|not done it yet||27/05/2022 22:01:54|
|6809 forum posts|
I still don't really get why if a bearing is made to the spindle it will be used with it needs "scraping in".
This is an easy one. Precision and longevity. Replacement bearings may need to accommodate less than perfect shafts. Can you guarantee your shaft to the n th degree? Mass produced shafts, mass produced bearings over umpteen batches over many years of production - some may match.but likely not all. Particularly replacement bearings, where wear has obviously taken its toll on the original bearing (which may have been changed previously.
Even with automotive engines, parts are chosen to match. Pistons (weight , for balance), bores (different ‘grades’ of pistons fitted), piston protrusion (different head gasket thickness for compression and/or not contact when running), bearing shells (particularly for competition engines), valve seat inserts (for correct valve recession) just as obvious examples.
Regarding running in - some items, in the past were ‘run in’ by tightening the bearing caps to such an extent that the bearing surfaces almost melted - then clearance shims were fitted. Those bearings were most certainly ‘relieved’ on the sides to allow lubricant to be picked up as the shaft rotated. Likely lightly scraped to provide oil capture/retention, too.
These processes were clearly needed and understood by both the manufacturers and the engineers of the day. Most motor mechanics became just that - mechanics, not engineers.
Although the average engine may only need a fitter to change service items, the engineering (to allow that) has progressed to that point over many years.
How many physically diagnose a problem these days? Not many. Fitters, to simply substitute old parts with new items, are cheaper than proper engineers. Most modern vehicles use computers to diagnose faults, so technicians can just look up any ‘fault codes’ and fitters can then substitute parts required. Good, but only to a certain point.
Think here of something like F1 technology levels where sensors, transmission and data processors are of paramount importance for absolute optimal performance of the unit for its short lifespan under very arduous conditions of operation. Your lathe is unlikely to perform to the same exacting standard as a F1 engine!
|Nicholas Wheeler 1||27/05/2022 22:21:18|
|930 forum posts|
One thing to consider is that the definition of 'fitter' has changed for the worse: now it means fit a new part out of a box and hope the job doesn't come back, instead of make/modify/adjust a part(using skills developed over several years) so that it works correctly. Engineer is another overused word - the man who can't diagnose, let alone repair your boiler isn't an engineer. He doesn't fit the definition of mechanic either, so we get the deliberately vague technician
6393 forum posts
Surface finish and alignment.
New bearings have bored holes up the middle. Rough as guts by tribological standards. Scraping removes the high spots and promotes consistent oil film thickness.
When you have two bearings like on a lathe, the outsides as well as insides are a machined finish. They may or may not be perfectly concentric and aligned. Scraping makes sure they are.
No idea if Myford scraped on new bearings at the factory though. If they were precision made bearings they might have got away with running as is. Seems like a typical Myford cost saving. They were a low cost hobby lathe in their day.
|Bill Pudney||28/05/2022 00:44:54|
|611 forum posts|
White metal bearings were involved a long time ago when decent ball or taper bearings hadn't been invited. The shells used for the use of engines (car, motorbike etc) require a VERY accurate and ground journal, they should not be scraped "in", they simply require that they are fit in the correct recess and have some appropriate oil. I think that Tony Vandervells concern invited them in the 50s.
Most good quality machine tools these days involve ball, roller or taper bearings, some involve ceramic balls, these are capable of very high r.p.m They can be eye watering cost, because the Careful Concern Company will use laboratory type clean room to achieve the cleanliness involved. Probably not a clean room were they could process surgery, or make electronics, but very very clean nonetheless.
It was my task to scrape in a 10" split front bearing, and a 8" back split bearing, from memory it took over a week, too get it fitted. But then it was VERY slow, maybe 100 r.p.m.
Which by the way is why "Fitters" are called "Fitters", because the make precision things"Fitted", not just use a new part in a cardboard box.
|Howard Lewis||28/05/2022 09:04:07|
|6104 forum posts|
The surface finish of the shaft is important. A rough finish will allow oil to drain away, allowing metal to metal contact, leading to failure.
In automotive engines with pressure oil feed, the steel backed shell beatings, of various compositions, (Aluminium/Tin, Aluminium/Silicon etc ) with normal clearances, the shaft needs to be dimensionally correct to within 0.001" ( 0.025 mm) on diameter and have a primary surface finish of no more than 16 micro inch Centre Line Average.
