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Posted by Bruce in Bangkok on April 24, 2008, 1:57 am
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On Wed, 23 Apr 2008 23:38:00 -0400, "Ed Huntress"
>
>>
>>
>>
>>
>>
>>
>> > When you cut threads by feeding straight in, you're cutting both flanks
>> > of
>> > the thread at once. That produces twice the tool load for a given depth
>> > of
>> > cut.
>>
>> > If you have a good lathe, your tools are sharp, and you're cutting
>> > aluminum,
>> > it may work just fine. But cutting clean internal threads depends on
>> > getting
>> > things pretty close to right, so the extra tool load is something to
>> > watch
>> > out for if you get some chatter.
>>
>> > --
>> > Ed Huntress
>> >Of more importance is that cutting on one flank loads the cutter
>> >against the lead screw. Cutting both flanks at once, the carriage can
>> >float instead on being driven by the leadscrew, and you can end up
>> >with a drunken thread.
>> >John Martin
>>
>> Maybe you can answer something that's bugged me for a long time, John: Why
>> do the traditional lathe books say you should advance the cut when cutting
>> Acme threads by plunging straight in, rather than on the appropriate angle
>> for cutting one flank at a time? I don't recall ever having seen an
>> explanation.
>>
>> --
>> Ed Huntress- Hide quoted text -
>>
>> - Show quoted text -
>
>>Some may say that, some don't.
>
>>Machine Tool Operation, Burghardt:
>
>>"5. The side of an Acme thread is 14 1/2 deg., therefore move the
>>compound rest 14 1/2 deg. to the right when cutting a screw, to the
>>left when boring an internal thread. Set the thread stop and feed for
>>successive cuts by moving the compound-rest handle."
>
>>Perhaps the audience has something to do with it. Most of the machine
>>tool books were written by and for guys using big, heavy lathes. The
>>heavier the carriage, the less likely it is to jump ahead due to soft
>>spots in the work or the alignment of the planets. With a lighter
>>lathe, and less friction and mass in the carriage, it becomes more a
>>factor.
>
>>John Martin
>
>Maybe. I just never could see why they'd recommend the angled cut for 60
>deg. threads, and then the straight-in cut for Acme. It seems to me that the
>Acme is going to produce a much heavier load on the tool.
>
>I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
>it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
>bigger, coarser ones would be a real challenge.
I have roughed out large acme threads using the compound and finished
them with a form tool. But my guess (for whatever it is worth) is that
the reason you usually plunge cut acme threads is because they are
easier to measure that way. If you grind the tool correctly you just
jam her right in there until the calipers just slip over root diameter
and you're right.
The 60 degree threads are easy to eyeball because when they get pretty
sharp they are almost done.
Remember that a lot of the procedures were developed when things just
weren't as accurate or easy to measure. One example: Once you chucked
up a piece of stock you never, never, never, took it out of the
machine until it was finished and parted off. It was just too damned
hard to get it back in the chuck running true.
Bruce-in-Bangkok
(correct email address for reply)
|
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Posted by Ed Huntress on April 24, 2008, 6:59 am
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> On Wed, 23 Apr 2008 23:38:00 -0400, "Ed Huntress"
>
>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> > When you cut threads by feeding straight in, you're cutting both
>>> > flanks
>>> > of
>>> > the thread at once. That produces twice the tool load for a given
>>> > depth
>>> > of
>>> > cut.
>>>
>>> > If you have a good lathe, your tools are sharp, and you're cutting
>>> > aluminum,
>>> > it may work just fine. But cutting clean internal threads depends on
>>> > getting
>>> > things pretty close to right, so the extra tool load is something to
>>> > watch
>>> > out for if you get some chatter.
>>>
>>> > --
>>> > Ed Huntress
>>> >Of more importance is that cutting on one flank loads the cutter
>>> >against the lead screw. Cutting both flanks at once, the carriage can
>>> >float instead on being driven by the leadscrew, and you can end up
>>> >with a drunken thread.
