Subject: Turning grooves in faces

From: [email-masked] (Bill of Lading)
Date: Wed, 12 Jun 1996 01:10:28 GMT

Hello All:

I am currently in the process of turning some brass disks to use as
the ends of some cylindrical RF cavity filters.  The disks are 1/4"
brass, 4" across.  The plans call for a  groove to be turned into one
side of the disk, with an outside diameter of 1 3/8"  and a cross
section of 1/8" square. A friend of mine with some machining
experience made  a tool for me, basically a 1/8" wide flat bit, with
sufficient relief on the bottom to clear the cut groove.  No matter
what I try, however, I get a large amount of chatter and the tool
keeps catching in the work and gouging itself in. The top of the tool
is flat (correct for brass, right?) and the front has about 15 degrees
relief. 

I see such grooves all the time in vacuum equipment and such, so there
must be a good way to do this cut. Am I on the right track?

Other factors: The brass is quite a hard brass. It was cut on a band
saw, but only with a great amount of pressure on the blade. This same
band saw went through steel like butter both before and after we had
cut the brass. We cut the brass into octagons, drilled a hole in the
center and mounted it in the 3-jaw chuck with a little home-made
mandrel comprising two pieces of steel rod with holes in them and a
nut, washer and bolt. Turining down the outer diameter of the disks
proceeded smoothly, even given the irregular starting shape. 

If someone could provide some insight, this rank amateur (obviously,
right?) would most appreciate it. 

Thanks,
Bill

-- 
   (The above information is NOT TRUE; it is merely a harmless joke).
                      "It's VMS! Soylent NT is VMS!"
[email-masked]                                           bmocweN lliB

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From: [email-masked] (Russ Kepler)
Date: Wed, 12 Jun 1996 17:03:25 GMT

>right?) would most appreciate it. 

You might need to work on the rigidity of the lathe to get a clean
cut.  Sometimes you might need to speed up the lathe or to slow it
down - I usually start at about 1/2 the turning speed.  Try different
feedrates as well, often if you *increase* the infeed the chatter
stops.  Make sure that the bit is on center, you'll really chatter if
it's even a little bit above center.

-- 
Russ Kepler posting from home  [email-masked]

                      Please don't feed the Engineers

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From: [email-masked] (Jonathan M. Elson)
Date: 13 Jun 1996 05:05:03 GMT

Bill of Lading ([email-masked]) wrote:
: brass, 4" across.  The plans call for a  groove to be turned into one
: side of the disk, with an outside diameter of 1 3/8"  and a cross
: section of 1/8" square. A friend of mine with some machining
: experience made  a tool for me, basically a 1/8" wide flat bit, with
: sufficient relief on the bottom to clear the cut groove.  No matter
: what I try, however, I get a large amount of chatter and the tool
: keeps catching in the work and gouging itself in. The top of the tool

Ah, yes.  This sounds all too familiar.  On a heavy 16" lathe, using a
tool like that would work just fine.  I have a 10" Atlas, a light lathe
even for 10".  I couldn't do any better than you are doing with that tool.
I would have to use a narrower tool, and work it in a little, say .050"
deep on one side of the groove, then move over a little and go .050" deep
on the other side.  Repeat until near the desired depth.  Make the two
sides total less than the .125" finished width of the groove.  When you
are near the full depth, then make a final pass - down one side, across
the bottom, and up the other side.  The tool should have a profile that
has a little relief on the side cutting edges, too.  Brass definitely has
that 'grabbing' tendency - something to do with instant work hardening
as the cut begins to gouge.  Aluminum can do this, too, but with much
less dramatic results.

I have made some changes to improve the rigidity of my lathe.  I adjusted
the babbit bearings to minimize free play in the spindle.  I changed the
sloppy lantern-style tool post to an Aloris knockoff by Phase-II, which
made the most difference.  I located a butchering job on the guides on the
bottom of the saddle, that permitted the saddle to lift up off the bed if
pressure was applied in the right direction.  This caused occasional
mayhem during cutoff work or other work involving heavy infeed pressure.
I re-shimmed the guides to prevent the saddle from lifting.

