I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm cut,
brazed carbide tool on cast iron).

It's not really practical to change or regrind the tool. I'm dead on center
height, and wondered whether changing the height might make it better by
changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and came
up with two different answers.

Thanks,


--
Peter Fairbrother

Peter Fairbrother wrote:

I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm cut,
brazed carbide tool on cast iron).

It's not really practical to change or regrind the tool. I'm dead on center
height, and wondered whether changing the height might make it better by
changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and came
up with two different answers.

Is this a BIG facing job? (more than a couple of inches in diameter?)
If so, you may have to increase the spindle speed to keep the cutting
speed in the desirable range as you move from or toward center.

Pete


--
__
Pete Snell
Royal Military College
Kingston Ontario


The reasonable man adapts himself to the world;
the unreasonable one persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man.
- George Bernard Shaw

Pete Snell wrote:

Peter Fairbrother wrote:

I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm cut,
brazed carbide tool on cast iron).

It's not really practical to change or regrind the tool. I'm dead on center
height, and wondered whether changing the height might make it better by
changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and came
up with two different answers.

Is this a BIG facing job? (more than a couple of inches in diameter?)

90 mm x 55 mm rectangle shape, and that may be a reason for it getting worse
at the middle - I can't get the speed up very high, as it's an odd shape
with the CoG off-axis in a 4-jaw independant and it vibrates too much.

I can get an okay finish with a very fine finishing cut or two, but I have
to do a few of these and they don't have to be measured, just flat.

If so, you may have to increase the spindle speed to keep the cutting
speed in the desirable range as you move from or toward center.

Sounds good when it's hissing, or actually more like air whooshing through a
carb in a sweet engine and it's one more nice point about having a
variable speed - you don't have to stop the lathe to change gear or even
stop the feed, you just turn the speed knob up until the swarf starts
burning.

Is that last bit right by the way? I've been doing it that way all day and
the carbide tool seems undamaged, I figured if the swarf wasn't burning the
tool wouldn't get too hot. No lube used (or needed), I'm cutting good
quality fine-grained grey cast iron, and the lathe doesn't have a coolant
system.

I'm afraid I'm abusing the poor wee lathe though!


--
Peter Fairbrother

Peter Fairbrother wrote in
:


I'm afraid I'm abusing the poor wee lathe though!



You are only abusing it if something breaks..otherwise, you are just
'using' it.



--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

What is the material? I have a hunk of steel round of indeterminate
ancestry (probably hot rolled...) that cuts like a piece of concrete and, no
matter what I do, I can't get a decent finish on it. OTOH, other materials
cut beautifully, using the same setup...

Jerry

"Peter Fairbrother" wrote in message
...
I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm
cut,
brazed carbide tool on cast iron).

It's not really practical to change or regrind the tool. I'm dead on
center
height, and wondered whether changing the height might make it better by
changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and
came
up with two different answers.

Thanks,


--
Peter Fairbrother

"Jerry Foster" wrote in message
om...
What is the material? I have a hunk of steel round of indeterminate
ancestry (probably hot rolled...) that cuts like a piece of concrete and,
no
matter what I do, I can't get a decent finish on it. OTOH, other
materials
cut beautifully, using the same setup...

Jerry


Chuckle!

Hot rolled? That's not a material, it's a method of producing material.
It could be 1020 or 4340 just as easily.

From the description, what you have is chrome-moly. It likes to tear and
will usually yield a good finish only when run fast enough with carbide.
If you're using carbide and getting a finish that varies between shiny spots
and torn spots, you're likely using the wrong grade of carbide and
experiencing premature tip failure.

Harold.

"Harold and Susan Vordos" wrote in message
...

"Jerry Foster" wrote in message
om...
What is the material? I have a hunk of steel round of indeterminate
ancestry (probably hot rolled...) that cuts like a piece of concrete
and,
no
matter what I do, I can't get a decent finish on it. OTOH, other
materials
cut beautifully, using the same setup...

Jerry


Chuckle!

Hot rolled? That's not a material, it's a method of producing material.
It could be 1020 or 4340 just as easily.

From the description, what you have is chrome-moly. It likes to tear and
will usually yield a good finish only when run fast enough with carbide.
If you're using carbide and getting a finish that varies between shiny
spots
and torn spots, you're likely using the wrong grade of carbide and
experiencing premature tip failure.

