Turning Prehardened 4140 at RC40 hardness, problems.

Wolfram Malukker · Jan 19, 2019

I am currently a home-only machinist, and as such I've got my nice old Logan 820 that is practically new. It's done everything I need it to do, and sometimes I had to do some learning before it would do it.

In this case, I believe that's what I need-is a bit of learnin' prior to doin'.

I have four 10" long bars of 4140 steel, that has been hardened and tempered to RC40, and OD ground to 1.75". They were bought to build custom lefty rifle actions on, and thus, they need to be drilled 0.650" end-to-end, and then further machined. I have some access to a bridgeport and all the carbide mills I can buy, so that's not too bad. But turning and drilling, I need to get a hole that I can then bore, or a hole that I can thread an EDM wire through. Even after EDM, I'll need to turn and internally thread at one end, so I'm not escaping the operation.

I have been using CNMG32.25 and 21.35 series inserts and while they are OK, they are not at all sharp and don't seem like what I should be using on this size machine.

Chucked up one today and center drilled it with a normal high-speed-steel center drill, which went fine. Then I set up to face the end off, and it was *almost* too hard for the carbide to cut, but it needed a LOT of pressure and took off only the finest shaving, with a little spark here and there. I think with PCD or cermet insert it would probably turn better, but I don't know.

What would you do, short of re-temper back to RC32-35? I could do that, sure, but this is a learning experience at the moment-Maybe the re-temper is the way to go here, maybe not. Stuff does leave a glass-smooth finish when cutting with the current carbides, but just barely taking a scraping will do that.

Gobo · Jan 19, 2019

Rc 40 is not what most people would consider hard. When set up correctly, carbide should cut that material like butter. It sounds like you are lacking rigidity in your setup. Minimum overhang of your workpiece and cutting tool is needed. Make sure your tool is on center, or just a tad below.

Comatose · Jan 19, 2019

I concur. With an AlTiN or similar coating and the right speeds and feeds it should cut extremely nicely. Maybe you're not feeding hard or deep enough?

DMF_TomB · Jan 19, 2019

Wolfram Malukker said:
I am currently a home-only machinist, and as such I've got my nice old Logan 820 that is practically new. It's done everything I need it to do, and sometimes I had to do some learning before it would do it.

In this case, I believe that's what I need-is a bit of learnin' prior to doin'.

I have four 10" long bars of 4140 steel, that has been hardened and tempered to RC40, and OD ground to 1.75". They were bought to build custom lefty rifle actions on, and thus, they need to be drilled 0.650" end-to-end, and then further machined. I have some access to a bridgeport and all the carbide mills I can buy, so that's not too bad. But turning and drilling, I need to get a hole that I can then bore, or a hole that I can thread an EDM wire through. Even after EDM, I'll need to turn and internally thread at one end, so I'm not escaping the operation.

I have been using CNMG32.25 and 21.35 series inserts and while they are OK, they are not at all sharp and don't seem like what I should be using on this size machine.

Chucked up one today and center drilled it with a normal high-speed-steel center drill, which went fine. Then I set up to face the end off, and it was *almost* too hard for the carbide to cut, but it needed a LOT of pressure and took off only the finest shaving, with a little spark here and there. I think with PCD or cermet insert it would probably turn better, but I don't know.

What would you do, short of re-temper back to RC32-35? I could do that, sure, but this is a learning experience at the moment-Maybe the re-temper is the way to go here, maybe not. Stuff does leave a glass-smooth finish when cutting with the current carbides, but just barely taking a scraping will do that.

.
harder the material usually have to take less depth of cut

Hazzert · Jan 19, 2019

Any shop that would do the EDM work for you would have a hole popper capable of getting a starting hole.

I would suggest switching to something like a CCMT or other positive insert with an appropriate chip breaker. I’ve had good luck with Iscar or Seco Duratomic. Your Logan is flat belt driven IIRC so minimizing cutting forces is your best option.

eKretz · Jan 19, 2019

Time out. What SFM were you running? What depth of cut? What feed? As mentioned above, at 40Rc, tungsten carbide tooling should eat this up for lunch all day long - at the correct speed.

Next question - are you certain they are only 40Rc? Who did the heat treat? Were they tested for hardness afterward?

eKretz · Jan 19, 2019

DMF_TomB said:
.
harder the material usually have to take less depth of cut

Not in my experience. Harder material usually means less feed, not less DOC. It always means less speed.

