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Posted by Robin S. on April 20, 2006, 3:10 pm
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> Robin,
> I suppose I am ruining the mill.
I specified that one can break a cutter and ruin a work piece by climbing.
The machine will jump but I'd be surprised if climbing could cause any
irreversible damage to the mill, assuming the gibs are tight.
> I've only been machining metal for 30 years or so
> and maybe you have more experience than me. Still, you should maybe
> watch me climb milling and see if it works well or not.
I've done it before and I that's why I cannot stress enough the danger
involved with climbing. Can you give an example of the type of cuts you're
taking?
For instance, I would run a 1" roughing endmill in O1 at about 200RPM (about
600RPM+ for aluminum), perhaps .3-.5" radial depth of cut, 1"+ axial depth
of cut, feeding until the spindle bogs down a bit (may require two hands to
apply adequate force on the crank to sustain the cut). As I said before, my
accidental climb cut broke the cutter and made the table jump in a similar
situation.
Do you never have issues with the table jumping and breaking cutters?
Regards,
Robin
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Posted by Eric R Snow on April 20, 2006, 4:33 pm
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wrote:
>
>> Robin,
>> I suppose I am ruining the mill.
>
>I specified that one can break a cutter and ruin a work piece by climbing.
>The machine will jump but I'd be surprised if climbing could cause any
>irreversible damage to the mill, assuming the gibs are tight.
>
>> I've only been machining metal for 30 years or so
>> and maybe you have more experience than me. Still, you should maybe
>> watch me climb milling and see if it works well or not.
>
>I've done it before and I that's why I cannot stress enough the danger
>involved with climbing. Can you give an example of the type of cuts you're
>taking?
>
>For instance, I would run a 1" roughing endmill in O1 at about 200RPM (about
>600RPM+ for aluminum), perhaps .3-.5" radial depth of cut, 1"+ axial depth
>of cut, feeding until the spindle bogs down a bit (may require two hands to
>apply adequate force on the crank to sustain the cut). As I said before, my
>accidental climb cut broke the cutter and made the table jump in a similar
>situation.
>
>Do you never have issues with the table jumping and breaking cutters?
>
>Regards,
>
>Robin
>
Robin,
Any screw with any clearance will make the table jump in a sort of
stick/slip scenario. Unless the table has drag on it. Using a
ballscrew will cause all kinds of problems unless it too is run with
drag. On a CNC machine the servomotor provides this drag. It's not
generally looked upon as drag, but it does prevent the pressure from
making the screw turn enough to lose position within the parameters of
the machine control. So if a climbing cutter pulls the table into the
cut then the ballscrew will be rotated to move the work out of the
cut. Then the direction reverses so the cutter is again climbing. Then
the screw reverses, etc. If effect dragging. On the manual machine
using acme screws the table locks are set to provide enough drag to
prevent the cutter from taking up the backlash in the screw. As
regards your method of milling it has been my experience that when
hogging material it works better, and gives longer cutter life, if the
cutter is fed into the work at almost the full cutter diameter. So
instead of using a shallow radial, deep axial cut, I use a deep
radial, shallow axial cut. This keeps more of the teeth cutting at one
time and is easier on the machine. BTW, I believe the rotating cutter
that removes the most material in the shortest time is a twist drill.
That is if all the material in converted to chips. Obviously an
annular cutter will remove more material, especially if the diameter
is large.
Eric
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Posted by Robin S. on April 20, 2006, 6:45 pm
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>>
> Robin,
> Any screw with any clearance will make the table jump in a sort of
> stick/slip scenario. Unless the table has drag on it. Using a
> ballscrew will cause all kinds of problems unless it too is run with
> drag. On a CNC machine the servomotor provides this drag. It's not
> generally looked upon as drag, but it does prevent the pressure from
> making the screw turn enough to lose position within the parameters of
> the machine control. So if a climbing cutter pulls the table into the
> cut then the ballscrew will be rotated to move the work out of the
> cut. Then the direction reverses so the cutter is again climbing. Then
> the screw reverses, etc. If effect dragging.
Are you sure the servo actively acounts for backlash? As I understand it,
the ballscrew nuts are preloaded to virtually eliminate backlash. I know
that in older worn machines, there are control paremeters that can account
for backlash, but because there is give in the (worn) screw, the fix is not
absolute. Interpolating arcs which cross 0º, 90º, 180º and 270º is
impossible and results in a slight flat is left on the work piece as the
axis reverses direction and the table is left floating.
Again, I'm no expert but I cannot imagine 3D machining would be possible if
not for a mechanically rigid machine which would include preloaded nuts.
> On the manual machine
> using acme screws the table locks are set to provide enough drag to
> prevent the cutter from taking up the backlash in the screw.
I am again interested in whether you have issues with the cutter grabbing
you work. I suppose I'm beating this subject to death, but you're basically
the only person I've ever heard specifying climbing on a manual machine.
> As
> regards your method of milling it has been my experience that when
> hogging material it works better, and gives longer cutter life, if the
> cutter is fed into the work at almost the full cutter diameter. So
> instead of using a shallow radial, deep axial cut, I use a deep
> radial, shallow axial cut. This keeps more of the teeth cutting at one
> time and is easier on the machine. BTW, I believe the rotating cutter
> that removes the most material in the shortest time is a twist drill.
