Guerrilla guide to CNC machining, mold making, and resin casting
Copyright (C) 2013 by Michal Zalewski (lcamtuf@coredump.cx)
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7. Workshop safety topics

We're done with the technical stuff, but stay with me for a bit longer: even for DIYers, workplace safety is no laughing matter. You will be dealing with power tools and reactive chemicals, and so if you want to pursue this hobby, there are inherent risks you simply have to understand and accept. It's entirely possible to lose an eye or set your house on fire; there are certain common-sense steps you can take to minimize the probability of mishaps, but the odds will never go down to zero.

It is also your sole responsibility to investigate and follow all the applicable HOA rules, city ordinances, and other regulations - especially as they relate to purchasing laboratory supplies and chemicals, the use of power tools, and the disposal of unreacted chemicals. In many places, the laws are sensible or non-existent; but exceptions happen. If you accidentally ignore a piece of obscure, boneheaded legislation - well, nine out of ten times, nobody will know or care, but do you really want to be that one unlucky guy?

OK. With all that in mind, this chapter provides a quick but non-authoritative overview of some of the most obvious safety considerations and health risks for CNC machining and resin casting. There are no guarantees that this information is accurate, complete, or up-to-date, so do your own research. Do not blindly trust one random dude on the Internet!

7.1. Working around milling machines

Small CNC mills are fairly safe, compared to most other power tools - but you still need to read and follow the instruction manual, and in general, exercise common sense in everything you do. The primary risks are getting your hand pinched between the rotating tool and the workpiece, or between any other moving parts; having loose clothing or hair caught by the spindle; or being hit by any debris ejected during the cutting process.

Becaue of this, you should really stay clear of the machine while it's running a job - after all, you have no business fiddling with it. If you really need to make some sort of an adjustment, or perhaps simply vacuum off accumulated shavings, it's imperative to pause the cutting process first. It is also a very good idea to wear basic eye protection at all times; although the odds of a broken tool or other sharp material hitting your eye are very low, the damage can be significant.

All that aside, high-speed CNC lathes and high-power CNC mills (over 1 kW or so) have their own safety rules that I will not cover here. These devices are often powerful enough to break bones, sever limbs, or eject a loose workpiece with a speed sufficient to kill or maim. If you have one of these, the use of robust safety covers made out of sheet metal, polycarbonate, or wood, is definitely a must.

7.2. Dust considerations

Any type of dust can aggravate existing respiratory conditions; prolonged and heavy exposure may also lead to the development of brand new ones. That said, most of the noteworthy materials give off relatively little airborne particles when machined in a sensible way - and so, your overall exposure to nuisance dust will probably not change in an appreciable way.

When working with prototyping boards such as RenShape 460, airborne dust is produced in modest amounts, and has no known, specific health risks; the same goes for many other common plastics. Covering the mill with plastic sheeting for the duration of the job (well clear of any rotating parts and inlet vents) is a good practice, even if just to keep your workplace tidy; building or buying a proper enclosure is an option, too. In truly extreme cases, a simple N95 particulate respirator (link) is good to have at hand.

That said, there certainly are materials that pose an elevated risk; to avoid really unpleasant surprises, always investigate the substances you are milling, sanding, or otherwise dispersing in the air. You should be particularly wary of anything that contains crystalline silica (quartz), asbestos, elemental metals, toxic pigments, and any other harmful substances that may be absorbed through the lungs.

In essence, be sure to handle any powdered fillers and pigments with care, and do not mill, sand, or cut rocks, and mystery plastics of unknown composition - and you should be fine.

Note: whie crystalline silica (quartz) is a significant danger when inhaled, there is no compelling evidence that amorphous glass products, such as milled glass fibers or 3M glass microspheres, share the same risk. You still don't want to breathe any of that stuff in, but these fillers should be safe to use in their intended way - and a tiny amount of dust should be OK.

7.3. Vacuum and high pressure

Vacuum isn't particularly dangerous to work with, but you should be aware of the risk of implosion. Plastic vacuum chambers should be inspected for cracking or crazing, and should never be exposed to high temperatures, substantial quantities of solvents, strong acids and bases, and so on. Sketchy-looking or homebrew chambers can be secured with packing tape, shrink-wrap foil, fabric, and similar materials that will retain debris if things go wrong. Other than that, you have very little to worry about; oh, if you are working in a confined space, it may make sense to vent the vacuum pump to the outside, so that the relatively small quantities of gases liberated from the resin - as well as the fine mist of mineral oil from the pump itself - don't end up right under your nose.

