Parting tool chatter

[Richard Hed]

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!

Just one question on that: If you are going at a slower speed (revolutions/m), then are u slowing the engine or is it a geared (or belt drive) that is changed? I'm wondering because it seems to moi, that a geared or belt driven system would have very high torque because it is still got the same horse power behind it, just going slower. so each rev. may be take longer but the power, proportionally, should be much higher. but if it is driven by Variable speed machine, then all bets are off.

 

[Richard Hed]

It will be interesting to know just how many people are actually making a Worden grinder.
Apart from me---

I am not familiar with the Worden. Does u have a photo? Are the plans in public domain? I needs a specialized grinder and would like to build one.

 

[goldstar31]

I am not familiar with the Worden. Does u have a photo? Are the plans in public domain? I needs a specialized grinder and would like to build one.

Google 'Hemingwaykits'

 

[Steamchick]

Hi ENGINEMANPK. I agree. Alternative cuts to make a kerf - wider than the cutting blade - can alleviate some side stresses on the blade and workpiece (=heat) but when the parting blade is true to the alignment of the lathe (perpendicular to the axis) and the cutting end of the tool has no bias, I find it isn't necessary. Try the hacksaw blade alignment to the chuck face that I showed in an earlier photo. The manufactured precision of the blade and un-used teeth give a visual guide that my old out-of-calibration eyes can see. But if you use a parting blade that has the top face angled away from the horizontal - per the wedge-clamp mounting in the holder, then the blade will develop side thrust and your trick will cause the cut to wander sideways. Been there and got it wrong!
But often we have to do what our tools and set-up will allow, and overcome imperfections and difficulties that the text-book in its perfect world doesn't consider. So thanks for the reminder/advice.

 

[Steamchick]

Just one question on that: If you are going at a slower speed (revolutions/m), then are u slowing the engine or is it a geared (or belt drive) that is changed? I'm wondering because it seems to moi, that a geared or belt driven system would have very high torque because it is still got the same horse power behind it, just going slower. so each rev. may be take longer but the power, proportionally, should be much higher. but if it is driven by Variable speed machine, then all bets are off.

Hi Richard, you are correct about the torque developed by the motor being translated by the gearing to a higher torque available on the wrapper at lower (geared) speed. Also, that some (not all) variable speed systems drop torque with motor speed.
However, in my badly explained diatribe, I was referring to the torque developed by the cutting (shearing) of material during parting -off. If you have a constant feed with (say) 1 thou per second feed, you will develop a certain torque at high spindle speed (thin swarf), but higher torque at lower spindle speed (thick swarf). My hands give a fairly constant speed of feed, not spindle speed related, as I do not have a power feed on the cross-slide. So I must be careful not the feed too fast when parting-off as on my small lathe I can stall the shaft (belt-slip). So it fails-safe. But on an older lathe I had, my hand-feeding could over feed the cut to the point where I could see the whole lathe twisting with the torque of the cut! It had a much larger motor and gearing than originally made. (And a ham-fisted operator!). Thus the torque developed by the cutting operation is higher when the cut is heavier... as developed wth a slower spindle speed where the cut is not directly controlled by the spindle speed, all other factors beiing the same.
I was simply trying t explain why, on smaller lathes, feeding the parting cut "hard" may not work because you can over-work smaller tools. I have seen many utube videos with experts using larger (stiffer) lathes with heavier cuts than I can consider on my small "hobby" lathe.
Therefore , the amateurs seeking advice must be cautioned by experts, as to pitfalls that can mean working practices from one set-up cannot be applied to the other.
Sorry for any confusion...

 

[BaronJ]

Here you go Richard, Well worth a read !

I am not familiar with the Worden. Does u have a photo? Are the plans in public domain? I needs a specialized grinder and would like to build one.

http://www.jeffree.co.uk/Pages/worden/worden.htm

 

[goldstar31]

Here you go Richard, Well worth a read




http://www.jeffree.co.uk/Pages/worden/worden.htm

John

Don't get me wrong, it's a good write up but it is NOT the present Mark. I sort of guess it is Mark2 wheras mine is Mark3.

Again, the Mark 3 will do the following- with attachments available separately and-- at more cost.
4 facet tool grinding
rounding off things like lathe tools
re-grinding slitting saws
using a diamond wheels

OK- I have seen it all before as I have the forerunner- the Kennet. Worth a look at Lathes.co.UK.

To sort the 'doers' from the 'talkers', there is the Quorn- now in its 3rd lease of life.
That should keep Richard occupied- for years to come. Martins Models in the States also has castings

Bored- waiting for food and the arrival of a new set of castings for the GH Thomas parting tool kit etc

Cheers and that

Norm

 

[holmes_ca]

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!

Steamchick, I do understand what you are saying but your post is too much for me to absorb at my age, I have done machining professionally for over 45 years and another 23 years as a hobby, I realize that chunk of metal was too much for my little lathe and normally I would not attempt parting off a billet of that size, it just happened to be laying around and was convenient at the time, my preference would be to bandsaw it off to length first, that would be a faster and safer operation in my situation, and it might also be beneficial in eliminating a possible shattering of a larger parting tool on a larger machine that has more than enough torque and which more than likely has a geared head, that could be another part to the equation, I do depend a lot on my hearing with any type of machining, But I do understand the point you are making,

Edmund..........Alberta

 

[Balta]

I have been toying with this idea for a while and finally got it done. The cutoff toll you see in the pictures is a piece of a woodworking carbide tipped saw blade. the holder is mild steel with a V grove top and bottom. Originally I was going to use a 60 deg dovetail cutter but the one I have is 3/4" and that would have been too big for the tool holder, so I ended up running a 6mm end mill holding the holder at 45deg (you can see this in picture "cutoff III"
Works like a charm and there is no chatter... and I haven't even ground the tip properly.
Incidentally, I paid $1 for the blade at a flea market and I have enough for several more.

