Materials and Fabrication

1/12/2012

Various tidbits learned from SF TechShop classes

Oxy-Acetylene Torch

Starting: Open the acetylene valve to the torch first, then the oxygen valve.
Stopping: Close the oxygen valve, then the acetylene.
Gas valve open/close order mnemonic: POOP — Propane, Oxygen, Oxygen, Propane.

Oxygen blown by the cutting head of the torch actually burns the steel / iron which is being cut. The extra oxygen combusts the material which turns into slag (~oxide) falling off onto the floor.

Acetylene pressure in the hose needs special attention: acetylene detonates above 15 PSI. The explosive velocity is ~2270 m/s which puts it nearly in range with low-end high explosives. For comparison, TNT detonates at 6900 m/s, while RDX (C4) detonates at 8750 m/s. More info here.

Video demo of an oxy-acetylene detonation

Flashback is when the gas flame disappears into the hose, which may end up igniting the tank. Sometimes there might be safety features within the welding set that prevent such a catastrophic failure. Though one cannot rely on these. A loud whistling noise coming from your welding equipment is usually a sign that this has happened. Shut all valves off immediately. Panicking is useless as you will not be able to outrun such an explosion.

Basic Metal Shop

Bandsaw blades should be installed correctly. How to tell if it's not the right blade for the material? Multiple teeth should always engage along the cut of the workpiece. If fewer than 3 teeth on the bandsaw blade are simultaneously touching the stock material, you will have difficulty cutting it. Using the wrong blade also wears the blade out much faster due to the fact that there are less teeth per cut to uniformly absorb cutting forces. An individual tooth would needlessly bear more force.

Coolant valve on the horizontal bandsaw should be adjusted to completely coat the blade while it's cutting.

Disk sander is most safely used on its right side; it spins clockwise so if the workpiece slips out of your grip, the table should stop it from flying off. Outer edge of the disk has a higher velocity, meaning material placed there will be ground down faster than at a spot towards the center. Never lean into your workpieces on this machine.

MIG Welding

To burn hotter, increase the wire feed speed. Voltage only determines the arc length, not the energy delivered to the torch tip.

MIG Welding 2 - Make Better Welds

As with soldering, welding is about heat transfer. The key is knowing which work pieces to deliver sufficient heat to, for creating a balanced and uniform weld. Weld should comprise enough of each work piece for maximum strength. Thus, the tip angle you point should be directing the heat into the piece which most requires it; larger piece means more bias for that area in the welding top angle.

Laser Cutting and Etching

Laser cuts are always an exercise in pointilism. Raster prints on the laser translate to dithered cuts, much like an inkjet printer.
Cutting chloride polymers will release (gasp) chlorine gas. Never do this unless the goal is to poison yourself.

Metal Lathe

Align work piece mounts, 3 tooth chuck holds and centers cylindrical objects like poles, etc, while 4 piece can hold things of arbitrary shape.

Safety is a big concern with using the metal lathe, it can pull things into the spindle and tear them apart very easily if anything happens to be caught.

Gears and speed are adjusted according to material being cut, much like with drill press or other cutting tools.

NextEngine 3D Scanning

Occluded surfaces need to be mapped correctly, adjust the axis of rotation to minimize number of occluded surfaces.

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TIG Welding

TIG welding is much like gas welding where the torch merely heats up the workpiece to form a puddle for the filler metal to weld with. Gasket and tungsten tip should stick out about half the radius of the gas cup. Tungsten should be sharp, though ball-tipped to make sure the cone of where the arc will form is predictable.