MTConnect: Different Devices, Common Connection Changing ...

INDUSTRY PROFILE: ENGINEERED FINISHING CORP.
The first step for minimizing is
to reduce the costs for individual
finishing and roughing passes for the
various depths of the cut. Considered
in the next step is the combination of
depths of the cut for the finishing and
roughing passes. Depth of cuts, along
with an optimal number of passes,
are combined with the stepover path
distance to minimize run times and
cost. The stepover value determines
whether the surface finish on a
component is rough or smooth.
For example, when using a
flatbottomed tool such as an end
mill, the stepover value is normally
around 70 percent of the cutter
diameter in combination with depth,
feed, and speed rates. Raster passes,
radial passes, spiral passes, morph
passes, and boundary passes are the
methods of choice for true surface
machining. Allocating expensive CNC
machine time for cleanup and surface
finishing should be considered overprocessing
and costly when measured
in machine time (throughput dollars)
and the achieved results. Abrasive Flow
Grinding (also known as abrasive flow
machining) is a cost effective machining
process for finishing and polishing a
surface, including difficult-to-reach
surfaces and internal passages.
The Abrasive Flow Grinding process
involves three principle elements: a
tooling fixture, the machine, and the
abrasive. The typical Abrasive Flow
Grinding process uses two opposing
cylinders to push the media in two
directions, i.e., back and forth with a
controlled flow pressure. Tooling is
used to direct the media flow across the
surface of the part and through internal
passages and intersecting passages. Flow
grinding action occurs wherever the
media comes in contact with a surface.
The flow pressure as well as the number
of finishing cycles (back and forth media
grinding/flow) are controlled.
A workpiece fixture is used to hold
a part or multiple parts for processing.
The fixture directs and controls the
volume and pressure of media flow
across surfaces and through internal
passages. Lightweight fixtures are placed
on and removed from the lower media
Figure 3
cylinder by the machine operator. The
lower media cylinder (the processing
station) is located in the center of the
worktable. Higher production and/
or heavier fixtures can be positioned
automatically from the loading station to
the processing station by a hydraulically
actuated dual fixture rotary table. In the
way of increasing throughput dollars,
flow grinding reduces other costs by
reducing and/or eliminating handwork
that may not be as uniform, repeatable,
or predictable.
The flow grinding media is made of
a flowable polymer carrier mixed with
one or more sizes of abrasive grain. The
media viscosity range is from a soft,
almost grease-like consistency, to a firm,
putty-like material consistency. With
a pure grinding and polishing backand-forth
type of action, the grinding/
abrasive media flows across surfaces
or through internal passages to refine
surfaces and edges.
The most commonly used abrasives
are silicon carbide and aluminum oxide.
These provide a good balance between
high performance and moderate cost.
Typically, the particle sizes of abrasives
used range from a coarse 20 mesh
abrasive (average particle size ~0.9
mm) to a very fine 600 mesh abrasive
(average particle size ~0.9 microns).
Abrasive media will contain around 25
percent up to around 67 percent, by
weight of the abrasive grain.
Abrasive Flow Grinding has helped
us achieve polished and lapped external
and internal passages characterized by
their measured Ra values. Tools that are
used together can help to reduce waste
as measured in throughput dollars and
over processing. All production practices
promote the reduction of waste,
reduction of over-processing, and the
control of flow for increasing throughput
dollars. The benefits of milling a finished
surface probably are not worth the cost.
Several examples of surface finishes we
have accomplished with abrasive flow
grinding are shown below.
September | October | 2012 2012 PRECISION MANUFACTURING | 19| 19