Parting tool chatter

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Tim Wescott

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Just for reference, here's a picture of my super-duper cutoff tool holder. I'd never want to use this in production because you basically have to hand-assemble it each time. But thanks to the combination of super-high-tech materials, it does hold things rigidly enough that I can part off 1 1/4" diameter steel (as demonstrated by the last shot, if you can just believe that I did it).

The tool itself is purchased; it's nicely "cutoff tool" shaped. The aluminum block in there is tapered just right so that when I press the tool in from the side it's at the correct angle -- the super high tech top & bottom holders then transfer the force of the screws to the tool, and hold it in the correct position.

The feed has to be right -- too little and it chatters, too much and it stalls the lathe.

(And per all the "who has experience" subject -- I design electronics and write software for pay; my formal education in machining ended with a freshman combined wood, metal, and electronics shop class back when I was 13 years old).

holder_back.jpg
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it_works.jpg
 

Tim Wescott

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Ah yes-- but does his humour travel, please? A bit like Gerard Hoffnung.
"Heath Robinson" is "Rube Goldberg" in British.

And yes -- it is very much thrown together. But the thing just insists on working, and working, and working. So it lives on, doing all sorts of parting-off jobs quite nicely, thank you.
 

Steamchick

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Maybe because I was taught machining in the 1960s.. at a machine shop turning cast-iron cross-heads 10 1/2" diameter and weighing over 150lbs/ casting... all the tool set-up and clamping was just like this. I remember that the deformation in the micrometer (due to its own weight and stiffness - or lack thereof...) meant I had to check the OD with Mic vertically (2 ways), horizontally from above, then from below, and take a mean value... to get the parts "right" to within a thou or so. It was where I learned the different sounds of cutting... good, bad and various materials. Does anyone else remember all the different smells from different metals? - Aluminium, cast-iron, forged steel, etc. all sound and smell different and that is part of the "memory" joy for me, when machining.
On parting, my "commercial tool holder and tool" seems to work best at a tiny bit below centre, but I do find that excess pressure makes the tool regress into the holder. Also, the tapered top edge needs to be dressed "flat" to avoid the blade wandering under cutting forces on the top face. See picture for how I set it square (the hacksaw blade is a convenient size and the teeth give me a good view of full contact on the chuck face - see how the reflections of the teeth points meet the actual teeth? - It is easier to detect out-of-alignment than a with a simple straight-edge to my eyes!) and the height of the tool. (A true vertical blade in line with the square would be exactly centre). Sorry about focus, or shake, as my camera didn't like being so close to the job. (This is about as good as I can see anyway, without a good magnifier!). The back-tool-post isn't very stiff, but is better at larger diameters than the 4-post tool holder. However, I can only cut about 3/8" deep with this tool before I have to change to the smaller commercial tool on the front tool post.
Hope you find something useful in this? (e.g. the hacksaw trick against the chuck?)

Also some carbide tips I have ground with ordinary carborundum, as I didn't know you must use diamond wheels.... some are home-made with old lathe tools and broken bits of carbide tips. Takes a bit of time and patience, but it can be done. (Time is free for me).

And on another matter, not all suppliers of tools send good ones.... sometimes you get the ones from the scrap-bin!
 

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goldstar31

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Well, I'm or was a bean counter- I left that World for longer than I worked, I never listened. Actually, there was a canteen full of electrical engineers who laid out moore high voltage cable there than they ever laid for 1.6 million who benefited from the time when they were - not theorising ;)
But the pension- like my other ones is acceptable-- ho, ho, ho! Poor things, I spend hours sweetly mourning their last vist into an electric oven---:p;)

And so to my 'Moonbeams from a Lesser Lunacy'. Without a formal clue about parting off, why use the top slide to hold a FRONT parting tool? A top slide is merely what some character out of his tiny mind, thought that he knew a man who knew a man.
Answers please?

Norman

N.B. as a sort of addendum:)D👷‍♂️ I've saved money in the Covid-19 and sold my last car and saved a horrendous car insurance.
So I've added more bits in kit form to my grinder projrct and-- wait for it💂‍♀️- both another rear parting tool kit- because I really don't feel that it is sensible to unbolt and swop tooling between my indoor and out door little workshops

More anon if the Grim Reaper keeps away!