DMF_TomB · Jan 19, 2019

some inserts are coated and the edge often are not sharp especially after coating. that is on a medium soft material it also tends to burnish surface a bit and give a better surface finish.
.
very hard steel and very soft material like plastic material, inserts need to be honed so edge sharper as inserts even new out of the box act dull. hard to describe but i have seen change of insert edge type made a big difference

JST · Jan 19, 2019

I cut prehardened 4140 on a similar machine without problems. Usually it will be a bit less hard than that, possibly at RC35 or so.

Even HSS will do the dutting, but it wears too fast to be practical.

Bobw · Jan 19, 2019

I have been using CNMG32.25 and 21.35 series inserts and while they are OK, they are not at all sharp and don't seem like what I should be using on this size machine.

Who cares what size and shape the insert is? WHAT IS THE GEOMETRY OF THE CUTTING EDGE.. then substrate
and coating...

You need to buy the correct insert for the application.. Sounds like you have a heavily honed roughing
insert that is meant to be fed at a very high rate, at a speed that you probably aren't comfortable doing
on an open machine.

Carbide inserts aren't just "Carbide".. There is a reason that those catalogs are 2" thick..

How to choose the right insert?? Hell if I know.. I've been at this for a long time, and I *USED* to
try and pick the right insert, and then I realized that there are people out there that do this for
a living. People that KNOW what to use for what application.. So I let them choose..

Try this guy out, he's a member here, and he's never steered me wrong.. "Hey Curtis, I need a DNMG432 that
will do BLAHBLAHBLAH". And it shows up on my doorstep, and it works, and I like the prices.. I don't
have to waste time digging through catalogs, trying to figure out what the hell is supposed to do what I
need.. It just shows up and it works.. He goes by Exkenna on this site.. Ex-Kennametal salesman...
Smart dude, he won't screw you...
\
tools tooling for hobby lathes

And what you are trying to turn.. Not that big of a deal. Just need the right inserts, the right
speed and the right feed...

DMF_TomB · Jan 19, 2019

eKretz said:
Not in my experience. Harder material usually means less feed, not less DOC. It always means less speed.

.
i often mill 65 rockwell hard material thats fully hardened. i can mill for hours with carbide end mills if taking only .001 or .002 depth of cut. if i take a lot more depth it just breaks the corners off the mill.
.
at least thats been my experience the last few hundred times i have done it over many decades
.
if 45 rockwell C hardness you can take more depth of cut but not as much depth as if its soft 1018 steel. i would start with .005 or .010" depth of cut and watch the corners are not breaking off the tool

eKretz · Jan 19, 2019

DMF_TomB said:
some inserts are coated and the edge often are not sharp especially after coating. that is on a medium soft material it also tends to burnish surface a bit and give a better surface finish.
.
very hard steel and very soft material like plastic material, inserts need to be honed so edge sharper as inserts even new out of the box act dull. hard to describe but i have seen change of insert edge type made a big difference

Again, I disagree. When turning very hard steel the insert needs a more blunt edge to hold up. The best success in my experience with hard steel is achieved by blunting the edge with a micro radius and/or creating a t-land. Upsharp edges are great for non-ferrous materials or plastics. Not for very hard steel. Regardless, the gentleman with the problem isn't dealing with very hard steel, more middle of the road stuff.

eKretz · Jan 19, 2019

DMF_TomB said:
.
i often mill 65 rockwell hard material thats fully hardened. i can mill for hours with carbide end mills if taking only .001 or .002 depth of cut. if i take a lot more depth it just breaks the corners off the mill.
.
at least thats been my experience the last few hundred times i have done it over many decades
.
if 45 rockwell C hardness you can take more depth of cut but not as much depth as if its soft 1018 steel. i would start with .005 or .010" depth of cut and watch the corners are not breaking off the tool

We are not discussing milling operations in this thread, as far as I'm aware. Totally different situation. Not relevant.

DMF_TomB · Jan 19, 2019

eKretz said:
Again, I disagree. Very hard steel needs a more blunt edge to hold up. The best success in my experience with hard steel is achieved by blunting the edge with a micro radius and/or creating a t-land. Upsharp edges are great for non-ferrous materials or plastics. Not for very hard steel. Regardless, the gentleman with the problem isn't dealing with very hard steel, more middle of the road stuff.

