Construction Methods,
Materials, and Fabrication
Methods in FRC
By Akash Rastogi, Eric Drost , Alex Mills &
Connor McLaughlin
FRC 11 - Mt. Olive Robotics Team
Objectives
1. Learn about forces that structures have to withstand
2. Discuss different construction methods used by many FRC teams and the
benefits of each kind
3. Discuss materials commonly found on FRC robots and their different uses
and applications
4. Learn about different fabrication methods commonly used in FRC with
emphasis on resources currently available to MORT
Forces Acting on
Structures
There are different types of forces that act in different ways on structures such
as bridges, chairs, desks, buildings, robot frames, etc...
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Static Loads: Student on desk or chair
Dynamic Loading: force in motion
Forces Continued...
Tension & Compression - when the ends of an object are pulled apart from
both ends versus pushed towards the center from both ends respectively.
Forces Continued...
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Shear: applied parallel or tangential to a face of a material
Torsion: occurs when an object, such as a bar with a cylindrical or square
cross section is twisted
Why do triangles work the best in structures? Why do we use trusses and
triangles in FRC? See the following link:
http://legacy.mos.org/etf/force.html
See 'Types of Forces' and 'Design Strength' tabs. Skip 'Build a Skyscraper'
Construction Methods
Tubing
Round
Square
Welded Tube
Gusseted Tube
Tube + Sheetmetal
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C-channel, Right Angle, and other extrusions
Hex, round, and square bars
C channel, U channel, Right angle
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Sheetmetal
Bent
Flat
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Plate
Thicknesses
Pocketing plate
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Tubing Construction
Tubing is an extremely versatile and strong construction method which can
create light parts without sacrificing too much strength.
We use 6061-T6 aluminum tubing with varying wall thicknesses from 1/16" to
1/4" We have also used other materials in extruded tube form such as
polycarbonate, fiberglass, and carbon fiber. Even very thin wall steel works
in small quantities.
Tubing provides:
much stronger frame than just flat or right angle extrusion
more mounting surfaces than flat plate
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Round and Square Tubing
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Round Tubing
Difficult to mount to
Easy to create custom bent shapes
Thinner wall round may be stronger than
square in many cases (arms)
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Square Tubing
Very easy to mount to
Easy to machine
Easier to weld/gusset
Connecting Tubing
Gussets/Sheetmetal
Welding
Using gussets is incredibly easy,
especially with CNC equipment.
Thin or thick plate can be used in
different shapes along with rivets
or bolts to connect a frame
together.
Welding tubing has a high risk of
warping the frame due to the heat
introduced to the part.
Sheetmetal parts are bent in a way
which allows tubing to be fastened
at the corners. Most sheetmetal
parts have gussets designed into
them.
Welding must be done by a skilled
professional or a well practiced
mentor/student
TIG welding is needed - Tungsten
Inert Gas
Connecting Tubing
Continued...
C-Channel, Right Angle,
and Other Extrusions
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Hex, round, and square bars
C channel, U channel, Right
angle
All of these extrusions are used in
various parts of an FRC robot.
Hex and round are used for shafts
and standoffs which can be
aluminum, delrin, polycarbonate,
or several other plastics.
C-channel, U-channel, and right angle
extrusion are commonly used for
structures.
Sheetmetal Construction
Sheetmetal construction relies on strategic
bends in the plate which creates stiffer
parts.
Thicker sheetmetal parts are made of a
softer aluminum alloy 5052-H32 to
avoid cracking of the material
Thinner sheet <.090" can be 6061 and can
be bend without cracking
When many bent sheet parts are riveted
together, they are extremely robust,
even when using thin material.
Extremely easy to mix with tubing
Bent (flanged) > Flat sheet
Plate Construction
Aluminum, steel, polycarb, delrin, and
even wood can all be used on various
parts of robots. Let's focus on
aluminum for now.
6061 plate can be used in many
thicknesses for frames and gearboxes.
1/16" sheets to 1/4" sheets are common.
Thinner sheets do not need pocketing
to save weight.
Usually better to use a thinner material
unpocketed than pocketing a slightly
thicker material.
If pocketing, create triangular truss
patterns
Fasteners
Most common ways to fasten parts in FRC are bolts, rivets, or
welds.
Bolts
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Steel, heaviest option
Easily removable
Should be standardized
(10-32) for easy of
working in pit
Bolt pattern in frame can
make changes easy
Bolts are used to keep
assemblies
removable/modular
1/4 20 is overkill for the
majority of situations
Welds
Permanently attaches
parts
Lightweight and strong if
done by an experienced
welder
Difficult with most
aluminum
Heat can warp
frames/parts
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Rivets
Nearly "free," in terms of
weight
Semi-permanent
Use rivets of the same
material you are fastening.
