Part built Allchin 1.5 inch

[derek blakeParticipant @derekblake72550]

Hi Paul

the crank has a tiny wobble, and seems to wear brass or bronze bush after an hour or so, the cranks is a one piece crank.

I fit new bush and seems ok but get worse and worse.

very frustrating….

[derek blakeParticipant @derekblake72550]

I’ve just read that cast iron makes a far better bearing than bronze or brass, but not sure if it will help me,

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Paul Kemp on 31/05/2020 23:37:11:

[…]

What is the nature of the wobble on the crank? Is it built up or machined from one piece? ..?

.

Not in my comfort zone, Paul, but … curiosity led me to this photo from Derek’s album:

**LINK**

https://www.model-engineer.co.uk/albums/member_photo.asp?a=49637&p=818310

MichaelG.

[derek blakeParticipant @derekblake72550]

Any suggestions on how to fully true up the crank shaft or us this near on impossible not it’s finished? I’m concerned that I make new bearings but because of the runout it egg shaped the bearings yet again.

i hasn’t measured the run out/wobble yet but it’s very small, maybe this is normal and acceptable?

kind regards

Del

[Paul KempParticipant @paulkemp46892]

Derek,

Looking at the picture Micheal kindly linked too there are two possibilities that occur to me. Either it was cut form flat bar and machined or it was a casting. I am guessing this came ready made when you got the engine and you are not aware of its origin? I am guessing that originally it would have been turned between centres and if it was a casting it would have had a support cast in opposite the big end for turning the outer journals which would then be cut out to set it over to turn the large end. If it is out of true there are a couple of possibilities, if it was a casting the outer ends were finished to size, the ligament cut and at that point stress was released that induced a bend. If cut from bar stock possibly there was too much pressure from the tailstock when the large end was turned that induced a bend.

The crank for my half size (many times larger) was an SG iron casting. Order of play was to put in the two sets of centre drillings in the end blocks, set between centres and rough turn the outer ends and leave for a few weeks to relax. Turn the outer ends almost to size, cut out the ligament and set up between centres with wood blocks between the webs and the end blocks to give support from bending and finish turn the large end journal. Then reset to the original centres with a packer between the webs to support and skim the outer details to finished size. It did change shape very slightly between steps (after leaving to relax and after cutting out the ligament) but following that process meant the final item was straight and true.

How you can or can't deal with this depends really on the material of the crank, if it's cast bending it back isn't likely to be an option, cos it will likely snap! If it's bar stock you have more chance.

First thing is to identify against a clock where the error likely is. If the run out is all in the plane of the large end heating and tweaking if it's cast might correct it. Very first crucial thing to understand is exactly what is wrong with it, then you can make an informed decision on how or if to correct it. If it were mine I think I would set up a couple of thin vee plates at the bearing locations and rotate against a clock and mark the high / low spots at the ends or set between centres and clock the bearing locations to get a picture of what is going on.

Paul.

[Michael GilliganParticipant @michaelgilligan61133]

An important detail is missing from that photo, Derek … So, I must ask:

Are the two ends of the crankshaft centre-drilled ?

A job like that should have been turned between centres: But it may be that the original builder was over-confident about what they could do in a chuck.

MichaelG.

.

Edit: Good to see that Paul got-in with some informed advice whilst I was typing 

Edited By Michael Gilligan on 01/06/2020 12:36:26

[Paul KempParticipant @paulkemp46892]

Michael,

it may have been turned between centres even if there are no centres! It's usual for cast cranks to have blocks or webs at the ends to take the various centre drillings, once machining is complete they are cut off! I have put the centres back in after as they can be useful for other ops like tailstock support for spline milling but some may not do this.

Paul.

[derek blakeParticipant @derekblake72550]

Hi Guys

thanks very much for responses, it has holes drilled in the end so must have been done between centres.

i think I need to set it up as Paul suggests to get an idea how far out it is, I’ve never noticed the run out before which is odd.

is it normal to have any sort off run out or would all cranks be totally spot on?

[derek blakeParticipant @derekblake72550]

It’s possible it runs out by 0.1mm but I need to confirm if I can later.

[Paul KempParticipant @paulkemp46892]

Derek,

Depends who made it and how well! Normally you would expect a crank to be "spot on" certainly within half a thou. However they are tricky things to machine being spindly and long and easy to put a bend into. Machining the large end needs a long tool overhang with a fairly slender tool so it's easy to get chatter, dig ins etc! It's a case of sharp tooling, right speed and patience. Also of course adopting the right sequence if it's cast as it's likely to have internal stresses which can be released as machining progresses, hence the roughing and finishing processes and setting aside during for it to relax and find its line.

Paul.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Paul Kemp on 01/06/2020 12:37:45:

Michael,

it may have been turned between centres even if there are no centres! […]

.

Yes, I realise that Paul ; but it seemed a reasonable first line of questioning.

… Happy to back-out of the discussion now.

MichaelG.

[derek blakeParticipant @derekblake72550]

Thanks Paul, all very interesting! I think I will avoid ever trying a crank!

looks like mine could be far more than half a thou out, I will attempt to check tonight.

[derek blakeParticipant @derekblake72550]

Michael, I’m very glad if you have any ideas how to remove a few thou run out?

i know in the book it says to make the crank from bar stock, but I’m unsure how mine was made.

[Paul KempParticipant @paulkemp46892]

Derek,

Maybe check commercial suppliers and see if anyone offers a cast crank? If not you can be confident it is probably bar stock in which case it's a good candidate for straightening. If you have a decent size lathe, stick it between centres, clock it, put the clock opposite the bend, lump of brass in the tool post and give it a tweak with the cross slide. Don't do this in a watch makers lathe though! A good size lathe should be fine.