>>> >John Martin
>>>
>>> Maybe you can answer something that's bugged me for a long time, John:
>>> Why
>>> do the traditional lathe books say you should advance the cut when
>>> cutting
>>> Acme threads by plunging straight in, rather than on the appropriate
>>> angle
>>> for cutting one flank at a time? I don't recall ever having seen an
>>> explanation.
>>>
>>> --
>>> Ed Huntress- Hide quoted text -
>>>
>>> - Show quoted text -
>>
>>>Some may say that, some don't.
>>
>>>Machine Tool Operation, Burghardt:
>>
>>>"5. The side of an Acme thread is 14 1/2 deg., therefore move the
>>>compound rest 14 1/2 deg. to the right when cutting a screw, to the
>>>left when boring an internal thread. Set the thread stop and feed for
>>>successive cuts by moving the compound-rest handle."
>>
>>>Perhaps the audience has something to do with it. Most of the machine
>>>tool books were written by and for guys using big, heavy lathes. The
>>>heavier the carriage, the less likely it is to jump ahead due to soft
>>>spots in the work or the alignment of the planets. With a lighter
>>>lathe, and less friction and mass in the carriage, it becomes more a
>>>factor.
>>
>>>John Martin
>>
>>Maybe. I just never could see why they'd recommend the angled cut for 60
>>deg. threads, and then the straight-in cut for Acme. It seems to me that
>>the
>>Acme is going to produce a much heavier load on the tool.
>>
>>I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
>>it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
>>bigger, coarser ones would be a real challenge.
>
> I have roughed out large acme threads using the compound and finished
> them with a form tool. But my guess (for whatever it is worth) is that
> the reason you usually plunge cut acme threads is because they are
> easier to measure that way. If you grind the tool correctly you just
> jam her right in there until the calipers just slip over root diameter
> and you're right.
>
> The 60 degree threads are easy to eyeball because when they get pretty
> sharp they are almost done.
>
> Remember that a lot of the procedures were developed when things just
> weren't as accurate or easy to measure. One example: Once you chucked
> up a piece of stock you never, never, never, took it out of the
> machine until it was finished and parted off. It was just too damned
> hard to get it back in the chuck running true.
>
>
> Bruce-in-Bangkok
> (correct email address for reply)
'Sounds possible. I'll have to look into the old books sometime to see if
there's any comment on it. I really miss having access to the old
McGraw-Hill library, which contained all of the _American Machinist_
references back to the 1870s. I don't think their metalworking library even
exists anymore.
--
Ed Huntress
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|
Posted by John Martin on April 24, 2008, 8:59 am
Please log in for more thread options >
>
>
>
>
>
>
>
> > > When you cut threads by feeding straight in, you're cutting both flank=
s
> > > of
> > > the thread at once. That produces twice the tool load for a given dept=
h
> > > of
> > > cut.
>
> > > If you have a good lathe, your tools are sharp, and you're cutting
> > > aluminum,
> > > it may work just fine. But cutting clean internal threads depends on
> > > getting
> > > things pretty close to right, so the extra tool load is something to
> > > watch
> > > out for if you get some chatter.
>
> > > --
> > > Ed Huntress
> > >Of more importance is that cutting on one flank loads the cutter
> > >against the lead screw. Cutting both flanks at once, the carriage can
> > >float instead on being driven by the leadscrew, and you can end up
> > >with a drunken thread.
> > >John Martin
>
> > Maybe you can answer something that's bugged me for a long time, John: W=
hy
> > do the traditional lathe books say you should advance the cut when cutti=
ng
> > Acme threads by plunging straight in, rather than on the appropriate ang=
le
> > for cutting one flank at a time? I don't recall ever having seen an
> > explanation.
>
> > --
> > Ed Huntress- Hide quoted text -
>
> > - Show quoted text -
> >Some may say that, some don't.
> >Machine Tool Operation, Burghardt:
> >"5. The side of an Acme thread is 14 1/2 deg., therefore move the
> >compound rest 14 1/2 deg. to the right when cutting a screw, to the
> >left when boring an internal thread. =A0Set the thread stop and feed for
> >successive cuts by moving the compound-rest handle."