Rivets are used to hold planes
together, they are extremely
strong when many are used
together.
When riveting metal to plastic,
the "blind" side of the rivet
should be on the metal side.
Larger diameter rivets require
a pneumatic gun, smaller
diameter rivets can be done
by hand rivet guns
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Bolt Pattern Examples
Common Materials In FRC
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Wood- ply, teak, birch
Aluminum- typically 6061-T6 aircraft, 5052 sheetmetal, 7068 & 7075 high
strength
Steel- try to avoid due to weight
HDPE- high density polyethylene- impact resistance, light weight, low
moisture absorption, and high tensile strength, high rigidity
UHMW- Ultra-high-molecular-weight polyethylene- low moisture
absorption, self lubricating
Nylon- blocks and slides
PVC tubing- polyvinyl chloride
ABS Plastic - stronger than PVC
Polycarbonate- NOT PLEXIGLASS
Delrin- Polyoxymethylene- high strength acetal
Fiberglass- poltruded fiberglass
Composites
Aluminum and Steel Used
in FRC
Aluminum Alloys:
6061 - All purpose, very strong and used for plates, housings, axles, pretty
much anything. Good machinability
2024 - Stronger than 6061 for axles and standoffs. As strong as a mild steel
7075 - Extremely strong aluminum alloy, used for axles, gears, sprockets.
Common on aircraft
5052 - Easy to use in bending applications and is also extremely easy to weld
Steel:
4140 - Multipurpose steel, used to create shafts, gears, sprockets, etc...
4130 - Easy to weld and used for making fasteners, gears, sprockets, and
structures
HDPE - High Density
Polyethylene
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Tensile Strength: Poor
Impact Strength: Good
Slippery
UHMW - Ultra-High-Molecular-Weight
Polyethylene
UHMW is tough enough to resist the scuffs, scrapes,and strikes that other plastics can't. In addition to
excellent abrasion, wear, and impact resistance, it has a slippery surface that is good for bearings,
bushings,machine guards, and chute and hopper liners.
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Tensile Strength: Poor
Impact Strength: Excellent
Polycarbonate
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Not Plexiglass - acrylic shatters on impact
Polycarb is strong, fairly light, and easy to use.
Used for windows and is bullet resistant
beginning at 3/8"
Can be heated and bent for many applications
Tensile Strength: Good
Impact Strength: Excellent
Fabrication Methods
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CNC and Manual MIlling (both available in
house for MORT)
CNC and Manual Turning (manual available in
house for MORT)
CNC Plasma Cutting (available in house for
MORT)
Waterjetting (available via sponsor)
Laser cutting (available via sponsor)
3D Printing (available via sponsor)
CNC/Manual Milling
Use high speed spinning milling bits to cut away material. CAD files are used directly for CNC milling
and manual machining requires detailed CAD drawings as instructions. Most CNC milling is high
precision but can be slightly slower than other methods depending on part thickness, material
being cut, and the sharpness of the cutting tool.
CNC/Manual Turning
Uses high speed lathe which spins a part to shape it against a sharp cutting
tool. Lathes are also used to size, drill, and tap parts such as standoffs,
shafts, and wheels, pulleys, gears, and sprocket hubs.
CNC Plasma Cutting
Not as high precision as other CNC tools. Puts a lot of heat into the plate being
cut (localized hardening or annealing). Bearing bores must be finished up in
a mill and small holes must be drilled out.
CNC Laser Cutting
Works extremely well for cutting sheets thinner than 1/4" but cannot cut many
plastics due to fumes produced. Puts heat into part but is high precision.
Can create custom sprockets and plate gears very easily.
Waterjetting
Uses water or water + abrasives to cut away material. Can be used to cut very
thick pieces of materials such as steel, aluminum, rubber, polycarbonate,
etc...
Does not put heat into part and creates a nice edge finish. Can create very
intricate parts which can undergo secondary machining on a mill.
3D Printing/Additive
Manufacturing
Materials such as ABS plastic is layered and printed then solidified either using heat or
some type of epoxy. Creates very intricate parts that may take hours or be impossible
for conventional machining. CAD to reality in a matter of hours. Used to light load
applications with most machines, but technology for stronger parts is available and is
growing every day. Great for spacers/sensor mounts/any prototypes.
Sources & Links
http://www.mcmaster.com
www.onlinemetals.com
www.facebook.com/RoboMemes
www.chiefdelphi.com