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Hard Milling over 60Rc? Coolant/Air

Houdini16

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Joined
Nov 28, 2017
So I wanted to bring this back up, after so many years on this subject.
I had never hard milled before so,
I bought some Harvey Hard Mill Cutters $$$$, called the mfg, they said use wet.
Looked at some Maritool Hard Mill Cutters , says wet or dry,
Sandvik said use dry.
So I have ran a bunch of these over 62rc parts, minimal machining, but am getting a lot of parts per end mill running coolant.
I didn't want all the little dry chips blowing all over with an air gun in the machine, So I tried coolant first,
with all the parts I'm getting I'll stick with it but,
what's the popular consensus with those who have done "actual/not opinion" production and tried the 2 different wet/dry and noticed significant differences?
 
I used to get all freaked out about coolant vs air. It started for me when we got some ceramic tools at work in the early 2000's that couldn't be run with coolant. Then suddenly sales reps started saying that other tools need air blast only, or I would see in a youtube video, etc...

After a decade of experimenting, I've started to solidify my thoughts in the last couple of years. Basically IME, there are only two reasons why it would be better for the cutting tool when run with air instead of coolant:

1 - if thermal shock will break the tool
2 - if air blast is required to ensure chips aren't being recut

I have had a few programs over the years that had really aggressive parameters, which ran great with air blast. Decided to switch all of them to through spindle coolant where possible. In almost every case, tool life went up. With one micro tool in mild steel, the tool life tripled.

I can only recall a single instance with a regular carbide endmill where thermal shocking seemed to break the tool. It was in really aggressive roughing, where the SFM was sky-high. With coolant the tool would break in minutes instead of hours. I decided to slow down the SFM and change the process to be coolant friendly, then low-and-behold, the new "slower" process took less time because it allowed more tool engagement.

Back to the question - I think you made the right call. If the tools aren't seeing high thermal loading, coolant is only going to improve your overall process stability.
 
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I used to get all freaked out about coolant vs air. It started for me when we got some ceramic tools at work in the early 2000's that couldn't be run with coolant. Then suddenly sales reps started saying that other tools need air blast only, or I would see in a youtube video, etc...

After a decade of experimenting, I've started to solidify my thoughts in the last couple of years. Basically IME, there are only two reasons why the cutting tool should be run without coolant:

1 - if thermal shock will break the tool
2 - if air blast is required to ensure chips aren't being recut

I have had a few programs over the years that had really aggressive parameters, which ran great with air blast. Decided to switch all of them to through spindle coolant where possible. In almost every case, tool life went up. With one micro tool in mild steel, the tool life tripled.

I can only recall a single instance with a regular carbide endmill where thermal shocking seemed to break the tool. It was in really aggressive roughing, where the SFM was sky-high. With coolant the tool would break in minutes instead of hours. I decided to slow down the SFM and change the process to be coolant friendly low-and-behold, the new "slower" process took less time because it allowed more tool engagement.
Looking at it through a physics lens, this was my thought also, that it depended, was there actually enough heat to cause the oscillating extremes for thermal shock to take place, if not coolant was better.
In my instance (mfg specs) >
3/16" dia 7 flute tool, .06 DOC .02 WOC 50SFM 1000RPM 3 IPM .0004 chip before calculating chip thinning, I don't see the heat here.

I could see if I was hogging with a 1/2" in a file, with lava spewing out of it
 
Can I go with just that this is great question?
Should I throw fire or not? Will the tool base and coating live with that?
Am gonna guess this what you are doing is coated carbide and not CBN?
Most thinking here is about thermal shock to the base carbide as in the cut it gets super hot and out the cut it sees coolant.
This makes for micro cracks but this is not always true. This is not breaking a tool, it is edge wear and tool life.
Mostly I am way in favor of dry and big air. This is my go to. BUT have seen flood cool work better.
Bob ( I am of no use)
 
I've always used air blast when hard milling to prevent the part from getting hot.

Typically we only hard mill off about .003" per side in order to maintain a .0005" tolerance or less on a dimension.

If you're hard milling like that video above, you're creating a ton of heat and won't be able to hold a tight tolerance.
 
There are so many variables at play. Thermal cycles on the substrate vs. enough heat to activate the coating vs. putting too much heat into the workpiece vs. cutting strategy vs. workpiece material...

Years back I did a job milling open pockets out of the edge of thick slabs of HSS that were blades for something or other offshore. Maybe a massive pipe beveller but I can't recall for sure. The pockets were to clear a bolt head that had been "field engineered" and the blades would no longer fit.

It was quite a lot of material to remove and I was tasked with doing it quicker than sparking them. I had all sorts of trouble with heat and tried coolant on and off. Endmills failed very fast with coolant in that application. They didn't last very long with air either, but still about 100% better tool life compared to coolant. I don't recall the specific endmill I used but it was a 12mm 6 flute optimised for hard milling from wnt/ceratizit.

Like boosted I have done a lot of alternating between through coolant and air. One application I found where tool life was drastically reduced by running coolant was high feed milling in 316 stainless.
 
