|
Finding
the Sweet Spot in Microtool Woodworking
The
first question that most people ask when using carbide mini
and micro tools to cut wood (and other soft materials) for
the first time, is, "What are the best speeds and feeds?".
What they really want to know is, "How fast I can cut
without breaking a bit? What are the optimum cutting conditions
with my equipment?"
As
with most things in life, a precise answer to these questions
is usually very difficult to come by. However, there are a
few techniques that one can use to determine a very good approximation
to the ideal conditions for machining the material at hand.
The
following discussion assumes that you have measured
the runout (TIR) of your spindle and found it to be less
than 0.001" (0.025mm), that your spindle bore and collet
/ tool holder have been cleaned
with ColletCare and, finally, that the backlash on both
the X and Y axes of your CNC router are less than 0.001"
(0.025mm).
In
any material, tool performance, and longevity are primarily
influenced by:
- feedrate
ramp or acceleration
- feedrate
- spindle
RPM
Acceleration
Acceleration
is defined as the rate that a velocity changes. The acceleration
that we are most concerned with is how fast an XY movement
goes from 0 to the selected feedrate (F) once the computer
tells the controller to move. In the graph below, the acceleration
(also known as ΔV/ΔT) is the slope of the ramp of
the red velocity plot. The steeper the ramp (higher acceleration),
the quicker the axes get up to speed. The shallower the ramp
(low acceleration), the longer it takes to get to "F". |
|  |
| From
the point of view of higher performance, higher acceleration
means shorter cycle times and snappier customer demos. Unfortunately,
as acceleration is increased, more transient transverse stress
is exerted on the bit. If the stress exceeds a certain threshold
(transverse rupture point), the bit will break. This is like
whacking something brittle with a hammer (or your boss's head
with a new idea). A little tap might not be a big deal, but
a resounding smack will send shrapnel flying. In the case of
cutting wood with small diameter carbide tools, it turns out
that there is a pretty good compromise between higher performance
and longer tool life. If
you set your acceleration to:
1
inch per second per second, or
60 inches per minute per second, or
25mm per second per second, or
1.5m per minute per second
your
bit will think that you are coddling it like a wee little
baby and not break as soon as it starts to move. This is especially
important as the bit starts to get dull and cutting resistance
increases. Of course, if you set the feedrate (F) too high,
the rupture point will eventually be exceeded and the bit
will break anyway, but more about this in a moment.
Feedrate
and Spindle RPM
We
consider the feedrate (FEED) and the spindle RPM (SPEED) together
because, in the case of cutting soft materials like wood and
plastic, it is their combination into a single parameter known
as "CHIP LOAD" that matters the most. As the name
suggests, chip load is the amount of material (load) each
flute cuts during each revolution (every chip). Another way
of looking at it is how far the bits chews into the material
every time it rotates one full turn.
As
the chip load increases (deeper penetration per revolution),
the transverse stress on the tool increases. Clearly it is
important to keep the stress below the breaking point of the
tool. On the other hand, at very low chip loads, not much
material is being cut so there is nothing to carry heat away
from the bit. Below a certain limit, the tools gets too hot
and abrasion rolls away the cutting edge, rendering the bit
useless.
There
is another aspect to chip load that is often overlooked. As
the bit turns and starts to cut, material "flows"
across the outside and inside faces of the cutting flutes.
If this material "flow" is too high on the outer
surface (low chip load) the cutting edge rounds over. It the
material "flow" is too high on the inner surface
(high chip load) the cutting debris cannot be evacuated quickly
enough causing it to back up and pack. With nowhere to go,
the impacted material seals off the flute and the bit breaks.
When the outside and inside "flow rates" are balanced,
edge erosion is symmetric and the bit stays sharper longer.
We call this the "sweet spot"
What
we need to do is find this magical point where feed and speed
are balanced in such a way to provide optimum cooling, and
balanced material flow to maintain the sharpness and integrity
of the cutting edges. Happily, in the case of many soft materials,
this is relatively simple to do. The figure below shows a
simple pattern that can be used to derive the optimum machining
parameters for any combination of wood and cutting tool. |
|  |
The
program is quite simple.
- A
set of parallel slots 1" long are spaced apart 3 times
the diameter of the bit that you are testing (S = 3 X Bit
dia.)
- For
a given RPM (SPEED), an initial feedrate (FEED) is chosen
so that the bit cuts 0.0005" per revolution. For example,
if your speed is 20,000 RPM then a feed of 10 IPM (inches
per minute) will yield a full bit chip load of 0.0005"/rev.
(just divide the feed by the speed).
- Plunge
as deep as you intend to cut and make the first 1"
slot.
- Pick
up the tool, move to the top of next slot.
- Increase
the feed by 1" (25mm) per minute.
- Cut
the second slot.
- Pick
up the tool, move to the top of next slot.
- Again
increase the feed by 1" (25mm) per minute.
- Continue
in this fashion until one of two things occurs. Either
the bit will break or the quality of the cut will markedly
deteriorate.
- Whichever
happens, stop the test and record the feedrate where the
bit started to fail.
- Multiply
this value by 0.75 to get the sweet spot for these cutting
conditions.
Schematically
the cut sequence would like something like: |
 |
| "BREAK
THE BIT? Are you out of your mind?" you scream. Well, maybe
a little bit, but not in the way that you mean. The reason for
this test is to show that the optimum cutting point is MUCH
closer to bit breakage than it is to the point where the feed
is so low that friction from the spinning bit actually
burns the wood. The good news is that, once you
have done this a couple of times with different diameters and
a variety of materials, you will be able to calculate the approximate
the sweet spot for any other bit / material combination. But
that is a topic for another tutorial. |
Todd
Reith (Reith
Guitars) offers a very valid objection to this testing
scenario. He points out that lifting the tool between each cut
does not accurately reflect the cutting dynamics encountered
in most machining operations. He proposes the alternate cutting
plan comprising:
- A
set of parallel slots 1" long spaced apart 3 times
the diameter of the bit that you are testing (S = 3 X Bit
dia.)
- For
a given RPM (SPEED), an initial feedrate (FEED) is chosen
so that the bit cuts 0.0005" per revolution. For example,
if your speed is 20,000 RPM then a feed of 10 IPM (inches
per minute) will yield a full bit chip load of 0.0005"/rev.
(just divide the feed by the speed).
- Plunge
as deep as you intend to cut and make the first 1"
slot.
- Cut
to the right a distance equal to S.
- Increase
the feed by 1" (25mm) per minute.
- Cut
the second slot.
- Cut
to the right a distance equal to S.
- Again
increase the feed by 1" (25mm) per minute.
- Continue
in this fashion until one of two things occurs. Either
the bit will break or the quality of the cut will markedly
deteriorate.
- Whichever
happens, stop the test and record the feedrate where the
bit started to fail.
- Multiply
this value by 0.75 to get the sweet spot for these cutting
conditions.
|
 |
Information
brought to you by:
|
