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Coolant Flow Rates

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Nigel Graham 206/06/2019 12:47:17
386 forum posts

Well, I'm not exactly of trophy-winning standard but not a beginner... but this has puzzled me for a while.

Conventional machine tools in our own workshops, not the massive swarf creators now used in industry.

Using pumped or gravity-fed coolant, and not unduly loading the machine, which is better: large flushing rates or a thin trickle / dripping at about the rates typical of manually brushing the suds on?

I recall a professional centre-lathe turner who'd set the suds to mere dripping, yet his work was all of the high standard needed, whether in mild-steel, stainless-steel or aluminium-alloy. Yet his colleagues all used at least thin streams of the soluble oil.

One reason for asking is good practice ideas.

Another, I am considering a simple coolant system centrally-pumped, or portable between machines. I wonder if the Hozelock garden "micro"-irrigation system pipes and fittings would be ideal, with adequate filtering.

Also, with brush or spray, the heat and spinning tend to push the suds uselessly along the bar, slightly ahead of the cutting itself.


I once used a simple gravity system using a one-gallon can with a drip-feed tap and hose above my EW 2.5" lathe bolted to a steel bench lacking a proper drain.

My main machines are Myford 7 and Harrison L5 lathes, Myford VMC mill; and when re-commissioned a Denbigh H4 horizontal mill. Brush- or squirt- application would be adequate for the bench-drill, EW lathe and a modern, small Chinese-made lathe .

Andrew Johnston06/06/2019 13:03:35
4855 forum posts
544 photos

I mostly turn and mill dry, albeit it with carbide tooling. When I use HSS tooling on the lathe and horizontal mill I use flood coolant, ie, enough to provide cooling without the excess flow being sprayed up the wall or over the floor. I use a water/oil mix; to cool you need flow, drops here and there don't do anything worthwhile. An exception is the CNC mill. On that I use flood coolant at full flow for all materials except cast iron, brass and plastics. On the CNC mill the flood is as much to wash away swarf as it is for cooling. While not industrial quantities the CNC mill can generate a lot of swarf:

A Lot of Swarf


Thor06/06/2019 13:15:00
1120 forum posts
31 photos

Hi Nigel,

I too usually turn and mill without using coolant. When using HSS tools without coolant I run at a somewhat slower speed than the tool manufacturer recommends as their speeds are for industrial machines with flood coolant.


Nigel Graham 206/06/2019 20:46:26
386 forum posts

Thank you. Umm, I had specified conventional, not NC, machines.

Nick Hulme06/06/2019 21:03:31
703 forum posts
37 photos
Posted by Nigel Graham 2 on 06/06/2019 20:46:26:

Thank you. Umm, I had specified conventional, not NC, machines.

The provision of cooling and the removal of chips to prevent re-cutting is the same principle on manual or CNC, especially in the absence of a full enclosure.

Nigel Graham 206/06/2019 21:15:28
386 forum posts

I appreciate that, and in many cases chip removal is one of the main problems, such as working in deep holes.

I wondered though how much is cooling and how much is lubricating the cutting action, where flushing is less important such as in external turning.

I have not seen anything on this topic anywhere, beyond which materials to cut dry and which to cut wet, but are there any rules-of-thumb, or is largely a matter of trial-and-error for the task in hand?

Andrew Johnston06/06/2019 21:37:08
4855 forum posts
544 photos
Posted by Nigel Graham 2 on 06/06/2019 20:46:26:

Umm, I had specified conventional, not NC, machines.

Ooopsie, my mistake. I'll modify my previous post in the light of the stated conventional machines and not pushing them. You don't need coolant as nothing will be getting hot enough to need it. thumbs up

Information on coolants and lubrication is out there, but you'll need to be reading professional literature. Start with Machinery's Handbook.


Hopper07/06/2019 05:52:00
3712 forum posts
73 photos

Usual practice with coolant on manual machines is to use a small but steady stream so that it actually cools the cutting tip and the job. But not so much that it sprays everywhere. HSS benefits more from coolant than carbide, mostly in that it lasts longer between resharpening. On small home machines such as Myfords I don't bother with coolant at all, HSS or carbide. Bit of neat cutting oil or motor oil for screwcutting is about all. And I have a squirt bottle of diluted cutting oil for HSS parting tools, which is dribbled on for lubrication more than cooling. Stops microwelding etc.

Nigel Graham 207/06/2019 08:41:04
386 forum posts

In many cases it is definitely cooling as well as lubricating the cut, but is former function very dependent on the latter?

For screw-cutting and turning small areas I tend to use a stickier lubricant, such as soluble oil used neat, as the cuts are shallow and at usually at moderate speeds. Similarly for parting, normally now from a rear tool-post.