At 18 CLA, the shell will begin to show a polish
At 20 CLA the bearing will begin to wipe (Localised melting ).
By 22 CLA failure is certain!
With Nitrided, or Tufftrided shafts, rather than Induction hardened, these limits become even more important.
Once failure begins, "healing" is MOST unlikely, usually the damage increases exponentionally. Often the root cause is impossible to find, because the damage to shaft, bearing, and housing is so extensive. Often the steel shell, if any is left, will have been blued and hammered to a thickness of less than 0.001" The whilte metal will have been distributed by the pressurised oil all over the surroundings, in minute flakes..
The "White Metal" bearing needs to be of similar finish, a variation of 0.00001" (0.00025 mm 0.25 microns! ) will show itself as a polished band on the surface of the bearing surface. More than that leads to failure..
This kind of failure can happen at speeds of 2,000 rpm or below, with rubbing speeds of as low as 1,500 f p m..
So shaft and bearing finish and clearances need to be correct to minimise the risk of wear or failure.
|77 forum posts|
As I am not likely to be machining parts for Red Bull Racing or the aerospace industry any time soon and this lathe runs at maximum speed of 640 RPM I am going with Old Marts method.
I am experienced in engine repair and assembly but machining is just a hobby that I am not so good at.
Due to my stupid comparison the engine side of the discussion has really taken over and that was not really my concern but thanks for all the contributions.
|Howard Lewis||28/05/2022 10:25:27|
|6104 forum posts|
Don't worry, Ivy
We all learn as we go along, every day.
Experience allows us to recognise the mistake, the next time that we make it!
With a low speed, drip fed, fairly lightly loaded application you can afford to minimise clearances.
When Sentinel made expansion engines for British Oxygen, the large whitemetal bearings used to be hand scraped. Being a big, single cylinder engine, the pile of swarf on the floor was big!
For accuracy you need the spindle to be kept as closely aligned as possible anyway.
Edited By Howard Lewis on 28/05/2022 10:28:40
8691 forum posts
A chap on youtube eh? You can always trust the internet!!! Old Mart is much closer to the mark, his method is respectable. He tests and adjusts.
The problem with Youtube's 'he showed the results of the change including running temperature at the bearing caps' is it proves nothing. Even a sick bearing can pass that test. Unless a bearing is very bad, it's performance isn't measured by running a machine for a bit. The real test is how many running hours the bearing can do before it wears out and has to be replaced. Years.
Well fitted bearings last much longer than badly fitted ones. Takes time for the damage to appear - see Howard's post - and for a long time it's invisible, Bottom line, it's easy to fink a new bearing has been fitted satisfactorily, when actually it's a bodge.
It may not matter! As machines go lathes are lightly loaded and slow. And most hobby lathes aren't run for hours on end day after day. So if a bodged fix lasts longer than the owner he will never know or care that he messed up! He might even post a convincing internet video suggesting he knew what he was doing.
Kawsaki motorbikes are different! They're are ridden hard by millions of customers and fitting poor bearings soon becomes bleeding obvious due to excessive warranty failures and loss of confidence in the product.
|77 forum posts|
A chap on youtube eh?
I didn't want to name him without his knowledge. There are lots of youtubers that really know their stuff and are happy to show it and discuss.
You can always trust the internet!!!
Isn't this the same internet?
|Tony Pratt 1||28/05/2022 11:50:01|
|1963 forum posts|
If you post on YouTube then it’s in the public domain as intended, I think we are talking about Steve Jordan?
|old mart||28/05/2022 18:41:49|
|3772 forum posts|
Ivy was right about starting an argument, wasn't he.
|Pete Rimmer||28/05/2022 18:48:57|
|1233 forum posts|
TBH I would not put too much weight on advice given by someone who demonstrates lapping in cross slide ways with loose abrasives. It might make good sense to the unwitting, but lapping is the very last thing you should do to finish any moving way.
8691 forum posts
I'm not thinking of anyone in particular, just noting that perhaps the majority of internet videos are flawed. It's because video makers mark their own homework and are easily pleased! As any fool can post a video on any subject, or he might be an 'influencer', it's best for viewers to watch critically. Question everything.
The best internet videos are peer reviewed and go through some sort of editorial process. Most don't.
The forum is different because it allows question and answer. Whenever anything is posted here that's ambiguous or wrong, other members can and do point the issue out. As a result the quality of forum advice is generally high, but you do have to read it all carefully and make your own mind up!
|77 forum posts|
SillyOldDuffer / Dave.
