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Part built Allchin 1.5 inch

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derek blake01/06/2020 14:25:37
556 forum posts
140 photos

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 blake01/06/2020 21:26:48
556 forum posts
140 photos

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 Kemp01/06/2020 21:40:15
477 forum posts
18 photos

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 blake01/06/2020 21:52:15
556 forum posts
140 photos

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 Kemp01/06/2020 22:17:47
477 forum posts
18 photos

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 blake01/06/2020 22:30:44
556 forum posts
140 photos

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 blake10/06/2020 20:55:21
556 forum posts
140 photos

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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