Making Prototypes

  • tags: fab projects Rapid prototyping of prototyping machines
  • Author: #Dan Gelbart
  • Videos: Making prototypes watch learning
  • Resources
  • Water jet
  • Pressbrake
    • Use U bends for sheet metal instead of bending from all sides as it can be easily bent without changing the die arrangement.
    • Sheet metal body with spot welded springs,
    • Nitinol cross bend springs provide large movement because of larger elasticity.
  • Spot welder
    • use smaller time to set up temporary welds increase cycle times to get permanent ones.
    • You want to set the max current and adjust the time to get the welds, the faster the current can reach the less time it has to seep sideways and the less energy is needed. Only adjust current if the smallest time is too big for the weld.
    • Only weld conductive metals, the less conductive metals, the higher the current and the more heat that is generated.
    • **Min 15-25 Kva, 15Kva **is about 3mm (12 guage)
    • This is perfect for welding rods a 15Kva unit can weld even half inch dia rods. The reason is the contact area of the weld is small and there is a large current density. So it is very efficient to weld rods.
    • Spot welders are good for captive hardware like hinges, nuts, rivets, latches, stand offs. Spot welding nuts, have a small shoulder, Higher the surface area higher the current needed
    • You have to make electrodes for the standard sizes to match the hardware you are welding.
    • It is very useful to **weld balls **at the end of bars for like accurate positioning and kinematic joints. You need a conical adapter that can house the balls, low current is needed as surface area is small.
    • To make invisible welds, by textbooks you need to use a regular electrode and a flat electrode but what happens is that as the molten metal cools, it pulls the metal in, leaving a small mark, and you have to sand a lot to get it out. The best way is to take one flat electrode and made a small hole in the center, a few millimeters in dia and about 50 Microns deep. This creates a dimple and the metal is pulled out into it, and when it cools, it pulls back in.
    • Spot welding difficult to weld materials like nitinol, sandwitch it between two pieces of stainless steel and then weld through it. Try to avoid any abrupt changes in shapes.
    • Make your own electrodes, buy copper rods/squares and just bend them into shape, copper becomes work hardened but can be brought back by annealing. The tips need to made from a copper alloy Tungsten, buy them in the rod and machine a small taper into them and press them into the copper.
    • The neck of most welders are long, the longer the neck, the less the current as, when you put a ferro magnetic sheet into it, it creates a large inductance.
    • The voltage is safe, about 2.5V but there is 5000A of current flowing through it. So if you are holding something magnetic, it will be attracted into the machine.
  • TIG Welder
    • needs a lot of skill to weld thin stuff because it burns a hole. It is perfect for stuff with thick materials
  • Presswork
    • Press are one of the most dangerous machines in the lab, they are deceiving as they move very slowly, but they can store a lot of energy.
    • The first rule of presswork is never to press something that is hardened. Sometimes when pressing ductile materials, the tool maybe hardened, and the press itself is hardened. Best to use a half inch polycarbonate shield.
    • As similar to a truck spring, when you press steel, it stores energy. A block of steel can store more energy than the truck spring of same mass, as the spring was not loaded until failure.
    • Steel is a brittle material, when compressed beyond failure, it shatters in all directions, and the pieces have more energy than a bullet form a hand gun.
    • If you take a dime and press it with the full force of the press (100T) then you will damage the press, because the full force is applied on an area about 2 Sq Cm. This by far will exceed the Yield strength (20T/sq cm) of the best steel. Best not to press something with an area less than 10 sq cm.
    • Waterjet and press are a great combination as you can easily make tooling for the press with the waterjet.
    • Increasing stiffness for making any high-performance part like in Aircrafts of any part its best to press closed form ribs into it.
    • If you take a normal strip which is kept flat, then you can easily bend it, but when you press a rib into it, then immediately the moment of inertial goes up and so does the rigidity.
    • But the best thing is closed form, if you press ribs into a strip, then it is stiff in lateral loads, but is weak in torsion. In order to get Torsional Rigidity you need a closed form shape, press the ribs into the strip and spot weld the edges. You have a part with the same weight but much higher stiffness.
    • The best materials for Light weight structures is some form of stainless steel that can be heat treated after forming. You press the ribs in and then put it into the oven and then after the heat cycle, it comes out with a much higher elastic range, so even if you press it it comes back. Eg -17-7 stainless steel Which can be hardened by heating it, no need to quench.
    • Useful thing for press is making cylinders, inorder to make cylinders you have to have round covers, that needs a bit a raised lip, so that you can spot weld them. Cut out the die for the ribs in the Waterjet, but give a bit higher clearance, like for 1mm thick sheet five 1.5mm or 2mm and if clearance is small then you will not be able to separate them again.
    • How to align male and female parts of a die? There is trick to pressing with male and female dies, in order to align them, you use a male die on one side and use a hard rubber like polyurethane as a backing a press an indentation on it.

