CNC Coolant System: Mist, High Pressure, Through Spindle, Flood

Is Manual Machining Faster than CNC for Simple Parts?

CNC Coolant System: Mist, High Pressure, Through Spindle, Flood [ Lesson 10: F&S Email ]

CNCCookbook’s Feeds & Speeds Master Class

Let’s get off Feeds and Speeds for a minute and talk about Coolant. It’s a pity that coolant is called “coolant”, because it makes us ignore the more important roles of using coolant.

Here’s a quick video intro I did for Cutting Tool Engineering Magazine on the role of coolant:

We actually use coolant for three distinct reasons:

The Role of Coolant in Machining

1. Chip Clearing: Spraying a liquid at the cut helps move the chips out of the way of the cutter. Clearing chips minimizes recutting the chips and means there are fewer chips for the cutter to remove. Recutting chips destroys surface finish and dulls tools much faster. In the worst case, a cutter down in a slot or hole can get clogged with chips and get much hotter or even break.

2. Lubrication: Some materials, like aluminum or some steels, are sticky. They have a chemical affinity for the cutter and will try to weld themselves to the cutter. Lubrication makes things slippery so the chips are less likely to adhere and weld on. Clearing chips makes adherence less likely too.

3. Cooling: Liquids are capable of carrying much more heat away from the cut than air. Plain water conducts heat 25 times more efficiently than air, for example.  So water-soluble coolants are better at cooling than oil-based.

I put cooling dead last for a reason.  While not unimportant, cooling is probably the least important role of coolant!

In fact, I was very pleased when Haas released a video called, “Haas Chip Clearing and Tool Lubrication.” The video puts the emphasis where it should be and not on the cooling aspects. Finally, the word is getting out about what Coolant is all about!

How to avoid flood coolant

If you don’t have a full machine enclosure, flood coolant is messy and almost unusable.  Fortunately, the cooling aspect is the least important thing about coolant, so if we can manage chip clearing and lubrication, we can dispense with flood coolant for many applications.  A strong compressed air blast will handle chip clearing.  Adding a lubricant to the air blast to make a mist takes care of lubrication.

There are some materials where flood coolant still reins supreme.  Stainless Steel and Titanium are two examples.  It’s hard to do a good job on these with mist.

Let’s take a closer look at each of these three critical coolant functions.

Chip Clearing

Chip clearing is by far the most important function. I cringe every time I see a cut where the chips pile up.

Sure, it’s easier to photograph the action, but those piling chips are very hard on your cutter’s life and can even lead to breakage.

You’re much more likely to experience built-up edge (BUE) where chips weld onto the cutter if the cutter must recut the same chips over and over and can’t get rid of them. If there is insufficient chip clearing in your machining operation, you may use up all the chip clearance your cutter has available.  The already cut chips have to compete with newly cut chips, which can lead quickly to cutter breakage.  

At the very least, it means you can’t cut as fast.

If your machine has no flood coolant, rig up an air blast or mist to get the chips out of there. Get paranoid about having too many chips around.

Think about it this way: most tooling manufacturers recommend you turn off the flood coolant when surface speed goes above a certain point and you will increase the life of your tooling. If it was all about heat, that shouldn’t be the case as more surface speed means more heat.

Lubrication

Lubrication helps the tool to cut more easily so it generates less heat. As the face of the tool slides across the workpiece, it rubs while cutting. As the chip curls up, it also rubs on the tool, generating more heat. All that rubbing will produce less heat with a little lube, as like any sliding fit would.
 
Reducing heat is an important role for lubrication, but it’s not the most important one.
 
A much bigger role for lubrication is reducing the likelihood of Built Up Edge (BUE). This is a big deal as anyone who has seen a big wad of aluminum get welded to their cutter will attest to. Things stop working quickly when that happens!
 
Fortunately, BUE is material-specific. It applies mostly to Aluminum and low-carbon or unalloyed Steel. Titanium is another material that has a reputation for being sticky.
 