> >Perhaps the audience has something to do with it. =A0Most of the machine
> >tool books were written by and for guys using big, heavy lathes. =A0The
> >heavier the carriage, the less likely it is to jump ahead due to soft
> >spots in the work or the alignment of the planets. =A0With a lighter
> >lathe, and less friction and mass in the carriage, it becomes more a
> >factor.
> >John Martin
>
> Maybe. I just never could see why they'd recommend the angled cut for 60
> deg. threads, and then the straight-in cut for Acme. It seems to me that t=
he
> Acme is going to produce a much heavier load on the tool.
>
> I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
> it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
> bigger, coarser ones would be a real challenge.
>
> --
> Ed Huntress- Hide quoted text -
>
> - Show quoted text -
Not at all sure who your "they" is, Ed. As I quoted, Burghardt - who
is certainly among the top few most widely read of the machine tool
writers - said to set the compound to the flank angle and feed with
it.
Bruce may be on to something with regard to the measurements. For 60
degree V threads, with the compound set to 30 degrees, it's pretty
easy to figure the compound feed necessary to hit full thread depth -
it's the same as the pitch. No fussing with trig tables.
Again, though, it seems that not all the writers said the same thing.
John Martin
|
|
Posted by Ed Huntress on April 24, 2008, 9:31 am
Please log in for more thread options
>
>
>
>
>
>
>
>
> > > When you cut threads by feeding straight in, you're cutting both
> > > flanks
> > > of
> > > the thread at once. That produces twice the tool load for a given
> > > depth
> > > of
> > > cut.
>
> > > If you have a good lathe, your tools are sharp, and you're cutting
> > > aluminum,
> > > it may work just fine. But cutting clean internal threads depends on
> > > getting
> > > things pretty close to right, so the extra tool load is something to
> > > watch
> > > out for if you get some chatter.
>
> > > --
> > > Ed Huntress
> > >Of more importance is that cutting on one flank loads the cutter
> > >against the lead screw. Cutting both flanks at once, the carriage can
> > >float instead on being driven by the leadscrew, and you can end up
> > >with a drunken thread.
> > >John Martin
>
> > Maybe you can answer something that's bugged me for a long time, John:
> > Why
> > do the traditional lathe books say you should advance the cut when
> > cutting
> > Acme threads by plunging straight in, rather than on the appropriate
> > angle
> > for cutting one flank at a time? I don't recall ever having seen an
> > explanation.
>
> > --
> > Ed Huntress- Hide quoted text -
>
> > - Show quoted text -
> >Some may say that, some don't.
> >Machine Tool Operation, Burghardt:
> >"5. The side of an Acme thread is 14 1/2 deg., therefore move the
> >compound rest 14 1/2 deg. to the right when cutting a screw, to the
> >left when boring an internal thread. Set the thread stop and feed for
> >successive cuts by moving the compound-rest handle."
> >Perhaps the audience has something to do with it. Most of the machine
> >tool books were written by and for guys using big, heavy lathes. The
> >heavier the carriage, the less likely it is to jump ahead due to soft
> >spots in the work or the alignment of the planets. With a lighter
> >lathe, and less friction and mass in the carriage, it becomes more a
> >factor.
> >John Martin
>
> Maybe. I just never could see why they'd recommend the angled cut for 60
> deg. threads, and then the straight-in cut for Acme. It seems to me that
> the
> Acme is going to produce a much heavier load on the tool.
>
> I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
> it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
> bigger, coarser ones would be a real challenge.
>
> --
> Ed Huntress- Hide quoted text -
>
> - Show quoted text -
>Not at all sure who your "they" is, Ed. As I quoted, Burghardt - who
>is certainly among the top few most widely read of the machine tool
>writers - said to set the compound to the flank angle and feed with
>it.
I don't remember which books, John. It may have been the Atlas or South Bend
lathe books. If I think of it I'll look for it and see what they say. It's
been years since I've looked.
>Bruce may be on to something with regard to the measurements. For 60
>degree V threads, with the compound set to 30 degrees, it's pretty
>easy to figure the compound feed necessary to hit full thread depth -
>it's the same as the pitch. No fussing with trig tables.
>Again, though, it seems that not all the writers said the same thing.
