Cutting Oil

[Rod999]

  My lathe has a built in cutting oil system which I filled with cutting oil. Over time it gets lost, mostly on my shirt and hands. My question might seem strange but, is there a good reason NOT to top it up with the drain oil from the car or lawn-mower. It would for sure never get to a 50-50 mix, I still have at least half of a 5 gallon bucket of the cutting oil, just trying to make it last in this economy~

[gldavison]

That oil's got to have a lot of contaminants in it plus all kinds of metal flakes and filings and you name it. That would not be good on the wear points of your lathe unless you can find some way to run it through a pretty fine filter first.

[samc88]

Used oil can be ironically corrosive. However I have had good results with using clean EP90 gear oil as cutting oil in a pinch (I have gallons of the stuff with the land rover and did a bit of experimenting one day). Smells great when hot

[john mills]

what is your cutting oil  i would think it should be a soluble oil  just top up whit water what do you want to use old engine oil for it is not cutting oil

[AVTUR]

This is well documented. There are two options:

1. "Soluble" oil, actually a water oil emulsion. White in colour and popular for ordinary industrial use. It does go off with time and bacteria can breed in it. It should be cleaned off any metal when the lathe or mill is put to bed.
2. Neat cutting oil. This is not used with water. It is the costly option but does not suffer from ageing or bacterial growth. It does not attack iron/steel and can be safely left on the machine. I am aware than my sump of neat cutting oil under my lathe is slowly being contaminated by other oils and white spirit.

Old engine oil, or any engine oil, is not an option. Over the last nnn years oils have been progressively tailored for specific jobs.

I could go on but the bathroom fitters are about to arrive.

AUTUR 

[Rod999]

What's in it now is MOBILMET 426 non active cutting oil. But I asked for reasons not to and I got them~