> That is if all the material in converted to chips. Obviously an
> annular cutter will remove more material, especially if the diameter
> is large.
I've always shared your observation (and have read articles which support it
as well). I remember there were quite a few articles written about "plunge
roughing" a couple of years ago. I don't do work which requires that type of
roughing but I can imagine it would be very efficient if applied correctly.
Regards,
Robin
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Posted by Peter Wiley on April 20, 2006, 8:51 pm
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> >>
> > Robin,
> > Any screw with any clearance will make the table jump in a sort of
> > stick/slip scenario. Unless the table has drag on it. Using a
> > ballscrew will cause all kinds of problems unless it too is run with
> > drag. On a CNC machine the servomotor provides this drag. It's not
> > generally looked upon as drag, but it does prevent the pressure from
> > making the screw turn enough to lose position within the parameters of
> > the machine control. So if a climbing cutter pulls the table into the
> > cut then the ballscrew will be rotated to move the work out of the
> > cut. Then the direction reverses so the cutter is again climbing. Then
> > the screw reverses, etc. If effect dragging.
>
> Are you sure the servo actively acounts for backlash? As I understand it,
> the ballscrew nuts are preloaded to virtually eliminate backlash. I know
> that in older worn machines, there are control paremeters that can account
> for backlash, but because there is give in the (worn) screw, the fix is not
> absolute. Interpolating arcs which cross 0º, 90º, 180º and 270º is
> impossible and results in a slight flat is left on the work piece as the
> axis reverses direction and the table is left floating.
>
> Again, I'm no expert but I cannot imagine 3D machining would be possible if
> not for a mechanically rigid machine which would include preloaded nuts.
>
> > On the manual machine
> > using acme screws the table locks are set to provide enough drag to
> > prevent the cutter from taking up the backlash in the screw.
>
> I am again interested in whether you have issues with the cutter grabbing
> you work. I suppose I'm beating this subject to death, but you're basically
> the only person I've ever heard specifying climbing on a manual machine.
I wouldn't do it on either my B/port or my horiz mill except maybe for
very light finish cuts, full depth, to remove tool marks.
I *have* broken cutters and ruined workpieces when trying this many,
many years ago. With a heavy enough machine and enough drag on the
table, I'm sure you could get away with it, but I don't care to try.
PDW
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Posted by John Martin on April 20, 2006, 9:08 pm
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Peter Wiley wrote:
>
> > >>
> > > Robin,
> > > Any screw with any clearance will make the table jump in a sort of
> > > stick/slip scenario. Unless the table has drag on it. Using a
> > > ballscrew will cause all kinds of problems unless it too is run with
> > > drag. On a CNC machine the servomotor provides this drag. It's not
> > > generally looked upon as drag, but it does prevent the pressure from
> > > making the screw turn enough to lose position within the parameters of
> > > the machine control. So if a climbing cutter pulls the table into the
> > > cut then the ballscrew will be rotated to move the work out of the
> > > cut. Then the direction reverses so the cutter is again climbing. Then
> > > the screw reverses, etc. If effect dragging.
> >
> > Are you sure the servo actively acounts for backlash? As I understand i=
t,
> > the ballscrew nuts are preloaded to virtually eliminate backlash. I know
> > that in older worn machines, there are control paremeters that can acco=
unt
> > for backlash, but because there is give in the (worn) screw, the fix is=
not
> > absolute. Interpolating arcs which cross 0=BA, 90=BA, 180=BA and 270=BA=
is
> > impossible and results in a slight flat is left on the work piece as the
> > axis reverses direction and the table is left floating.
> >
> > Again, I'm no expert but I cannot imagine 3D machining would be possibl=
e if
> > not for a mechanically rigid machine which would include preloaded nuts.
> >
> > > On the manual machine
> > > using acme screws the table locks are set to provide enough drag to
> > > prevent the cutter from taking up the backlash in the screw.
> >
> > I am again interested in whether you have issues with the cutter grabbi=
ng
> > you work. I suppose I'm beating this subject to death, but you're basic=
ally
> > the only person I've ever heard specifying climbing on a manual machine.
>
> I wouldn't do it on either my B/port or my horiz mill except maybe for
> very light finish cuts, full depth, to remove tool marks.
>
> I *have* broken cutters and ruined workpieces when trying this many,
> many years ago. With a heavy enough machine and enough drag on the
> table, I'm sure you could get away with it, but I don't care to try.
>
> PDW
I think you both are missing what Eric said. He typically cuts using
almost the full diameter of the end mill. Do it that way and there is
almost no difference between conventional and climb milling. Bury the
full diameter of the cutter and there is no such thing as climb milling
or conventional milling.
My initial thought was that, if he is setting the table locks so tight
that the cutter won't move it, he's setting the table too tight, but
again - if he's using the full cutter diameter there is less force
needed to keep the table from jumping. Especially with four-flute and
more cutters.
John Martin
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> I suppose I am ruining the mill.