The difficulty in actually lubricating and cooling the cutting action tends to be in turning, in long cuts at higher speeds, where the coolant, especially soluble oil, tends to creep along the bar towards, and even onto, the chuck. In doing so it does a little cooling but is otherwise rather ineffective.

I have found the worst spray from a lathe is rarely from the work, but from the chuck.


In milling, simply brushing a soluble oil onto a vertical face and the cutter means does not achieve very much as most of it just flows off without washing the swarf away. Soluble oil used neat is one answer though doesn't clear the swarf. Most of the viscous cutting-lubricants I've found are really for low-speed, low-pressure work like tapping and sawing, and anyway just keep the chips in place.

Perhaps then, a pumped coolant system is more important on a milling-machine on a lathe, though delivering enough to wash away the chips even on a conventional machine, means arranging suitable screens to control the spray.


The most awkward but sometimes most important for lubricating / cooling, especially cooling, is deep drilling and boring. The only answer is withdrawing the drill frequently to brush the chips off and lubricant on.

Industrial NC machines use special insert-tipped drills with coolant channels down them, for large diameters. This is not feasible in small diameter drills but the principle might work in lathe boring-tool holders down to fairly small sizes.

SillyOldDuffer07/06/2019 10:25:46
4713 forum posts
1010 photos

Back in the day with Carbon Steel cutting tools it was vital to keep the tool cool. Carbon-steel loses its hardness at surprisingly low temperatures, perhaps 180°C. The easiest way to keep Carbon-steel cool was to flood it with coolant. It was also found that lubricating the work improves finish. White emulsion is a mix of water - an excellent cheap absorber of heat - with an oil that does the lubricating. The flow of liquid tends to remove swarf, which left to get under the tool will ruin the finish. Cutting fluids may emphasise lubrication more than cooling, or improve both.

HSS stays hard up to about red-heat, but - worked hard - it too requires fierce cooling. In production, speed is often everything because stopping machines to swap out blunt tools can be a financial disaster.

Carbide is much sterner stuff than HSS, and its cooling needs - if any - are different. Splashing coolant on carbide is likely to crack it so either flood cool or not at all.

The material being cut matters. Most obviously Aluminium needs help - it tends to weld itself to tools, especially hot fast carbide. A bit of paraffin works wonders! Mild-steel is fairly soft, but other varieties are harder/tougher and need help. Work hardening stainless needs plenty of cutting fluid and aggressive cutting.

Much of the commercial advice about coolants is driven by the needs of high-volume production, not what goes on in a small workshop. In production a tool lasting 20% longer, with a 5% reduction in poor-finish rejects, and a 10% reduction in the electric bill make cooling and lubricating essential. Chaps making one-off's at quarter-speed wouldn't notice the difference.

Flood cooling with emulsion has many disadvantages. The original mixes used to 'go off', stink, and cause horrible infections when they got into a cut. (Before antibiotics!) Later formulations fixed that problem, but flood cooling is messy; the water can cause rusting, dumped down the drain the fluid is polluting, and the lubricant costly. The machine tools are modified to collect waste, and a system installed to recover clean lubricant. Not worth it in a small workshop.

Flood cooling is so obnoxious there are many alternatives. Mist systems are popular - these spray a measured dose of coolant direct on to the cutting point. The dose is carefully regulated so that the machine doesn't get wet. Mist systems are complicated to set-up and introduce a new health hazard - breathing the mist. So they too may need to be shielded or fitted with an extractor system. Expensive. Not many amateurs would need a mist system.

In the small sample of Machine centres I've read about, compressed air, Nitrogen or Argon is favoured. It's directed at the tool tip which it cools without thermally shocking the carbide and it blasts swarf away from the work and into into a collector. An interesting feature of these machines is how they manage the ton or so of swarf produced per shift. Conveyor belts... A key advantage of compressed gas cooling is that it's clean. The swarf is easier to handle, and being uncontaminated, has top recycling value.

In my workshop, for what it's worth, I use cutting fluid/coolant only:

  • When drilling (CT90 sprayed or brushed to extend the life of the drill)
  • When tapping (CT90 sprayed to reduce the chance of breaking the tap and to reduce the chance of tearing the thread, taps and dies)
  • On Aluminium (to stop it sticking to the tool . Brushed or sprayed WD40, CT90 or paraffin, which is cheaper)
  • When hacksawing by hand (CT90 Sprayed. Because I need all the help I can get with physical work and the blades last longer)
  • Flood cooling on my mill only when roughing out a lot of steel. (To extend the life of my HSS cutters) I find flood cooling too messy to be worth the trouble on small jobs. I flood with neat cutting oil due to a prejudice against water based emulsions which I fear might cause rust. With little reason!
  • When the finish is important and applying cutting fluid improves it. ( I mostly use carbide, and good finish is often best achieved by taking deeper cuts. My modest lathe isn't ideal for carbide, so when needs must HSS and lubricant are good alternatives.)

