Used motor oil as cutting fluid

Motor oil does the exact opposite of what you want when cutting, it's SUPPOSED to keep the various bits from digging in to each other. Sulfurized and chlorinated compounds in the various cutting fluids promote that. About all you'd get is a coolant effect and water-based compounds would be cheaper for that and probably less toxic. Lots of nasties in used motor oil.
Stan

Oh damn, I almost forgot this episode.
Long ago I worked for the CHEAPEST Swamp Yankee on Earth. He had a machine shop with a gas pump out front. You were supposed to stop in mid-cut to run out front and pump gas because the gas sales paid the rent. Every time he did an oil change for someone, we all got new "cutting fluid". Since it was in a not-too-rich area, the oil was always long overdue for changing. The day he handed out oil after an oil change in a Mercededs Diesel was my last day there. I walked out. My wife had stopped by to bring me a lunch, looked around, sniffed, and said "QUIT NOW!"
We went though a LOT of cutting fluid because all the end mills were dull. ALL his machinery, he bought from junk dealers by the pound. They were RUSTY. Because he had less than seven employees, he did not have to have a toilet there. Employees were expected to alk down the street to the Police station and use theirs. He was also not required to have Workmens' Comp Insurance. He used to get kids right out of Vocational School. They'd leave shortly, disillusioned, and he'd just go back and get more.
He got his one year, though, since a town full of bored pissed off kids decided it would be funny to break in while he was on vacation shutdown. They took the chucks off the lathes and the kellering Threading Mill and ran carbide in to spindles and generally wrecked the place. Not that it was any loss.
The short answer is "Don't be like HIM".
You can get a gallon of soluble oil or synthetic from McMaster, etc reasonably and it lasts a long time. Don't do this to yourself. Old motor oil is full of carcinogens and additives and its characertistics are shot. Even new motor oil or transmission fluid or spindle oil are not that great for cutting...After all, are they not formulated to PREVENT metal removal?
For steels and the stainlesses I have had good results with the heavy duty soluble oils. The Amine based "Synthetic biodegradable" work VERY well with aluminum and copper, etc.

If you have to be cheap on cutting oil, try bacon grease. Filter it through a coffee filter and brush it on with an acid brush. Cut it with mineral spirits to keep it liquid in cold weather and from going rancid.
I've used it with verygood results, it's said that the salt in the bacon will rust the work/tools but I've not found that to be true..the smoke smells like frying bacon till you add the mineral spirits. ED

I use it, seems to work fine. I think they all probably work pretty well. I used to like Tap-O-Matic when it had nasty stuff (trichloroethylene) in it. Now it has become nicer, considerably more expensive and it doesn't work as well.
For ally: Ernie reported some time ago that d-limonene works like magic as a cutting fluid for aluminum. It can be found for 20 bux or so -- but I've found that "Goo-Gone" works just fine. You can find liquid Goo-Gone in about any Dollar Store. I don't know how much limonene it contains, but it sure works fer me.

The advice here is a lot better than the last time this subject came up. As someone said, motor oil and cutting oil are almost opposites; tools actually can skate a bit before cutting into the work when you use motor oil, which will dull edges on HSS tools in a hurry.
Lard oil, with sulfur and chlorine in it or not, is good for smaller or older machine tools, where power is low and cooling is not the issue. On those machines, reducing cutting force is the issue. Water-soluble (miscible) oils are good, but they're really engineered to cool more than to lubricate the cut.
The ideal is an oil that resists high-pressure penetration of the oil film (thus the sulfur) but without the extreme high-pressure lubricity of modern motor oils.
Lard oil is the same thing as bacon fat but without the stearin that makes fat so thick and viscous. It works better with the stearin out of there. For ordinary cutting on small or old machines, using HSS tools, plain lard oil is hard to beat (I use Buttercut, which is plain lard oil). The chlorine in some formulations is supposed to make it easier to cut harder steels; I don't have enough experience with it to tell. Sulfur really isn't necessary until you're cutting with some horsepower.
-- Ed Huntress

*need* is a pretty strict word to use here... We have one cutting fluid here that we use for drilling, tapping or reaming all metals... But then again, my jobs are relatively simple and done on small drill press machines or by hand. Anything more complex than that, we do someplace else and they use the proper stuff..
Point being... If it's a hobbyist, a bottle of motor oil is probably fine. Even 3-in-one will probably work for most jobs... If it's a professional shop doing lots of stuff, then that's another issue and *need* may be more properly applied to their situation's description.

No...I always used the soulble oil types. I figred if the guys at work used them on the CNC's with stainlesses, titanium (Both 6-4 and CP) and Zirconium, then it would handle anything I could throw at it.
I use this water soluble oil: Medium Duty 6 1020K11 25.11
I was manufacturing parts from 303, 303, and the 400 types of stainless, as well as cold-rolled and 4140. The coolant also worked well with other alloys, like that horrible stringy-chip 316,pure nickel, and Invar.
For aluminum and even some plastics like delrin I use
1-gal. Container 4 10705K54 26.86 The cooling effect of the water is needed for some plastics. Aluminum loves this stuff. Even if you have not set up a recirc system and just use a wash bottle or a brush or a squirt bottle, I think these are better. The water content not only improves cleanup, it REALLY cools better. For titanium this is critical. The cutting tools love it too. One of the things that kills tools is the local microscopic heating at the cutting edge and adjacent to it as the hot ship curles and rubs against it.. Imagine an end mill with its flutes and reliefs, tapering to a razor sharp point. The place where the heat is generated has to get rid of that heat by conducting it away from the edge. If you feed too fast or run too fast, it cannot. Some tools steels do not have to get red hot before they lose their temper. This is why resharpened/ground end mills usually do not last as long as a new one. Heat damage.
I used to use the old fashioned tallow oils for turning and milling and threading. I STILL use Tap Magic for hand tapping, though.
$25.00 a gallon seems pricey till you think about adding it into water (6:1 or so for heavy use, depending on the oil.)

