Sand Blasting by Daryl Anderson
Copied from spanner. June 2001
As a Monumental Mason (maker of headstones/memorials to the departed) we use gritblasting to engrave the inscription into the stone, and I have 28 years experience in this. I build the gritblasters we use and all the dust extraction facilities so feel qualified to answer any questions. Gritblasting is the term we use, but the word sand is the term the layman uses.
Firstly there are sandblasters and there are sandblasters. The largest models are used for cleaning bridges, the smallest for fine models etc. For Meccano parts cleaning something in-between is needed because of speed of removal. Sandblasting is the method used to clean large structures as it's the most efficient and to me (but I am biased) the same is true for cleaning Meccano parts in preparation for re painting.
Different media (sand) is used. Raw beach sand can be used but there are two main pitfalls. One is the silica content of the sand, which kills the operator over time (silicosis). I don't believe the silica content hurts the job you are doing. The second problem with beach sand is the salt content, which, if used raw, leaves a deposit, which rusts under the shiny new paint. So beach sand must be washed thoroughly before use. Beach sand generally breaks down on use and can only be used once or twice, unless your local sand is very hard. This is another problem, as the sand will vary too much even in a space of a few miles.
We use aluminium oxide, which is what most grinding wheels are made of. Silicon Carbide is an alternative although more expensive, lasts longer and there are many other materials without getting into walnut shells and glass beads, which are used, for specialised applications.
Now we come to the sandblasting tools of which there is pressure fed, gravity fed and suction. In pressure and gravity types the media is stored in a vessel with a T-junction in the bottom, mixed with air flowing from one side, taken through a hose and out the nozzle. Simple in theory, but in practice it's more complicated. A gravity fed is unit is just that, the vessel is open at the top, you fill it up with media, air rushing through the bottom mixes with media and out the hose. The pressure of gravity pushing it down limits the amount of media that can flow, and if too much air is flowing then it will take the easy path up through the media instead of out the nozzle. So this type is not capable of rapid removal of rust paint etc., simply because it can't. These are the types you see in local tool shops. We call them toys but they work, slowly. The suction type uses a venturi to draw media up a hose and is mixed with the air rushing through, exactly the same way as a spray paint gun. This is less efficient then the gravity fed. The only serious type is the pressure pot. You get what you pay for.
In the industrial pressure pot type the media holding container is pressurised, so the media is forced out then mixed with air as above. The advantage is more media can flow so the rust removal is quicker. This pressure vessel MUST be designed, manufactured and tested to a standard. Welding of pressure vessels is NOT a job for the home handyman with a welder. I don't do this part, but take the vessel from there and make the controls. There are controls for the overall pressure in the pot, the airflow across the bottom and the amount of grit flow. There are other controls, but I am trying to keep it simple.
The air controls are simple valves and regulators, but the grit control is more specialised. This is usually made with a rubber liner or insert, with the rubber compressing to stop the grit flow. This wears and must be replaced. In our larger main pot used for a few hours every day, this is replaced every 2 months or so.
The size of compressor needed is governed by the size of the nozzle used. The larger the hole the more air needed. Bridge blasters may be using a one inch holed nozzle, with massive hoses and compressors. We use a 1/8" inch or 3-mm when new. Our compressor is 30 cubic feet per minute (CFM) with a 10 horsepower motor. When we blast lettering on site in the cemetery, we use a smaller nozzle with 2 mm hole and 7.5 horse power (petrol motor) compressor which of course is slower to cut. My advice would be to use a 2mm nozzle powered by a 2-3-hp compressor and 80 - 100 grit (mesh) Aluminium Oxide. In New Zealand this grit costs about $5 per kilogram, so it must be recovered. This grit can also be used in the tumbling machine (or concrete mixer!) to clean your small parts.
Distortion of parts when blasting is caused by local heat generated by the friction when the media hits the steel, so the bridge blaster is going to distort smaller thin items as he is using a greater flow, coarser grits and larger pressure. We do a lot of metal blasting as a side line, up to car bonnet (hood size) with no distortion as we are gentle enough but efficient too.
