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Posted by Gunner on May 13, 2008, 2:06 pm
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On Tue, 13 May 2008 07:29:54 -0500, "Lloyd E. Sponenburgh"
<lloydspinsidemindspring.com> wrote:
>HeffaLump <"Percussion Engineering"> fired this volley in
>
>> I've developed a problem when facing on my lathe, I do a lot of small
>> dia disks (20-40mm) in 6082 Aluminium and when it comes time to polish
>> the face I'm noticing concentric circles appearing on the finished
>> surface.
>>
>> They're not to pronounced, I'm able to polish them out easily enough,
>> but I'm worried it might be indicating a developeing problem.
>>
>> I've tried cutting at 0.5mm down to 0.1mm with little differance both
>> manually & on powerfeed, I've adjusted to gibs to the cross slide
>> (tried it loose, spot on & too tight), varied the RPM's, differant
>> cutting tools/holders and I can see that the saddle isn't moving from
>> the DRO.
>>
>> I've put a dial guage against the surface with no discernable movement
>> when rotating !
>>
>> The circles are easy to see but impossibly difficult to photo, you
>> have to catch the light just right but they appear to be 2-3mm apart.
>> I cannot tell if they are truly concentric or spiralling.
>>
>> I'm thinking 2 things . . . either play in the head bearings or the
>> crossfeed leadscrew. Please don't let it be the head bearings but I
>> can't see how the leadscrew can affect the finish if the gibs are
>> correctly set????
>
>I'm thinking chip relief/breaking. If any part of the chip contacts the
>work, it will burnish a circle there.
>
>Have you changed your tool or the grinding of it, or is it building a
>false edge?
>
>LLoyd
One assumes he didnt have the problem before, and now does, and hasnt
changed brands/type of inserts?
Gunner
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Posted by Gunner on May 13, 2008, 2:05 pm
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On Tue, 13 May 2008 12:35:30 +0100, HeffaLump <"Percussion
Engineering"> wrote:
>I've developed a problem when facing on my lathe, I do a lot of small
>dia disks (20-40mm) in 6082 Aluminium and when it comes time to polish
>the face I'm noticing concentric circles appearing on the finished surface.
>
>They're not to pronounced, I'm able to polish them out easily enough,
>but I'm worried it might be indicating a developeing problem.
>
>I've tried cutting at 0.5mm down to 0.1mm with little differance both
>manually & on powerfeed, I've adjusted to gibs to the cross slide (tried
>it loose, spot on & too tight), varied the RPM's, differant cutting
>tools/holders and I can see that the saddle isn't moving from the DRO.
>
>I've put a dial guage against the surface with no discernable movement
>when rotating !
>
>The circles are easy to see but impossibly difficult to photo, you have
>to catch the light just right but they appear to be 2-3mm apart. I
>cannot tell if they are truly concentric or spiralling.
>
>I'm thinking 2 things . . . either play in the head bearings or the
>crossfeed leadscrew. Please don't let it be the head bearings but I
>can't see how the leadscrew can affect the finish if the gibs are
>correctly set????
>
>Any ideas?
Use a tenths indicator and check for play in your spindle bearings,
both laterally and longitudinally.
Btw..that is one of the signs of increasing backlash in
ballscrews/leadscrews in CNC machines.
About .0004-.0006 will start showing the rings
A herringbone pattern on a side turn will often indicate excessive
bearing clearance
Plain or ball/roller bearings?
Gunner
Gunner
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Posted by Richard J Kinch on May 13, 2008, 2:09 pm
Please log in for more thread options HeffaLump writes:
> The circles are easy to see but impossibly difficult to photo, you have
> to catch the light just right but they appear to be 2-3mm apart. I
> cannot tell if they are truly concentric or spiralling.
I second the suspicion about the cutting tool and/or coolant, rather than
the rigidity. Sounds more like a finish problem than a positioning
problem.