I understand what you mean about some youtubers and I don't automatically assume the worst or best in people that's one of the reasons I asked here. Some don't communicate very well or at all, some are brilliant at editing, with all the doubt that adds and some make excellent teachers for the likes of me.
I acknowledged the bad comparison I made with the IC engines but unfortunately that took over and started to come across as a, "I know more about engines than you" competition. And I also know from other forums (motorcycle related and predominately American} that they can be very cliquey if you don't blindly follow the script.
My requirements are very crude by some standards and although Old Mart is the only person to offer an alternative to scraping I am sure his method will serve me more than well enough for the rest of my life. Fortunately I have another lathe that turns out good work by my standards so nothing ventured nothing gained.
|not done it yet||28/05/2022 21:14:33|
|6809 forum posts|
If the YouTuber was the person that Tony suggested, I would, like Pete, not take too much notice of his videos.
His main application seems to be as a paid ‘influencer’ to persuade people to buy a particular brand of rubbish. A lot of the lower 50% are likely impressed by his reviews.
|Neil Lickfold||28/05/2022 23:45:42|
|862 forum posts|
My Myford headstock is fitted very close to its bronze tapered bearing. The scraping like been said allows for the capillary action via the wick to lubricate the bearing. Its clearance is just enough for a very thin film of oil, that it actually runs on. Model enigins by comparison have very large cleanaces for the pin diameter, from 0.04mm to 0.08mm on diameter clearance on 5 to 6 mm pins.
6393 forum posts
He is the guy who machined the top off his topslide to accommodate a different toolpost etc then when commenters pointed out he had weakened the toolpost stud anchoring area, he built up the lower side of the area with a dollop of JB Weld to "strengthen" it again.
I think his knowledge and experience are limited so I would check around a bit first before assuming an unscraped set of bearings is OK because they are not running hot or noisy and he says so. Five generations and more of millwrights/fitters have not been scraping bearings for the past 200 years for nothing.
There is real danger in getting engineering advice off YouTube. I have seen some shocking clangers, including by some of the most reputable on there. EG Blondie sticking a dial indicator on a job right next to the set up fixed steady to "check it's running true". Lots of Harley "gurus" doing the same thing with flywheel truing stands, putting the dial indicator on the shaft right next to the supporting bearing instead of on the end of the shaft. And the lathe chuck rebuilding guy who flogs the back plate off his chuck with a shifting spanner and screwdriver instead of using the provided jacking screws and tapped holes. When queried, his response was he used the shifter because his hammer was outside in the back os his truck. And even lifelong skilled machinists like Joe Pie do things like blow swarf with compressed air -- enough to get you sacked some places I worked at. So it always pays to shop around for info.
Edited By Hopper on 28/05/2022 23:56:18
367 forum posts
My rough calculator for how much knowledge/skill a ytuber has is to look at how many glossy colorful images he has on the front of each video.
If it looks all splash and colorful, like some sort of storefront display, with lots of shiny bits, that seems to indicate a rank amateur who is only there for the views.
The more shiny videos they have, the less knowledge they seem to have.
Its like some sort of linear relationship in math(s).
Some of the least visually impressive channels have some of the best technical material.
I guess it is a matter of whether you focus on what your videos appear to look like from the outside, or the depth of knowledge your video actually contains.
Reminds me of what we use to call "sleeper cars" in high school.
The bright and shiny cars were always were the slowest.
The cars with somewhat of a rough paint job, and a few dents here and there were the ones to watch out for, since they often concealed a 500 hp Hemi or something like that, and would leave you very quickly in a very humiliating cloud of dust in an impromptu street race.
Edited By PatJ on 29/05/2022 01:00:34
|77 forum posts|
There is real danger in getting engineering advice off YouTube
Trouble is as an amateur hobbyist with no more than CSE metalwork for experience and no access to real world help I watch lots of youtube and read forums if I get stuck on something. The alternative is to give up.
Someone mentioned Blondie, I think she is great, may not be text book or 100% in everything she does but always says, "this is how I did it", mistakes and all and that is a great help. This old Tony and Doubleboost are great entertainment too.
Edited By Ivy on 29/05/2022 07:54:43
|not done it yet||29/05/2022 07:52:40|
|6809 forum posts|
One particular instance I remember was of some idiot using a cheap plastic sandwich box as an enclosure for mains electrics - and with no cable restraints AND locating it outdoors!
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.