Intro to Mechanics & Fabrication

Mechanics

Simple Machines

What are Simple Machines? Do you remember from your Middle School Physics class? Simply put, they utilize mechanical advantage to multiply force. There are six different kinds:

  • Lever

    • Composed of a beam/rod that pivots at a pixed fulcrum point. By changing the location of the fulcrum, you can change the mechanical advantage.
  • Wheel & Axle

    • Consists of a wheel attached to a smaller axle, so that when you rotate the wheel, the force is transferred to the axle, and vice versa. You can change the mechanical advantage by changing the size of the wheel.
  • Pulley

    • Supports movements by changing the direction of a cable wrapped around a Wheel on an Axle. Mechanical advantage can be changed by adding one or more pulleys to create a Block and Tackle.
  • Inclined Plane

    • Also know as a ramp. Mechanical advantage is changed by varying the ratio of length to height of the ramp.
  • Wedge

    • A portable Inclined Plane. It converts forces applied to its blunt end into forces normal to its sloped faces. You can change Mechanical Advantage by chaning the ratio of slope to width.
  • Screw

    • Converts rotational motion and torque to linear motion and linear force. You can vary Mechanical Advantage by changing the pitch of the screws threads

What can we do with Simple Machines? Anything! They’re the fundamental building blocks of every complicated mechanical machine we see in our lives.

507 Movements

Exercise 1.1: Analyzing Mechanisms

  1. Go to 507 Movements and find an interesting mechanism
  2. Determine what simple machines compose the mechanism
  3. How could you use the mechanism as part of a larger system?

Screws

Screws are a great way to attach things to other things. We use them a lot, and you see them in almost every device you use on a daily basis. There’s a few things to be aware of when speccing a screw for use.

Screw or Bolt?

In general, bolts are used to join two unthreaded pieces together, with the help of a nut. Screws combine things when one of those things is internally threaded. You would also refer to a screw as a screw if it creates its own threads while being fastened (a deck screw going into wood, for example).

Head Type

There are a ton of different head shapes for screws and bolts. Here are some examples:

(a) pan, (b) dome (button), (c) round, (d) truss, (e) flat (countersunk), (f) oval

Additionally, there are even more types of screw drive shapes you can choose:

(a) slot, (b) Phillips, (c) Pozidriv, (d) Robertson, (e) square, (f) hex, (g) hex socket, (h) security hex socket, (i) Torx, (j) Tri-Wing, (k) one-way, (l) Pentalobe

Material & Finishes

The type of material you pick can be exteremely important. Stainless Steels are corrosion resistant, but expensive. Steel is low-strength, but cheap. Brass is electrically conductive, Aluminum is lightweight, and Plastic is inexpensive and corrosion resistant, but weak.

The most common material, Steel, is usually coated with another material to help it resist corrosion. Black Oxide is a good default that makes the screw Black, easier to thread, and mildly corrosion resistant. Zinc Plated screws are another common coating, and make the screw silver and resistant to wet environments.

Size

Screws and Bolts come in either Metric or Imperial sizes.

Metric dimensioning is easy. An M2 x 12mm screw, for example, has a thread diameter of 2mm and a length of 12mm. Metric screws can come in two different thread pitches: course and fine. Course threading is standard.

Imperial/Inch sizing has a little more variety. They can be specced by gauge or by diameter in inches. An 8-32 x 1/2" screw, for example, has a wire-gauge thickness of #8, a 32 thread per inch pitch, and is 1/2″ long. A 1/4-20 x 2-1/2" bolt is 1/4″ in diameter, has 20 threads per inch, and is 2 and a half inches long. Sometimes you’ll see Imperial wood or sheet metal screws referred to by just their gauge size.

Wire gauge diameters are a function of how much electric current a wire can safely carry, and doesn’t have an easy conversion to Inches or Metric. Instead, you should rely on charts to determine the thickness of a numbered screw.

Nuts & Washers

Nuts and washers are necessary when using bolts. Nuts fasten a bolt from the other side, and washers distribute the load of that nut (or the bold head) across a wider area.

Nuts come in a wide variety of types from the standard hex nut, to locking nuts that won’t come undone due to vibration or manual loosening. There’s also a variety of special nuts, like Flange nuts (which have a build in washer), Wing nuts (which can be tightened by hand), and Cap nuts (which provide a decorative finish and protect the bolt threads).

Drills & Taps

When creating holes for your bolts to pass through, you have to make sure they’re sized correctly. Special charts exist to tell you exactly what diameter that hole should be. Looking at the chart above, for example, we can see that a 8-32 bolt should have a .1770″ diameter (or a #16 drill) hole drilled for a normal fit.

Tapping a hole is the process of cutting threads into it. If we wanted to tap a hole in acrylic for that 8-32 screw, we would look at that chart and determine we need to drill/cut a hole with a diameter of 0.1360″ (or a #29 drill), and then tap it with a 8-32 tap.

Exercise 1.2: Sourcing a screw

  1. Go to the McMaster website
  2. Add a box of M3 x 12mm, Socket Cap, Black-Oxide Steel screws to your cart
  3. Add the correct size Drill bit to your cart
  4. Add the correct size Tap to your cart

Gears

Gears are wheels with teeth that mesh with other gears (in terms of Simple Machines, they are Levers mounted on a Wheel & Axle). They are used to change direction, torque, and speed of a moving part.

Spur gears are what you think of when you think of a gear. A great generator is below, for creating gears you can laser cut:

Gear Generator

Another type if gear you might make in this class would be Rack and Pinion gears, gears that convert radial movement to linear movement.

