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Posted by Joseph Gwinn on May 1, 2008, 11:30 am
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I think I've figured out a big piece of the mystery of what I
generically called chatter in the "Clausing 5914 Chatter ..." threads.
Tightening the lathe dovetails up helped a great deal, and is a good
idea in general, but didn't really solve the problem.
It came to me in the shower Wednesday morning - self-feeding would
explain just about everything seen, and that if one reverses something,
the sign of the mechanical feedback loop can be reversed. Now the
feedback is positive, yielding the vicious loop that causes visible
tilting, flake chips, and breakage. By contrast, negative feedback
causes stability.
The key experiment was to try face grooving a 2.125" diameter 6061
aluminum slug held in the 4-jaw chuck, and using the BXA-6 toolholder
with the huge overhang (~5" from cutting edge to toolpost center) but
with the tool bit held upsidedown and the lathe running in reverse.
Running backwards cases the self-feeding effect to change sign, becoming
a self-unloading effect as the toolpost tilts away from the workpiece,
reducing the bite of the tool bit.
Now I can peel very thin chips off the plate while face grooving,
although it is still necessary to use the back gear, and there was mucho
squealing. I did it dry, with too much overhang, and without properly
shaping the tool bit. But it worked anyway. The difference is
night-and-day.
So I now think it was the tendency to self-feed that caused all the
problems, even if the self-feeding effect wasn't so powerful as to cause
visible tilting of the toolpost. The squealing will go away with a
stiffer method of holding the tool bit, and perhaps a better-shaped bit.
This also applies to parting off, and specifically explains why using an
upsidedown blade coming towards the back of the workpiece, or coming
from the front with the lathe running backwards, works. (BXA-7R)
I never really believed the theory that better chip removal was why
upsidedown cutoff blades worked better, because I had problems even when
there were no chip wads to be found and the groove sidewalls were clean.
Actually, the galloping chatter tended to throw thick flake chips far
and wide, so they were everywhere but in the groove.
To summarize, there are two elements that are necessary for face
grooving, trepanning, and parting off on the Clausing 5914 lathe: Use
the back gear (for tortional rigidity), and use an upsidedown tool bit
(for stability).
As for use of a coaxial boring bar to hold a grooving/trepanning
toolbit, it turns out to be a common method. I found it mentioned in US
Patent 5,640,890 as prior art to be improved upon. The patentee is
trepanning stainless steel hydraulic hose fittings in production, and
needed a method that worked in a screw machine of some kind.
Joe Gwinn
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Posted by DoN. Nichols on May 1, 2008, 8:38 pm
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> Joseph Gwinn wrote:
>> I think I've figured out a big piece of the mystery of what I
>> generically called chatter in the "Clausing 5914 Chatter ..." threads.
>>
>> Tightening the lathe dovetails up helped a great deal, and is a good
>> idea in general, but didn't really solve the problem.
>>
>> It came to me in the shower Wednesday morning - self-feeding would
>> explain just about everything seen, and that if one reverses something,
>> the sign of the mechanical feedback loop can be reversed.
[ ... ]
> I had some real problems doing these types of operations on
> Atlas/Craftsman lathes. The 5914 is a MUCH sturdier lathe with
> a much heavier bed casting, so I'm surprised you were having
> this sort of trouble.
As am I. I've got the 5418 (similar bed profile, but manual
change belts), and have very little chatter.
> I finally traded up to a Sheldon R15-6,
> which is an even heavier lathe, in the 3000 - 3500 Lb class.
> You can stand on the bed with no measurable deflection (and I
> have precision electronic levels that are REALLY sensitive).
> It is amazing what a difference stiffness of the lathe makes.
> Now, another comment is that you may have a poor "fit up"
> between your carriage and bed.
You know -- there is one other possibility which occurs to me.
I presume that your lathe, like mine, has the hollow level adjusting
screws in the feet of the bed through which pass the bolts to lock it
down to the stand. If you have something like the near side bolt loose
and the level adjusting screw a little clear of the stand, the bed would
wind up under torque, which could give you similar behavior.
Did you get a proper sensitive level and adjust the bed to
proper level at both headstock and tailstock end? (I did.) After that,
did you make sure that the hold-down bolts were tight (and didn't
disturb the leveling)? *This* may be where your flex is -- especially
since you have a longer bed than I do (I believe), and thus more total
flex possibility.
