I understand how much everything on a lathe, including the work can flex, but
this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a half inch
and dial in 0.01" of feed and measure the before and after diameter with a
micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will remove more
than 0.002" of diameter. For example if I need to remove 0.003" of diameter,
feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the crossslide dial
in one direction- there no backlash weirdness going on.

Why might this happen?

Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a
half inch and dial in 0.01" of feed and measure the before and after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will
remove more than 0.002" of diameter. For example if I need to remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going on.
This is spring. The stack of carriage, cross slide, compound, toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the spring
unloads and drives the cutter deeper than you wanted. All machinists
need to learn to compensate for this.

Generally, the more rigid and massive the lathe, the smaller this effect
gets, but at some level it shows up on any lathe.

Jon

"Jon Elson" wrote in message
...
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can
flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends
only a
half inch and dial in 0.01" of feed and measure the before and
after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it
will
remove more than 0.002" of diameter. For example if I need to
remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty
close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going
on.
This is spring. The stack of carriage, cross slide, compound,
toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the
spring
unloads and drives the cutter deeper than you wanted. All
machinists
need to learn to compensate for this.

Generally, the more rigid and massive the lathe, the smaller this
effect
gets, but at some level it shows up on any lathe.

Jon

After taking and measuring the 0.01" cut, run the bit back over the
work and see how much more it removes.

On my old lathe the first fine cut after roughing down removes more
than the dial shows, but subsequent finishing cuts match the infeed.

jsw

Lloyd E. Sponenburgh lloydspinsidemindspring.com wrote:
Cydrome Leader fired this volley in news:kjkivb
:

Why might this happen?

hogging. Try reducing the backrack of your cutter.

Hmm forgot about this. I've been fiddling with toolpost height, setting
the cutter so that a facing pass doesn't leave a nub at the end of
something. Too low definetely makes the parting blade very unhappy as the
work starts to climb and chatter like crazy.

I'm able to compensate for this behavior, it just seems opposite of what
I'd expect.

My first guess is that a deeper cut would tend to be less than you dialed
in as the machine everthing is flexing apart. I'd then assume a wimpy fine
cut would be about what the handwheel would indicate there's less stress
on the machine, work and everything in between. Going over what I dialed
in would be possible due to backlash if the cutter is being pulled INTO
the part.

Is that about right?

Jon Elson wrote:
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a
half inch and dial in 0.01" of feed and measure the before and after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will
remove more than 0.002" of diameter. For example if I need to remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going on.
This is spring. The stack of carriage, cross slide, compound, toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the spring
unloads and drives the cutter deeper than you wanted. All machinists
need to learn to compensate for this.

I was moving cutter over to the right past the work, so there wasn't a
buildup of tension or springiness that way. The post about backrack makes
sense though. There's nothing to keep the cutter from being pulled into
the work on a sherline. Mine has about 2-3 mil of backlash after some
adjustments, but there's no real positive lock of any sort, just tension
on a wedge shaped plastic gib. You can only get it so tight before it just
acts as a brake and nothing moves at all anymore.

Generally, the more rigid and massive the lathe, the smaller this effect
gets, but at some level it shows up on any lathe.

Jon

It's interesting process to try to tame thing thing.

Jim Wilkins wrote:
"Jon Elson" wrote in message
...
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can
flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends
only a
half inch and dial in 0.01" of feed and measure the before and
after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it
will
remove more than 0.002" of diameter. For example if I need to
remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty
close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going
on.
This is spring. The stack of carriage, cross slide, compound,
toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the
spring
unloads and drives the cutter deeper than you wanted. All
machinists
need to learn to compensate for this.

Generally, the more rigid and massive the lathe, the smaller this
effect
gets, but at some level it shows up on any lathe.

Jon

After taking and measuring the 0.01" cut, run the bit back over the
work and see how much more it removes.

material does some off again when you go backwards. I was measuring after
this second pass and the tool cleared the work so I could fit the
micrometer in place.

On my old lathe the first fine cut after roughing down removes more
than the dial shows, but subsequent finishing cuts match the infeed.

That sounds about like what I'm seeing.

Cydrome Leader fired this volley in news:kjknae
:

Is that about right?