0.1 (0.004"&nbspis too much I would say!

Michael,

Wasnt decrying your question re centres, not everyone appreciates how the throw and the centres are achieved in the first place. Just pointing out that whatever centres are present now may not be those it was machined on. In fact as later additions the may not even be "in the centre" depending on how and why they were put there lol.

Paul.

Bloody smiley thing!

Edited By Paul Kemp on 01/06/2020 13:07:00

[derek blakeParticipant @derekblake72550]

Many thanks, I shall give that a try 👍

[Michael GilliganParticipant @michaelgilligan61133]

My first move would be to mount the crankshaft between dead centres, and measure the bearing surfaces as thoroughly as possible [taper, run-out, eccentricity, surface finish], so that you understand the magnitude of the problem, and where it lies.

BUT … As Paul so rightly emphasised, this pre-supposes that those centres are nominally the centres of the main bearings.

MichaelG.

[derek blakeParticipant @derekblake72550]

Hi Michael

thanks very much, yes I agree with the centre holes.

i will set it up in the large tonight and try and work out what’s wrong.

the flywheel runs beautifully true so maybe it the opposite end and the end close to the bearing that’s wearing.

[derek blakeParticipant @derekblake72550]

Hi Guys,

well to be honest I feel like I’ve wasted four hours of the precious evening on the crank shaft.

i feel it was around 0.4mm out of true and I feel it’s around 0.1mm now. It’s soft like cheese you push just ever so slightly and you go miles out but I guess we are talking small number.

i don’t think I can get it any better, I know it needs to be but it’s beyond my skills to make it any better.

im tempted to pay for a newly made crank shaft to solve the issue but there maybe no machine shops open presently.

i will make two new bearings once the materials arrive but if it knocks after that I think I’ve exhausted all my skills let alone patience 😩

[Paul KempParticipant @paulkemp46892]

Derek,

Hardly a waste of time old chap. Firstly you now know more about making crankshafts than you did this morning and you are some way to understanding what may be wrong and even to a degree putting it right. Shaft straightening never mind crank shaft straightening is an art and not something you walk into a novice and 5 mins later emerge as an expert.

Assuming it was 0.4mm out and now it's down to 0.1mm then you have a 75% improvement, so for four hours, a bit of head scratching and some tinkering surely that's not bad? I would say that 0.1 is still quite a lot on such a small crank but it's a lot better than 0.4 and it might just solve your problem. If not rather than looking for someone to make one, have a crack yourself! Lump of structural black steel from the local steel fabricators, hacksaw, drills and files and of course the lathe. Think of the satisfaction when you have done it!

Paul.

[derek blakeParticipant @derekblake72550]

Thank you for the kind words Paul, you have an art of bringing the mood back up and that’s great as I felt very defeated.

i agree 0.1 is a lot, anywhere else it would probably be fine but on the crank and it’s bearings maybe not.

i will however machine two new bearings, where in my next question please I’ve read different advice some say cast iron are the best bearings to make, some brass and some bronze.

ive ordered all three types of material but what would you consider the best? I’m hoping that at least some of the knock would disappear after all this work.

the slightly straighter crank should help, I wasn’t exactly sure how to straighten the crank such as which direction to push, it was trial and error but I probably should have asked the basics.

Del

[Paul KempParticipant @paulkemp46892]

Derek,

Problems are there to be solved not to be shied away from, it's from problems and getting over them that you gain experience and confidence! I have straightened parrallel shafts in the past and really there is no substitute for doing it, it's not something that can be readily book learnt. It's really down to an intuitive understanding of where the bend actually is and judging where to apply a force to correct it and how far to push it! Also depends on the material and its resilience. I would say from your short foray into the subject you didn't do too bad. Look on the bright side, you didn't make it worse!

Bearings then; Yes cast iron is a good bearing material paired with steel, in fact many older small lathes have plain cast iron headstock bearings. Not so good for high shaft speeds but the big advantage is cast iron contains a fair amount of free graphite which on its own is a lubricant also cast iron due to its grain structure tends to "hold" oil (try painting an old bit of cast iron machinery that has been well coated in oil over its life!). Traditionally from full size these bearings would be bronze. Bronze although soft can actually be a hard wearing material and where low carbon steel un hardened shafts are run in bronze bearings it is often the shaft that wears faster than the bearing! Brass as a bearing material in an application like this – forget it! Fine for slow moving items like clocks, for use on a fast revolving shaft it will wear very rapidly. In the days of yore bearings were often called brasses – big end brass- in fact they were bronze unless white metalled and force fed oil.

So yes feel free if you want to use cast iron as long as it's a free graphite (grey) iron and you fit good lubricators. Personally I would use bronze cos that's what was always done and you generally find with this type of thing the old boys knew a thing or two but no harm in stretching the envelope and trying something different.

Final thoughts, engineering is generally a pursuit of patience and tenacity. The man who never made a mistake, never made anything and if these things were easy to build everyone would be doing it! Keep at it, you'll get there in the end even if there are a few hiccups along the way.

All the best,

Paul.

[derek blakeParticipant @derekblake72550]

Thanks Paul, that’s excellent information and kind words.

i will make some new bronze bearings from scratch and give them a go.

i will let you know how I get on,

Thanks again,

Del

[derek blakeParticipant @derekblake72550]

Hi Guys

for anyone who’s interested I’ve sorted the dreaded knock, for now at least as I’ve not run the engine for long.

I replaced the cotter pin in the cross head with a push fit, and it runs sweet as a nut at the moment, far better than before so I’m happy.

if the Knock comes back at least I know what it is now, it was a tiny amount of slop but enough to vibrate through the crank.

regards,

Del

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