CNC Machining Design Guide
Apr 22, 2019
Apr 22, 2019
A comprehensive guide to help you achieve the best results for
your CNC machined products and components.
Though physical parts come to life on the shop floor, the
manufacturing process actually begins long before the CNC
milling and turning machines are turned on. It all starts with
the design process.
Not only is the design process an essential step in coming up
with a product or part’s dimensions and structure, but it can
actually ease the manufacturing process down the line by
accounting for certain limitations inherent in the production
In order to ensure that our clients are getting the most out of
our manufacturing services, we’ve compiled a complete design
guide for CNC Machining filled with helpful design practices,
tips and dos and don’ts.
In the CNC Machining Design Guide we will cover the basics of
what CNC Machining is, the process’ design limitations, our
specific CNC services, how to choose the best material for your
project and more. Our CNC Machining Design Guide can help you
not only maximize your product order but also keep costs low.
In the simplest terms, Computer Numerical Control (CNC)
Machining is a subtractive manufacturing technology that
produces finished parts by removing layers from a solid block of
material, known as a blank. CNC Milling, one of the main types
of CNC Machining uses cutting tools that rotate at a rate of
several thousand rotations per minute (RPM) to precisely remove
material until a net shape is achieved. In addition to milling,
CNC Machining can also use lathes and drills to produce parts or
As a computer-controlled manufacturing process, CNC Machining
produces parts based on a Computer Aided Design (CAD) model that
is sent to the CNC Machine via a Computer Aided Manufacturing
CNC Machines operate on a variable number of axes. A traditional
3-axis CNC machine is able to manipulate and work parts along
the three linear axes: X, Y and Z. The technique is most
effective for relatively simple, shallow parts. A 5-axis machine
is capable of working along the three linear axes as well as
around two rotational axes. This enables more complex and deeper
cuts to be made, broadening the design potential of parts.
Because of CNC Machining’s precision, speed and automation, it
has become one of the most widely used subtractive manufacturing
processes. One of the technology’s most common uses is the
production of metal prototypes, as it is currently one of the
most effective ways to produce small series or one off metal
parts. CNC Machining is also highly scalable, meaning it can be
used for one-off production and prototyping as well as medium-
to high-volume production.
CNC Machining offers a number of advantages to manufacturers,
including a high level of automation, excellent reliability,
reproducibility and high geometric accuracy. The process also
offers a high quality of surface finish for parts and provides
Despite all these advantages, however, there are certain limitations to CNC Machining. Fortunately, if these limitations are taken into account during the design process, production times and costs have the potential to be reduced and the manufacturing process can be optimized.
CNC Machining is a popular manufacturing process because of its ability to produce a broad variety of parts, ranging from simple shapes to complex structures. Like every manufacturing technology, however, CNC Machining does come with some design limitations. We’ve broken them down below to ensure that your product is optimally designed to meet our CNC Machining’s capabilities.
When submitting a design for production, it is helpful to
include a tolerance, or the acceptable range for a part’s
dimensions. Tolerances must be determined to meet the required
functionality of a given part.
Though CNC Machining is capable of achieving extremely tight
tolerances, it is important to keep in mind that tolerances
tighter than our service’s standards can often incur longer
production times and higher costs.
If no specific tolerances are indicated on the product’s design,
RapidDirect maintains a standard tolerance of +/- -.005” for
metal parts and +/-.010” for plastic parts. Tighter tolerances
are possible, though can be more time consuming to achieve as
RPM rates must be decreased to account for the higher
With CNC milling, it is important to note that all interior
vertical edges or walls will have a radius, rather than a right
angle. This is because the cutting tools used in CNC milling are
cylindrical, meaning they can not produce a straight edge. This
type of angle is called an inside corner fillet.