There's an awful lot that can go wrong on a light "hobby" lathe. Banquer
said it right, about having to "baby" it to prevent flex and spring. The
other responses about spring in the system were also spot-on.

Also, if you're using a lantern toolpost, it's hard to control the rake
without re-grinding the cutter. Raising and lowering the tool only serve
to alter it's "cut per increment".

It's not "kosher" on aluminum to use a zero back-rake, but it will tell a
tale if you try it. The finish won't be all that great unless you use
high speed and have a VERY sharp cutter, but if the problem goes away, it
indicates that hogging (due to spring in the system) is the problem.

A zero back-rake springs away from the work -- if it's precisely on the
centerline.

You might also check your mounts. If the whole bed can spring and twist,
it will give similar disgusting results.

Lloyd

On Thu, 04 Apr 2013 16:07:22 -0400, Jim Wilkins wrote:

"Jon Elson" wrote in message
...
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can
flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only
a
half inch and dial in 0.01" of feed and measure the before and after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will
remove more than 0.002" of diameter. For example if I need to remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty
close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going
on.
This is spring. The stack of carriage, cross slide, compound,
toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the spring
unloads and drives the cutter deeper than you wanted. All machinists
need to learn to compensate for this.

Generally, the more rigid and massive the lathe, the smaller this
effect
gets, but at some level it shows up on any lathe.

Jon

After taking and measuring the 0.01" cut, run the bit back over the work
and see how much more it removes.

On my old lathe the first fine cut after roughing down removes more than
the dial shows, but subsequent finishing cuts match the infeed.

Note too that with some material/tool combinations there's a minimum
depth of cut that you can achieve. This mostly happens with materials
that have a lot of flex for their strength, and tools that are not as
sharp as they need to be for that material.

I mostly have this issue when machining plastic, or when I have not been
diligent about sharpening my HSS tools.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

On Thu, 4 Apr 2013 19:06:19 +0000 (UTC), Cydrome Leader
wrote:

I understand how much everything on a lathe, including the work can flex, but
this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a half inch
and dial in 0.01" of feed and measure the before and after diameter with a
micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will remove more
than 0.002" of diameter. For example if I need to remove 0.003" of diameter,
feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the crossslide dial
in one direction- there no backlash weirdness going on.

Why might this happen?

Perhaps your cutting tool has too much "draw" and is pulling into the
work. Try locking the gib screws on the cross-slide before making the
cut.

Tim Wescott wrote:
On Thu, 04 Apr 2013 16:07:22 -0400, Jim Wilkins wrote:

"Jon Elson" wrote in message
...
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can
flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only
a
half inch and dial in 0.01" of feed and measure the before and after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will
remove more than 0.002" of diameter. For example if I need to remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty
close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going
on.
This is spring. The stack of carriage, cross slide, compound,
toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the spring
unloads and drives the cutter deeper than you wanted. All machinists
need to learn to compensate for this.

Generally, the more rigid and massive the lathe, the smaller this
effect
gets, but at some level it shows up on any lathe.

Jon

After taking and measuring the 0.01" cut, run the bit back over the work
and see how much more it removes.

On my old lathe the first fine cut after roughing down removes more than
the dial shows, but subsequent finishing cuts match the infeed.

Note too that with some material/tool combinations there's a minimum
depth of cut that you can achieve. This mostly happens with materials
that have a lot of flex for their strength, and tools that are not as
sharp as they need to be for that material.

are there any sort of rules of thumb for cutting vs rubbing depths?

I mostly have this issue when machining plastic, or when I have not been
diligent about sharpening my HSS tools.

I have noticed that the generic looking 1/4" HSS tools with all sorts of
bevels behave quite differently than one indexable HSS insert (little
diamond shaped things)holder I have where the top of the insert is
completly flat. The only bevel is on the edge of the cutter itself.

It seems to leave a nicer finish on aluminum under pretty much any
condition. Is this possibly due to the less backrake?

"Tim Wescott" wrote in message
...
...
Note too that with some material/tool combinations there's a minimum
depth of cut that you can achieve. This mostly happens with
materials
that have a lot of flex for their strength, and tools that are not
as
sharp as they need to be for that material.

