Midland
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Post by Midland on Apr 27, 2015 20:05:11 GMT
Hi All I know what it is in its many senses but sticking to the lathe, I thought that if everything is rigid enough it would not occur. But it does, why, oh masterful engineers??
Cheers David
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Post by Deleted on Apr 27, 2015 20:18:13 GMT
Hi David
I think you'll find that anything will chatter no matter how rigid if feed rate and speed are wrong...also blunt cutters and size of machine doing the job...the job can be as rigid as possible to the machine but if the machine itself is too light for the job in hand then it will chatter.
cheers
Pete
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Chatter
Apr 27, 2015 20:20:22 GMT
via mobile
Post by vulcanbomber on Apr 27, 2015 20:20:22 GMT
There's many causes and many cures, without knowing your particular circumstances it's hard to help out....
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SteveW
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Post by SteveW on Apr 27, 2015 20:21:50 GMT
It's all about the spring in your system. No matter how rigid your system is it'll bend a little under stress in the same way a very big bell will ring when struck.
It is very difficult to damp a system at all frequencies. Your rigid system may not chatter at low frequencies because of its rigidity but it will at other/higher frequencies. Perhaps change the speed and you'll move away from the current resonance.
A while back I gave my blunt TCT tip a quick sharpen with a diamond lap. Result: it sang like a top. A more considered sharpen and it was quiet again.
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Post by Deleted on Apr 27, 2015 20:24:44 GMT
Hi All I know what it is in its many senses but sticking to the lathe, I thought that if everything is rigid enough it would not occur. But it does, why, oh masterful engineers?? Cheers David Chatter can be simple instability caused by lack of rigidity in tool mounting , slides et al but as often it is a caused by a harmonic torsionwise instability in the lathe spindle . In many cases though there are a dozen things happening at the same time and it is hard to assign a specific cause . Since chatter is usually a forced resonance condition it will usually be bad at specific speeds and cutting loads and less troublesome at other speeds and cutting loads . Chatter once initiated tends to be self perpetuating and a step change of speed and/or tool load is needed to cure problem .
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Midland
Elder Statesman
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Post by Midland on Apr 27, 2015 20:48:41 GMT
I think I understand that in a sort of way. My solution is to back off a bit and try again! Or try another way to do the job but thanks for that, you have given me something to think about. And seriously and with no disrespect I will have to look up the techincal terms you used!!!! I might add that my first job was fleet manaser of a fleet of leased tank cars in Canada and after a meeting with the engineers I had to dive into technical books to find out what they were talking about half the time. Quite soon, our mechical repairs manager became my translator. Then I got another job and I picked an enginer to be our Chief Engineer and told him speak to me in english but by then I was the boss and he had to!!!!! never did really understand but he was reliable so why bark when you have a dog is the expression I think. The trouble is, now I have to learn!! So thanks for your lucid description, . . . and the harmonics and the distortion are the same as in music!!!
Cheers David
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Post by donashton on Apr 27, 2015 21:09:39 GMT
and the harmonics and the distortion are the same as in music!!!
Hi David, But in music the harmonics and distortion are essential parts of the creation. Like this lot in the engineering scene the issues can be complex, and if I have sought a particular 'distortion' (or blend) I have to know how to create it, and not how to 'cure' it! Either way, you have my sympathy. Best wishes, Don.
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Post by Deleted on Apr 27, 2015 21:15:57 GMT
Sorry David !
Better explanation of some of it :
(1) Harmonic torsionwise instability in the lathe spindle .
The whole drive system from the motor through to the workpiece in the chuck is flexible in torsion . This means essentially that in the rotational sense it can wind up and unwind a bit while turning at a nominally constant speed . Under unfavourable conditions this winding up and unwinding can become systematic - essentially a rotational resonance is set up which manifests itself as chatter .
Imagine trying to use a rubber shaft to drive between a motor and a flywheel - it drives ok but flywheel isn't always going at input speed - it will usually be slowing down and speeding up a little relative to input speed all the time .
(2) Forced resonance .
Two types of resonance - forced and unforced .
Unforced is where a system is disturbed once and then oscillates for a long time without further input .
Forced is where a system needs constant or periodic input otherwise oscillations rapidly die away .
(Never well defined - usually some of each in real systems)
Roughly same as difference between a plucked string and a bowed string .
Regards ,
MichaelW
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Midland
Elder Statesman
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Post by Midland on Apr 27, 2015 21:49:06 GMT
Perhaps I should come clean! About two years ago, Ed, yes dear EJP made me a set of brake hangers for my Aspinal tender by some sort of wire system. This was after ten years of utter frustration. (Bless you Ed) They are bloody good. I have slowly hacked them into some sort of shape. But events have intervened as in becoming chairman of Southampton, took on newsletter of SF and have various grandchildren etc etc, so progress has been slow. (And I might add, I have a Princess to keep running and a surrogate Compound to keep in service, and they are brilliant!) I have to turn the little bit at the bottom of these things and I tried it in the four jaw chuck and used the dial thing but I never got it quite right. I was not brave enough to tell EJP about that! But then I read the Vulcan bomber thread (he is one third my age!!) and he had all sorts of bits that he milled away to make some brake hangers and I thought that if I was to make a disc with a pin the middle that I could put Ed’s hangers on, I could turn the final bit just fine. Some years ago I got some brass from a scrap yard and I put this chunk of two inch diameter brass into the lathe and started to part off an inch wide disc to make a work holding thing so I could turn Ed’s bit. This is what VB did. Now I used a revolving centre in the other end to hold it firm but the whole bloody lathe banged away and I thought this was chatter!!! I have now cut off the bit I want off with a hack saw (have just mastered this bit of technology) and am now going to turn it so to produce a spigot to hold the bit I want to turn and the pin to hold it still on the disk. And then maybe I will produce what I am supposed to produce. God this is an uphill battle. I thought model engineering was fun!!!! Anyway, this is a bit of a catharsis to reveal my complete inadequacies as all the contributors to these threads are so bloody marvellous and I stand in illustrious company. I enjoy learning so please do not let up!!! So guys (and girls), my question about chatter. Perhaps you could recommend a good home? Cheers David PS I have not had a bottle of wine tonight!!!