Get it chattering and feel each foot with the adjacent finger on
the stand so you can detect relative motion between them. If you find
any, you'll need to tighten the hold-down bolts, and probably re-level
the bed.
Good Luck,
DoN.
--
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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Posted by Joseph Gwinn on May 1, 2008, 11:25 pm
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> > Joseph Gwinn wrote:
> >> I think I've figured out a big piece of the mystery of what I
> >> generically called chatter in the "Clausing 5914 Chatter ..." threads.
> >>
> >> Tightening the lathe dovetails up helped a great deal, and is a good
> >> idea in general, but didn't really solve the problem.
> >>
> >> It came to me in the shower Wednesday morning - self-feeding would
> >> explain just about everything seen, and that if one reverses something,
> >> the sign of the mechanical feedback loop can be reversed.
>
> [ ... ]
>
> > I had some real problems doing these types of operations on
> > Atlas/Craftsman lathes. The 5914 is a MUCH sturdier lathe with
> > a much heavier bed casting, so I'm surprised you were having
> > this sort of trouble.
>
> As am I. I've got the 5418 (similar bed profile, but manual
> change belts), and have very little chatter.
>
> > I finally traded up to a Sheldon R15-6,
> > which is an even heavier lathe, in the 3000 - 3500 Lb class.
> > You can stand on the bed with no measurable deflection (and I
> > have precision electronic levels that are REALLY sensitive).
> > It is amazing what a difference stiffness of the lathe makes.
> > Now, another comment is that you may have a poor "fit up"
> > between your carriage and bed.
>
> You know -- there is one other possibility which occurs to me.
> I presume that your lathe, like mine, has the hollow level adjusting
> screws in the feet of the bed through which pass the bolts to lock it
> down to the stand. If you have something like the near side bolt loose
> and the level adjusting screw a little clear of the stand, the bed would
> wind up under torque, which could give you similar behavior.
Do you mean the bolts between cabinet and the floor, or that hold the
headstock to the bed? I assume you mean between headstock and bed.
> Did you get a proper sensitive level and adjust the bed to
> proper level at both headstock and tailstock end? (I did.)
I do have such a level (Starrett model 98-6), and did level the bed by
adjustment of the leveling feet between cabinet and floor.
> After that,
> did you make sure that the hold-down bolts were tight (and didn't
> disturb the leveling)? *This* may be where your flex is -- especially
> since you have a longer bed than I do (I believe), and thus more total
> flex possibility.
Hmm. This I did not think of, or check. But I will.
I don't see why bed length would matter, given that while trepanning all
the action happens within a foot of the chuck face.
> Get it chattering and feel each foot with the adjacent finger on
> the stand so you can detect relative motion between them. If you find
> any, you'll need to tighten the hold-down bolts, and probably re-level
> the bed.
What I'm not quite sure of is which bolts you mean. On the 5914, the
headstock is bolted to the bed with four solid 3/8-16 hex socket cap
screws and two clamp bars, and the bed is bolted to the head pedestal
foot, which is in turn bolted to the cabinet, all with solid
non-adjustable bolts. The only hollow bolts I know of are in the
leveling feet between cabinet and floor, which doesn't seem relevant to
chatter (versus inability to turn a cylinder due to bed twist).
I looked the the 5418 manual. Now I understand. The designs of 5914
and 5418 are very different in this area. But there is no harm in
making sure that all those bolts are nice and tight, especially those
holding headstock to bed.
I suspect that on the 5914 they just machined the mating surfaces of
cast iron components close enough that adjustment wasn't needed, and
that the two pedestal feet isolated the bed and headstock sufficiently
from the sheet metal cabinet and chip pan that adjusters were not
needed. Unlike the 5418 manual, in the 5914 manual, there is no
procedure for adjusting how the cast iron stuff rests upon the sheet
metal stuff.
Joe Gwinn
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Posted by DoN. Nichols on May 2, 2008, 12:28 am
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>> > Joseph Gwinn wrote:
>> >> I think I've figured out a big piece of the mystery of what I
>> >> generically called chatter in the "Clausing 5914 Chatter ..." threads.
[ ... ]
>> > I had some real problems doing these types of operations on
>> > Atlas/Craftsman lathes. The 5914 is a MUCH sturdier lathe with
>> > a much heavier bed casting, so I'm surprised you were having
>> > this sort of trouble.
