Finishing
Ultra Small Holes
At
RTSI we utilize much of our time and energy in training and
application with Citizen Swiss type sliding spindle turning/ milling
machines. These machines are very suitable for micro machining of
small type parts, as sometimes used in the medical industry. A
typical small part can have an outside diameter of 0.025 with an
overall length of 0.450 and an inner diameter of 0.022, resulting in
a thin wall part. We
recently had the task of turning and drilling this part from 316
stainless steel.
A
part this small can be drilled with some difficulty.
What can be done if a superior surface finish is required in
the inner diameter? We’ve
recently been introduced to a super finishing process called
wire-DSM. Schlafli
Engineering (www.schafli.com) in
Switzerland
developed this
process for use in the watch making industries and now this
technology is helping in other high-tech, micro machining areas like
electronic and medical industries.
The
basic process can be described a “wire honing”.
A high precision wire (coated with diamond paste) is fed
through the bore feature of a rotating part.
Typical
applications for this method can be found in the bearing industry
with the finishing of sapphire and ceramic bearings.
Both ceramics and sapphires are very hard but with the
application of the diamond cutting medium, wire-DSM can achieve
roundness tolerances of 0.00004 and surface finishes of 1 micro inch
Ra.
Another
interesting application is in the production of water-jet nozzles
from a zircon stabilized ceramic material. The water jet process
requires a delivery nozzle that can withstand the abrasive effect of
the water medium. The machining of the bore feature in this highly
resistance material is a good application for the wire honing
process.
The
coated wire is coil fed through the bore and allows finishing of
holes as small as 0.002” diameter with a length up to 12”
overall. The process can finish longer tube type parts or with the
aid of special fixtures numerous parts can be clamped together and
honed in batches. The part or parts are rotated as the wire is fed
through the bore. |
The
honing process uses a precision wire that is between 3 – 15 feet
in length.
This
wire can be as small as 0.001 diameter. The insertion of this small
wire into a part can present some problems. The wire has a slight
taper that is used in conjunction with the diamond paste to machine
the part. A major factor in the success of this method is rotating
the parts to assist in the honing process.
Typical
cycle times can be fairly quick. The above water-jet nozzles have an
overall length of 0.040” with a through hole of 0.010” diameter.
The initial hole size was increased to 0.014 during the honing
process. That is a large amount of material to be removed from such
a small initial hole.
Another
situation where this honing method might be useful is in finishing
thin wall tube-type parts. Many
applications call for high quality ID finished, especially in the
medical industry.
The
challenge in producing such small diameters normally lay in
manufacturing the initial tube prior to diamond honing. Parts can be
drilled with micro drills, produced with EDM or a laser. The quality
of the initial manufacturing process will effect the time required
to finish the hole with the wire honing method.
The
future of micro machining is here with the production of smaller and
smaller parts. Micro honing allows for the production of both close
tolerance holes with superior surface finishes. As parts keep
getting smaller, demand for this type of high-quality finishing will
only get bigger.
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