I mostly have this issue when machining plastic, or when I have not
been
diligent about sharpening my HSS tools.

Good point. I use my cutoff bit for finishing to tenths (0.0001") and
was thinking of mentioning that here. So I went down and cleaned up
the rough-sawn end of some Delrin rod with it, 1 - 2 thousandths per
pass, the bit angled 9 degrees toward the face (Multifix toolpost),
and had no trouble with the bit sliding before cutting.

I hollow-grind the end of the cutoff bit so it can be touched up
easily with a stone, which contacts and cuts at only the very top and
bottom.
jsw

Cydrome Leader wrote:

There's nothing to keep the cutter from being pulled into
the work on a sherline. Mine has about 2-3 mil of backlash after some
adjustments, but there's no real positive lock of any sort, just tension
on a wedge shaped plastic gib. You can only get it so tight before it just
acts as a brake and nothing moves at all anymore.
SHERLINE??!!

I now have a 3500 Lb. Sheldon, and I STILL have to worry about things like
this, but somewhat less. You CERTAINLY have to be VERY aware
of cutting forces on a light machine like the Sherline or Taig (or ghastly
Chinese mini-lathes.)

Jon

Cydrome Leader fired this volley in news:kjkt4s
:

Is this possibly due to the less backrake?


That is a zero back rake tool (assuming the tool holder isn't tilted)...
usually used for brass, bronze, and cast iron, but suitable for testing
the principle. As you can see, back rake makes a big difference in how a
tool hogs -- especially on a fairly flimsy machine.

LLoyd

Reminds me of turning a 1" rod of 6060 in the lathe - dry as a bone -
and as it heated up Al expands and you cut more. Then you come off it
and it soaks into the chuck and air - shrinks back to where it was and A
slot I cut not a smoothing cut!

Al needs coolant and plenty of it to stay cool so it won't expand.

Martin

On 4/4/2013 2:06 PM, Cydrome Leader wrote:
I understand how much everything on a lathe, including the work can flex, but
this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a half inch
and dial in 0.01" of feed and measure the before and after diameter with a
micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will remove more
than 0.002" of diameter. For example if I need to remove 0.003" of diameter,
feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the crossslide dial
in one direction- there no backlash weirdness going on.

Why might this happen?

3500 - wow nice heavy. I have a 11x44 Sheldon myself.
Martin

On 4/4/2013 6:19 PM, Jon Elson wrote:
Cydrome Leader wrote:

There's nothing to keep the cutter from being pulled into
the work on a sherline. Mine has about 2-3 mil of backlash after some
adjustments, but there's no real positive lock of any sort, just tension
on a wedge shaped plastic gib. You can only get it so tight before it just
acts as a brake and nothing moves at all anymore.
SHERLINE??!!

I now have a 3500 Lb. Sheldon, and I STILL have to worry about things like
this, but somewhat less. You CERTAINLY have to be VERY aware
of cutting forces on a light machine like the Sherline or Taig (or ghastly
Chinese mini-lathes.)

Jon

"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote in message . 3.70...
Cydrome Leader fired this volley in news:kjkt4s
:

Is this possibly due to the less backrake?


That is a zero back rake tool (assuming the tool holder isn't tilted)...
usually used for brass, bronze, and cast iron, but suitable for testing
the principle. As you can see, back rake makes a big difference in how a
tool hogs -- especially on a fairly flimsy machine.


The main advantage of having an exaggerated positive rake angle is reduced horsepower usage per unit material removal....if your tool is "hogging in" then I would suggest tightening up the slide lock..

Disadvantage of exxagerated rake being it results in a fragile tool edge, which will be more prone to premature wear via micro fracturing.

Additional issue is material property; a soft flexible bar that bends fairly easily is more likely to climb up onto the tool under negative or neutral rake than under positive.

Martin Eastburn wrote:

3500 - wow nice heavy. I have a 11x44 Sheldon myself.
R15-6. A VERY advanced lathe. It looks like a geared head, but the
spindle is driven by a belt drive (3 matched belts) that runs
on a separate input shaft suspended on its own bearings surrounding,
but NOT running on, the spindle, so as not to transmit vibration to
the spindle. It drives the spindle by two clutch dogs that are
hand-scraped to put exactly matched torque to the spindle.
The gears are on the motor, down below.