I use CT90 because my local has it on the shelf, not because I've tried anything else. Other potions available!



Edited By SillyOldDuffer on 07/06/2019 10:31:11

Clive Foster07/06/2019 11:51:00
1840 forum posts
59 photos

Dave and Andrew have it nailed.

For typical home shop cutting rates the amount of cooling and lubrication needed is very small. I have ex-professional Bijur Spraymist systems on my machines. The big issue is getting reliable lubricant delivery at sufficiently low rates. Unless you are cutting hard, which should be in an enclosed machine, there shouldn't be enough oil in the air to cause a problem. Bad atmosphere means far too much lube and too high delivery pressure. True mist rather than droplets, lots of bounce back and stuff flying everywhere. No doubt that lubrication makes for more reliable working, better finish and longer tool life without having to work at it.

Decent air blast is handy when milling to clear chips but the oil delivery should be even smaller. My considered opinion is that the Bjur systems don't, and can't work, as they are supposed to. Massive over delivery of lubricant seems designed in. Mine can approach being OK but need careful tuning along with careful aim of the blast. The swan neck tube isn't very good at precise positioning.

A one day some day maybe (if I live long enough) project is to investigate home brew microdrop systems. Import mist delivery systems are around a tenner from the usual auction site so thats the basic hardware sorted. My Bijur reservoir units are fine and dandy so really for me its a matter of figuring out how to drip tiny drops of lubricant into the airstream without driving it into mist. Maybe jiggering around with the innards of a fuel injector off a car would work. Or maybe something could be done with an ex printer inkjet nozzle and driver. Certainly an inkjet nozzle or two looks to in the right volume region. Probably need drilling out tho'.

For air supply on a full DIY build perhaps one of the cheap, low pressure high volume paint / fence protection sprayer pump units would be a good starting point as they are supposed to run happily for long periods. I think modern units run at around 10 to 15 psi. Fine for lubricant delivery as you just want the drops to flop onto the tool and job with no bounce back but maybe not enough for milling. Ideal would be the old Binks-Bullows Hydrovane type units. About 45 psi max and, apparently, built to run for ever. I hook mine up to the Bijur systems when I can't be bothered to run the big shop compressor. Air blast alone is sufficient to clear chips at my milling rate. Trouble is the are getting a bit of cult status. Over £100 being asked on E-Bay. Mine was "dunno what it is mate but I'll take a fiver" at a boot fair.

The 1024VSL has flood coolant which tends to get used about every second blue moon on obdurate materials, usually when threading. Seriously messy and getting the stuff out from under the saddle inspires "creative" commentary.

Bottom line is that I still work dry most of the time.


Edited By Clive Foster on 07/06/2019 11:51:30

Edited By Clive Foster on 07/06/2019 11:52:16

Nigel Graham 207/06/2019 11:53:17
386 forum posts

Thank you for that Dave - a very comprehensive answer!

CT90 - I'll look out for it.

I do use WD40 sometimes for aluminium, also occasionally for tapping. I'm a bit wary or using too much WD40 or paraffin on a machine-tool because it is very good at washing out lubricants from slides and bearings.

My Myford and Harrison lathes, and the Mill, are equipped to take flood cooling but it may be that by the time enough has accumulated in the chip-tray that you lose as much as you would by just brushing it on.

The milling-machine, equipped with some screens to control the splashing, might the better candidate for a flood coolant system to help clear the chips from the cutter.

On the lathes I might simply get away with a basic gravity arrangement, and placing a tray within the main chip-tray to catch most of what runs off.


I occasionally used the lathe in the workshop of a small electronics contracts firm I worked for years ago. (It had to make special enclosures etc. too, for much of its work). This had a large suds tank in its cabinet and sure enough every now and then I'd have to rake off the bacterial mat that formed on the liquid.

When later I worked in the metals store of another company, the hacksawing machine used a different type of soluble oil that was evidently not tasty to micro-organisms, and might even have contained a disinfectant.

Andrew Johnston07/06/2019 14:32:47
4855 forum posts
544 photos
Posted by Nigel Graham 2 on 07/06/2019 08:41:04:

Industrial NC machines use special insert-tipped drills with coolant channels down them, for large diameters. This is not feasible in small diameter drills but the principle might work in lathe boring-tool holders down to fairly small sizes.

Drill Service sell ordinary twist drills with thru hole coolant down to 1mm diameter. I understand that thru coolant tooling needs high pressure to be really effective, hundreds or thousands of psi.