Balta

 

[OrangeAlpine]

An interesting experience the readers of this thread might enjoy.

I did my best to incorporate the many suggestions, especially concerning rigidity. Some improvements, but not earth shaking. The tool had a tendency to hog in and we all know where that ends.

Fast forward to last week. I tackled a project that required parting 1 1/16" water hardening drill rod (annealed state) drilled to 7/16". I could see nothing good coming from this exercise. But to my amazement, I encountered zero problems. The tool would chatter (high frequency) for the first 1/16", then settle down to some steady cutting. Made eight partings using hand drip cutting oil and never resharpening the tool. About 75 rpm. The water hardening drill rod is not an easy machining steel, but cuts cleanly, the chips were never a problem and it parts like a dream! Much better than the leaded stuff that I had trouble with.

Who would have thought?

 

[goldstar31]

I think that the reason is that your technique has vastly improved.
Long may it continue
Regards

Norman

 

[Steamchick]

I agree with Goldstar. Also, you will now appreciate that the high frequency chatter is a cutting speed related phenomenon, as with the reducing diameter the cutting speed also reduced. I have a variable speed drive and a small tweak of reducing speed stops the chatter if I encounter any, then I always increase speed as I approach half diameter and below to "keep up a good cut".
Keep up the good work,
K

 

[Steamchick]

That should read "increase rotational speed with reducing diameter" to "keep up a good cutting speed".
Sorry,
K

 

[SmithDoor]

Any update
If you findcan books per 1925 It give how to sharpen cutters.
After that time they change the angle so ever one would buy new lathes and mills.

Dave

What causes it? I've made a very rigid parting tool holder that mounts directly on the compound and makes a very rigid setup. Usually works like a dream, but sometimes get chatter. It really seems to be a random event as it comes and goes with the same setup. Sometimes touching up the tool helps, but not always. The tool is spang on center and flat on top. I've tried varying the grind angle, sometimes it helps, usually does not. When it does smooth the cut, the next cut may chatter. Using a .090" wide blade. Chatter starts the moment the tool touches the part.

Using a WWII era HSS blade.

Any suggestions?

Bill

 

[goldstar31]

Any update
If you findcan books per 1925 It give how to sharpen cutters.
After that time they change the angle so ever one would buy new lathes and mills.

Dave

You are probably referring to Robert H Smith's Advanced Machine Tool Work( 1925)
Splendid book and available on the 'net.
Sadly in a fit of generosity I gave my coppy away

However, I'm hanging on to my copies of the various volumes of Holzapffel. They 'say it all!
I mean where does one get a design for a honing jig-- or how to use sharkskin or a walrus hide?

The bit of 'Smith' that I like is how to calculate how to hit a lathe tool angle using mathematics.
Oh, Hell, I've got a Quorn tool and cutter grinder which will do almost everything apart from the tide tables in HongKong harbor-- but you get the idea.

 

[mnay]

"parting is such sweet sorrow"
Quote from a machine shop

 

[goldstar31]

"parting is such sweet sorrow"
Quote from a machine shop

'I am Chronicler'-----
Romeo and Juliet, please!

Shakespeare also wrote in Midsummer Night's Dream about ' Rude Mechanicals'

And All the World's a Stage and all the men and women merely players. They have their exits and their entrances and -- one man in his time plays many parts--------------------------------

Exeunt!!!!!!!!!! Perchance to Dream

 

[mnay]

Shakespeare, Machinist? Pretty close...………...

 

[SmithDoor]

I was look see the old old machine tools could cut and not chatter.
I finely found in a old old book. It is the rake angle .
Even today the ads say low rake angle.

Dave

You are probably referring to Robert H Smith's Advanced Machine Tool Work( 1925)
Splendid book and available on the 'net.
Sadly in a fit of generosity I gave my coppy away

However, I'm hanging on to my copies of the various volumes of Holzapffel. They 'say it all!
I mean where does one get a design for a honing jig-- or how to use sharkskin or a walrus hide?

The bit of 'Smith' that I like is how to calculate how to hit a lathe tool angle using mathematics.
Oh, Hell, I've got a Quorn tool and cutter grinder which will do almost everything apart from the tide tables in HongKong harbor-- but you get the idea.

 

[Steamchick]

Agree that a couple of degrees of rake helps, on my hand-ground tools. But with my long-blade-in-a-holder commercial tool, there is zero rake and sometimes that is best... why? I don't know. But almost always when it chatters I can cut better if I withdraw the tool, slow down the motor by 20% or some and re-introduce the tool to the workpiece. I have cheated sometimes by cutting a mm or so with a hacksaw blade in the middle of the parting groove, then the parting cut works OK. I suspect because I have reduced the cutting load on the tool. Possibly why the commercial blade works, as it is narrower than my hand-ground tools?
But my final advice is "Sharp tools and wits, cut slowly and carefully, make good parts and bits"
K