The threading leadscrew has double universal joints at the left end
so it doesn't transmit any radial torque to the carriage and make
waves in the parts. Carriage and crossfeed drive is done by
metal plate clutches so they can slip if you have a crash, no
broken gears.

The QC box has 80 feed settings! They have two big drums full of
gears that can be rotated by cranks to select the desired feed or
thread.

There's more, LOTS more, sophistication in the design of this machine,
I was just in awe of it when I did the rebuild. Every time I took
something apart I was amazed at the great ideas that went into it.

As for use, it works amazingly well after my rebuild of the main bedways
and applying Moglice to the bottom of the carriage. I did some
1"- 20 TPI threads in 1018 steel in 3 passes, and could have done it in
TWO! I also made some 1" ball joint sockets with a quarter-round form
tool. I drilled to 7/8" (my largest Morse taper drill) and then
just plunged the form tool in, expecting at least a little chatter.
This would have made my 12" Atlas dance and the neighbors would have
heard the noise, but all I heard was a faint crinkling sound as the
chips crumpled inward on the cutter.

I'm embarrassed to have such a fine machine and use it so little!

Jon
Jon

Jon Elson wrote:

Martin Eastburn wrote:

3500 - wow nice heavy. I have a 11x44 Sheldon myself.
R15-6. A VERY advanced lathe.
Oh, forgot to mention 2.25" spindle through bore, and D1-6
camloc spindle mount. I sure love it!

Jon

On Thu, 04 Apr 2013 20:38:07 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Cydrome Leader fired this volley in news:kjkt4s
:

Is this possibly due to the less backrake?


That is a zero back rake tool (assuming the tool holder isn't tilted)...
usually used for brass, bronze, and cast iron, but suitable for testing
the principle. As you can see, back rake makes a big difference in how a
tool hogs -- especially on a fairly flimsy machine.

LLoyd

Its probably because the tool nose radius is bigger on the insert tool

On Thu, 04 Apr 2013 18:19:31 -0500, Jon Elson
wrote:

Cydrome Leader wrote:

There's nothing to keep the cutter from being pulled into
the work on a sherline. Mine has about 2-3 mil of backlash after some
adjustments, but there's no real positive lock of any sort, just tension
on a wedge shaped plastic gib. You can only get it so tight before it just
acts as a brake and nothing moves at all anymore.
SHERLINE??!!

I now have a 3500 Lb. Sheldon, and I STILL have to worry about things like
this, but somewhat less. You CERTAINLY have to be VERY aware
of cutting forces on a light machine like the Sherline or Taig (or ghastly
Chinese mini-lathes.)

Jon

Very very true!

My 1500 lb Hardinge Tool Room Lathe..an HLV-H has to be watched on
some stuff..and it has so little backlash as to be zero.

Gunner

Gunner Asch fired this volley in
:

Its probably because the tool nose radius is bigger on the insert tool




Gunner, a larger radius might give a better surface finish (with
everything else rigid and strong), but it will also increase the forces
causing a toolholder to spring relative to the work.

I don't know if I follow your contention...

LLoyd

On Sat, 06 Apr 2013 06:35:17 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Gunner Asch fired this volley in
:

Its probably because the tool nose radius is bigger on the insert tool




Gunner, a larger radius might give a better surface finish (with
everything else rigid and strong), but it will also increase the forces
causing a toolholder to spring relative to the work.

I don't know if I follow your contention...

LLoyd

Yes..it does take more force. As do negative rake tools. But thats
why industrial machines are made tough, strong and rigid.

Im sure that most of you here are familiar with negative rake tools?

They are not for every material..but work very very well if you have
the hp and rigidity for it. My 7.5hp Clausing 1500 gets used a LOT
with negative rake tools. My Hardinge HLV-H..does not. Zero rake is
best for it in most materials.

Id be using positive rake tools on light, flimsey machines. And have
a corresponding loss of surface finish.

As for tool nose radius....

http://www.sandvik.coromant.com/en-g...s/default.aspx

Some interesting discussions on TNR

http://www.homemodelenginemachinist....-radius-16526/

Gunner