Start Model Engineering

An air compressor is a dual edged sword in the workshop. A handy tool for blasting away swarf and chippings on the lathe or mill and a convenient source of power for running our little engines. I already have two largish compressors and one small airbrush type compressor housed in a carpet lined cupboard in an attempt to reduce the noise factor. A silent compressor would be a welcome addition for demonstrating my engines at shows or indoors.

A recent magazine article described the building of a ’silent’ compressor based on a pump from a discarded ‘fridge and an empty fire extinguisher as an air holding tank. Well a pump has been acquired and the hunt is on for an empty 2 litre fire extinguisher. I have put a bid in on fleabay for a ‘full’ extinguisher. I am rather hoping I don’t win it as it has dawned on me that discharging the contents could be very messy ! My opening bid of a fiver was sufficient to secure me my fire extinguisher.

No worries. A bin liner was placed in a large plastic bucket. The discharge pipe inserted into a small opening in the bag. A few tentative squeezes of the trigger and the job was done - well nearly ! After the pressure had dropped, I unscrewed the brass outlet fitting and discovered there was still quite a lot of powder remaining inside. Once this was tipped out the cannister was washed thoroughly inside and out with hot soapy water and left to dry.

This project has taken me once again into the confusing world of BSP threaded fittings.  Check sizes of components especially when you are ordering on line against this handy chart.

My extinguisher cannister brass fitting  was fitted with a schrader valve, presumably for pressurising the cannister at manufacture and maintenance. I discounted this as a non-return inlet valve for my purpose as I doubted the ability of my ‘fridge pump to overcome the power of the spring. A quick Google revealed an abundance of low cost non-return and safety valves.

Silent compressor undergoing a trial run. 25 psi recorded, need to eliminate one or two leaks on connections then should be good for 35 psi or more.

Well the project took a step forward following a visit to a compressor specialist who gave me enough encouragement to persevere with this project.

A trip to my local Plumb Centre furnished me with the necessary connectors to link things up including a check valve which even under very low pressure allows air through one way but won’t let it out.

I have also modified a safety valve from Machine Mart to function at just 50 psi by replacing the coil compression spring with a weaker alternative. A bit of trial and error is required to arrive at the correct tension.

Now I have reassured myself that everything works it’s a case of mounting everything to form one compact unit.

The final ‘icing on the cake’ would be a pressure control valve but so far I have been unable to  track one down that cuts in and out at the pressures I require - or at a price I am willing to pay !