Diamond and cutting
tools |
| What is
diamond? |
|
Diamond is composed of pure carbon
atoms, arranged in a very special crystal orientation that gives it its unique
physical properties. The key element that allows for the special crystal
structure is the “sp3” bonding of the carbon atoms—and that is
where sp3’s company name comes from. |
|
| What materials are
best suited to machining with diamond? |
|
Abrasive non-metallic materials,
non-ferrous metals, and abrasive non-ferrous metals. |
|
| Why can't you machine
ferrous metals with diamond? |
|
Diamond is unaffected by almost
every other chemical or compound in nature. One exception is hot iron. The
carbon atoms in diamond will dissolve into the iron, quickly eroding the
diamond surface. Iron wheels are used for polishing natural diamond. |
|
| How thick is the
diamond on sp3 tools? |
 |
Thickness is tailored to the
application, and can range from 5 to 50 microns. |
|
| How do coating
thicknesses in microns relate to inches? |
|
1 micron is 1 millionth of a
meter, or 0.00004 inch. A coating thickness of 10 microns is therefore 0.0004
inch; 40 microns is 0.0016 inch. |
|
| How do you grow
diamond? |
|
The diamond
coating is grown in a vacuum chamber using feed gases of hydrogen and methane.
Usually the gases are fed into the chamber in a 50:1 mix, predominantly
hydrogen. A very high temperature element in the chamber causes the deposition
process to take place.
sp3
uses fine filaments energized to a temperature of approximately 2200°C. The
high temperature breaks down the methane into carbon and hydrogen. The carbon
atoms will nucleate and grow tiny crystals of diamond. In time these tiny
diamond crystallites will grow into a continuous diamond film. |
|
| How long does it take
to coat a tool? |
|
Diamond grows slowly, about 0.5 to
1.0 micron per hour. A typical reactor run to grow 40-micron films on inserts
can take almost two days; most round tools are coated overnight.
Because of the slow
diamond growth rate sp3 finds ways to maximize the number of
tools that are coated in every reactor run. |
|
| How many tools can be
coated in a reactor? |
|
The sp3 reactor
provides a uniform diamond coating over an area of about a square foot. This
enables coating 200 to 300 inserts at a time, or 65 to 100 round tools. |
|
| Why are diamond-coated
tools so expensive? |
|
Primarily because of the slow
growth rate of the diamond film—0.5 to 1.0 micron per hour. Another factor
is pretreatment of the substrate to guarantee good adhesion. As pretreatment
and deposition processes are further developed costs will drop, as they do in
most maturing technologies. |
|
| How do you make the
diamond coating stick? |
|
Left to itself, diamond has almost
no chemical bond to cemented carbides. The trick is to pretreat the carbide
surface, roughening it so the diamond can grow into the surface and attach
itself through the mechanical interlocking of the carbide and the diamond. |
|
| How sharp is the
cutting edge on a diamond-coated tool? |
|
The coating will naturally tend
to round the edge somewhat. Sharpness is a function of the sharpness of the
uncoated tool and the thickness of the film. Thin (10 micron) films over a dead
sharp edge are still quite sharp. Thicker films, e.g., 40 microns, will
approach an “A” hone in sharpness. |
Custom Coating |
| What carbide grades can be
coated? |
|
This can’t be answered in
just a few words. In general, typical C2 nonmicrograin carbides are the best
candidates. See specific grades accepted by
sp3. |
|
| Can sp3 coat
micrograin carbides? |
|
sp3 can coat
down to about a 1 micron carbide grain size. If the carbide is too fine it does
not respond to the preparation, which requires a rough surface for good
adhesion. |
|
| If I send sp3 a
tool, can they coat it? |
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If it is in an approved carbide
grade we an coat it. See specific grades accepted by
sp3. |
|
| If I send sp3 a tool
in an acceptable grade for coating, how long will it take? |
|
sp3 can
typically turn around custom coating requests in about 10 working days. |
|
| Can tools coated with other coatings
such as TiC and TiN be coated with diamond? |
|
Unfortunately not.
sp3 needs a bare carbide tool of the proper grade to insure a
successful bond between the diamond and the carbide. |
|
| Can tools be diamond coated a second
time? |
|
sp3 does not
coat tools that have already had a diamond coating for two reasons. The first
is that the diamond deposition process alters the carbide at the surface, and
the required surface preparation will be less predictable a second time.
The second reason is
the uncertainty of the integrity of the ground edges. Often a tool is run until
the diamond film is worn through in one or more spots. The carbide substrate
will wear very quickly once the diamond is gone, leaving a notch or groove in
one or more spots on the tool. These defects are difficult or impossible to
remove with further grinding. |
|
| Can HSS tools be coated? |
|
During deposition the substrate
temperature is about 850°C. High-speed steel will not survive these high
temperatures. |
|
| Can tools with brazed carbide tips be
coated? |
|
The normal deposition temperature
is 850°C. Typical brazing will not survive these high temperatures. |
|
| Can you braze diamond-coated tools?
|
|
This is feasible if the braze is
done in a non-oxidizing atmosphere such as argon, and temperatures are kept as
low as possible. However, one consideration is that the tool cannot be touched
up or ground to size after brazing. You would be grinding a thin diamond film,
and it would be very difficult to grind accurately without the possibility of
breaking through the film. |
Machining with diamond tools
|
| Can I expect longer tool life than
carbide with diamond and, if so, how much longer? |
|
Yes, you can. How much longer
depends a great deal on the application. In very abrasive metals such as 390
aluminum a 10x life increase in not uncommon. In graphite, lifetime increases
between 30x and 50x are common. In glass fiber filled materials 30x is
common. |
|
| What are the advantages of diamond
other than longer life? |
|
Accuracy: The diamond
coating is very thin, but very hard, and tools don’t change significantly
in size during their life. For instance, the radius on an endmill will change
by about 10 microns (0.0004") from when new to the point that it is worn
out.
Speed: Diamond tools can typically be run at two to three times
the surface speed of carbide tools.
Dry cutting: Diamond tools can
often convert an operation from wet to dry machining, providing a significant
saving in overall machining costs. |
|
| Should you use coolant with diamond
tools? |
|
Diamond tools can often be run
dry. In some cases coolant can shorten tool life due to thermal shock.
If coolant is used
for chip removal, it can provide a better finish in some applications. Coolant
also contributes some lubricity which can be helpful in some applications.
In general, we recommend to try running dry or with mist coolant. Use flood
coolant only when chip removal or finish demands it. |
|
| How do diamond-coated tools perform
with respect to surface finish? |
|
The application of a diamond film
to a tool has the effect of making the tool less sharp. What is the consequent
effect on surface finish?
First, the question
must be asked—what do we mean by finish? Is it an Ra reading using a
profilometer, or is it surface sheen or brilliance that is important?
In terms of an Ra
reading, CVD diamond tools often provide a finish similar to carbide or PCD
tools.
With
respect to visual finish, diamond films are composed of millions of tiny
crystals, which will cause micro scratches on the surface of some materials,
leaving a matte finish rather than a mirror finish. If a mirror finish is
required, stay with upsharp carbide or PCD tools. |
|
| What is the difference between DLC and
diamond coatings? |
|
DLC is an amorphous carbon film
called Diamond Like Carbon. It is not diamond because it is not
crystalline. DLC films are typically 1 to 2 microns thick, and do not hold
up in very abrasive materials such as silicon aluminum. In graphite thay will
typically last about 10 to 15% of the life of a diamond tool. See a
comparative test result. |
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