A bundle of 20 x 12.5" strips would take 5-10 minutes and a single 5 1/2" x 2 1/2" flanged plate about 1 minute. It's not the blasting time with Meccano, it's holding down of parts so they don't blow away. I have found the best way is simply to hold them in a gloved hand rotating as necessary. It is possible to clean 2 ½" parts, but easier limiting to 5 ½" as the smallest than can be held and rotated in a gloved hand. We have a hole in the booth just big enough to pass a hand inside.
We recycle the grit, never discarding but adding new material as the media breaks down. An extraction system is necessary to remove the small particles or it is best done outside. The advantages of inside in an enclosed booth are dust and media containment. The best type of extraction for the home would be a vacuum system, either a domestic vacuum cleaner or a woodworking dust extractor with bags. Be careful if the bag system is used that the bag is fine enough to trap the particles. If you use your domestic vacuum cleaner, don't blame me if the fine dust wears out its bearings!
Our main Blaster uses a water spray filter. The contaminated air is forced into a chamber where there are many showerhead sprays, which wash the particles from the air then they are collected and disposed of.
Now onto nozzles where we have a choice of three. In the beginning there were steel and cast iron which is what we used when I started work, cost if I remember rightly about $2 each and would last about 2 hours. Then ceramic came on the market, which cost $10 and lasted a week. About 15 years ago we started to use tungsten lined. These cost $100 each and lasted 2 months. Then about 4 years ago I heard of a new type of tungsten, these came highly recommended and cost $500 each. As they were imported from USA I bought 3 and we are still on the second one which is just about worn out now. So you can see this has been a huge cost saving. It's important to use a slow wearing nozzle because as the hole gets larger you use more air (which you may not have in reserve), but more importantly the characteristics of the Blaster changes i.e. you have to advance a control more than you used to make it work efficiently. If your nozzle only lasts a week, you are constantly adjusting the controls.
As I said above you need controls for air and grit flow. Also needed is an overall regulator to govern the air in the system. When the pot is connected directly to the compressor, as the air governor works when the tank reaches pressure, then this will effect the mixtures. So if your compressor reaches 100psi and stops, and the motor starts again at 90 psi, then regulate the pressure to 80psi to create a constant pressure in the pot. To start blasting, pressurise the system, open the air valve a little to force air through the bottom of the 'T' with the grit flow off and clean air out of the nozzle. Then slowly open the grit control to let some grit flow. When you see grit coming out the end of the nozzle, open the air some more and play with the controls to get a good 'rich' flow. If not enough grit flows (as in the gravity and venturi types) there is not enough media to cut properly; only air. If too much media is flowing then what will happen is there will be a stream of media, much like water coming out of a hose. This won't cut at all. You will soon get the knack.
A nozzle is needed to concentrate the flow. Without a nozzle nothing happens when you point it at a 52a. Take the nozzle off and try it, you can hold your hand in the flow. What the nozzle does is concentrate and accelerates the flow so it cuts, but the nozzle wears. An essential part of this is water traps. When the compressor squashes the air, moisture in it doesn't compress, as we all know you can't compress water. When the air reaches a larger vessel being the main holding tank or your pressure pot, the water drops out. Some collects in the main tank but the air is still hot from being compressed, so you must have water traps on and before the pots to extract as much water as possible before the air goes into the pot. If you don't then the water that gets in the pot flows to the bottom and clogs the grit flow. If you recycle the media it will have more dust and fine particles in it so this is more of a problem that the bridge Blaster as his media is new every time. There are 3 water traps on our system after the main tank on the compressor. The last one removes very little which means the air is basically clean.
All prices quoted in NZ$. If you have any questions just ask, refer them to Spanner or privately to firstname.lastname@example.org
Kind regards, Daryl Anderson, Hawera, New Zealand.