>John Martin
--
Ed Huntress
|
|
Posted by Bruce in Bangkok on April 24, 2008, 9:51 am
Please log in for more thread options On Thu, 24 Apr 2008 05:59:54 -0700 (PDT), John Martin
>>
>>
>>
>>
>>
>>
>>
>>
>> > > When you cut threads by feeding straight in, you're cutting both flanks
>> > > of
>> > > the thread at once. That produces twice the tool load for a given depth
>> > > of
>> > > cut.
>>
>> > > If you have a good lathe, your tools are sharp, and you're cutting
>> > > aluminum,
>> > > it may work just fine. But cutting clean internal threads depends on
>> > > getting
>> > > things pretty close to right, so the extra tool load is something to
>> > > watch
>> > > out for if you get some chatter.
>>
>> > > --
>> > > Ed Huntress
>> > >Of more importance is that cutting on one flank loads the cutter
>> > >against the lead screw. Cutting both flanks at once, the carriage can
>> > >float instead on being driven by the leadscrew, and you can end up
>> > >with a drunken thread.
>> > >John Martin
>>
>> > Maybe you can answer something that's bugged me for a long time, John: Why
>> > do the traditional lathe books say you should advance the cut when cutting
>> > Acme threads by plunging straight in, rather than on the appropriate angle
>> > for cutting one flank at a time? I don't recall ever having seen an
>> > explanation.
>>
>> > --
>> > Ed Huntress- Hide quoted text -
>>
>> > - Show quoted text -
>> >Some may say that, some don't.
>> >Machine Tool Operation, Burghardt:
>> >"5. The side of an Acme thread is 14 1/2 deg., therefore move the
>> >compound rest 14 1/2 deg. to the right when cutting a screw, to the
>> >left when boring an internal thread. Set the thread stop and feed for
>> >successive cuts by moving the compound-rest handle."
>> >Perhaps the audience has something to do with it. Most of the machine
>> >tool books were written by and for guys using big, heavy lathes. The
>> >heavier the carriage, the less likely it is to jump ahead due to soft
>> >spots in the work or the alignment of the planets. With a lighter
>> >lathe, and less friction and mass in the carriage, it becomes more a
>> >factor.
>> >John Martin
>>
>> Maybe. I just never could see why they'd recommend the angled cut for 60
>> deg. threads, and then the straight-in cut for Acme. It seems to me that the
>> Acme is going to produce a much heavier load on the tool.
>>
>> I've only cut 10 tpi Acme threads (because I have only one Acme tap, and
>> it's 10 tpi), and it's not a big deal on my South Bend to cut those. But
>> bigger, coarser ones would be a real challenge.
>>
>> --
>> Ed Huntress- Hide quoted text -
>>
>> - Show quoted text -
>
>Not at all sure who your "they" is, Ed. As I quoted, Burghardt - who
>is certainly among the top few most widely read of the machine tool
>writers - said to set the compound to the flank angle and feed with
>it.
>
>Bruce may be on to something with regard to the measurements. For 60
>degree V threads, with the compound set to 30 degrees, it's pretty
>easy to figure the compound feed necessary to hit full thread depth -
>it's the same as the pitch. No fussing with trig tables.
>
>Again, though, it seems that not all the writers said the same thing.
>
>John Martin
Shoot man! you don't need to measure it just eye ball it :-)
Reminds me of the part time job I took with a gunsmith in Shreveport
LA. The first job he gave me was to make a couple of stub axles to
weld in a piece of sch 80 pipe to make a boat trailer axle. I roughed
out the first one and set up the threading tool. Roughing pass,
finishing pass - upps just a little tight so made a light cut to
finish. Parted it off and started the second one. Got it ready to
thread, set the tool; first pass, second pass, tried the nut and
parted it off. Finished!
An hour or so later the Boss sidled over and sort of out of the corner
of his mouth asked me, "how'd you know those nuts were going to fit on
them axles?" I looked him right in the eye and said, "experience."
I tell you, from that day on if I said it went this a way then the
Boss pointed and said Go!
Probably if I'd a told the truth and said "blind luck", he wouldn't
have been so impressed :-)
Bruce-in-Bangkok
(correct email address for reply)
|
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