Use of sharp cutters with very high rake angles (positive rake is your friend!) can help reduce adhesion, but it’s not enough.
 
Many coatings used on tools can also provide lubrication. But coatings are fragile. They wear off or chip and shouldn’t be the primary answer to BUE. In the end, a little bit of mist can deal with this problem as well as flood coolant, so it isn’t the end of the world.
 
By now, you should realize you need to do something about lubrication before that wad of aluminum welds itself in all the wrong places.
 
I’ve heard many professional machinsts flat out declare you can’t machine aluminum without some form of lubrication. Even a little spritz of WD-40 makes a big difference with aluminum.

Cooling (and its Evil Twin Shock Cooling)

Which brings us to our next issue, Cooling. The temperature of the tool is the biggest factor affecting tool life.
 
A little heat is good, as it softens the work material, making it easier to cut.
 
A lot of heat is bad. It softens the tool, which means it wears, becomes dull, and cutting forces skyrocket. That makes it get hotter, and soon you have a vicious cycle.
 
Note that the amount of heat allowed is dependant on the tool material and coating. Carbide takes a lot higher temperatures than HSS. Some coatings, such as TiAlN, need the higher temperatures to do their job.
 
The benefits of TiAlN aren’t even present until there is enough heat to “activate” it. So TiAlN is often used without coolant.
 
In fact, turning the coolant off can often increase tool life under the right conditions.
 
Carbide develops micro-cracks under the thermal shocks of uneven heating and cooling. Imagine submerging glass heated by boiling water into ice water. It’s going to crack.
 
This “Shock Cooling” can impact tool life in higher performance applications.
 
Sandvik, in their cutting tool study course, recommend either no coolant or copious amounts of coolant to avoid this problem.
 
Too much heat causes thermal expansion. This is not helpful to accuracy, as it makes your workpiece change size.

Let’s also talk about coolant type. There are water soluble coolants, and there are oil-based coolants. From a cooling standpoint, the water soluble coolants win. How much? Consider this data:

Coolant
Specific Heat of Coolant

Steel A (tempered)

Temp Decrease %

Steel B (annealed)

Temp Decrease %

Air
0.25
   
Compound oil, high viscosity
0.489
3.9
4.7
Compound oil, low viscosity
0.556
6
6

Aqueous solution of wetting agent

0.872
14.8
8.4
Aqueous “soda product” solution, 4%
0.923
13
Water
1.00
19
15
 
The efficiency of the various coolants at removing heat corresponds to the specific heat of the coolant.
 
Second thing to notice is that air is pretty lousy, about 1/4 as good as water. That’s actually not as bad as predicted, since water carries 25x more heat than air.
 
The reason for the difference is the coolant doesn’t make efficient contact everywhere. Also, if you’re using the right cutting parameters (e.g. feeds and speeds), most of the heat is carried away in the chips, not the coolant.
 
Oil-based coolants are about half as effective as water-based at cooling the tool and workpiece. Between that and the health considerations, its no wonder a lot of shops have gone to water-soluble coolants–they cool better.
 
On the flip side, the oil lubricates better (natch), and there are still some applications where machinists may prefer oil (usually turning) to the water soluble coolants.
 
One last thing about flood coolants. Above a certain critical surface speed, they all start to work about the same. The faster you go the less cooling effect they have.
 
At higher speeds, there isn’t time for a big gout of coolant to make it’s way into all the nooks and crannies. Cooling becomes less and less consistent. This also contributes to the shock cooling effects that make coolant hard on carbide life above certain speeds.

Material Considerations

There are two material-specific considerations for coolant. The first is the tendency for BUE. Preventing chips welding onto the cutter through lubrication is important.
 
The second is the ability of the workpiece material to absorb and transfer heat. Some materials do not transfer heat very well–titanium is a good example.
 
Those materials are often more dependent on the coolant for cooling to offset the inability of the material to carry heat. That inability makes it harder for the chips to carry away heat. And also harder for the workpiece to stay cool without changing size due to excess heat. Titanium further compounds the problem by producing relatively small chips.
 