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Posted by Jon Elson on May 13, 2008, 4:02 pm
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HeffaLump wrote:
> I've developed a problem when facing on my lathe, I do a lot of small
> dia disks (20-40mm) in 6082 Aluminium and when it comes time to polish
> the face I'm noticing concentric circles appearing on the finished surface.
>
> They're not to pronounced, I'm able to polish them out easily enough,
> but I'm worried it might be indicating a developeing problem.
>
> I've tried cutting at 0.5mm down to 0.1mm with little differance both
> manually & on powerfeed, I've adjusted to gibs to the cross slide (tried
> it loose, spot on & too tight), varied the RPM's, differant cutting
> tools/holders and I can see that the saddle isn't moving from the DRO.
>
> I've put a dial guage against the surface with no discernable movement
> when rotating !
>
> The circles are easy to see but impossibly difficult to photo, you have
> to catch the light just right but they appear to be 2-3mm apart. I
> cannot tell if they are truly concentric or spiralling.
>
> I'm thinking 2 things . . . either play in the head bearings or the
> crossfeed leadscrew. Please don't let it be the head bearings but I
> can't see how the leadscrew can affect the finish if the gibs are
> correctly set????
>
> Any ideas?
Could be half a dozen things. It can be built-up edge of workpiece
material on the cutting edge, variations in feedrate from your hands
(try using the power feed at a very low advance rate). Oh, OK, I see
you've already tried that! It could be looseness in the gibs allowing
the cross slide to weave back and forth slightly as you turn the crank.
If the spacing of the rings match the thread pitch of your crossfeed
screw, then the connection is nearly certain. remeber that no machine
is totally rigid, everything is slightly flexible. And, even with the
gibs "too tight", there is still an ability of the upper slide part to
rock just a little. It is possible there is a misalignment between the
screw and the nut that is applying sideways forces on the nut, thus
rocking the slide as the screw turns.
Jon
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Posted by Don Young on May 13, 2008, 10:16 pm
Please log in for more thread options
>
>
> HeffaLump wrote:
>> I've developed a problem when facing on my lathe, I do a lot of small dia
>> disks (20-40mm) in 6082 Aluminium and when it comes time to polish the
>> face I'm noticing concentric circles appearing on the finished surface.
>>
>> They're not to pronounced, I'm able to polish them out easily enough, but
>> I'm worried it might be indicating a developeing problem.
>>
>> I've tried cutting at 0.5mm down to 0.1mm with little differance both
>> manually & on powerfeed, I've adjusted to gibs to the cross slide (tried
>> it loose, spot on & too tight), varied the RPM's, differant cutting
>> tools/holders and I can see that the saddle isn't moving from the DRO.
>>
>> I've put a dial guage against the surface with no discernable movement
>> when rotating !
>>
>> The circles are easy to see but impossibly difficult to photo, you have
>> to catch the light just right but they appear to be 2-3mm apart. I cannot
>> tell if they are truly concentric or spiralling.
>>
>> I'm thinking 2 things . . . either play in the head bearings or the
>> crossfeed leadscrew. Please don't let it be the head bearings but I can't
>> see how the leadscrew can affect the finish if the gibs are correctly
>> set????
>>
>> Any ideas?
>
> Could be half a dozen things. It can be built-up edge of workpiece
> material on the cutting edge, variations in feedrate from your hands (try
> using the power feed at a very low advance rate). Oh, OK, I see you've
> already tried that! It could be looseness in the gibs allowing the cross
> slide to weave back and forth slightly as you turn the crank.
>
> If the spacing of the rings match the thread pitch of your crossfeed
> screw, then the connection is nearly certain. remeber that no machine is
> totally rigid, everything is slightly flexible. And, even with the gibs
> "too tight", there is still an ability of the upper slide part to rock
> just a little. It is possible there is a misalignment between the screw
> and the nut that is applying sideways forces on the nut, thus rocking the
> slide as the screw turns.
>
> Jon
>
It seems possible that the leadscrew may be slightly bent. This can easily
rock the saddle enough to form noticeable rings.
Don Young
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