4140 Finish issues

Rogue_Machinist · Apr 16, 2021

So ive been doing this Machining thing for several years. I usually dont have a problem with getting a good finish on 4140 but this most recent batch of 4140 doesnt seem to want to give me a good finish. Im leaving .045" for a finish pass. Im using CSS its a 3/8" diameter to that ends at 1.250". But all I seem to be getting is tearing. every calculation Ive done shows 4074RPM @.375". My machine isnt capable of doing that. Max I can run it is 3000 which I started at. Im cutting at .006"/rev. And im still getting a ratty finish. Ive experiemented with various feeds and still nothing.

What am I doing wrong. This didnt happen with my last batch of Mat'l.

HuFlungDung · Apr 16, 2021

Sounds like the difference between a batch of annealed versus a batch of heat treated, stress relieved. Complain to the supplier and try a different heat.

Milland · Apr 16, 2021

Along HFD's thoughts, do you have access to a hardness tester? If you can check between an older piece and the new material, that might confirm the issue.

mkd · Apr 16, 2021

Yeah, this is common. It's the insert.

Bobw · Apr 17, 2021

When you can't get the surface speed you need, and you are tearing,
the first trick I pull out of the bag is to shut the coolant off
during the finish pass.

If your .045 is on diameter, try that on radius, that can help.
Maybe feed a little faster.

If the simple stuff doesn't do it, then its time to change the
insert. Go to something sharper. I'm a big fan of the ground
polished up sharp inserts for aluminum. They are awesome in
gooey steels that tend to tear when you can't get the
surface speed you need. They are kind of like a piece of
sharp HSS, but you can run them faster.

If you don't have any of those in stock, pull out the sharpest thing
you've got.

One last stupid thing, I can NEVER get a good surface finish using
CNMG's especially on smaller diameters, no matter what the insert
geometry is... I don't know why, and I don't even try to figure it
out anymore, I just do most of my finishing with DNMG's.

Rogue_Machinist · Apr 17, 2021

Milland said:
Along HFD's thoughts, do you have access to a hardness tester? If you can check between and older piece and the new material, that might confirm the issue.

I do. I will be checking this monday morning. Because im using the previous programmers notes as he has passed away. Just seems very strange. Ok. im gonna test it. Thank You.

Milland · Apr 17, 2021

nissan300ztt said:
Because im using the previous programmers notes as he has passed away.

Bummer. Stressful job, my condolences to his family.

Rogue_Machinist · Apr 17, 2021

Milland said:
Bummer. Stressful job, my condolences to his family.

Yeah we havent made this particular part in 20 years. Long before I started at the shop. And he left very little info about them. So I made similar parts last year with great results in 4140. These just dont see to be cutting well. I will check hardness on 2 different pieces of material from different batches and see what they show me.

cnctoolcat · Apr 17, 2021

4140 pre-hard is your best bet for a good surface finish. Annealed is a crap shoot!

Also, try a cermet finishing insert...and like Bobw says, either in DNMG or VNMG, with a "1" (.016"), or even a "0.5" (.008") nose radius.

ToolCat

DouglasJRizzo · Apr 17, 2021

Bobw said:
One last stupid thing, I can NEVER get a good surface finish using
CNMG's especially on smaller diameters, no matter what the insert
geometry is... I don't know why, and I don't even try to figure it
out anymore, I just do most of my finishing with DNMG's.

Agreed. I do the same and have been for years. I frequently use a DNMG with a sharper radius insert also.

Rogue_Machinist · Apr 17, 2021

cnctoolcat said:
4140 pre-hard is your best bet for a good surface finish. Annealed is a crap shoot!

Also, try a cermet finishing insert...and like Bobw says, either in DNMG or VNMG, with a "1" (.016"), or even a "0.5" (.008") nose radius.

ToolCat

Already using a .015" tool nose radius. Brand new.

Guest · Apr 17, 2021

Bobw said:
One last stupid thing, I can NEVER get a good surface finish using
CNMG's especially on smaller diameters, no matter what the insert
geometry is... I don't know why, and I don't even try to figure it
out anymore, I just do most of my finishing with DNMG's.

Weird, I am pretty much the opposite, the only DNMG's I currently have are from an Ebay bargain buy ages ago that I have almost used up. As for 4140 I recently ran a lathe job out of all the remnants I had laying around
it was a mix of PH and ANN 2 different round sizes and even some hex, oddly every part looked the same. So far I haven't had any 4140 issues, luckily.