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Midland
Elder Statesman
Posts: 1,870
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Post by Midland on Apr 27, 2015 21:57:19 GMT
Sorry David ! Better explanation of some of it : (1) Harmonic torsionwise instability in the lathe spindle . The whole drive system from the motor through to the workpiece in the chuck is flexible in torsion . This means essentially that in the rotational sense it can wind up and unwind a bit while turning at a nominally constant speed . Under unfavourable conditions this winding up and unwinding can become systematic - essentially a rotational resonance is set up which manifests itself as chatter . Imagine trying to use a rubber shaft to drive between a motor and a flywheel - it drives ok but flywheel isn't always going at input speed - it will usually be slowing down and speeding up a little relative to input speed all the time . (2) Forced resonance . Two types of resonance - forced and unforced . Unforced is where a system is disturbed once and then oscillates for a long time without further input . Forced is where a system needs constant or periodic input otherwise oscillations rapidly die away . (Never well defined - usually some of each in real systems) Roughly same as difference between a plucked string and a bowed string . Regards , MichaelW Now that is brillant,understand complety Thanks so much, David PS Understand the cause, up to me to solve. Thank you Michael, perhaps I should go the Engineering school!! PS See my much less briliant confessional!
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Post by Deleted on Apr 27, 2015 23:10:52 GMT
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Post by ejparrott on Apr 28, 2015 8:34:12 GMT
Is it chatter, or is it just bouncing off the intermittent cut? Slow it down. Never be afraid to drop right down to the slowest speed on a job that's going to hammer the tool
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Post by Roger on Apr 28, 2015 11:00:25 GMT
Ed's advice usually sorts out chatter and it's always my first port of call. There is no bottom speed for machining, you can pull the chuck over by hand if you want, I've had to do that on a form tool on a large diameter. The more rigid something it, the higher the natural frequency, so small tool and work overhangs are good, as is the use of a tailstock support the work even though you might think it isn't necessary. Setting the sharp tool dead on centre is also a big help, and not easy to get spot on without some sort of measuring/setup device. Some plain bearing head lathes are very light and there's no cure for that I'm afraid. You may find that you can adjust a plain bearing but there has to be clearance. My bigger Warco is a world apart from my old Southbend and that really shows the kind of difference a more meaty machine can make, even if it's not that well made.
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Post by Deleted on Apr 28, 2015 11:05:40 GMT
A lot of the problems on small lathes originate from spongy belt drives . Always far less problems with bigger gear driven machines .
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jma1009
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Post by jma1009 on Apr 28, 2015 11:16:36 GMT
hi david,
i am not too sure ive understood the bit you are trying to remove, but if you are machining the bosses top and bottom of the brake hangers, then it would be best to secure both ends. perhaps by bolting to a block of steel that could itself be bolted to the faceplate or held in the 4-jaw. i would remove most of the metal in the mill first. intermittent cuts on my lathe is not something i look forward to, though i wouldnt call it 'chatter'.
it is worthwhile doing the loco brake hangers at the same time as the tender hangers.
when i get round to making Stepney's brake hangers, i will silver solder the bosses on. the brake hangers are otherwise quite straightforward being strip steel and already cut to size.
cheers, julian
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Midland
Elder Statesman
Posts: 1,870
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Post by Midland on Apr 30, 2015 21:24:48 GMT
Julian I may have misled you. I was in the process of making two new safety valves for my Compound. Now I may have committed the cardinal sin but I had some brass from the local scrap merchant (acquired some years ago) that appeared to be marine brass. It was marked LN 2374 or something like that. It looked tough enough. As I turned the hex off it chattered something rotten. But then it settled down quite nicely. As I turned down the dia it chattered occasionlly and then settled down. I realised that the amount of over hang was an issue. But now it works fine. This was more an intellectul question than anything specific, just to learn a bit. When you are stupid as whot I am, you can ask dumb questions and get away with it!!!
Cheers david
PS You sould see my Princess of Wales, running gorgeously! Especially as I come from Glamorgan, historically!!
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jma1009
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Post by jma1009 on Apr 30, 2015 21:38:27 GMT
hi david, turning tough bronze in large sizes always causes me problems unless i drop the speed and sharpen the tooling and hone it. i dont like turning tough bronze at too high a speed as the swarf is apt to burn a hole in my shirt or workshop dust coat! cheers, julian
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Midland
Elder Statesman
Posts: 1,870
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Post by Midland on Apr 30, 2015 22:27:35 GMT
Julian The problem with this forum is that we talk from miles apart. I would to have to have you a couple of miles away and we could go to the lathe and do these things together! Yes the swarf was red and angry and that bothered me butI did it slowly to lower the distress. I ended up with a niCe little safetly valve reports latyr D
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Post by andyhigham on Apr 30, 2015 23:30:07 GMT
Some chatter can be caused by the inherent roughness of single phase motors
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Post by Deleted on Apr 30, 2015 23:42:11 GMT
Some chatter can be caused by the inherent roughness of single phase motors It's one of the factors that can just make that little bit of difference when you are in a position where work might chatter or might not . There is no doubt that three phase and just physically bigger motors do perform better than small single phase motors when really smooth running is important .
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