For the occasional Friday when the air is down due to maintenance.. Harbor Freight.
It REALLY REALLY REALLY REALLY helps if you could say how much air you need..
If you have 3 air purge spindles that are using 14cfm each, a POS Harbor Freight compressor
for $150 isn't going to do it for you.
If you are changing tools on all your machines every 20 seconds probably not going to do it
for you..
Seriously.. I've run 3 Fadals and a Mazak lathe on a 21 gallon $150 compressor. If I'm doing a
lot of tool changes, it doesn't work so well.. so its 2 compressors. 1 for the lathe which has an
air chuck and an air over hydraulic turret, and air something or other going on in the tailstock, and
air to change gears. and one for the mills.
My big compressor died 3 years ago.. Actually the motor died, and I bought a new motor, but while waiting
for the motor to arrive I was using my little Craftsman backup, and that wasn't enough, so I bought a
POS 21 gallon 2.5hp from HF.. Then the craftsman puked.. And what do you know, the single HF POS ran
everything.
3.5 years later, I've bought 1 new motor for the big compressor (still not installed for $386?), and 3 21 gallon
POS's from HF, and 2 8gallon POS's from HF.. for a total of $650 in POS compressors.. Small compressors running
at a bit lower pressure.. I've taken $150 off my electric bill a month for 3.5 years.
A backup compressor(s) for occasional use doesn't need to be the latest and greatest and bestest thing.. It just
has to give you air to make it through the day. I would suggest hooking up your airlines so you still have access to
an air dryer, with adequate shutoffs and bypasses so you aren't trying to fill a million gallon tank with a small compressor.
I would suggest shut offs and bypasses no matter what.. Ever had to shut the entire shop down for 3 hours because the
air dryer froze up and you didn't plumb in a by-pass, and the back up compressor was also upstream of the air dryer?