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When the economy
slows down and orders become fewer in number and far more competitive
in terms of pricing, it is important to find ways to reduce the cost
of manufacturing. In the routing industry there have been numerous technological
advances in machines, materials, and tooling aimed at raising the efficiency
(and therefore lowering the cost) of production. In the last five years
alone there have been great advances in many of these areas. Machines
have more than tripled their rapid traverse rates and at the same time
managed to increase their rigidity through advancements such as ceramic
bearings and HSK tool holders. Materials have increased their resistance
to crazing, scratching, and breakage while being offered in more colors,
sizes, and composite formulations than ever before. New tooling technology
has brought about increased surface finishes, longer tool life, faster
feedrates and less induced stresses on cut parts through better materials
and engineered geometries.
While these improvements have undeniably helped to increase the state
of the art for plastic routing, the unfortunate truth is that most application
troubles are still the result of basic and fundamental problems. These
troubles, if not found and eliminated, invariably cause increased cycle
times, cutter usage, and higher machining costs. Because they are core
machining principles, the solutions to these problems haven’t
changed since plastic CNC machining started its rapid growth in the
mid 1980s. For that reason, it is always a good idea to occasionally
get back to the basics and review what makes for a successful CNC routing
operation.
The four core concepts that require addressing for successful and profitable
routing operations are: Material, Rigidity, Tooling, and Programming.
By reviewing the fundamentals involved with each of these aspects of
the routing operation before a job begins the shop owner, machine programmer,
or machine operator can find opportunities to produce better parts at
a lower cost.
Material
With so many plastic grades in the marketplace today, it makes no sense
to ignore the applications support that plastic manufacturers can supply.
The fabricator, acting as an intermediary between the customer and the
material supplier, has the greatest opportunity to select a material
which meets the customer’s specifications but is easier to machine
than similar formulations. There are numerous resources for machining
information available and they should be utilized as early as possible
to help the customer specify the best material grade for all aspects
of the design, manufacture and final use of the product.
Whether published or residing in the experience of a seasoned applications
engineer, router manufacturers typically have a wealth of knowledge
in regards to plastics machining. It is always a good idea to contact
the machine manufacturer for some up front advice on different material
styles.
Tooling companies can provide very timely information on some of the
newer materials as well as the old standbys. Due to the expendable nature
of their product, tooling companies typically have more involvement
in the day-to-day machining of products and may see a more varied spectrum
of successful applications. Knowledge of a material’s benefits
and limitations can an invaluable resource to resourceful inquisitors.
Lastly, and most importantly, material manufacturers can be the best
resource when it comes to pre-job material selection. They typically
have specialized materials that are designed with machining in mind
and applications personnel can assist both the fabricator and the end-user
in the selection of materials that are router friendly. Most larger
companies publish drilling, routing, and sawing specifications for their
popular materials and many of them have joined the www.PlasticRouting.com
website to help develop a centralized database allowing the comparison
of material machinability characteristics between manufacturer, grade,
color, and thickness.
By reviewing the characteristics of materials before a job begins in
terms of formability, resistance to damage, machinability, and customer
specifications, the fabricator can help reduce production costs while
increasing the chances of a satisfied customer.
Rigidity
Realistically, material specification can be a difficult parameter for
the fabricator to control. Many times the specification is already decided
when the job is offered to the fabricator. Fortunately, this is not
the case for the next most important variable in the job: Rigidity.
This parameter is entirely up to the router operator and is the most
critical parameter over which the operator has control.
Rigidity applies both to the machine itself and to the fixturing of
the components to be cut. (It can also be applied to the tooling, but
that subject is better dealt with in the Tooling section.) A router
that is poorly maintained will never be capable of achieving the results
of even the oldest machine that has been properly kept up. Plastics
machining is entirely different than the routing of other materials
in that the feedrates are typically much faster than in standard metal
milling and the finish requirements are much more precise than in wood
routing.
Properly lubricated and maintained machine slides and drive systems
are essential to maintaining optimum feedrates. Since plastic is so
sensitive to the relative motion of the cutter, any backlash or worn
track or ball areas can have a visually noticeable effect on the part.
Any play in the table or spindle mounting systems can cause erratic
marring of the work surface. Failure to follow a preventive maintenance
schedule with the spindle can cause concentricity problems so severe
that no tooling will produce an acceptable finish. It is important to
remember that routers are not milling machines. They are typically much
larger than a standard horizontal mill and are built with speed as a
primary focus and rigidity as a second focus (albeit, still a critical
one). Routers are a viable method of production if the operator understands
the limitations imposed when using a 10 foot aluminum table versus a
3 foot steel bed as a worksurface. Preventive maintenance of CNC routers
is critical to long term operation when part surface finishes are critical.