When designing a part, taking the inside corner fillets into
account can result in a more efficient production process. For
instance, it is advisable to specify a non-standard radius to
allow the cutting tool the freedom to cut and turn the corner
without having to stop and reposition. Additionally, fillets
with a larger radius are faster and cheaper to mill than smaller
As features that cannot be produced using a standard milling
tool, undercuts present a number of challenges when CNC
Machining and should, if possible, be avoided. For one, if an
undercut geometry does not have standard dimensions, it will
require the production of a custom machining tool to produce,
which will increase machining time and costs significantly.
Therefore, if an undercut feature is necessary in your part,
ensure that it has standard dimensions.
Second, because CNC tools are limited in length, undercuts
should not be made too deep. Undercuts that are too deep or
placed in a hard-to-reach area will be impossible to produce as
the CNC tools will not be able to access them.
With CNC Machining, as with other manufacturing processes, it is
important to ensure that your part design complies with minimum
wall thicknesses. Generally, it is advisable to opt for the
thickest possible wall thickness and to avoid very thin walls or
features. This is because thin or fine features are susceptible
to vibrations caused by the CNC tools, which can cause breaks or
A standard minimum wall thickness for CNC Machined metals is
0.030” (~0.76 mm) and 0.060” (~1.5 mm) for plastics.
Once a part has been machined to its specified dimensions and
form, it is possible to add threads using cut taps, form taps or
thread mills. When designing your part, it is helpful to choose
the largest thread size possible, as smaller taps have a higher
risk of breaking during production.
If possible, also avoid deep threads, as deep holes in your part can incur higher production costs (if a custom tool is necessary). Similarly, sticking to custom thread sizes in your part’s design can help keep costs and production time down.
At RapidDirect, we offer a range of CNC Machining services,
including one of the most common CNC processes: CNC Milling. In
the CNC milling process, a material blank is mounted onto the
machine bed, where rotational cutting tools, such as endmills,
cut away at the raw material. These tools rotate at extremely
fast rates of thousands of rotations per minute (RPM) to remove
material resulting in a final part.
To make the CNC milling process as efficient as possible, the
initial material to be removed is cut away by larger tools at a
higher RPM rate. This enables the quick removal of the bulk of
the material. When the part is nearly cut to shape, higher
precision tools with fewer RPM come in to finish the part.
Once the part has successfully been cut to size, it is carefully
inspected to ensure that required tolerances are met and that
the quality is up to standard. From there, the part can be
post-processed and delivered to the client.
In the CNC Turning process, a material blank is mounted onto a
rotating chuck, which moves the part as stationary cutting tools
do their work. CNC Turning is well suited for producing parts
with symmetry along their central axis and is often faster and
cheaper than CNC milled products.
CNC Turning tools, or lathes, remain stationary as the raw
material is rotated at high speeds, coming into contact to
precisely remove material until a final geometry is achieved.
Because of the nature of CNC Turning, the process is mostly used
to produce cylindrical parts, however it is also possible to
produce non-cylindrical profiles using multi-axis CNC turning
and milling systems.
As a subtractive manufacturing process, CNC Machining builds
parts from a block of raw material—either metal or plastic. This
block is known as a material blank. No matter what material you
are working with, choosing the right dimensions of material
blank is crucial before the production process. Generally, it is
wise to choose a blank that is at least 0.125” (~0.3 cm) larger
in each dimension than the final part’s dimensions, to account
for any inconsistencies in the raw material. At the same time,
it is important not to use too big of a blank, to minimize
Another crucial thing to keep in mind before ordering your CNC
machined part is that your material selection will impact
production time and cost. This is because certain materials have
better machinability than others, meaning they are easier and
faster to process. The faster machining rates ultimately
translate to lower production costs.
As a general rule, metals that are highly ductile are easier to
machine, as the CNC mills can cut them more easily and achieve
higher RPM rates. Brass, for instance, is one of the easiest
metals to machine because of its good malleability. Aluminum
alloys are also well suited for CNC Machining and result in fast
Steel, on the other hand, is a strong metal that has a far lower
machinability rate than aluminum, meaning that production rates
will be slower and higher in cost. It is important to keep in
mind, however, that different grades of steel have different
machinability levels depending on their carbon content. Steels
with with very low and very high carbon contents are typically
challenging to machine.
