, to get a thin smear. I understand too thick can give errors.

3) [pics 4+5] I'm concrend both transfers show ribbing - his pictures show a much smoother transfer. Is this because spindle journels are themselves ribbed - I did notice it's very slight with fingernail - but didn't want to bugger about with emery paper.

4) [pic 5] What is that clear band?! It's clean acros the whole circumference. Surely it's due to the highlighted lump. That would explain blue gathered around its base.

5) Following from Q4, I assume the lump should be scraped away?

6) Overall: Looking at these transfers, is any scraping needed? Can I jump straigh to shimming?

7) I struggled to know how tight teh caps hshould be. His "light finger tight" seems to cause both lower halves to have better transfers. Suggest caps weren't seated over spindle properly. Does this show I need to re-do with caps tigher?

Edited By choochoo_baloo on 14/06/2020 02:32:44

I tried this on my ML7 as well. Not as easy as I expected but a good learning experience. Here's a form thread about it. LINK

It's always hard to tell from photos but I'd say you could get away with removing shims and no scraping on these. Seems to be fairly even distribution of blue in both directions. If you only had three thou shake, the bearings are not too worn so most likely don't need scraping.

Looks like maybe a little too much blue on the spindle and maybe rotating the spindle too many times. Try one half turn rotation and see how that looks.

That one clear band could maybe use a little scrape right where that bit appears to be embedded or whatever. Also check the spindle there to see if there is something raised there.

Pete B.

The shiny bands are where oil is not remaining between the surfaces so you're not getting proper lubrication. You'll need to scrape those lightly, but imagine that you are scraping the divots in a golf ball - don't join them up.

The dimple in pic 5 is a result of the above - the spindle has picked up the bearing slightly. You'll need to scrape that little island down before you can get a sensible print. It doesn't matter if you crate a divot, just don't have that lump there or leave at atol when you've scraped it.

What blue are you using? Looks like it's got bits in it.

Having spent a lifetime as a mould and die toolmaker I have done my share of bluing the dies together. Over the last few years before I retired I found the "Stuarts" blue to be to wet and gave poor results. This was overcome by applying with a small brush with the bristles cut short, dabbed in the blue and then rubbed out on newspaper. You will get enough blue on the job this way to see what you are doing without false results.

It's in the nature of plain bearings to wear! Provided there's plenty of oil they spin efficiently because the metal parts float on a film of oil - there's no metal to metal contact.

Unfortunately, because plain bearings can't float on oil when the lathe is stopped, there's an awkward moment or two whilst stopping and starting during which the spindle grinds briefly against the bearing metal. Even though the shell is designed to minimise the effect, it's unavoidable.

Poor maintenance is probably the main cause of plain bearing failure. They wear rapidly if the operator fails to provide oil, or it's too thin or too thick, or the oil-ways are choked. Gross neglect might cause a bearing to seize; more likely it's working life is quietly slashed. A bearing that would have lasted 30 years suddenly has 5 years wiped off due to faulty lubrication. Not an obvious problem, but damage caused by lubrication failures adds up. Over-speeding and overloading is also likely to collapse the oil film.

Ball bearings were developed to address the shortcomings of plain bearings and they have considerable advantages. However, plain bearings remained popular on lathes for several decades because they're easy to make and vibrate less than early 'not quite round' ball bearings and even modern clapped out ball bearings. A plain bearing lathe wears smoothly, whereas rumbling rollers affect finish. The advantage has largely gone now thousands of identical perfect spheres, rollers, tapers and races are mass-produced.

Is a plain bearing a good choice in a Myford lathe? Yes. But no surprise to find signs of wear in an older plain bearing, even if it's been lightly used and well looked after. Are plain bearings a good choice for modern machine tools? Probably not - avoidable maintenance is always unwelcome when time is money.

Does Choochoo's bearing need to be scraped because of the scratches? I've no idea; my book is good on theory, much less help on judging when attention is needed, and when its best to leave well alone. Not smart to scrape bearings unnecessarily because scraping is wear too. If it ain't bust, don't fix it! I'd be inclined to shim, but it's just a guess. What do the experts think?

Dave

The lump needs removing for sure. It will be causing a false reading on the entire remainder of the bearing.

