Fabrication Tools Selection - Compressors

Many people with off road vehicles consider other work on their vehicles which is over and above the basic servicing and maintenance, this is usually a variety of modifications which involves fabrication works. Fabricating requires a lot of expensive equipment, which is often purchased over a longer period of a person’s life, in this section we will look at this equipment and determine the most suitable equipment for our hobby. We will decipher some of the technical jargon and some of the misconceptions which are prevalent, and hopefully allow people to make an informed decision when purchasing expensive equipment and not purchase based on advertising myths.


Compressors are a very useful piece of equipment, they can power a range of tools and equipment which may range from the very basic tyre inflators and blow guns, right through to spray guns for painting, impact wrenches, and applicator guns for Schultz and underbody treatments. Many other tools are available, DA’s or dual action sanders for preparing vehicle bodies for spraying, die grinders for fine finishing on metal, and even saws or nail guns are available for those woodworking are now cheap to purchase.
Compressed air is the most expensive form of power, it converts electrical energy into compressed air with a pump, these vary in type and style; we will concentrate on the piston pumps as they are the most common, and the most likely sort used at home. Compressors come with a receiver, this stores the air under pressure until it is used, connected to this are a regulator which regulates the compressed air pressure as it leaves the compressor to the desired air tool. Compressing air leads to water or moisture in the air becoming separated, this forms in the bottom of the receiver or tank; the regulators often incorporate a separator/regulator as a combined unit to remove water or oil droplets as it leaves the receiver. This ensures a clean supply of air for the air tool, especially important if spraying vehicle paints or other finishes is undertaken. Water collects over time in the receiver, this must be drained so receivers incorporate a little drain valve on the underside to drain this off: air pumps have air filters to clean the air, much like an engine, so check it is a cleanable type and not a replaceable element.

Many advertisements mislead, this is most often by displaying the air quantity the compressor produces, this is quoted as CFM or cubic feet minute, and this is the actual amount of air the compressor pump provides to the receiver. This is actually the theoretical piston displacement based upon the size of the piston/pistons, and is not the measurement we need to match tools. The measurement we need is called FAD or free air displacement, FAD is the actual amount of compressed air the receiver provides for the air tool, this can often be up to one third less than the quoted CFM. This information and its understanding is vital to allow us to match the air output to the tool we require air to power, many people believe they should use the CFM rating and become disappointed when they match a tool to this rating and it does not perform correctly through insufficient air supply. This is especially important when high consumption tools are used such as HVLP (high volume, low pressure) spray guns, DA sanders, or larger impact wrenches with high torque ratings are used.

Compressors are controlled by a pressure control switch, there are two types of these; the basic type switches the motor on and off depending upon the pressure you set it to, basically it is the compressor’s on and off switch. The second type is one which allows you to set the pressure, and the working range the compressor will switch on and off at. Most tools require a pressure of 90 PSI (pounds per square inch) so we can set our pressure switch to cut in at 95PSI to provide sufficient air pressure. Setting the adjustable range means we can alter the maximum and minimum cut in and cut out pressures, therefore we can set our compressor to cut in at 95PSI and adjust the range so it cuts out at 120PSI, ensuring adequate working pressure at all times. This means we have a longer working time with our selected tool before the pressure drops sufficiently to allow the motor to cut in again, alternatively we can reduce our range if small air consumption tools are used. If we use a tool with minimal air consumption we may elect to set the range from 95PSI to 105 PSI to save power consumption from not compressing air to too high a pressure.

Working pressure is important, this is usually quoted as “Bar”, one bar = 14.7 PSI or one atmosphere to those old enough to remember working in atmospheres, a typical small and cheap compressor may only provide 6Bar (88.2PSI) of working pressure. This would not be sufficient to drive most air tools so always aim for the highest working pressures you can afford, the common type is the 15Bar (147PSI) unit, working at the 90-120PSI range means it is not working flat out.