If the material you’re cutting transfers heat poorly relative to aluminum (which is an excellent conductor of heat), steel (a decent conductor), or other common materials, make sure you’ve got a good flood coolant setup and are using it well.
 
Work hardening materials also benefit because coolant can reduce work hardening. Stainless Steel is a good example.

Performance Recipe: Aiming the Coolant for 5% Higher MRR

Where you aim the coolant matters, whether for chip clearance, cooling, or lubrication. But how many machinists take time to aim the coolant after each tool change? Different tools have different lengths. Different machining operations may also change the best aim of the coolant.
 
Adjusting the coolant nozzle is a productivity burden. Avoid it by using multiple nozzles preset to a range of heights. With three nozzles, you can cover a pretty decent range.
 
The problem with this approach is you now only have 1/3 of your coolant available at the ideal spot. With enough volume and pressure there is a wall of coolant. That’s why the high pressure and volume option available for most machines is worthwhile.
 
Lastly, you can retrofit your machine with an automated nozzle that allows you to pinpoint your coolant by twisting a knob on the control panel. These programmable coolant nozzles can track your tool changes and change the aim automatically.
 
What’s good aim worth? Spider Cool says its good for 5% better MRR, and they have a free trial to prove it. Might be worth checking into your coolant aim.
 
Our G-Wizard Feeds and Speeds Calculator will make the allowance for a Programmable Coolant Nozzle in its Feeds and Speeds Calculations.

Performance Recipe: Through Spindle Coolant

The most difficult chip removal job is down in a hole. For this case, through spindle coolant (TSC) is a huge benefit and sometimes the only way to drill deep holes. The performance increases possible with high pressure through spindle coolant are pretty amazing. Check out our full support for calculating feeds and speeds with high pressure TSC in G-Wizard.

Another advantage for TSC machines: you can often run more flutes, particularly when profiling.

Horizontal Mills and Lathes Have Gravity Helping Clear Chips

Don’t overlook the benefits of gravity for machining. On vertical mills, gravity makes it harder to pull chips out of deep holes. On lathes and horizontal mills, gravity makes it easier. Kinda makes you wonder why nobody has a machine that cuts from underneath. You’d need a pretty crazy palette loader so you could drop the workpiece onto the table and then flip it for cutting. Too far out, I thought, but then I discovered such machines actually exist. They’re called “Inverted Spindle Lathes” and are a potent alternative to bar fed lathes.

Here is a link to an MMSOnline article about them.

Exotic Recipe: Alcohol as Coolant

Datron uses an ethanol alcohol mist as coolant for its HSM machines. They make a good case for it:

It is ideal for high-speed, micro-tooling of non-ferrous metals and some plastics due to a thinner-than-water viscosity that allows the ethanol to quickly cover and cool more of the surface area on fast-moving parts. The low evaporation point of ethanol makes it an
efficient cooling and lubricating solution. Since the ethanol simply evaporates, disposal, recycling and their associated costs are a thing of the past. Plus, ethanol coolants leave no residue on machined parts, which makes costly secondary operations, like de-greasing, obsolete — maximizing throughput, increasing efficiency and ultimately improving a manufacturer’s bottom-line.

Datron alcohol mist coolant

Conclusion

For many high performance applications, machinists can focus on chip clearance and lubrication and ignore the cooling issues. Above a certain surface speed, many tooling manufacturers recommend turning off the flood coolant and using an air blast (perhaps with mist for lubrication) to clear chips. Materials that don’t transfer heat well like titanium will require flood coolant regardless.

Additional Information

Selection of Cutting Fluids in Machining Processes: Good overview of some of the factors involved in selecting coolant type.

DIY Flood Coolant: Need to put together your own flood coolant system? Check this article out.

Tramp Oil Skimmer: Easy to build but powerful tramp oil skimmer.

Turning Down the Heat in a Cut

CNC Coolant System: Mist, High Pressure, Through Spindle, Flood
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