Guest · Apr 17, 2021

nissan300ztt said:
Already using a .015" tool nose radius. Brand new.

What brand and carbide grade?

Rogue_Machinist · Apr 18, 2021

Dualkit said:
What brand and carbide grade?

Seco and dont know the grade. But I ran these last time I did a similar part that it gave me an amazing finish.

Rick Finsta · Apr 18, 2021

Call your local Seco apps guy. I've found them to be a great help when I can't figure out something like this.

Guest · Apr 18, 2021

nissan300ztt said:
Seco and dont know the grade. But I ran these last time I did a similar part that it gave me an amazing finish.

The only Secos I have are some ceramic ones DNMG with a huge radius, never tried them on 4140. With 4140 I have had good luck with both ANN and PH using Sumitomo CNMG 432EMU grade AC510U and KENNAMETAL CNMG431MS Grade KC5010 on my fixed headstock lathe with 3/4 shank tools and on my Swiss with 1/2 shank tools I use Kennametal CPMT32505LF grade KC5010 and CCMT32505FP grade KCU25.

I write down on the insert box what materials the tools have had success with, not the greatest system but works for me.

HuFlungDung · Apr 18, 2021

Talking about not being able to finish with a C shape insert: it depends: got to keep the feed quite high, maybe .010" It really doesn't result in much of a scallop and because you're done sooner, then you can have more time to polish it

C shape is particularly bad at trapping a chip between the insert and the work, and scuffing the hell out of it. But C works pretty nice if you can arrange to have the leading edge angled back 9 degrees (Multifix has its uses!

) But then you can't finish the shoulders. Bummer.

I've never liked the .015" radius tip for lathe turning. Always use the .032" My preference to finish with is a T insert. Probably D is ok too, but I don't have more than a boxful of those around, because they never really solved something that I couldn't deal with, with a T insert.

Strange fact: the old Kennametal K090 ceramic insert with T land (on a T insert) will skim cut 4140 about a thou to half thou at a time, and produce a very shiny, smooth surface, without running particularly faster than normal with carbides. The chip shoots straight up in the air, red hot, and comes off like wool. Since the ceramic insert has no chip forming, it's not really an all around solution, but skim cuts are a fact of life.

CarbideBob · Apr 18, 2021

Interesting the successes of C vs D, rotate holder and choice of rad size and then a triangle. I guess much good here in that three (or six) corners to use in one tool.
You are in double neg holder with a cylinder radius. Everything goes so wrong after the tool tip and you have to cut here to make the cusp.
What can a C do that a T can not and one has more cutting tips per insert.
Technically a T not as strong but how many are hammering the entire tip off of a carbide insert with today's grades?

Everyone has a what works for me from experience or testing and all I see correct.

.006 per rev here and tearing or welding.
Bob

Rogue_Machinist · Apr 19, 2021

So the hardness was the exact same as the previous batch. And the Insert didnt seem worn. But I changed out the insert with a fresh one and increased my feed to .008"/rev and she cut amazingly.

Rogue_Machinist · Apr 19, 2021

HuFlungDung said:
Talking about not being able to finish with a C shape insert: it depends: got to keep the feed quite high, maybe .010" It really doesn't result in much of a scallop and because you're done sooner, then you can have more time to polish it

C shape is particularly bad at trapping a chip between the insert and the work, and scuffing the hell out of it. But C works pretty nice if you can arrange to have the leading edge angled back 9 degrees (Multifix has its uses! ) But then you can't finish the shoulders. Bummer.

I've never liked the .015" radius tip for lathe turning. Always use the .032" My preference to finish with is a T insert. Probably D is ok too, but I don't have more than a boxful of those around, because they never really solved something that I couldn't deal with, with a T insert.

Strange fact: the old Kennametal K090 ceramic insert with T land (on a T insert) will skim cut 4140 about a thou to half thou at a time, and produce a very shiny, smooth surface, without running particularly faster than normal with carbides. The chip shoots straight up in the air, red hot, and comes off like wool. Since the ceramic insert has no chip forming, it's not really an all around solution, but skim cuts are a fact of life.

I usually rough turn all workpieces with a .032" Tool Radius because I can hog off lots of material quickly. In this case its roughed @.120" doc and leaves .045" for finish. The little .015" rad usually works great. But I got it figured out. Must have been a bad insert even though under the microscope it looks almost brand new. Just a light heat mark.

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