Probably the steel, possibly the tool, maybe a combination of both. Would be worthwhile sourcing a bit of known easy-machining steel as your first option. I'd also check adjustment of the gibs, need to be snug but not too tight.

Rob

Emgee types quicker than me

Edited By Grindstone Cowboy on 11/02/2021 23:47:17

Yes, you are right, you scallop out the centre of the bar with it supported and then take the very lightest cuts with the bar unsupported before measuring them with a micrometer. If you can get it to within a thou. you will be doing fine. It's really not that important to me, at least, because it's only on a 'flimsy' lathe like the Myford that any adjustment by flexing the bed is possible. Don't get me wrong, the Myford is a fine machine in its class, I would never part with mine, but you certainly could not make any such adjustment on, say, a Colchester because the bed is like the Rock of Gibraltar so you are governed by the inherent accuracy of the machine or not.

With the greatest respect, like 'Inspector Meticulous' I think you may be being a little too concerned with getting it precisely correct. In the real world it is very unlikely that your chuck will close precisely concentric or the un-ground stock you put in it for most jobs will be exactly round. Most turning jobs are about just reducing diameters over quite short lengths that is why beds wear most nearest the chuck, so even if your lathe is cutting .005 thou. taper over the length of the bed over an inch at the headstock end the run-out is all but undetectable.

I accept I may be criticised for holding such opinions, but I am only an amateur pottering around in a back shed workshop, however well equipped it may be, and everything I make usually fits together or works effectively enough but we all have disasters from time to time and learn from them. I do not work in the tool-room at Rolls-Royce where I would be required to achieve world-class production standards, and rightly so.

A length of tough steel protruding 6 inches from the chuck as you say, and without further support as I think you say this was, is pretty severe test of lathe, tool and material.

I'd suggest the problem here is a combination of tool, the particular steel, and that overhang.

I've found sometimes the development of chatter varies with diameter, but in an odd way, worse at some intermediate depth than at the starting or near finishing diameters. I don't know why this would be.

One-inch diameter bar can't be set back much on an ML7 as it won't fit in the spindle, nor indeed on a Harrison L5 about twice the size, for the same reason; and even with free-cutting steel on the latter I'd be wary of machining a long length without a tailstock centre or a steady.

To test the lathe to produce a parallel cylinder,tailstock centres and steadies cannot be used, sugest use leaded en 1A mild steel and a really well ground HSS toolbit, check that the chuck jaws are not bell mouthed, I have a Colchester master with a well used 4 jaw chuck and brand 4 jaw. Try turning a length of 2 inch en 1a leaded in the old chuck and it will chatter like mad,use the same material ,tooling and lathe settings in the new chuck and the finish is excellent. To find if the ML7 chuck is causing the chatter,try turning the bar between two centres,not chuck and tailstock centre.If it turns with reduced chatter or no chatter then its the 3 jaw chuck jaw at fault.Also grind the HSS toolbit to a sharp point and just stone a small rad on the point.

Assuming the lathe is in good order, three areas go wrong:

1. The Operator! It's necessary to judge cutting-speed, rpm, work-holding, type of tool and the general approach to a job. Confidence is no substitute for basic competencies. I think of my lathe as a musical instrument that has to be played properly. It doesn't 'just work' when I fire it up and twirl the controls.
2. The tool. Sharpening HSS to the correct profile is another learning hurdle and not everybody is good at it.
3. The material. Don't assume that a metal-lathe will cut any old metal stuck in it. Starting out I was grossly misled by the contents of my junk-box: all the scrap I'd collected was vile. For a few months I was convinced mini-lathes are hopeless. Truth was, none of my scrap was machinable. Designers choose from a wide range of material properties, and there's a good chance machinability isn't important in manufactured goods at all. Many Aluminium alloys are formulated to be rolled and extruded and are too soft and sticky to cut well. Any hardened or toughened steel will give HSS a very bad time. Alloys are often intended to be ground, stamped or forged rather than cut. Ordinary mild-steel cuts OK rather than well, which is why EN1A is recommended, or even better the Leaded version EN1A-Pb. Leaded EN1A is about 3 times more machinable than ordinary structural mild-steel, but it too has a disadvantage – it can't be welded. Finding the right material can be a beginner challenge.

I think von dutch has spotted what's wrong. He's trying to cut hard steel which bends away from the knife rather than cut. His home-ground HSS cutter may be wrong. He's inexperienced and the lathe is new to him and may be worn or out of adjustment. If the work and/or tool are set up with excessive overhang, and the tool is applied with wrong depth of cut, feed-rate or surface speed, then chatter ensues. Each flaw in the set-up multiples the likelihood of trouble, even if the lathe is in perfect condition.

The cure is to eliminate potential problems and try again. Most obviously, replace the hard steel with a material known to be machinable: buy new with 'Free Cutting' or 'Good Machinability' in the product description. Mount the work so the minimum sticks out of the chuck, no more than 2.5x diameter without a steady and/or tail stock centre. Aluminium alloy (of the machinable type) is good for experimenting, and I prefer Brass to Steel, but EN1A is fine. Get a copy of Sparey's The Amateur's Lathe.

With luck decent metal and a bit of practice will fix it. If not, ask again. Could be as simple as loose gibs, or more troublesome the headstock bearing is worn.

Although random scrap can cause a lot of trouble, it's possible to use it once the operator has developed a feel for his machine. Experience reveals the difference between difficult but doable metals, those needing carbide, and the downright impossible. After a while you recognise the symptoms and sort of automatically 'know' what to try to improve the cut as it progresses. It involves finding a combination of speed, depth of cut, feed-rate and lubrication that cuts effectively. (Where 'effective' is a suitable mix of metal removal rate and finish).

As always it pays to learn to swim at the shallow end before tackling the English Channel!

Dave

Edited By SillyOldDuffer on 12/02/2021 16:11:17

Well said, Dave! (Both )

The books by L H Sparey, Ian Bradley, Harold Hall etc all provide good basic information.

Once the basic principles have been grasped the novice can soon gain experience.

Without knowing and understanding the basics, disappointment, frustration and disillusionment are likely to follow.

Zeus charts and Tubal Cain's "Model Engineers Handbook" are invaluable references.

I have seen a mini lathe ruined by trying to take 6 mm cuts with a high feed rate. The repair bill will be nearly half the cost of a new machine!

Yet posters on here produce superb work on machines that are a century old. A former colleague makes things on a pre WW1 Drummond that I would struggle to do on a 2003 machine, or an even newer mini lathe..

My turning instructor, who had earned his living on piece work, used to be frustrated by our taking 0.010" cuts on, by hobby standards, heavy industrial lathes with 5 or 10 hp motors. But we did not injure the machines or ourselves. As we became more proficient, we were able to utilise more of the machine capacity and facilities, with swarf coming off "Like clock springs" to his delight!

We learn every day. This is what the Forum does for us, exposing us to the knowledge of folk with other experiences, on which we can now draw.

Experience allows us to recognise the mistake the next time that we make it!

Howard

Edited By Howard Lewis on 12/02/2021 12:04:52