For instance, Stainless steel 304, which has a relatively low
carbon content, is difficult to machine as it becomes gummy and
hardens too quickly. Additives such as sulphur and phosphorus
can make stainless steel easier to machine, as shown with the
better machinability of Stainless Steel 303.
To put the machinability of these metals into perspective,
carbon steel typically takes four times longer to machine than
aluminum, while stainless steel can take double that amount of
At RapidDirect we offer the following metal alloys for CNC
Though it is possible to machine thermoplastics, the material
properties of polymers can present challenges with CNC
Machining. For one, because of poorer thermal conductivity
rates, many thermoplastics melt or warp when they come into
contact with CNC mills or drills. Still, for parts that do not
require the strength and rigidity of metals, thermoplastics can
offer a less expensive alternative.
Within the scope of thermoplastics, Delrin (POM), high-density
polyethylene (HDPE) and ABS have good machinability.
Though popular for their strength and durability, materials such
as PEEK, Ultem, Nylon and many composites are more challenging
At RapidDirect we offer the following thermoplastics for CNC
The final step in the CNC Machining process is post-processing.
At RapidDirect, we offer a number of surface finishing
treatments to complete your final parts and bring them up to
your specific requirements. It should be noted that with CNC
Machining, post-processing is optional, as the quality of
machined parts is already high (see As-milled).
Anodization provides excellent corrosion resistance to machined
parts, improving the hardness and wear-resistance of the surface
while dissipating heat. Anodization is the most common surface
treatment for painting and priming as it provides a high quality
surface finish. RapidDirect offers two types of anodization:
Type II, which is corrosion resistant; and Type III, which is
thicker and adds a wear-resistant layer. Both anodization
processes allow for a broad range of color finishes.
As-milled surface finishes, which does not entail a
post-processing treatment, offer the fastest turnaround for
parts. The surface finish of an as-milled part is comparable to
125 µin Ra, while the requirements can be increased to 63, 32,
or 16 RA µin. Minor tool marks might still be visible on the
In the powder coating process, powdered paint is sprayed
directly onto the machined part. The coated part is then baked
in an oven, forming a durable, wear-resistant and
corrosion-resistant layer. A wide variety of color options are
available in the powder coating process.
Bead blasting consists of blowing small glass beads against the
surface of the machined part in a controlled manner. This
technique results in a smooth surface with a matte texture. In
bead blasting, various materials—such as sand, garnet, walnut
shells and metal beads—are used to clean parts or prepare them
for subsequent surface treatments.
If possible, keeping a part design as simple as possible will
often result in faster production times and therefore lower
costs. This is because the creation of complex structures and
surfaces uses many small cuts, which take longer to execute than
simpler, larger cuts on planar surfaces.
Following on the keep it simple tip, making sure that your part
design integrates consistent dimensions for internal corner
radii and holes can drastically cut down on machining time. In
other words, using the same dimensions where possible will
reduce the number of tool changes needed and every tool change
tacks on extra time and costs.
As stated earlier in the guide, material selection can have a
big impact on CNC Machining production times and costs. If
possible, choose a material with good machinability, such as
brass or aluminum. For applications where the rigidity and
strength of metals are not required, CNC machining a plastic
material can also help bring costs down, as the material blanks
With CNC Machining, higher tolerances and thinner walls often
entail higher costs, because of the time associated with
achieving higher precision cuts. If your product or part has an
allowable tolerance range, opt for the lower tolerance to reduce
production time and costs. The same goes for wall thicknesses:
where possible, choose the larger wall thickness allowance.
The last stage of the CNC Machining process, surface finishing,
can also affect the cost of your overall project. Choosing a
single surface for your part or product can lead to better time
and cost efficiency. If more than one finish is necessary, it is
still helpful to minimize the number of different surface