While machine rigidity is critical to consistent performance, fixturing
is equally important to individual performance per part. As has been
stated in previous articles, surface finish for metals and plastics
is typically measured in millionths of an inch. Consider that even .001”
of part movement is 50 times the magnitude of what is generally considered
a good surface finish. With such a low margin of error, it is essential
that everything possible be done to allow the machine and cutting tool
a chance to produce optimum finishes.
Fixtures should be rigidly built and mounted to the worksurface. Vacuum
supply should be oversized whenever possible and hard fixturing should
be securely mounted and without slop. When dealing with 5-axis fixtures,
unsupported overhangs should be minimized and vacuum distribution should
be brought as close as possible to the area being cut. Friction enhancements
such as rubberized coatings or gasketing sheet foam are always a good
idea.
Tooling
With the thousands of available choices for tooling, this could seem
to be a difficult parameter to optimize. However, the contrary is actually
true. The reason for the large selection of available tooling is the
fact that it has become so specialized over time. The best methods for
specifying tooling for a particular job is either published resources
or vendor representatives. Published resources can be recommendations
from material suppliers, empirical test data such as www.PlasticRouting.com,
or vendor catalogs with tool selection cross references. Experienced
vendor representatives or applications engineers can also be of infinite
value because of their knowledge of similar applications and the pitfalls
to avoid.
The goal for best performance is to find the tool geometry that was
developed specifically for the type of material being cut and the machine
being used (i.e. 3-axis, 5-axis, carving, etc.). Additional factors
that should be considered are:
- Tool Material:
Carbide for finish, steel for sharpness, diamond for life.
- Tool Diameter:
Is 1/4 inch required or can 3/8 inch be used to produce a better finish?
- Cutting Length:
Are stub length tools available for better rigidity?
- Shank Diameter:
Cutting diameters smaller than the shank can lead to tool breakage.
- Helix: What
are the part hold down parameters? Should low helix, high helix, or
straight cutters be used?
Programming
Once material, machine rigidity, fixturing, and tooling have all been
selected for best operating practices the final step is programming
the part path. There is a tremendous amount of material published concerning
this process, but by just focusing on the basics a good probability
of success can be assumed. Some general rules of thumb for routing of
plastics:
- Cut Direction
Matters: In almost all cases the conventional cut or climb cut side
will produce a better finish than its counterpart. The best method
for determination is trial and error. Compare both the finished part
and the scrap for edge quality. If the scrap is better, reverse the
cut direction. For empirical data, once again www.PlasticRouting.com can be consulted. As a starting
point, larger tool diameters typically work better in a conventional
cut presentation and smaller diameters are workpiece specific.
- Chiploads:
Chipload is the size of the chip being formed. It is the result of
the number of cutting edges, the spindle RPM, and the feedrate. Router
bits work best at a very specific chipload and can perform quite poorly
even a few .001 inch from the optimum value. Consult with the tooling
manufacture for a good starting point and then vary feedrates or RPMs
to determine the best cutting zone for the particular job.
- Cutter Entry:
Router bits that plunge directly into the workpiece can wrap long
chips, deform part edges, or melt the surrounding surface. Always
ramp or helically plunge into a scrap area and rout to the part edge
to prevent these problems.
- Scrap: Try
to minimize the amount of unsecured scrap and thin wall scrap that
is present. Poor scrap control can lead to part ejection, vibration,
and broken cutters.
A thorough review
of each of these fundamental areas – Material, Rigidity, Tooling,
Programming – can eliminate many of the problems that arise every
day in the routing industry. Far too often fabricators have spent money
and time fine tuning programs and part paths for optimum cycle times
when building better fixtures or selecting different tooling could have
doubled feedrates with minimal additional effort. Poor programming has
been solved many times with custom (i.e. expensive) tooling when rewriting
cutter entry cycles or part paths could have resulted in immediate solutions
and the use of off-the-shelf tooling.
Perhaps the worst example and unfortunately the most common is the application
where the machine must be run at a minimal feedrate to prevent chattering
of the part. If machine condition is preventing fast feedrates, fix
the machine – do not decide that the maintenance cost is prohibitive
or the downtime is not possible. Once machine wear begins, it accelerates
quickly. It must be caught in the beginning and fixed promptly to prevent
a myriad of problems later.
With a solid return of Back to the Basics fundamentals in the routing
process, delays can be eliminated, cycle times optimized, and costs
reduced.
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