Using whatever is necessary, be sure narrow tools are well supported,
and either put another tool blank under small tools or minimize tool
overhang from the toolpost.  This grooving operation is similar to the
conditions present with a cutoff tool, ie. a flat-front tool cutting a
wide chip deep in a groove.  This used to give me fits.  A slow speed,
consistent infeed, minimum tool overhang and LOTS of LUBE seem to be
the secrets, as told by many contributors in this group when the subject
came up about 6 months ago.  Using that collected wisdom, I have had the
best results doing cutoff work on my Atlas.  I have also been fixing
problems in the lathe, so I don't know which avenue has led to the
improvement, but I think it applies to your problem.

In doing cutoff work, the recommendations were: slowest non-back gear
speed.  Since your groove is 1 3/8" diameter, you might want to use a
back gear speed.  I think the lots of lube advice helped more to clear
the rolled-up chip that forms on the cutoff tool in steel, but you should
at least try it.  I have always observed that any turning which
creates a wide chip is prone to vibration.  You can cut a fairly DEEP chip
without vibration, but the very thin, wide chip will vibrate severly!
This builds up once started, as the wavy surface the vibration caused 
comes around under the tool for the second (third...) time.  I'm pretty
sure that the natural frequency of the lathe structure is involved,
because whether I'm turning really fast (over 1000 RPM) or in back gear,
the frequency of the sound the lathe makes during vibration always seems
to be about the same.  Once when using a long boring bar, mounted at its
center, I was getting a high frequency vibration.  Noting the different
frequency, I stuck a large turning tool in the open position on the
OTHER END of the boring bar, and the vibration stopped!  A very
interesting discovery!  The free end of the boring bar was ringing like a
one-armed tuning fork.  (Just a little anecdote to get you thinking about
causes of vibration.)

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From: [email-masked] (Peter Drumm)
Date: 12 Jun 1996 23:26:12 GMT

In message <[email-masked]> - [email-masked] (Bill of
Lading) writes:
:->
:->Hello All:
:->
:->I am currently in the process of turning some brass disks to use as
:->the ends of some cylindrical RF cavity filters.  The disks are 1/4"
:->brass, 4" across.  The plans call for a  groove to be turned into one
:->side of the disk, with an outside diameter of 1 3/8"  and a cross
:->section of 1/8" square. A friend of mine with some machining
:->experience made  a tool for me, basically a 1/8" wide flat bit, with
:->sufficient relief on the bottom to clear the cut groove.  No matter
:->what I try, however, I get a large amount of chatter and the tool
:->keeps catching in the work and gouging itself in. The top of the tool
:->is flat (correct for brass, right?) and the front has about 15 degrees
:->relief. 
:->
:->I see such grooves all the time in vacuum equipment and such, so there
:->must be a good way to do this cut. Am I on the right track?

Try a negative rake on the tool, ~7-10 degrees, and turn the piece as
slow as possible(my lathe goes down to 39 RPM, which is still too fast
sometimes). Make sure you hold the bit as close as possible to the
cutting end, to eliminate flexing.

[email-masked]
Using Warp/Neologic News/PMMail/iLink2/WebEx
Wausau, WI. 44d 58m 00s N x 89d 36m 45s W
All opinions are my own, even if they are wrong!

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From: [email-masked] (Carl Grinnell)
Date: Thu, 13 Jun 1996 18:05:39 GMT

From your description it sounds like the disk is flexing and this
is the cause of your chatter.

    There are many ways to mount a disk so that it won't chatter.

Use double sided carpet tape to mount the disk to a heavier plate.
Solder it to a plate.
Loctite might work??
Make a presser, a disk with a center drilled hole in it that can
press the disk against a backup plate mounted in the lathe chuck.
The presser will be just smaller than the groove that you have to cut
and the presser and backup plate can have a dowel pin to align them
and the brass disk.

                    Carl Grinnell