Harold.


I understand "hot rolled" is a method, not a specific material. My basic
point was, however, that different materials cut VERY differently and, if
you try to use materials of "indeterminate ancestry," you never know quite
what you are getting into. And, for the benefit of the original poster of
the question, unless you know what the specific material is, it is difficult
to suggest an approach to machining it (or even if it can be satisfactorily
machined for the application...).

But hot rolled steels are, in general, much less uniform in hardness (hard
spots and soft spots) and tend to machine poorly. Most of the more
"machinable" steels are cold rolled. Your friendly metal dealer will tell
you he mostly sells cold rolled steels to machine shops and (cheaper) hot
rolled steels to welding shops...

Jerry

"Jerry Foster" wrote in message
om...

"Harold and Susan Vordos" wrote in message
...

"Jerry Foster" wrote in message
om...
What is the material? I have a hunk of steel round of indeterminate
ancestry (probably hot rolled...) that cuts like a piece of concrete
and,
no
matter what I do, I can't get a decent finish on it. OTOH, other
materials
cut beautifully, using the same setup...

Jerry


Chuckle!

Hot rolled? That's not a material, it's a method of producing
material.
It could be 1020 or 4340 just as easily.

From the description, what you have is chrome-moly. It likes to tear
and
will usually yield a good finish only when run fast enough with carbide.
If you're using carbide and getting a finish that varies between shiny
spots
and torn spots, you're likely using the wrong grade of carbide and
experiencing premature tip failure.

Harold.


I understand "hot rolled" is a method, not a specific material. My basic
point was, however, that different materials cut VERY differently and, if
you try to use materials of "indeterminate ancestry," you never know quite
what you are getting into.

Agreed. While you may encounter 1020, it's likely it is something else,
especially if it has been machined. That's why industry has color coding
of materials, and certification that is available when the specific alloy
must be known and used.

And, for the benefit of the original poster of
the question, unless you know what the specific material is, it is
difficult
to suggest an approach to machining it (or even if it can be
satisfactorily
machined for the application...).

Hard to argue with that! However, unless a part is subjected to heat treat
or welding, as long as it can be machined, the material choice usually makes
little difference. I can't help but think that's why so much material comes
from scrap sources for the home shop types. Economy must be a chief
concern.

But hot rolled steels are, in general, much less uniform in hardness (hard
spots and soft spots) and tend to machine poorly.

I've machined one hell of a lot of 4130/4140 and 4340 in my years in the
shop and have to say I've never encountered that problem. In fact, I'd
suggest to you that the material tends to be very uniform. Didn't say it
machined well, just machined consistently the same. Because such material
usually is accompanied by certs, it tends to be of high quality. If you'te
talking about turning an axle from an auto, you're more likely talking about
tool failure from heat treated steel than material variations.

Most of the more
"machinable" steels are cold rolled. Your friendly metal dealer will tell
you he mostly sells cold rolled steels to machine shops and (cheaper) hot
rolled steels to welding shops...

Depends on the nature of the work, wouldn't you say? If you're engaged in
making parts from alloy steels, the vast majority of it *is* hot rolled
material. Machine shops may very well consume a larger volume of hot
rolled material than cold processed materials (rolled or drawn). Screw
machine shops are notorious for choosing free machining steels,
1117,1213,1215,12L14, 11L17, all of which lend themselves to such
operations. You might say that work of that nature migrates towards those
that have the equipment. You can't generalize much beyond that.
Further, if you're doing work for people that know what they need, they
typically provide material specs that must be met. You generally have
little choice of how the material is produced.

One material is particularly good machining *because* it is cold processed.
Stressproof. LaSalle Steel developed the process by which it is produced.
Surface finish is generally very good, and it cuts without tearing.

A lot of shafting isn't cold processed (drawn) at all, but ground and
polished hot rolled. You can't generalize.

Harold

Jerry Foster wrote:

What is the material?
[..]

"Peter Fairbrother" wrote
I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm
cut, brazed carbide tool on cast iron).

Cast iron. Grey, fine-grained, not scrap.

[..]
It's not really practical to change or regrind the tool. I'm dead on
center height, and wondered whether changing the height might make it better
by changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and
came up with two different answers.

What I really wanted was a quick rundown on how changing tool height might
change the effective rake, and why. I'm sure I've read something about it,
but I can't remember where.