Receivers come in many styles and sizes, there is one basic rule to follow; always buy the unit with the biggest receiver you can afford, many come with very basic 25 litre units, and the largest units suitable for home use have up to 300 litre units. The larger the receiver, the longer the working time as it contains a much larger quantity of air, this prevents the compressor continuously running, always aim for 150 litre or higher receivers if space or finances are restricted.

Motor types are crucial, many of the cheaper units are an integral motor/pump unit, these are cheap because of their design; these are best avoided; the best type are the separate pump and motors, connected with V belts These are the professional types and if the major components become worn or damaged they can be replaced individually, the integrated types invariably mean purchasing a new compressor, which is much more expensive.

Many compressors connect to a 240 volt supply, this means they consume a lot of power and require a minimum 30 amp supply of 240 volts to operate, this means they cannot run from an ordinary domestic plus. This means a dedicated supply rated at, or above 30 amps is required in the garage or workshop to run them without continuously blowing fuses; the maximum size compressor which can run from a 240 volt supply is 15CFM piston displacement. Above this size you would require a separate three phase supply to be installed, 15 CFM is adequate for most home users and light professional use as it will provide 10-12 CFM of FAD (depending on make) which runs most things we require.
Many manufacturers recommend, or even specify a “direct online starter”, this acts to give the compressor motor a power boost upon starting, this is essential when a compressor is running as it is always starting and stopping on load. Basically it has to have sufficient kick to begin compressing air which is already compressed to around 95PSI; many incorporate overload protection, this saves the motor from burning out which is a common problem in domestic use. Many manufacturers specify an online starter with overload protection as part of the machine’s warranty, if they specify it then fit it, if they do not specify it then fitting one is a good idea.

Connecting your compressor to your tool means pipes and connectors, flexible pipes are available from 5 metres to 30 metres long, it is personal choice as to which length you choose, most come with a ¼ BSP thread fitted to each end. These connect to the compressor in two ways, simply screw toe pipe to the regulator/filter output pipe with a ¼ BSP nipple, and do the same to the tool. This is inconvenient where multiple tools are used, many ranges of snap connectors are available, these are a standard range of connectors which simply allow multiple tools to be connected and disconnected by sliding back a sleeve and they separate.
My preferred choice is the “PCL” as they are the most common type, simply fit a PCL female connector to the end of your hose, and fit males to all your air tools, they come in a variety of thread sizes to fit most air tools, and a number of accessories are available. Perhaps you want two tools connected, simply buy a two way branch and connect it to your hose and plug in two tools at the same time.
Hose size is crucial, most hose is 6mm or 1/4” bore, this is the most common size and suits most applications; some tools may require a larger bore hose to allow the required quantity of air to reach the tool without restrictions. Always check the hose size a tool requires as some may need hose sizes of up to 12mm bore.
Most people prefer to purchase a number of hoses, this is a good idea if you may periodically need a long length of hose, this means you have a couple of spares for most applications and you simply connect two or more together for a longer length.

Remember the golden rules:

Buy the largest output compressor you can afford.
Check its FAD output measured in CFM is sufficient for your tools
Get the largest receiver (minimum 150 litre)
Get a machine which is industrially rated if possible.
Ensure it has a pressure control switch with adjustable pressure and range settings.
Unsure it has a filter/regulator unit, if not buy a separate unit to filter your air.
Ensure the air filter is a cleanable type.
Buy a unit with separate motor and compressor pump.
Ensure it can run from a 240 V 30 Amp supply and install a 30 amp supply.
Standardise your connectors to one type, PCL or other.
Install an online starter with overload protection.

Always follow these simple rules and maintain your compressor, it only requires oil changes for the compressor pump, air filters cleaning, and water draining periodically; belt drives will require the belts tensioning and replacing periodically. This will ensure it will last for many years, and recoup your original purchase costs many times over, mine is over 30 years old and as good as the day it was bought, always buy quality and it will last.