.
i often have cutters not bite in and cut and rub cause inserts are coated and the edge are rounded a bit from the coating (edge prep not correct for material). can often see material being pushed around or burnishing action. burr forming even seen sticking up .0005" facemilling where 2 parallel mill passes over lap. on some materials this burnishing action can give a mirror like finish. on incorrect material it works badly
.
i dont have any problems machining hardened steel. i find a resharpened end mill where it has no coating cause it was ground off resharpened cuts better. i literally have often milled fully hardened steel for hours that way. i often have drilled right through broken taps too. usually resharpend carbide drill with no coating and sharper edge works better in my experience
.
not talking rake or more acute angle. i talking the actual edge if rounded or honed too much it doesnt bite it but wants to just rub

eKretz · Jan 19, 2019

DMF_TomB said:
.
i often have cutters not bite in and cut and rub cause inserts are coated and the edge are rounded a bit from the coating (edge prep not correct for material). can often see material being pushed around or burnishing action. burr forming even seen sticking up .0005" facemilling where 2 parallel mill passes over lap. on some materials this burnishing action can give a mirror like finish. on incorrect material it works badly
.
i dont have any problems machining hardened steel. i find a resharpened end mill where it has no coating cause it was ground off resharpened cuts better. i literally have often milled fully hardened steel for hours that way. i often have drilled right through broken taps too. usually resharpend carbide drill with no coating and sharper edge works better in my experience
.
not talking rake or more acute angle. i talking the actual edge if rounded or honed too much it doesnt bite it but wants to just rub

And again, milling is not turning. A milling cutter is much more picky about tool pressure since finish is imparted over a wide area rather than the tiny one with a turning tool. With milling cutters the edge prep radius must be on a micro rather than a macro level, and in a specific shape for milling applications to work well. But again, that has nothing to do with the O.P.'s problem.

Limy Sami · Jan 19, 2019

Guys, we're talking about a lathe that's near enough a 10'' Southbend, which hasn't the balls for the type of tooling the OP's using.

eKretz · Jan 19, 2019

I should think it would still be fine for a light facing cut. Agreed that it certainly could not push the tooling anywhere near where it should be for roughing cuts though.

Matt_Maguire · Jan 19, 2019

I didn’t know anybody sold 4140 at 40Rc and called it prehard, quenched & tempered to customer specs yes... Is this a specialty company selling “to the trade”???

Agree with Hazzart, on the change to positive rake tools mentioned above. Steel going from 20Rc to 40Rc takes about a 45% HP bump with turning. Some quick calcs for 300FPM @ 1.75” to 1” dia goes from 650 to 1150RPM. With the CCMT tools at .006” feed and .030” depth of cut you should be able to break chips. That works out to a little over 1 horsepower.

If that doesn’t work drop to .025” infeed & the HP drops to 3/4HP.

On edit, In my time working with old stuff they tended to work better pushing them hard (taking the slack out of EVERYTHING). It was always the delicate stuff they failed at...

Good luck,
Matt

Wolfram Malukker · Jan 19, 2019

The CNMG inserts and toolholders were given to me by the old hand at the last shop I worked in. So, they may just have been the right "size" and not really the right tool, I've never really had good luck with them.

As to the flat belts, I can stall a 1.5HP motor with the belt drive that's setup on it, and not have the belt slip. I re-machined the pulleys for 6 groove micro-v belts. Yes, it's a bit more hassle to swap speeds. But with the VFD on the motor, that isn't such a big deal.

Turning speed was 680RPM, which was quite fast but also the speed where it actually started cutting. Slower and it just wanted to rub and chatter.

The hardness is RC40 on the nose, I knew it wasn't "4140PH" but instead 4140 that had been hardened, and so when it arrived with a ground OD I took it to work and checked it. Like I said-it drilled OK, but slow, with a 0.200" diameter drill. It just acted VERY hard when I started trying to turn with the carbide inserts.

The toolholders are inexpensve Shars 1/2" shank stuff, and I was using the bigger CNMG inserts in the big holder today. I also have CCMT 21.51 tooling, which is a smaller insert, and I think I have a CCMT 32.51 inserts and tooling as well.

I found this chart:
Insert Designation Chart - provides ANSI and ISO designation code definitions for carbide insert shapes, relief angles, tolerances, chipbreaker codes, hole types, size values, thickness values, radius values, wiper lead angle, wiper clearance angle,

and it looks like the CCMT's should have a 7* clearance angle, but that may not be what the holder is supposed to use-this could be part of my problem.

If I can get the turning problem solved, these will go back in the box and the drawings will get finished based on this size stock. Once the drawings and cut list are made, I'll have another go with them.

CarbideBob · Jan 19, 2019

Wolfram Malukker said:
.....
I have been using CNMG32.25 and 21.35 series inserts .......
. Then I set up to face the end off, and it was *almost* too hard for the carbide to cut, but it needed a LOT of pressure and took off only the finest shaving, with a little spark here and there. ......

There just has to be some hints and clues hidden in here.....
:popcorn:

Bob

Turning Prehardened 4140 at RC40 hardness, problems.

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