--
Peter Fairbrother

"Peter Fairbrother" wrote in message
...
Jerry Foster wrote:

What is the material?
[..]

"Peter Fairbrother" wrote
I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for
a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm
cut, brazed carbide tool on cast iron).

Cast iron. Grey, fine-grained, not scrap.

[..]
It's not really practical to change or regrind the tool. I'm dead on
center height, and wondered whether changing the height might make it
better
by changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and
came up with two different answers.

What I really wanted was a quick rundown on how changing tool height might
change the effective rake, and why. I'm sure I've read something about it,
but I can't remember where.


--
Peter Fairbrother


Changing tool height has a bit of an affect on rake, but very little. You
generally can't change the height of the tool enough to make much of a
difference. It has a profound affect on front clearance, however,
especially on small diameter cuts. If you're experiencing the tendency to
hog, reduce the front clearance a little, which will lower the hogging to
some degree. You can achieve a delicate balance where the feed rate is
just slightly less than the amount of clearance, so the tool isn't dragging,
but if it hogs, it bumps into the cut below the tip and stops the advance.
That's not a great way to go, but on a light machine it can be a useful
dodge. Good luck.

Harold

"Harold and Susan Vordos" wrote in message
...

"Peter Fairbrother" wrote in message
...
Jerry Foster wrote:

What is the material?
[..]

"Peter Fairbrother" wrote
I'm doing facing cuts and the tool is grabbing because the rake is
too
positive (I think. There's a graunchy noise and the feed goes slack
for
a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5
mm
cut, brazed carbide tool on cast iron).

Cast iron. Grey, fine-grained, not scrap.

[..]
It's not really practical to change or regrind the tool. I'm dead on
center height, and wondered whether changing the height might make it
better
by changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused,
and
came up with two different answers.

What I really wanted was a quick rundown on how changing tool height
might
change the effective rake, and why. I'm sure I've read something about
it,
but I can't remember where.


--
Peter Fairbrother


Changing tool height has a bit of an affect on rake, but very little. You
generally can't change the height of the tool enough to make much of a
difference. It has a profound affect on front clearance, however,
especially on small diameter cuts. If you're experiencing the tendency
to
hog, reduce the front clearance a little, which will lower the hogging to
some degree. You can achieve a delicate balance where the feed rate is
just slightly less than the amount of clearance, so the tool isn't
dragging,
but if it hogs, it bumps into the cut below the tip and stops the advance.
That's not a great way to go, but on a light machine it can be a useful
dodge. Good luck.

Harold



One more thing. Cast iron responds fairly well to negative rake, so if
you're having trouble with hogging and chatter, consider reducing your rake
to 0 degrees. I wouldn't go negative on a light duty machine, but no rake
would likely be quite useful.

Harold

On Fri, 18 Mar 2005 08:57:31 +0000, Peter Fairbrother
wrote:

Jerry Foster wrote:

What is the material?
[..]

"Peter Fairbrother" wrote
I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm
cut, brazed carbide tool on cast iron).

Cast iron. Grey, fine-grained, not scrap.

[..]
It's not really practical to change or regrind the tool. I'm dead on
center height, and wondered whether changing the height might make it better
by changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and
came up with two different answers.

What I really wanted was a quick rundown on how changing tool height might
change the effective rake, and why. I'm sure I've read something about it,
but I can't remember where.


Deviating from dead centre height only makes
sense when turning at approximately constant diameter. It
makes an equal change change in both front clearance and
front rake angles

This angle change is almost directly proportionate to
the cutting radius (arcsin h/r) and so would be extremely
variable with a facing cut. It has no effect on the side
clearance and side rake angles.

Its only real use in a facing cut is to set the tool a
few thou high so that, at dead centre, it pushes off the
tiny residual stub rather than passing underneath it and
leaving a pip.

With cast iron, sufficient packing under the tool to
achieve about 10 deg negative rake is a possible improvement
but the quick and dirty method is to stiffen the setup :-

Lock the carriage to the bedways.

Fully tighten the topslide gib strip to lock down the
topslide (or remove it altogether!).

Overtighten the cross slide gibstrip to the point where
substantial force is needed to produce crossfeed.

It sometimes helps to start the facing cut from the centre
rather than the periphery.

Let us know how you get on.

Jim\