Construction Methods, Materials, and Fabrication Methods in FRCBy 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 kind3. Discuss materials commonly found on FRC robots and their different uses

and applications4. Learn about different fabrication methods commonly used in FRC with

emphasis on resources currently available to MORT

Forces Acting on StructuresThere 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:

http://legacy.mos.org/etf/force.htmlSee 'Types of Forces' and 'Design Strength' tabs. Skip 'Build a Skyscraper'

Construction MethodsTubing• 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 ConstructionTubing 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 TubingRound 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

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

Connecting Tubing Continued...

• 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.

C-Channel, Right Angle, and Other Extrusions

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

Sheetmetal 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

Plate Construction

Fasteners

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

Most common ways to fasten parts in FRC are bolts, rivets, or welds.

Bolt Pattern Examples

• 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

Common Materials In FRC

Aluminum and Steel Used in FRCAluminum 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

• 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 MillingUse 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 TurningUses 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 CuttingNot 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 CuttingWorks 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.

WaterjettingUses 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 ManufacturingMaterials 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