The indentation will not give you sharp corners, but it is enough, now you have something to register the two halfs of the die, place the male and female corresponding to the indentation and press the part.. You can also go for a deep draw, but then it will have to be annealed. - If you want to press it in one step with sharp detail, you have to press with lead maybe half inch thick, lead can be reused by melting it. - In order to use a press to punch out a part, you need a hardened die with sharp corners and press it onto rubber or lead.

  • Surface Coatings
    • Sand Blasting
      • They are simple and effective tools for preparing surfaces for gluing and for painting.
      • It is recommended to replace the normal window with a diamond coated window like in the supermarket window.
    • An adhesive bonds to a surface through some mechanical and chemical bonding, mechanical depends on the roughness of the surface and the chemical bonding by means of surface energy or true chemical bonds between the surfaces.
    • The normal way of preparing surface by means of rubbing with a sandpaper and using a solvent is that, no matter how well you wash it afterwards, you are just diluting the hydrocarbons in the solvents. There will always be a monolayer (one molecule layer) left. This will prevent true chemical bonds. But bathroom cleanears like ajax will decompose the hydrocarbons and activate the surface.
    • The standard test paint is you take a painted sample use a knife to put a grid of about 4x4, get the strongest tape you can find and then stick it on top of it and peel it off. If any of the squares have paint lifted off them, then the test has failed.
    • The **best way to prepare a surface **is by #Sand Blasting the only cases when you cant do that are when the material is very big or it’s sheet metal less than 0.5mm.
    • #Sand Blasting sheet metal less than 0.5mm thick will put compressive stress on one side of the piece which will cause it to bend. like shot peening.
    • Next best use of sandblasting is in bluing steel. It involves cleaning the surface and then heating the metal to about 300 C. Or scrubbing it with bathroom cleaners like Ajax or comet, the reason is not only do they remove any oils, but they also oxidize and activate the surface.
    • Test to check if a surface is ready for painting or bonding is called #Wetting Test. In the Wetting test you place some drops of water on the surface of the metal and look at the beads, if the water rolls of the surface and forms small beads then its is not prepared.

In a prepared surface, the water will wet the surface properly and will not form beads. The angle which the water form with the surface is called Contact Angle and it is a directly measurement of the Surface Energy Where is the surface energy is high, then it will overcome the surface tension of the water and begin to pull it in and very low angle is formed.

When the surface energy is low, then the angle will be high and the surface will act hydrophobic. - A good way to prepare a surface for painting is to heat the part in an oven, any defects that could cause bubbles to come up in painting would disappear. like for example if there is some cutting oil left, you powder coat it and then bake it, then the oil will bubble out. - Everything has to be painted within 1hr of activating the surface, because the air contains hydrocarbons and by process of adsorption, a small layer is formed. One way to preserve the surface is by a process called Gumming, where you coat the surface and it protects it. - Surface energy is a product of the specific surface energy and the surface area. so even if the surface energy is small if you have a large surface area like when texturing the surface then even hard to stick materials like Teflon can be coated on a suface with sufficient area. - A good way to fix dents before painting is to heat up the work piece and put some of the powder paint into the dents, the heat will melt the paint and fuse it to the body. let the part cool and then sand it flat. The paint only remains on the dents. Then paint the body again.

  • Air Bearings
  • Materials
    • Everything you can make out of cold rolled mild steel sheets should be done that way. Its strong, cheap, easily bendable. most useful sheet thickness are 18 (1.2mm) and 10(3mm) Gauges. Also for stainless a cheap 300 series stainless would work with gauge 18 and guage 12 because its expensive.
    • Its useful to stock some stainless steel in spring temper, as it can be used for flexures and springs. eg type 301 full hard steel. The nice thing about the series 300 steel is that you can shear them with a shear, you can bend them if you have to. The much harder 400 series cannot be cut with a shear, not bendable.

As a general purpose spring material store series 300 stainless steel with either a full hard or a 3/4 hard. - Other type of spring temper is a 17-7 stainless steel. It comes annealed, so easily formable, so you can bend it spot weld it and do all operations, then you need to heat cycle it. Once after the heat cycle it becomes the hardest springiest of the stainless steel.