OK.. now sorry for a dumb question... Would it cause the lathe to rust? And does it become rancid after some time?
i

Some of them can get moldy, etc. if allowed to stand in the machine for a long time. No machines rust with soluble oils, though..that's the whole idea behind them. (Imagine the place I used to work having $300,000 multi axis machines RUSTING! People would die for that..these guys keep the machines like an operating room.)
Most are not really "Soluble", the mixtures are dispersions or emulsions, like the butterfat in milk. When the water dries, a thin protective film of oils remains on the machine surfaces. In the case of the triethanolamine based "Biodegradadable" ones, like the one I use for aluminum, they are clear solutions- that residue prevents rust, too The stuff is pretty harmless. You will see it listed on the labels of many shampoos and lotions.

OK... so it seems that I should not expect the lathe to rust from it. That's good. I can give it a go. How long does it take for them to "get moldy"? I use the lathe only occasionally, as is the case with all of my "equipment". So if that stuff would require changing out too often, it would not be great.
As an aside, I cleaned out the mess in the coolant sump yesterday. Actually found a bunch of lathe tooling there covered with dirty rags, such as live center, tool holder etc, as well as 3 lbs of dried grime. I think that the pump is fine. I can give the flood coolant a try, maybe I can limit it to a tiny stream.

Iggy,
How long is a piece of string? The moldy is a function of a lot of things and it may be a week or two or a month or two. It is basically stagnating, as in losing Oxygen. I use my big mill quite infrequently, sometimes a month or so goes by. Retired doncha know. :)
I use a small fish tank bubbler in the big mill sump. I have it on a timer that runs it for 2-hours each day. It is a small bellows pump, a length of PVC tube and a 12" Airstone. Check Wallys pet department and it will probably cost about 10-bucks all up.
The water does evaporate slowly and I top it up with the mix before I use it each time but I have not had to clean out the tank for several years.
You can also get tablets from MSC etc that are supposed to prevent the stagnation but the bubbler works for me.

I had such a bubbler with my fishtank.
So, to repeat, all you do is, you set the bubbler on a timer to pump air a few hours per day, and that keeps the coolant from stagnating? Plus you top it off to account for evaporation.
Is that right?

I can also vouch for this method after reading about it here. £10 down the pet shop for the cheapest kit comprising an air pump, tubing and air stone, and about £6 for a mains timer set for about an hour each day and the coolant still seems good about 5 years later. It gets topped up once or twice a year as necessary. I have run across rancid soluble oil coolant before and the stuff I'm using has been fine with this method.

The biggest problems with large coolant supplies in industry are bacteria growth, diminishment of anti-rust properties, and contamination with lubricating oil (tramp oil).
The first one is handled with bactericide additives, which, on smaller scales, come in the form of a big pill. The anti-rust additives usually are (or were) liquid. Tramp oil is skimmed off with a skimmer, or sometimes spun off with a centrifuge.
If the coolant stinks, you have bacteria, and that's the most common problem. It can be hard to fight, especially where guys pee into the sump. d8-)
-- Ed Huntress

I've been using Hangsterfer C500 soluble oil coolant for over a year now and the original 5-gal batch still has no odor. Tramp (way) oil that gets in the mix may be the primary cause of odor so it could help to remove that from your sump. Some of it can be soaked up with paper towels but a belt skimmer is more effective and less messy.

A good soluble oil does not separate out from the emulsion for some considerable time and should not allow any rust to form.
I used to work for BP until I retired 12 years ago. One of the products was Fedaro M, a soluble cutting oil. Two of the tests I had to do, to ASTM standards, were (1) rust, a cast steel square was polished and lathe or drill chips were placed on it and covered with oil emulsion, no rust or staining was permissible for either 24 or 48 hours, cannot remember which (2) emulsion stability, A 2% oil solution was mixed and had to remain stable for 24 hours with no oil separation.
I still have about 3 litres of Fedaro M left, should be enough for at least 5 more years.
I usually use 5% on a total loss system but have recently treated myself to a suds pump, just have to plumb it in to the lathe and mill, then I doubt if I will get any more blue chips
Mineral oil based products should not become rancid.

"Mike Henry" fired this volley in news: snipped-for-privacy@news1.newsguy.com: If you have an old piece of Tyvec material, stretch it and pull it until it gets a little "fluffy", then use that to wipe the surface of the coolant. Tyvec fibers will aggressively absorb oil, but will not pick up water or water emulsions. A "pig" oil sorbent pad will do the same.
LLoyd

Alan, would you say that Kool Mist 77 or 78 is about the same as products that you described?

As far as I know, it is not available in Oz, I have not heard of it so cannot comment. My local machinery pusher stocks some wierd brand name but as I do not need any I have not bothered to look at its specs.
Most of the major oil companies have a large range of machining products and have web sites explaining their uses. I gave away my book which showed which Shell/Mobil/Caltex etc. were equivalent to BP oils about 5 years ago so cannot help with comparisons. I know BP operates in some parts of the USA for fuels, but whether the lubricants are sold everywhere is unknown. There are a couple of companies I would not buy from, their specs. are as wide as a barn door, some +/- 10 or more % from nominal compared to the max +/- 5% I worked to.