The Importance of Spoilboards
in the Machining Process
The emphasis today in the
fabricating and machining of plastics is CNC — or computer numerical controlled
machines. These high spindle speed, high feed rate machines accomplish a tremendous
amount of work in cycle times, which enhance productivity and profitability.
A great deal of time is devoted to the selection of such machines and the appropriate
tooling and accessories, but the area of spoilboard methodology is given less
consideration. Without proper investment of time in this critical area, the
holding of parts to accomplish maximum productivity becomes challenging at best.
Types of spoilboards
The dedicated or discreet spoilboard system has traditionally been utilized
over the years to machine individual parts that are held by a gasketed vacuum
system. Unfortunately, many times the process of adequately constructing these
spoilboards has been ignored in the interest of time. The use of a piece of
MDF or particleboard with holes drilled inside an area encapsulated by self-stick
weather stripping does not meet the demands placed on the parts in a high-speed
application. Consequently, it is imperative to follow certain criteria when
building spoilboards to maximize the part hold-down procedure.
First, the selection of
gasketing material is foremost in the process of building substantial dedicated
spoilboards. This material should be quality closed cell foam, which has the
ability to return to its original configuration repeatedly under rigorous machining
conditions. Self-stick weather stripping, which is open cell construction, does
not possess such memory characteristics and should never be utilized. The gasketing
material represents the perimeter of the part configuration and must have the
resiliency and durability to maximize part rigidity and reduce vibration. A
good technical source for gasket material and usage is at www.allstaradhesives.com.
After the proper gasketing
material is selected, a channel should be routed into the spoilboard to establish
the outside perimeter of the part and as a recessed area for application of
the gasketing tape. This process allows the part to be solidly held to the table
surface, and prolongs the life of tape. The source of vacuum is port holes drilled
in the interior of the gasket perimeter. In order to generate a larger vacuum
surface area the holes should be connected to a groove routed just inside the
gasket perimeter. This provides an arterial flow to the outermost edge of the
part and substantially increases rigidity (Figure 1). Furthermore, the actual
spoilboard is often significantly impacted when constructed of double-sided
melamine board to reduce the leakage of vacuum associated with raw board materials.
The second type of spoilboard
is universal vacuum, which is also referred to as high volume, flow through
or suck through vacuum. This method distributes volume of vacuum throughout
the entire surface of low or medium density spoilboard, and has gained popularity
because of minimal setup time. The process is utilized to cut parts from whole
sheets of raw materials, and is particularly effective on larger parts where
part movement is not a major concern. However, smaller parts can become problematic
with this spoilboard approach and other techniques should be employed to avoid
part movement. Tab cutting and skin cutting techniques are especially effective
in dealing with small parts. This involves leaving a tab or a thin layer of
material on the bottom of the part to hold them together. The tab or skin portion
is then removed in a secondary operation. This process is slightly more time
consuming, but the final results are quality edges and less scrapped parts caused
by movement during the machining process.
Since the universal vacuum
approach involves high flow without the inherent benefits of dedicated spoilboards,
the opportunity for leaks and subsequent part movement is always present. In
order to minimize those kinds of problems, additional techniques can be applied.
Rubberized paint can be applied to seal spoilboard edges. Smaller diameter tools
will minimize cutting pressure and reduce larger open spaces on the cutting
area. Scrap parts or plastic sheet can be used to cover open unused areas of
the spoilboard when smaller than whole sheet raw material is utilized. Furthermore,
the spoilboard should be surfaced with a large diameter spoilboard surfacing
cutter. This process should be done initially to both sides of the spoilboard
before actual machining of parts to increase porosity and ensure the surface
of the board is flat. The cutter is continually used to maintain those two parameters
and eliminate rout lines caused by the cutting tool during the machining process
In addition to actual spoilboard,
the vacuum system should be constantly evaluated to guarantee optimum performance.
Rating this system should include the following criteria:
Does the pump have enough
- 400 cfm w/15in HG for
- 80 cfm w/25in Hg for
dedicated systems. Are supply lines large enough?
- 3 inch diameter minimum
for flow-through systems, 4 to 5 inches recommended.
- 3/8 inch diameter for
dedicated systems, 1/2 inch recommended.
Are there enough vacuum
How many bends are in the supply lines?
Are potential vacuum leak areas and unused areas sealed?
Regardless of which spoilboard application is utilized, it is imperative to
follow good construction and enhancement techniques to ensure parts are held
solidly. In the world of high-speed machining in plastics, it is the only method
to produce quality parts on a consistent basis.