Exercise 1.3: Creating Gears

  1. Use the Gear Generator to create a gear system that takes 20 RPM clockwise as an input, and turns it into 4 RPM clockwise, without making a gear with a larger Diametral Pitch than 1 unit
  2. Export each gear as an SVG

Linkages

A linkage is an assembly of levers connected together at strategic points to accomplish a specific movement.

Disney Research

Strandbeests

Adhesives

Everyone has used glue in their lives (I hope), but what you might not know is that there are specific types of glue you should use for specific purposes. ThisToThat.com is a great source for determining which type of glue to use.


In PhysComp, we often use Laser Cut acrylic. For attaching Acrylic to Acrylic, you should use Weld-On/SciGrip #3, #4, or #16 acrylic glue. You use this by butting the pieces together and brushing/injecting the glue onto the joint. It seeps into the joint via capillary action and creates a very strong bond. If you can’t find SciGrip, any type of CA (CyanoAcrylate) Super Glue will work (apply it as normal, not with the brushing method), just be careful with it.

Hot Glue is acceptable only if you have very good care and control of it. Your projects should not have any visible glue blobs or silky strings.

Collars, Couplings, and Bearings


When you want to mount something to a shaft, use a Shaft Collar



When you want to join two shafts together, end-to-end, use a Shaft Coupling



When you want to have a low-friction radial connection, use a Ball Bearing

Modeling — Rhino

Straight from the Rhino website, “Rhino can create, edit, analyze, document, render, animate, and translate NURBS curves, surfaces, and solids, point clouds, and polygon meshes. There are no limits on complexity, degree, or size beyond those of your hardware.”

Rhino is not free, but it is installed on the IDeATe Macbooks, and is relatively inexpensive for students. It also has a free trial, if you’re in a pinch. Other alternatives to Rhino are Sketchup, Inventor (free for students), and Solidworks.

A great resource for Rhino tutorials is available on the Rhino website. Lynda also has some good tutorials, as does the rest of the web. Today, however, we are going to go over how to use Rhino together in class.

Exercise 2.2: Modifying existing files

  1. Convert the downloaded SVGs from GearGenerator into files readable by Rhino, i.e., DXFs or EPSs
    • I generally do this step in Adobe Illustrator (and if my modifications are simple, I continue editing there), but you can use other tools to accomplish this if you don’t own AI. CloudConvert seems to be a pretty good service to do this.
  2. Open one of the files in Rhino
  3. Add a Circle to the gear on its existing crosshair
  4. Use SelDup to select all duplicate lines, and delete them (we don’t want the laser cutter to cut the same line twice)
  5. Make new CUT, SCORE, and RASTER layers, and change their colors to RGB(255, 0, 0), RGB(0, 0, 255), and RGB(0, 0, 0) respectively
  6. Put the lines you want to cut on the CUT layer, and the lines you want to score on the SCORE layer

Exercise 2.2: Make a Finger-Jointed box in Rhino

  1. Make a 3″ x 3″ Cube
    • Draw an equal-sided Rectangle, and ExtrudeCrv it the same amount
  2. Add thickness to the walls of the cube
    • Explode the cube to make each face separate
    • ExtrudeSrf each face (inwards) the measurement of the thickness of the material you want to cut
  3. Use boolean commands to make finger joints on all of the cube edges
    • Create new extruded rectangles
    • Place them on the edges of the cube
    • Use BooleanUnion and BooleanDifference to add and subtract those cubes to the existing geometry in a way that makes a finger-jointed box
  4. Disassemble all the parts, and lay them flat in a cuttable pattern
    • Use a combination of the Move, Rotate3D, and Orient3Pt commands to lay out all the pieces
  5. Remove the unnecessary 3D information to create a 2D cut file
    • Explode the parts
    • Select everything but the surfaces on the ground plane in one of the side views and delete them
    • Select the remaining surfaces and use DupBorder to create outlines
    • Select the remaining faces with SelSrf and delete them
  6. Change the line colors by putting them on special CUT, SCORE, and RASTER layers you create
  7. Export the files as .DXFs
  8. Check out Box Designer, a site that will make you feel like all that work was for naught

Laser Cutting

IDeATe has three Rabbit Laser Cutters, which are awesome machines that use lasers to cut through material. You accomplish this by giving them a .DXF cut file, like the one we just created.

In addition to cutting fully through a material, they can engrave with vector lines or draw filled-in areas with raster images. Our laser cutter can cut materials like Acrylic sheets, cardboard, paper, MDF, plywood, and fabric. Some materials are unsafe to cut due to fire or chemical hazards (never cut anything that contains Chlorine, like PVC). IDeATe has materials in stock, for purchase; options can be seen on their resources site.

You must have Fire Extinguisher training to use the laser cutters. Please sign up for it here, if you haven’t yet. Open hours for laser cutters are every weekday, 4:30-6:00; a monitor will be around to help you cut (and to cut for you, if you don’t have Fire Extinguisher training).

Exercise 2.3: Cut Your Cube

  1. Read the Laser Cutter Policies
  2. Follow the instructions at the IDeATe Resources page to load your file into the Laser Cutter computer
  3. Follow the instructions at the IDeATe Resources page to cut your file on the Laser Cutter

Sources:
Simple Machine
Linkages
McMaster Carr’s About Machine Screws
IDeATe Resources
Dave Touretzky’s lecture slides for the laser cutter from the 15-294 Rapid Prototyping mini-course