adjusting a blast cabinet for peak performance

 Where "peak performance" is defined as "the fastest material removal rate with the slowest media consumption rate".  

 Note I said nothing about air consumption.  Air consumption is (or should be, if the rest of your equipment is sized correctly) determined by just one thing: the size of the orifice in your blasting gun.  

 Your compressor must be size appropriately to deliver the necessary CFM to support that consumption.  

 Pro tip: unless you are a professional media blasting company (in which case, why are you reading this?) your compressor will almost never have enough CFM to support full-time, continuous blasting for long periods of time.

 However, having a high-pressure air compressor (which is then regulated down to the ideal blasting pressure for the abrasive in use) with a big tank will mean that your compressor will cycle on and off as designed (I am assuming the common, reciprocating-piston compressor).

 Using a blasting pressure higher than recommended for your operation will cause the media to exceed the maximum impact velocity, shattering every particle into non-useful fines on the first impact.  Instead, what we want is to gradually wear off the sharp edges of the media so that it lasts and lasts when being constantly recycled in an enclosed siphon type cabinet.

 For the aluminum oxide I will use most, this means no more than 90 PSI.  If you want to increase cleaning action for a given abrasive media, you increase the nozzle and orifice size, which increases the amount of air and media you are throwing at the work in a given amount of time.

  So our blasting pressure is set on the regulator we've installed at the cabinet.

 What about the orifice adjustment on the gun?  The best way to do this is with a manometer, and while you could use a fancy commercial one, you can also just use some clear plastic tubing filled with water, connected to the sand port of the gun, to measure the suction it is creating.  For any given nozzle and orifice combination, the orifice position must be adjusted to maximize suction.  Even if you change only the nozzle, you must readjust the orifice.

 When that is done properly, and if you keep the hoses as short as possible, you will be able to use the widest range of media weights.  With a really good setup, it may even be possible to use steel shot, something that is usually too heavy for siphon guns, requiring a pressur pot setup.

 The last adjustment is to the media metering valve at the bottom of the cabinet media hopper.  Starting with the valve closed (admitting no air), you should have almost no media being sucked up into the hose, unless the depth in the hopper is too shallow.  As you gradually open the air valve, more and more media will be sucked from the bottom of the hopper into the hose.

 The best way to determine where to set the valve - that is - when to stop opening it - is to actually blast some work.  At some point, you will notice that opening the air valve further is no longer increasing the speed of material removal.  So crank it back in until it just starts to decrease or just before that point.  Done.

 If you change media, everything has to be re-adjusted.  Failured to do so will mean you won't get the results you're expecting, or your media consumption will be higher than necessary, or other troubles.

 I wrote this up because nearly all of the instructions I found on YouTube for doing these adjustments were confusing, or didn't make sense.  Usually the ones that didn't makes sense were also just plain wrong in some way, as I found out when consulting actual media blasting industry reference materials.

 Hopefully, this article will save someone else the eighteen hours of research I just did.

 You can google for the properties of an abrasive you'd like to use, from whence you can find such useful pages as these:

https://www.raptorblaster.com/reusing-abrasive-media-blast-cabinets/

https://www.mediablast.com/blog/2017/07/06/8-rules-for-recycling-abrasive-in-blast-cabinets/

 Note that the general rule of thumb for steel shot (round for shot-peening of surfaces, cubical for cleaning) is that "you can't do it" in a siphon cabinet because it's too heavy.  This is absolutely true for small, cheap cabinets with shitty guns, undersized compressors, incorrectly-adjusted guns, and so on.

 And it works better the larger the orifice and nozzle, since that usually results in higher suction for a given pressure and media.

 However, it absolutely can be done in a siphon cabinet with a good quality gun and nozzles, adequate air flow, short hoses, an excellent metering valve, and so on.

 Furthermore, you can run a pressure-pot gun inside a cabinet, so long as you can find hoses flexible enough and a gun small / short enough to work inside your cabinet.