# Construction Methods, Materials, and Fabrication Methods in FRC

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Construction Methods, Materials, and Fabrication Methods in FRC
By Akash Rastogi, Eric Drost , Alex Mills & Connor McLaughlin FRC 11 - Mt. Olive Robotics Team

Objectives Learn about forces that structures have to withstand
Discuss different construction methods used by many FRC teams and the benefits of each kind Discuss materials commonly found on FRC robots and their different uses and applications 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... 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... 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: See 'Types of Forces' and 'Design Strength' tabs. Skip 'Build a Skyscraper'

Construction Methods Tubing Round Square Welded Tube Gusseted Tube
Tube + Sheetmetal C-channel, Right Angle, and other extrusions Hex, round, and square bars C channel, U channel, Right angle Sheetmetal Bent Flat Plate Thicknesses Pocketing plate

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

Round and Square Tubing
Round Tubing Difficult to mount to Easy to create custom bent shapes Thinner wall round may be stronger than square in many cases (arms) Square Tubing Very easy to mount to Easy to machine Easier to weld/gusset

Connecting Tubing Gussets/Sheetmetal
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. 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 Welding tubing has a high risk of warping the frame due to the heat introduced to the part. 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
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 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 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

Bolt Pattern Examples

Common Materials In FRC
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: All purpose, very strong and used for plates, housings, axles, pretty much anything. Good machinability Stronger than 6061 for axles and standoffs. As strong as a mild steel Extremely strong aluminum alloy, used for axles, gears, sprockets. Common on aircraft Easy to use in bending applications and is also extremely easy to weld Steel: Multipurpose steel, used to create shafts, gears, sprockets, etc... Easy to weld and used for making fasteners, gears, sprockets, and structures

HDPE - High Density Polyethylene
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. Tensile Strength: Poor Impact Strength: Excellent

Polycarbonate 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 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.

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.

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