I don't think that you can interpret these blue 'prints' because of two problems. Firstly, the 'blobby' appearance of the blue suggests that the bearing surfaces are contaminated - probably with oil. Get the journals and shells clean and dry, before applying blue. The blue film on the spindle looks thin enough. It should transfer to the high spots on the bearing shells almost as if it's dry - certainly without blobs or signs of flowing. Secondly, 3 thou is a lot of movement. This means that the spindle can rattle around when trying to get a print of the blue, so you are unlikely to get meaningful results. I suggest you start by taking out a shim (or two), so the bearing clearance is as small as possible, whilst allowing the spindle to be turned.

If the spindle surfaces are truly round (let's assume they are, to minimise complication at this stage), and the spindle's journals have been evenly blued, you should turn the spindle only about 1/8 turn, otherwise there will be more smearing than necessary, making interpretation difficult.

When you can get a repeatable blue print, you can start trying to make sense of it. No-one said it's quick...

Answer to questions 4 & 5 is 'yes'. I think we can be sure of this, even at this stage.

Shim removal will certainly be required, because you're going to scrape this, aren't you? - if only to find out how it's done, and 3 thou vertical play is excessive, even before scraping. Get a shim out, then start to fettle the bearings.

Edited By Kiwi Bloke on 14/06/2020 11:03:01

Thanks chaps. I am learning quickly! Some follow on Qs:

1. I agree there is probs oil residue on both shells and journals causing blobby. I tried kerosene and blue tissue. Seemed dry to the touch. Is there a better method?
   
2. Is it a good idea to both journals give a quick rub with emery strip anyway, just to knock back the tiny ribbing I felt with my fingernail (I'm cautious before diving in)?
   
3. Set the caps down was the [art I sturggled with most. Should I measure current shim thickness then minus say 2 thou, then nip up allen bolts to equal gap on both sides via feeler gauge? I suspect the caps were a bit sloppy. [Pete Barker says "light finger tight, but to measure gap with afeller gauge, which I found a tad ambiguous.]

Brake cleaner spray can will clean them up with no residue. Or petrol if you prefer to live dangerously. Or maybe even soapy water too. But Kero will leave an oily residue which might explain the beading in the blue in your pics.

Yes now is the time to gently smooth the tops off the ridges on the spindle with a strip of fine emery cloth wrapped 180 degrees around the spindle and the ends pulled back and forth as you lightly work it around the diameter. Just a quick once round should do the job. You don't want to remove much metal, just the very peaks. (Purists will call for crocus cloth here but its a steel shaft so abrasive embedding is not going to happen unless you grind it in hard under pressure.) Then clean the shaft afterwards to make sure no abrasive is left on there. Brake cleaner etc again. And paper towels. Repeatedly. And don't use any abrasive cloth on the soft white metal bearings themselves. Only the hard steel shaft.

Tighten the cap screws down by putting the long end of the allen key into the screw and turning the short end with finger pressure only - not the palm of your hand -- so the bearing is only just snugged up to the shaft but shaft is free to turn and the bearing blue is not squeezed out under pressure. No shims in place. You can if you like measure the gap each side with feelers to make sure the bearing cap is sitting on straight and not canted to one side.

Edited By Hopper on 14/06/2020 13:25:21

If these are white metal bearings, I would avoid use of any abrasive. It is all too easy for grit to become unintentionally embedded leading to premature shaft wear.

When I was young and didn't know any better I received a real tongue lashing from the fitting shop foreman for working with abrasive paper at a bench where a fitter was working on freshly made white metal journals for a marine diesel engine. I got a few black looks from the fitter as well. As penance I was given the job of overhauling the cylinder heads for the same engine. The foreman, with great ceremony, handed me a big tub of grinding paste, since I was so keen on abrasives, and sent me to the furthest bench away from the bearings. SIxteen cylinders worth and four on-board spares, each with four 100mm diameter valves to be ground in or replaced gave me time to reflect on the subject of abrasives. I had a couple more ships worth of this activity before the mantle (of shame) was passed to another unfortunate who had earned the dubious honour though engineering misdeed. I was cured of any passion for abrasives

I was eventually allowed to assist in making and fitting plain bearings, under the tutelage of the fitter who had given me black looks several weeks before. I was eventually ured of any passion for engineers blue as well.

choochoo_baloo, I've PMed you.