>>
>> As am I. I've got the 5418 (similar bed profile, but manual
>> change belts), and have very little chatter.
[ ... ]
>> You know -- there is one other possibility which occurs to me.
>> I presume that your lathe, like mine, has the hollow level adjusting
>> screws in the feet of the bed through which pass the bolts to lock it
>> down to the stand. If you have something like the near side bolt loose
>> and the level adjusting screw a little clear of the stand, the bed would
>> wind up under torque, which could give you similar behavior.
>
> Do you mean the bolts between cabinet and the floor, or that hold the
> headstock to the bed? I assume you mean between headstock and bed.
I see that you figured out what I was talking about from
examining the manual for the 5418. Since I don't have the manual for
yours, I had not realized how different they were.
>> Did you get a proper sensitive level and adjust the bed to
>> proper level at both headstock and tailstock end? (I did.)
>
> I do have such a level (Starrett model 98-6), and did level the bed by
> adjustment of the leveling feet between cabinet and floor.
Hmm ... not nearly as sensitive. The 98-6 (and the rest of the
98 series) have a sensitivity of 0.005"/foot, while the No. 199 "Master
Precision Level" has a sensitivity 0f 0.0005"/foot -- ten times the
sensitivity. But, of course, the 98-6 gets you close enough if you then
do the "turn two rings on a single bar and measure them" operation
afterwards.
>
>> After that,
>> did you make sure that the hold-down bolts were tight (and didn't
>> disturb the leveling)? *This* may be where your flex is -- especially
>> since you have a longer bed than I do (I believe), and thus more total
>> flex possibility.
>
> Hmm. This I did not think of, or check. But I will.
>
> I don't see why bed length would matter, given that while trepanning all
> the action happens within a foot of the chuck face.
O.K. As long as you are that close, it would not really matter.
Where it would matter is turning some distance from the headstock, or
running something like a 1" drill bit in the tailstock -- especially
with a long workpiece extension from the chuck.
>> Get it chattering and feel each foot with the adjacent finger on
>> the stand so you can detect relative motion between them. If you find
>> any, you'll need to tighten the hold-down bolts, and probably re-level
>> the bed.
>
> What I'm not quite sure of is which bolts you mean. On the 5914, the
> headstock is bolted to the bed with four solid 3/8-16 hex socket cap
> screws and two clamp bars, and the bed is bolted to the head pedestal
> foot, which is in turn bolted to the cabinet, all with solid
> non-adjustable bolts. The only hollow bolts I know of are in the
> leveling feet between cabinet and floor, which doesn't seem relevant to
> chatter (versus inability to turn a cylinder due to bed twist).
O.K. Quite different. The 5418 is supposed to have the cabinet
bolted firmly to the floor (I don't), and the bed leveled relative to
the stand and chip tray.
> I looked the the 5418 manual. Now I understand. The designs of 5914
> and 5418 are very different in this area. But there is no harm in
> making sure that all those bolts are nice and tight, especially those
> holding headstock to bed.
Agreed. Check the tailstock end too -- because that is what
would flex when the torque is transmitted from the spindle to the
carriage. It would wind up the bed (a little, at least).
BTW For parting there used to be a gooseneck parting tool. It went
in a lantern style toolpost, came out, turned up, formed an
Omega shaped arch, back down to where the parting tool is
actually clamped. The result is that excessive cutting forces
tends to move the tool away from the workpiece, thus eliminating
chatter. But your machine *should* be rigid enough to not need
this, especially with an Aloris style quick-change toolpost, and
the T-profile parting blades.
> I suspect that on the 5914 they just machined the mating surfaces of
> cast iron components close enough that adjustment wasn't needed, and
> that the two pedestal feet isolated the bed and headstock sufficiently
> from the sheet metal cabinet and chip pan that adjusters were not
> needed.
Perhaps the bed feet were machined flat on the bottom, then
bolted to a thick flat plate (say 4" thick or so), machined flat on the
top, the bed mounted on this, and the ways finish-ground this way, so it
could get all the twist out of the bed by adjusting the cabinet feet.
Hmm ... another thing to check. While you have chatter, see
whether there is any relative motion between the headstock and the bed.