The heat cycle is you heat it up to 800C, let it cool down then you heat it again to about 500C and let it cool down. No quenching, just double heating. - Both steel and stainless come in regular and free machining type. Best to spend a bit extra on free machining as you can save a lot of time. The free machining has some alloying components like lead or sulfur to help machinability. - Aluminum is best for large structures and for heat sinks. Good to store 3mm of 5052 Aluminum. The advantage of 5052 is that it can be bent without cracking. The most common one is the 6061 but they will crack when bent. - Spring wire is useful when making your own springs, stocking a couple of sizes of spring wire give the possibility of making many kind of springs. There are two types, one is the Music wire it is carbon steel, its cheap and easily available, but is not rust proof.

If you need rust proof springs then use the stainless steel alloy 17-7, which is available in wire form and make the spring. It is pretreated so only one heat cycle (480C for 1 hr) is necessary. - Loclite used for gluing parts like bearings to shafts, be careful when using it with some plastics as it may lead to cracking. - Wax Stick useful when cutting aluminum as lubricant. Particularly useful when cutting aluminum with wood working tools, put some wax on table saw to cut sheets, or on a router bit to prevent sticking. - Steel - The only hardenable steel you need to have is air harden able A2 steel. The advantage is there is no need to quench it. Quenching will introduce warps into the steel, and A2 is designed to have very low dimensional variations.

If you take regular or high carbon steel and quench it then it will have high dimensional changes, so you need to oversize it and then grind away the extra

Sometimes you will have to grind the surface when case hardening because the outer layer can De-carburize if it is not treated in an inert atmosphere. The best thing is to wrap it in paper and then stainless foil and seal the foil by spot welding. So when it is heated the paper absorbs the oxygen and carburized before the steel can lose the carbon. - Mild Steel has carbon as the main component.

    - [Surgical Stainless steel 440](/surgical-stainless-steel-440-5d8f9f){: .internal-link} Useful when dealing with medical devices, but not as corrosion resistant as the [series 300 stainless steel](/series-300-stainless-steel-d26230){: .internal-link}, cannot leave it inside the body as implants.
    - [High speed Steel](/high-speed-steel-c391ed){: .internal-link} You can make tools from HSS, you cannot anneal it, you can grind it.
    - [Carbon steel](/carbon-steel-57459d){: .internal-link} Cheapest hardenable steel, but need to quench it, lot more science involved.
- **How to tell the various steel apart**
    - Take a piece of the steel, grind it and look at the sparks,
    - [Mild Steel](/mild-steel-2521c4){: .internal-link} very little spark and don't have a star at the end of the spark
    - [Carbon steel](/carbon-steel-57459d){: .internal-link} Lot more sparks and at the end of the spark there is a star.
    - [A2 steel](/a2-steel-9f21ca){: .internal-link} Dull sparks and no stars at the end.
    - [High speed Steel](/high-speed-steel-c391ed){: .internal-link} No sparks at all and few sparks are red, then white.
    - [Surgical Stainless steel 440](/surgical-stainless-steel-440-5d8f9f){: .internal-link} No sparks at all, less than HSS.
- **How to Harden steel**
    - The best way is with an oven/Klim, but when its a small piece you can heat it up with a torch as well.
    - Its best to place the work piece over an insulator like fire brick, because it reflects the heat and work can be done 2X as fast.
    - For A2 steel its at 950c which is white hot and let it cool down in air.
    - After hardening it can become very **Brittle**. So we need to **Temper** it.
    - [Tempering](/tempering-6fde62){: .internal-link} is a **trade-off between hardness and toughness.**
    - Depending on the trade-off you need to heat it to between 220 to 300C. 250C will give you a hardness of Rockwell RC60, which is as hard as you need. But if you need **Impact strength** then you need to heat it to somewhere around 300C then it will have a hardness of RC56 but much higher toughness.
    - The range is 220-300C every 10C will make a huge difference, you need to be able to judge the temperature based on the colour.  Luckily stainless steel has a good colour range.

It goes from Straw colour **(200C) to dark brown, to **dark blue (270C) to light blue (320C) to light grey, which is overheated.

The Tempering range is from straw to dark blue, Straw which is full hardness. - One useful thing to get a Carbide tool is to take a piece of discarded carbide insert and braze it on to the steel, using an brazing alloy, so you can make custom tooling, drills, etc.

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