On Sat, 12 Nov 2005 09:25:13 -0500, wrote:
The other night I needed to whip out a bunch of 1" rollers to fix a
chip conveyor.
Well, I got them made but it did remind me that I really don't have a
good grounding in the proper use of a parting tool.
The tool was 3/32 hhs, and was ground to an angle so that the right
side entered the part first.
I didn't think that was right and likely the machinist on days may
have ground it that way for other purposes but it got the job done.
So, how about some tips on proper use/setup/feeds/ect?
Thanks in advance,
Wes
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Cutting off [or parting off for our GB readers] is one of the
more arcane and black-art aspects of lathe machining.

First about the angle grind. This was ground at an angle so the
the cut-off pip or tit would be to the left. While this may be
a "good thing" for high volume production I find that it causes
problems for for two reasons: (1) it is difficult to get the c/o
tool set exactly on center and (2) it generates a chip that is
wider than the width of the groove, which can be a major source
of problems.

Plain c/o tool blanks are a major PITA [at least for me] and I
have had much better results with the "T" style cutoff with a
radius in the top of the "T" as this tend to produce a chip that
curls in on itself thus is slightly less in width than the slot
allowing easier exit.

I suggest grinding the tool straight across with about a 5 degree
[effective] front clearance angle. The side clearance angles are
set by the shape of the tool blank. Because of the "dish" at the
top of the "T" there is a few degrees of effective side rake or
hook which seems to help a great deal.

A suitable holder will make a huge difference in performance. I
suggest a holder with 3 to 5 degrees of back rake. This will
require that you increase the measured front clearence by this
amount to get the suggested 5 degrees clearance.

As I indicated before the tool must be set to be exactly on
center and exactly at right angles to the axis of ther part. I
find it is wothwhile to keep a new "dead center" with a sharp
point just for use as a reference for setting the tool height.

A critical area which most people do not consider is the increase
in width of the c/o tool with heat. In normal turning this does
not cause a problem, but because of the c/o tool is cutting in a
deep slot and bounded on both sides, any appreciable increase in
heat and thus tool width will cause problems. A continuous
stream of lubricant/coolant is a necessity, both to keep the tool
cool and to flush out the chips. I find that for home use a
"windex" type spray bottle that can be adjusted to shoot a stream
is very handy. While you can use regular black sulphur [or other
heavy duty] cutting oil [try cutting with paint (not lacquer)
thinner [aka varsol] if too this is too thick], I find that water
soluable seems to work better in this application, most likely
because of its better cooling action. If you want to avoid
buying a 55 gallon drum you can get "water pump lubricant" at
most car parts stores/counters which is the same thing. This
will leave an oily protective film when the water evaporates.
One tip -- if you have hard water in your area use distilled.
This will help avoid the scum/deposit that hard water causes with
water soluable coolant.

One of the best things I have found to improve cutting off (in
addition to using the 'T" style cutter) is to fabricate an
"upside-down" tool holder which mounts at the rear of the cross
slide which incorporates 3 to 5 degrees of back-rake. The chips
clear the groove much better and the rigidity of the operation,
which is always important, seems much better.

You can see pictures of this at
http://www.mcduffee-associates.us/ma...g/rearcoth.htm
With this tool holder and a T-15 blade we have "parted off" 1.5
inch diameter pre-hardened [stress-proof] shafting with no
difficulty [but a large amount of "pucker"]. If it would be
helpful I will send you a dwg [autocad] file of this tool holder
sized to fit an Emco compact 10. The location of the slot for
the c/o blade must be adjusted for the center height of your
lathe and the desired amount of backrake. In our case a maximum
blade extension of 1_1/8 inches was used, allowing up to a 1 inch
depth of cut or a 2 inch solid bar.

You also need to remember that cutting off involves a thin tool
with a large amount of overhang which causes problems by itself.

Feeds and speeds are another thread. The heavier the feed the
thicker the chip and the more heat that is generated. The faster
the speed the more heat that is generated. With the upside down
holder and T-15 blades I have found that the highside RPM that
you would use for regular HS machining for the material and
diameter seems to provide a good starting point. Note that for
a given rpm the effective sfm will be the highest at the start of
the cut and decrease to zero at the exact center. The Emco
Compact 10 has only manual cross slide feed, but the general rule
seems to be to not let the tool rub as this cause heat, but to
keep the feed to the low side to produce thin chips and low heat.
[Stainless that workhardens is another topic] You should back off
the tool frequently to make sure the chips are clearing and to
allow the tool to cool down. Unless you are in a production
environment, the few minutes you save by maximizing the c/o
process are not worth the aggrivation.

While we used T-15 cutoff tools, this was because we received
several of these in some donated tooling from a screw machine
shop. Regular M2 seems to work as well for low volume usage and
has the benefit that it is easier to grind, is less brittle that
cobalt or carbide and thus less prone to corner chipping and
breakage. OBTW -- watch for corner wear and chipping/breakage
when parting off and sharpen the tool as soon as you notice this
or you notice that the machine seems to sound different or seems
to be working harder. It is *MUCH* quicker/cheaper to touch-up
the end of the tool and reset it than it is to correct a "crash"
which may involve a new tool and a scrapped part. Corner damage
is prone to occur at the start of the cut both because of
eccentric part location -- the tool digs in and because of
surface contamination -- rust. If you are using salvage
materials be careful of case hardened shafting.

I hope this helps.