Evening all. Tonighted I implemented the above helpful suggestions. Specifically:

1. journals lightly & quickly rubbed over with 600 grit 3M paper
2. journals and bearings cleaned with degreasant, then wiped out each time with a dry microfiber cloth (no oil residue!)
3. Stuarts ("Micrometer" from the tin) blotted on newspaper
4. 4 screws done up tight. Spindle seized. I then backed them of by 1 thou via a feeler gauge. Spindle turned smoothly.
5. Half spindle turn for print.

The bottom halves look near perfect. Scraping that island on RH did the trick - no streak.

So I restricted my later checks to top halves only. Here are 3 repeats. Separations set to same feeler gauge each time. Journals re-blued + bearings wiped clean each time.

You'll agree LH bearing never gave a good print. Why would just 1 of the 4 halves wear more? I realise it's diffciult to diagnose by photos, but given these prints, and crucialy the very good overall condition of the lathe, I'm reckon, a little scraping on left top half only.

Please comment chaps.

Forgot to mention one thing. I drifted the top LH bearign from its cap at the start of all this whole process (I thought a previous owner had flipped it around since the oil hole is off centre. Turns out no differnce, so soft hammered it back).

Could this have inadvertently canted the bearing, thus explaining why it's giving the wrost reading?

It could well do. You could try taking it out again and cleaning thouroughly then reassemble. Sit the flat surfaces on a solid bench or suitable block supporting the insert and tap down on top of the bearing cap with a soft hammer.

Again it's hard to tell from pics. But looking closer at your pics on the larger screen of my laptop, it looks like there is some blistering of the whitemetal at the lower left corner of the cap in the pics? Hard to tell from pics if its raised up or just discoloured.. But right where you have written #9 on the first pic looks like the whitemetal is blistered from overtheating maybe? Is that the area you had the burnished band and apparent embedded bit of metal in the other pics before?

That surface needs to be flat across there for the bearing and cap to seat correctly. If those blisters -- or whatever they are -- are raised up they could be stopping the cap from sitting down correctly and thus canting the cap to one side. Try running a smooth flat file across the surface and knock down any burrs etc. Same on the lower bearing housing mating faces. It would [pay to run a smooth file over all the mating surfaces anyway, to make sure a burr is not jacking that end of the cap up out of line.

Also, if there is some kind of meltdown damage there, it could also have affected the outer diameter of the whitemetal where it fits into the upper bearing cap. Similarly, it might need any "blisters" easing down there very carefully with a scraper or even a small file. (Never abrasive paper on the soft metal.) Did you notice any when you had it off?

You could also try cinching the cap down with the shims in place and see what kind of reading you get. Could be the bolt pressure pulls the cap down into position. You should get a reading of some sort if the clearances is not excessive, even if you have to lay the blue on a bit thick. If you have the factory shims, you can peel the layers off about two thou at a time.

The rest of the readings don't look too bad. You could try with a little less blue and a bit less rotation of the shaft to get a fine reading. But apart from the one area you have pointed out, the rest look like they would be best left alone if you are not an experienced scraper of bearings.

Edited By Hopper on 17/06/2020 02:42:10

You blue is too wet to get best reading results. Put some blue onto a paper towel or a piece of printer paper. The paper will wick away the excess oil. Then you will get a much better read of where to scrap. As you have it, it can wick to an area and create a false impression. You are on the right track with taking off only on the very tight spots.

Neil

I'm on my third interation of top LH bearing scraping. Wil report back later - almost done.

@Hopper there's one passage in your article I don't follow:

At the end [of scraping process], scrape a little extra clearnace about 1/4" either side of the shell join lines to provide oil space on these near verticle surfaces, where shimming will not add clearance.

Is this what you mean? What about the RH bearing with its existing pockets?

Edited By choochoo_baloo on 18/06/2020 13:37:30

Its just that if you scrape a bearing to fit closely all around the circumference, then you add a one thou shim to lift the cap up and provide clearance, it is lifting the cap vertically so it does not add any clearance for oil on the sides of the circumfrence. So A very light scrape in that area will ensure a bit of oil space. If you yours already has a recess there, may not be needed. One of those many things with the art of scraping that needs a bit of judgement case by case. I wouldn't worry about it unless you have scraped the whole surface down some and changed the original shape. What you don't want is to add extra unwanted sideways movement.