Turning 316L breaking inserts

[lucky-guesser]

We have machined stainless parts forever but historically our primary material was brass. People above me have decided that we are switching to 316L as our primary material. Over the past few years, issues with the machineability of the 316L have become more frequent and now that it is our go to material, it is a weekly/daily issue. So not a new issue by any means, just getting to be a bigger one. Parts are rough cast by our supplier and we final machine. We will get a batch of parts that machine well, then the next batch will be riddled with parts that are harder and noticeably more difficult to machine.

Standard practice is 95deg carbide inserts (CNMG 12 04 12-MR 2025) with -5deg lead, full coolant, .050 DOC per side. Our ID cuts are standard boring operations, but OD turning is compounded by being an interrupted cut (more air than metal on the interrupted section). OD is more susceptible to issues when cutting borderline material, but particularly hard parts cause issues on both OD and ID cuts.

The primary issue is chipping of inserts. Problem parts are needing inserts flipped/replaced every pass in extreme cases. When machining brass, we can typically do several parts per insert, but with a problem 316L part it is more like several inserts per part.



Using our PMI gun, a hard-but-not-awful part tested:
Si: .51%
Cr: 18.92%
Mn: .88%
Fe: 66.68%
Ni: 10.09%
Cu: .32%
Mo: 2.1%
The remaining .5% (including Carbon) in unknown.

Talking to a couple of our most experienced machinists, we have tried:
Numerous different inserts
DOC variations from .020-.200
Various RPM and feed rates (exact range is unknown, but you get the point)
Turning coolant off (OD interrupted cuts)
Annealing in our large oven. Some parts improved but others didn't.

I have instructed the machinists that when they get a particularly hard part again, to let me know and I will do a composition test to compare to the others as well as hopefully a hardness test. Our tester is of the "Telebrineller System" type which is a new one for me so we'll see if I get any meaningful results from that.


Any insight, experience, suggestions or off the wall conspiracy theories will all be considered helpful at this point. As I read and find another idea, I take it to the machinists and it's something they have already tried. Open to anything at this point, thanks.

 

[mfgenggear]

I would take up interesting issues like this.

Couple of question Was there failure analysis
met lab visual and tensile properties of the patent material of machinable vs problem parts.

Sample hardness of castings.

What type turning equipment

How is the casting held.
Is roughing and then finishing feeds and speeds.
Rough surface and interrupted cut is a pita.
Very slow rpm , doc ( depth of cut) required on initial cut to be at minimum. 030 inch.
Start out at standard feeds and speeds recommended by the insert manufacture.
316 SS is a tougher than brass. Feeds and speeds are a lot slower, and required more right
Work holding and insert tooling. It must be short and ridgit. Plus the the machinery require more Horse power (beefy) and more ridgit.
Depth of cut has to be deep enough on roughing and finishing. As not to work harden the parent material.
There sales engineers that specializes on tougher materials utilizing ceramics and special
Carbide . Round carbide is the more chip resistant and a stronger. But requires more force
Rigidity. Bigger radius inserts are second.
Round ceramics inserts are used for roughing
With incredible depth of cut, and high rpm.
Parent material literally floss of with incredible
Velocity. But 30 hp minimum, and must be rigit.
Mori seki cnc lathes were great machines.
But there are many. This is from actual experience. I sure lathes are better these days.
With the right geometry, carbide alloy, and right feeds and speeds for roughing as a starting points is important. And the rigity of the machine and tooling. Be prepared to spend money.
Contact several different suppliers, and keep going until the right combination works.
If this is no possible. Slow the feeds and speeds down as not to destroy the carbide inserts.
Slow rpm, slow feed with deep doc.

 

[mfgenggear]

Wanted to if after roughing on the finishing cycle
If a machinist is not use to work hardening materials , less than .010-.020 depth cut will work harden stain less. It will not cut correctly
And will destroy the insert. Being there done that
Rookie mistake.