F. George McDuffee wrote:


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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.

I get the wide chip part. The tool had a very steep angle by my
thingking likely 30 degrees or more.

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 want to make sure I understand plain vs T. The HSS blank is wider
at top for about an 1/8's or so of an inch and the rest of the blank
is narrower. Is the plain a rectangular profile?

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.

I didn't notice the dish but I will look tomorrow when I get to work.

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.

Thanks for that tip.

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.

Our cnc's use water soluable. I was using sulfur oil with a brush.

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.

Please send file. My anti spam address is at bottom. My other
address also works but I don't check that one also.

I saw the pictures on your site. The gent milling should have had
safety glasses on.

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.


Thanks,

Wes
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