Clive Foster07/06/2019 22:08:42
1840 forum posts
59 photos

Ha! I tamed the Bijur Spraymist.

Decent size alloy job up on the Bridgeport, nice finish needed on something not flycutter friendly. Groaned and set the spraymist up. As expected it very much wasn't playing ball or reliably spraying coolant.

So I went all Taz on it and shoved a number 60 drill up the nozzle. Result. At around 15 psi droplets on the work, pretty much no bounce back mist and still enough air oomph to shift the swarf.

So I suspect that one of the inexpensive import mister / mixers would work pretty well at around 15 or less psi if you run the coolant container at the same pressure as the air. Narrower airstream than the Bijur gives would probably be better. Maybe an annular shroud open both ends so some air is drawn from the rear to restrict the dispersion of the air and entrained coolant droplets would work.


Nigel Graham 208/06/2019 10:45:11
386 forum posts

This is very interesting to me, as I had not previously heard of mist-spraying coolant.

It's either manual brushing, squirting lubricant from an aerosol can or washing-up liquid bottle; or the all-out fire-hosing of suds on industrial CNC machine-tools.

I take it the basic principle is an air-blast to clear the swarf and cool the metal, and just enough liquid to help the cooling while lubricating the cutting itself?



Thank you for that information. I suspect those drills are not quite home workshop tools - probably very costly and definitely designed for fully-equipped NC machining-centres!

Perhaps more feasible for us would be a small-diameter tube clipped in some way alongside an indexable tool-holder to deliver pumped coolant / cutting-fluid into deep bores and recesses. It might not need play onto the tip itself as the confined conditions would distribute the liquid over the bore wall.

John Pace08/06/2019 14:34:05
156 forum posts
156 photos

Before embarking on air mist coolant systems it may be worth reading this,

whilst this is primarily aimed at industry having such a system under your nose
whilst machining may not be the smartest thing to do.
I use neat cutting oils and in one application involves neat cutting oil applied to high speed
spindles,i know from previous encounter some years ago it turns instantly to fog,
as of yet i have not decided on how to combat this.


Clive Foster08/06/2019 14:47:01
1840 forum posts
59 photos


A bit of googling will turn up quite a bit of information on mist coolant and microdrop lubrication / coolant systems.

Good picture of the basic operation principle here :- **LINK**

Cooling is by evaporation of the water in the coolant mix not, as in flood systems, by washing. Probably less effective in the limit but much less messy if you can stop the mist going everywhere. The amount of oil needed to to lubricate the tool and promote smooth escape of chips is tiny. Hence micro drop systems. If you can get a narrow stream pointed down drill hole they are pretty good when drilling too. Not in the same class as pukka systems but chips come out much better and the cutting edge stays clean. Even with super gum alloys.

Mine are Bijur Spraymist units as per picture here :- **LINK** . Basically a cast aluminium tank to hold the coolant mix with built in air pressure regulator and solenoid switch to control the low. Valve body with coolant and air inputs with needle valve to adjust how much coolant is delivered. Also usually fitted with non return check valve on the coolant input to stop it draining back when the air is off. Takes a while to fill the delivery tube otherwise. The silver swan-neck coming out of the valve has a coolant pipe down the middle running to a concentric jet at the end. Inner jettakes the coolant, outer part takes air. Airflow picks up the coolant coming out of the jet turning it into a mist which then sprays onto the work.

All very professional. Spendy too. Used units are usually gummed up to the wazoo and caked whit 'orris so budget for serious cleaning.

Import version for around £10 are like this :- **LINK** in many variations. Bring your own reservoir and air. Most have a simple tap rather than a solenoid.

Main issue in practice is too much mist in the air producing an unpleasant fog. Not nice to work in. Mostly due mishandling but, if my Bijur systems are anything to go by, basic air delivery rate is too high and output nozzle too small. Lots of word all over the internet about how to reduce fog but to my mind main issue is to reduce airflow and enlarge the nozzle to produce drops rather than a mist. Lower airflow reduces cooling effects and chip blowing but stops bounce back of coolant mist. As per previous post it seems to have worked.


Clive Foster08/06/2019 14:50:49
1840 forum posts
59 photos


Further to my previous post FogBuster effect is what you need in the home shop. See **LINK** for nice pictures of the difference between mist and entrained droplets.


Nigel Graham 208/06/2019 15:42:35
386 forum posts

It seems impressive but I can't help thinking you need the machine to be heavily enclosed so you don't merely leave a nice clean milling-machine with everything around it plastered with wet swarf and coolant.

I think for my modest means and cramped workshop, I'd better with a simple stream of coolant driven by a small pump, not the complexities of an aerosol system and compressor. It would still need guards to keep the chips and suds under control, but be a lot easier to set up, not needing all that attention to nozzle sizes etc..

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