Like some have said, we do a lot of hard milling and turning, but minimal depth of cuts (.002 - .003) and always use coolant. Combination of carbide and CBN (interrupted cuts while turning we prefer CBN), but we make tooling (not production parts) so we do it if we can eliminate grinding or EDM. But we have both those capabilities as well, so use whatever is faster or gives us a better quality tool. Sometimes there is no substitute for grinding.
 
I did forget about coating activation, I dont think with the parameters I mentioned above it would be getting activated in truth, everything is too slow.
 
Mostly I am way in favor of dry and big air.
Bob ( I am of no use)

Big Air needs big compressor.

I have a small compressor (relatively speaking 3hp 50 gallons) so use Small Air

I have been milling a lot of 12L14 and 303 recently with 3/8 end mills and have been using a carefully directed air blast which clears the chips nicely and doesn't use a lot of air. Works well,and is better then flood coolant I used before.

For the OP application I would be using air blast. If nothing else there's going to be a cleaner working environment.

-------------------------------

Running Air blast I can keep the shop doors/rollup doors closed, running coolant I typically have to open the big roll up door and run a 36" fan to pull the air out of the shop.
 
I did nothing but hard mill for a period of 8 to 10 months, and I learned a great deal. Like hardness had less to do with tool life than material. Even that same material, different bar, similar hardness, produced different results. Parts ranged from A2 at 54-56 hrc to some super secret german steel at 68-72.

Some cutters liked some materials better.

Ultimately it came down to closely, carefully, documenting and observing.

The coolant/air debate was constant, and what I found was no one had firm answers for either. The best founded argument was from a guy who did nothing but hard mill for close to 10 years. His answer was, yes.

Again, depending on part construction/type, material type, hardness, accuracy requirements, sometimes it was better to run coolant, sometimes air.

If you are running production, the key is documentation and experimentation.
 
The best founded argument was from a guy who did nothing but hard mill for close to 10 years. His answer was, yes.
(paper or plastic?, yep) His answer was yes to what?

If you are running production, the key is documentation and experimentation.
100%, like I said I am already getting a lot of parts per end mill with coolant, and I would rather run coolant, so ill stick to that on this "one particular job",
but experimenting is key if this was a returning thing, but it isn't its a one off, I would be testing it out fo sho.
 
@Houdini16
I've had cutters that I was told to run dry, with only high pressure air, and some could be wet, if the coolant was also high pressure.
I also think you did the right thing, as the thermal shock isn't showing up in what you're doing.

I prefer to run with coolant, as much as possible. High pressure, especially.
 
I did forget about coating activation, I dont think with the parameters I mentioned above it would be getting activated in truth, everything is too slow.
One type PVD end mill coat needs "activation".
Here we need to convert to Al2ox which for now can not be PVD applied. This is a different black coat done CVD so endmills NO.
So heat at the cutting tool edge does this.
Now is where it get fun. So flavors or this black/grey looking coat.
So we all know many will not run up to the temp to make this happen. Based on that we put in Tin or others in the blend.
Kind of a dual purpose thing.
What is the temp at chip sliding?
Not only surface footage as you drive it. Top rakes and tool geometry come into play here along with chip load max and min and effective.
It is all enough to drive you mad.
 
One type PVD end mill coat needs "activation".
Here we need to convert to Al2ox which for now can not be PVD applied. This is a different black coat done CVD so endmills NO.
So heat at the cutting tool edge does this.
Now is where it get fun. So flavors or this black/grey looking coat.
So we all know many will not run up to the temp to make this happen. Based on that we put in Tin or others in the blend.
Kind of a dual purpose thing.
What is the temp at chip sliding?
Not only surface footage as you drive it. Top rakes and tool geometry come into play here along with chip load max and min and effective.
It is all enough to drive you mad.
Yeppers, you really just need to take an (intelligence required)edumacated guess,
but even better, test the setup.
 
I work with 6 veteran mold makers that I program larger parts for and they all say it depends on the material / hardness and setup. I trust these guys!
 
I like using coolant with ball cutters as well. That tip of the ball is essentially dragging and not cutting. Coolant helps. With square and radius corner I usually cut dry. Unless there are pockets or features I don't want filled with little chips. Mold making mindset here. It may not be applicable to you.
 
I like using coolant with ball cutters as well. That tip of the ball is essentially dragging and not cutting. Coolant helps. With square and radius corner I usually cut dry. Unless there are pockets or features I don't want filled with little chips. Mold making mindset here. It may not be applicable to you.
tooling guy also, understand
 
I've always used dry and never broke an endmill/insert, but never had to do it in production. We only ever had 1-2 prototypes which always worked well - now you really got me thinking to try it wet. All I need is a big order that needs hard cut. Thanks for the post!
 
Apparently not that common / generally overlooked, but a very capable solution is MQL (minimum quantity lubrication) as it gives the greasing ability of the coolant without the thermal shock problem. The air is enriched with oil mist and sprayed on / through the tool. Best if your tool or at least the holder is capable of through coolant/air. And yes, i am aware of the risks in regards to explosive atmospheres. Please make sure you buy a system that uses less then 20ml/h or install a fire suppression system. This is not a small shop solution but very capable if you have the volume to make the investment viable.
We are running 3 mid sized horizontals with 42CrMo4 with 44-48 HRC parts. Tool life is double compared to same application with coolant.
 








 
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