If it is not clamped down firmly enough, or if there are chips trapped
between the headstock and the bed, that could introduce enough give to
create problems.
> Unlike the 5418 manual, in the 5914 manual, there is no
> procedure for adjusting how the cast iron stuff rests upon the sheet
> metal stuff.
O.K. This assumes that the sheet metal stuff is rigidly bolted
to the floor, I guess.
Good Luck,
DoN.
--
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
|
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Posted by Joseph Gwinn on May 2, 2008, 11:02 am
Please log in for more thread options
> >
> >> > Joseph Gwinn wrote:
> >> >> I think I've figured out a big piece of the mystery of what I
> >> >> generically called chatter in the "Clausing 5914 Chatter ..." threads.
>
> [ ... ]
>
> >> > I had some real problems doing these types of operations on
> >> > Atlas/Craftsman lathes. The 5914 is a MUCH sturdier lathe with
> >> > a much heavier bed casting, so I'm surprised you were having
> >> > this sort of trouble.
> >>
> >> As am I. I've got the 5418 (similar bed profile, but manual
> >> change belts), and have very little chatter.
>
> [ ... ]
>
> >> You know -- there is one other possibility which occurs to me.
> >> I presume that your lathe, like mine, has the hollow level adjusting
> >> screws in the feet of the bed through which pass the bolts to lock it
> >> down to the stand. If you have something like the near side bolt loose
> >> and the level adjusting screw a little clear of the stand, the bed would
> >> wind up under torque, which could give you similar behavior.
> >
> > Do you mean the bolts between cabinet and the floor, or that hold the
> > headstock to the bed? I assume you mean between headstock and bed.
>
> I see that you figured out what I was talking about from
> examining the manual for the 5418. Since I don't have the manual for
> yours, I had not realized how different they were.
Ahh. Yes. It will take a while, but I may scan my manual, which does
not appear to be copyrighted.
> >> Did you get a proper sensitive level and adjust the bed to
> >> proper level at both headstock and tailstock end? (I did.)
> >
> > I do have such a level (Starrett model 98-6), and did level the bed by
> > adjustment of the leveling feet between cabinet and floor.
>
> Hmm ... not nearly as sensitive. The 98-6 (and the rest of the
> 98 series) have a sensitivity of 0.005"/foot, while the No. 199 "Master
> Precision Level" has a sensitivity 0f 0.0005"/foot -- ten times the
> sensitivity. But, of course, the 98-6 gets you close enough if you then
> do the "turn two rings on a single bar and measure them" operation
> afterwards.
I have not yet done the bar turning test, but intend to, once the more
immediate problems are at least understood if not resolved.
I probably should repeat the leveling exercise, as based on the other
kinds of ignorance-induced problems I've already found, it's likely that
the lathe was left unleveled and thus twisted for years, and may have
taken a set that needs to relax out.
> >> After that,
> >> did you make sure that the hold-down bolts were tight (and didn't
> >> disturb the leveling)? *This* may be where your flex is -- especially
> >> since you have a longer bed than I do (I believe), and thus more total
> >> flex possibility.
> >
> > Hmm. This I did not think of, or check. But I will.
> >
> > I don't see why bed length would matter, given that while trepanning all
> > the action happens within a foot of the chuck face.
>
> O.K. As long as you are that close, it would not really matter.
> Where it would matter is turning some distance from the headstock, or
> running something like a 1" drill bit in the tailstock -- especially
> with a long workpiece extension from the chuck.
While using the 1" drill, the bed will most certainly wind up a bit. I
guess that what would resist permanent twist would be the leveling, with
the lathe resting on a concrete floor, as it now does.
> >> Get it chattering and feel each foot with the adjacent finger on
> >> the stand so you can detect relative motion between them. If you find
> >> any, you'll need to tighten the hold-down bolts, and probably re-level
> >> the bed.
> >
> > What I'm not quite sure of is which bolts you mean. On the 5914, the
> > headstock is bolted to the bed with four solid 3/8-16 hex socket cap
> > screws and two clamp bars, and the bed is bolted to the head pedestal
> > foot, which is in turn bolted to the cabinet, all with solid
> > non-adjustable bolts. The only hollow bolts I know of are in the
> > leveling feet between cabinet and floor, which doesn't seem relevant to
> > chatter (versus inability to turn a cylinder due to bed twist).
