I bought a bottle of general purpose cutting oil from machine mart. For general use. For alloy a lot use WD40 as it helps keep the cutting edge lubricated. With HSS tooling i usually use my cutting oil. If using insert cutters then not so much unless doing a finish pass. More experts will be along shortly with a more in depth answer.

Steve.

I always use a cutting fluid when turning steel, phosphur bronze and stainless steel but none on brass and cast iron.

My Boxford has a coolant tank and pump and I use Alusol from Castrol which is available in small amounts and I dilute with water at 10 to 1 to give good cooling and better cutting.

As its name suggests Alusol was developed for aluminium but it is also suitable for steels and someone on here recommended it so I gave it a go.

It doesn't develop a smell unlike the traditional suds.

For tapping and milling ops I use a straight cutting oil, also available in small quantities from our M.E. suppliers and applied with a plastic sauce bottle, the kind you get in cafe's.

Others will, no doubt, have their own favourites.

Brian

Like this Brian.

Steve.

Not a stupid question at all!

Good news, in a home workshop fluids aren't particularly fussy. Damp rather than soaking, but overdoing it doesn't effect cutting. I use a ½" paint brush, squirt from an oil can or squeezy bottle, or spray WD40 & CT90 from an aerosol. Basically a little of the right kind of liquid splashed on – enough to keep the cutting area wet.

Liquids flying off the spinning job are a good reason for keeping the volume down. Messy, and wildly overdoing it at high speed might get some in your eyes. Dabs, not pints!

Industry is different; they value high-speed cutting a perfect finish every time. They often flood cool, maintaining a continuous heavy flow over the tool and work. Partly because it increases tool life and partly because the flood washes swarf away. But flooding means dealing with gallons of dirty liquids and tons of wet swarf: a right pain unless it's needed, and what about the electrics! More recently industry have moved to compressed gases and carefully metered spray systems. Work very well but are expensive to set up and have the disadvantage that the machine should be enclosed to protect the operator. Breathing in coolant fog all day or being hit in the eye by jet propelled swarf is no joke. Always exceptions, but generally not worth the trouble in a small workshop.

Don't be afraid to experiment. Much depends on what's done in your workshop.

Dave

An addition to my earlier post.

On the milling machine I use a small decorator's paint brush to apply the cutting oil. The fluid is in a plastic bottle that is refilled from the lathe coolant/cutting fluid supply.

JA

+1 for all the above. for my uses …

Brass & Cast Iron – dry

Steels – with HSS Light cutting oil, SAE32 or similar. if using carbide inserts – mostly dry

Aluminium – WD 40 or parafin ( Kerosene ).

Threading – RTD or similar, I have used a heavier grade oil ( gearbox oil ) , on Aluminium I use WD40

Application – I use a kiddies non spill paint pot with 1/2 " brush ….

George.

I find just intermittent brushing of soluble oil is frequently very inefficient as it flows along the bar, away from the contact area.

So often, when using the self-acting feed, I use an old paint-brush about an inch wide, dipped into a small tin of suds then held from behind the work so the bristles trail over the work, and span the cut.

'

For lubricating deep drilling, I sometimes use a pump-action oil-can to squirt suds into the hole when I have withdrawn the drill to brush the chips off and cutting-oil on.

(Industry can use drills with internal coolant channels opening onto the cutting edges – obviously on machine-tools equipped for them)

'

I have used spray-can cutting fluid but this is very wasteful. I am not trying to machine the workshop wall!

Using taps and dies – cutting gels on steel, and a more liquid cutting-oil or WD-40 on aluminium.

Draw-filing steel – a little WD-40 or thin oil can prevent "pinning" and help produce that lovely satin sheen. (It was chalk when I did metal-work at school – blackboard "chalk", not the raw rock chalk, which can contain fine particles of silica a lot harder than a steel file).

Hacksawing – I have tried sawing with and without cutting-paste, with mixed results. Its main aid perhaps is in lubricating the blade flanks rather than the cutting edges, and may even help slightly worn teeth simply slip over the metal. I would not use any cutting lubricants on the bandsaw, with its rubber-covered wheels.

On light finishing cuts I sometimes use a squirt or two of WD40 when it seems advisable. For slow or heavyish cuts I sometimes use tapping fluid – as well as when tapping steel, of course.

But if the fluid is getting splashed about, cutting dry is an option in most conditions for model engineering. Even HSS will still cut at dull red heat, and you'd have to be doing something pretty cruel and greedy to be getting there.

I believe it's undesirable to use any fluid on cast iron. Some of the fines coming off the iron are hard and sharp, and can form an abrasive slurry with oil, which will stick to slideways and thread flanks. OTOH, the graphite inclusions will provide self-lubrication on dry-cutting, as per Bill Dawes' post above.

Edited By Mick B1 on 12/02/2021 18:30:33

In simple terms neat cutting oils provide lubrication and not much cooling, whereas soluble oils provide cooling and not much lubrication. Soluble oils are used for general machining whereas neat cutting oils are used for relatively slow but high pressure cutting, such as gear shaping. I only use soluble oils.

I never use coolant for plastics, brass, bronze and cast iron. On the centre lathe I normally cut dry (inserts and HSS) and only use coolant for drilling/reaming with HSS tooling. The exception is parting off – I always use coolant, apart from the aforementioned materials. On the repetition lathe I normally use coolant as tooling is all carbon steel/HSS.

On the vertical mill I never use coolant, apart from WD40 on aluminium as needed. That's partly due to not needing it and partly not having a drip tray underneath, so much of the coolant ends up on the floor. On the horizontal mill I mostly use coolant as I only carbon steel and HSS tooling with heavy cuts by hobby standards. On the CNC mill I mostly use coolant, but primarily to wash away the swarf rather than for cooling.

I always use coolant when cylindrical grinding to cool the work and reduce glazing of the wheel.

Hand and machine tapping is mostly done dry, with some exceptions. I use WD40 on aluminium and coolant on the repetition lathe where I'm tapping at speed – normally 500 to 1000rpm. If the tap is large, >3/4", or the material is difficult (stainless steel) I use Rocol RTD compound. I avoid RTD where possible as it's a pain to clean the work and tap afterwards.

I used to use Biokool14 from Hallett Oils, but now use Hysol XF from Castrol which is a general purpose coolant, formulated for hard water areas. Modern coolants are much better than those of old. I never have a problem with corrosion on the machine tools, but I regularly check the mixture with a refractometer and keep within the recommended dilution. I also never have a problem with nasty nifffs even though the coolant sits in the tanks for months, or years, at a time.

All my machine tools have their own pumps and flood coolant. Dabbing, or dripping, on by hand doesn't achieve anything useful.

Assuming that the OP is in the early stages of learning I'd say forget coolant. On a mini lathe it simply isn't going to achieve much, other than possibly paint a nice streak up the wall behind the lathe. There are many other things to learn that have a far bigger effect on the work produced than using coolant.

Andrew

Andrew –

Your "Dabbing, or dripping, on by hand doesn't achieve anything useful." made me smile.

When I worked as the Materials Storekeeper for a printing-machine manufacturer, I saw one of the two or three turners never do anything else; at least not on the free-cutting steel used for most of the turned parts.

He used the lathe's suds system, but always turned the tap down to deliver just a very gentle dripping much less than a paint-brush would apply!

Did you ask them why?

Andrew