Norma
 
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mcostello

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In The Home Shop Machinist magazine years ago there was an article from a Guy Who regularly parted off 11" and 18" bronze. A picture showed a piece of 11" being cut. He had a thin piece of metal wedged under the cut off tool. Not to burst anyone's bubble, including Mine.;)
 

tornitore45

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He had a thin piece of metal wedged under the cut off tool.
I have a parting blade holder that provides 5 degrees rake but also use a classic parting tool in a standard QT holder. To avoid grinding a top rake angle I place a suitable thick spacer under the tool front, presto structural rake without grinding.
 

goldstar31

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In The Home Shop Machinist magazine years ago there was an article from a Guy Who regularly parted off 11" and 18" bronze. A picture showed a piece of 11" being cut. He had a thin piece of metal wedged under the cut off tool. Not to burst anyone's bubble, including Mine.;)
The lathe was probably a bit larger and more rigid than a model engineer's one.
My two lahes swing 7" and 8" - respectably. :D

But, but I spoke the guy who makes a kit for the LARGER version of the Thomas design- when I ordered my bits for what is laughingly called MODEL engineering .
 

kinggt4

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Had a bit of trouble finding the Enots parting tool videos, so here is the one that compares tools:

And here is the one on grinding a parting tool:

In another video, I notice that he is using the insert tool type which I use. I have no trouble parting off with carbide inserts.

George
 

SmithDoor

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If you at video the carbide has a lot of rake around 10 to 15°.
This is secret of good cutoff with HSS or carbide.
The down side to carbide it is wider cut and cost.

Dave

Had a bit of trouble finding the Enots parting tool videos, so here is the one that compares tools:

And here is the one on grinding a parting tool:

In another video, I notice that he is using the insert tool type which I use. I have no trouble parting off with carbide inserts.

George
 

holmes_ca

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If I had a bigger motor I feel sure the operation in this video would work perfectly and on a small lathe like the Taig this op is over the top for such a small lathe, so the old fashioned spring holder is not at fault, in fact, it does the job well,

 

holmes_ca

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I think your lathe would have had a much easier job parting that steel if you used the lowest ratio on your pulleys.
Jack, thank you for the comment, trouble was the lathe was stalling and the motor was still turning and screaming on the smallest pulley so I went to the next bigger pulley for more grip, catch 22, I realize its not the best setup with the holder hanging out in a little block clamped to a 0.500 post with very little support but this operation is just a one-time thing my normal course would be the cutoff saw, I just wanted to show how an old ancient tool holder can still do the job,
 

Steamchick

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Hi. I would still advise caution from my "Engineer's" head.... A bigger motor, or even down- gearing, is seemingly a good idea because you ultimately want more torque applied. This is to overcome the shearing force exerted by the tool on the metal at the appropriate speed and feed of cut. 2 solutions, as the lathe (bed and slides) can only manage a limited torque. (If you don't know, the cutting torque is related to how much metal you remove = width and depth of cut, diameter at point of cut, and shear-strength of material being cut, so your lathe may be OK with a 0.0005in cut at 1in diameter, but not OK at 2 in diameter, or 0.001in cut because those both need twice the torque applied. Similarly a 2 mm tool takesc2/3rd the torque of a 3mm tool at the same diameter, feed-rate and workpiece material. )
So:
1: reduce the feed-rate, or increase speed of rotation so each revolution cuts less metal (reduced cut = reduced torque to within the capability of the lathe stiffness).
Or 2 : Reduce the width of cut - which is how a "Modified hacksaw" blade can work, when parting tools won't. Make a tool blade using a piece from a broken hacksaw blade - e.g. of un-used teeth just on the working side of the mounting hole. Select a tooth with a zero set as the cutting point. Make a precise holder so the blade is vertical. As the blade progresses into the cut the "kerf" from the set of the following blades will make the cut wider, but lubricate the cutting point as at the first tooth there is no side clearance - hence a lot of heat generated from the cutting shear, and side friction combined. I used this on a very small lathe, that had a big enough motor/gearing, but innadequate bed stiffness for cutting with my "regular" tool, on decent steel, at a larger diameter. But the feed was GENTLY, GENTLY! Not "stuff it in hard".
Sorry to be so long winded, but it is a complex problem, and £10,000 of lathe will have a much better (easier) capability than a £500 "hobby" lathe. I have a variable-speed cheap Chinese lathe and it is great! For what it cost. But at low speed it has hardly any torque, so like the spring loaded tool holder, it naturally stops the cut if I am too forceful for the stiffness of the whole machine. Not a problem as I have the time to work carefully within its limitations, and my local club has bigger industrial machines when needed.
Enjoy!
 

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