>
> O.K. Quite different. The 5418 is supposed to have the cabinet
> bolted firmly to the floor (I don't), and the bed leveled relative to
> the stand and chip tray.
The 5914 manual also speaks of bolting to the floor, but few people
actually do this I suspect.
> > I looked the the 5418 manual. Now I understand. The designs of 5914
> > and 5418 are very different in this area. But there is no harm in
> > making sure that all those bolts are nice and tight, especially those
> > holding headstock to bed.
>
> Agreed. Check the tailstock end too -- because that is what
> would flex when the torque is transmitted from the spindle to the
> carriage. It would wind up the bed (a little, at least).
The tailstock now clamps pretty firmly to the bed. This is one of the
first things I cleaned and adjusted, mainly because it was easy and
could be done without the manual.
> BTW For parting there used to be a gooseneck parting tool. It went
> in a lantern style toolpost, came out, turned up, formed an
> Omega shaped arch, back down to where the parting tool is
> actually clamped. The result is that excessive cutting forces
> tends to move the tool away from the workpiece, thus eliminating
> chatter. But your machine *should* be rigid enough to not need
> this, especially with an Aloris style quick-change toolpost, and
> the T-profile parting blades.
I vaguely knew (from reading old books) that there was such a tool, and
that it involved a gooseneck, but I had the "picture" upsidedown in my
mind, and couldn't see how it would work. I just googled it, and found
a book from 1910 that explained the principle as applied to use in a
planer. They did understand the self-feeding effect, saying that the
gooseneck would eliminate gouging the work, but a sufficiently rigid
machine didn't need gooseneck tools.
<http://books.google.com/books?id=SkdSAAAAMAAJ&pg=RA8-PA36&lpg=RA8-PA36&d
q=gooseneck+tool+lathe&source=web&ots=edrQWKc6hu&sig=QsKeq0Vn7zebOdjC5ydY
KtuqnkA&hl=en#PRA8-PA35,M1>
It occurs to me that a negative rake cutting tool could be set up to
develop enough outward force to at least partially counteract the
tendency to self-feed. Perhaps this is why one commonly stated cause of
chattering is too-sharp tools. When one carefully blunts such a tool,
one puts a little bit of very negative rake right at the cutting edge.
> > I suspect that on the 5914 they just machined the mating surfaces of
> > cast iron components close enough that adjustment wasn't needed, and
> > that the two pedestal feet isolated the bed and headstock sufficiently
> > from the sheet metal cabinet and chip pan that adjusters were not
> > needed.
>
> Perhaps the bed feet were machined flat on the bottom, then
> bolted to a thick flat plate (say 4" thick or so), machined flat on the
> top, the bed mounted on this, and the ways finish-ground this way, so it
> could get all the twist out of the bed by adjusting the cabinet feet.
It's certainly a plausible approach, at least for the rough machining.
But the machining will release stresses, causing warping when the bed is
unbolted from the 4" slab. The bed ways are ground, so what may have
been done is that well-aged castings were rough machined and then
further aged and/or normalized, and then were finished on a surface
grinder. The exact procedure may have been a trade secret, as it was
exactly such recipes that were the secret sauce of lathe manufacture.
> Hmm ... another thing to check. While you have chatter, see
> whether there is any relative motion between the headstock and the bed.
> If it is not clamped down firmly enough, or if there are chips trapped
> between the headstock and the bed, that could introduce enough give to
> create problems.
Hmm. Offhand, I don't see any safe way to do this, as my hand would
need to be very close to the spinning chuck, or to various gears and the
like.
I think I'll just torque all the clamp bolts, to see if any are loose.
I doubt that the headstock was ever unbolted, at least not by the people
who caused all the ignorance-induced problems. Taking the headstock off
would have frightened them, at least one fondly hopes that it would. In
any event, chips can only get in there is the headstock is unbolted.
The headstock is located on the bed ways by careful fitting of headstock
bottom to ways, plus a pair of steel dowel pins to prevent sliding.
> > Unlike the 5418 manual, in the 5914 manual, there is no
> > procedure for adjusting how the cast iron stuff rests upon the sheet
> > metal stuff.
>
> O.K. This assumes that the sheet metal stuff is rigidly bolted
> to the floor, I guess.
That would make sense. In the 5914, these adjustments have moved to the
cabinet-floor feet.
Joe Gwinn
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