Pressure casting is more dangerous; that's in part because the pressure differentials are much higher than for vacuum casting (especially if something goes wrong), and because all the energy is released outward if the container fails. Because of this, you should never try to rig your own pressure chambers or any other parts of the system, and not even think about removing any safety valves, regulators, and so on. Follow the instructions religiously, especially when it comes to draining the compressor; do not exceed manufacturer-provided pressure ratings; always double-check that the system is depressurized before trying to open the chamber; and disconnect everything completely when not in use. With common-sense precautions, catastrophic failure is unlikely - but rest assured, bad things have happened to quite a few people who should have known better.

Oh, one more thing: if you are inclined to get a a nitrogen tank for blanketing resin containers in storage, go for a small one - capacity of 1 cubic meter (40 cubic ft) is more than enough. Store it horizontally or attach it to the wall to make sure it can't fall over, and always use a regulator.

7.4. Silicone resins

Platinum cure silicones are believed to be essentially harmless; both the siloxane resin and the catalyst are non-reactive and show virtually no toxicity in animal studies. You should always read material safety datasheets for the specific product you are using, but chances are, the only risk you have to worry about is that the material is sticky, and spills can be annoying to clean up (naphtha and other nonpolar solvents can help). Beyond basic workplace hygiene, no special precautions should be necessary.

Now, I don't recommend using condensation cure silicones, but if you decided to choose that option, you should be aware that they often use a small amount of dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound, to catalyze the reaction. The substance is corrosive, and more troublingly, exhibits some chronic toxicity. It is present at around 5% in the catalyst component, and usually at less than 1% in the finished product - but since it will leach out in favorable conditions, you should probably not be using condensation cure silicones to make anything that is routinely worn or handled by humans, or that comes into contact with food. When mixing these resins, it's best to use latex or vinyl gloves; basic eye protection is not a bad idea, too. Storing the catalyst in a suitable cabinet and out of reach of children is important, too.

7.5. Polyurethanes

Polyurethane resins consist of several very distinctive components, and it's probably useful to discuss them separately. Here's what you can typically find in the MSDS:

As with any chemicals, you should avoid mixing polyurethanes - and isocyanates in particular - with potentially incompatible substances; as hinted earlier, this includes alcohols, water, but also strong bases, oxidizers, amines, and metal salts. Rapid polymerization, decomposition, or other funny developments may ensue. Isocyanates also decompose in a matter of minutes in dimethyl sulfoxide (DMSO), so do not use this solvent as a carrier for dyes, catalysts, and so forth.

In the same vein, don't mix unreasonable quantities of polyurethane resins in a single batch. Runaway exothermic reactions can become dangerous if the resin gets so hot that the mixing container melts away, or something catches fire. If you need to dispose of a large quantity of unwanted resin, polymerize it in batches; or at the very least, use a large, shallow pan and work outdoors.

Last but not least, because of their irritant properties, uncured resins should be stored safely, and kept out of reach of children and other creatures other than the intended user.

These considerations aside, cured polyurethanes are one of the safer, more stable, and least controversial plastics out there. Even if you mess up the mixing ratio, the unreacted isocyanates will simply eventually react with moisture, unreacted polyols will not pose a threat, and nothing bad should be leaching out of the material otherwise. Like most plastics, and organic materials in general, polyurethanes release a fair amount of toxic substances during thermal decomposition - carbon monoxide being by far the most significant problem - so try to resist the urge to burn them if at all possible.

7.6. Other chemicals

Throughout this guide, I have mentioned quite a few chemicals that won't be covered in this chapter in special detail. If you plan on getting any of them, you should take care to obtain and read the appropriate material safety datasheets, and understand all the associated risks. In no particular order, here are some of the things you should know:

It hopefully goes without saying that you should store all the materials discussed in this guide away from sources of ignition, excessive heat, and so on; that you need to keep them away from children; and that if you are pregnant or breastfeeding, you should avoid any non-essential exposure to exotic chemicals, no matter what the MSDS says.

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