Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.
Going to do my best here but have to be careful about the whole bias and marketing thing.
First here’s some critical data that we don’t have here (unless I missed it).
Grade of stainless:
This can change a lot in the chipload data for these tool. At a minimum stainless spans austenitic, ferritic, and martensitic (if we ignore duplex, which we are). These will change how you have to cut material as they have different hardening methods, composition, and specs (hardness, ductility, machinability, etc.).
4 flute AlTiN doesn’t really tell us a lot. Things like helix, rake, and relief start mattering more. Plus things like what’s it made out of. I’m going to assume a decent carbide and that we have at least decent geometry.
I’m going to assume here that you are not straight plunging. If that’s correct, what’s your ramping angle? Without knowing the tool I’d probably stick to around 2-3°.
Okay the above aside let’s go through some of this.
My first guess would be that you lost either an edge or the tip. When that happens you have a MUCH larger surface impacting the material and generating heat in addition to basically putting runout in the bit unless all the flutes fail in the same way. Could also be from a lack of chip clearing functional causing the same thing over time. Might also be runout related as you could have some skinny cuts that way too.
They almost all harden in some way. Although, the method and number of methods change with the material.
Don’t know the color to temp for stainless. But, as soon as you hit a decent amount of yellow in mild steel you are at 1200c. Keep in mind we are almost certainly hitting higher temps than visible from the outside (especially with mist or air on it). Additionally, the failure method will first be from the cobalt binder not the carbide itself. You will probably not have any glowing by the time you are embrittling the tool due to cobalt leaching.
It should. I’d use an oil based fluid if possible as it provides more forgiveness.
So with all the unknowns here’s my quick crack at it. There’s a LOT of assumptions and picking a middle ground here. A big assumption being machine runout which is a whole other mess to tack onto this.
First I’d drop the surface speed to no more than 250SFM (7640 RPM for an 1/8"). Again, we don’t know the geometry or material so this is going to be my best middle of the road guess.
1500mm/m at 8150RPM works out to a chipload of 0.046mm (0.0018"). However, we are only taking a 0.127mm WOC so we have chip thinning (4% stepover). The real chip ends up being 0.018mm (0.00071"). This might be okay but with a generic tool and unknown material I’d probably drop this to around 0.013mm (0.0005") to start with.
If we keep the WOC of 0.127mm on an 1/8" tool we need to times our desired chipload by 2.55 to get the chipload for the feed calculation. So (feed = desired chipload * 2.55 * RPM * Number of flutes) e.g. at 7500 RPM it would be 0.013 * 2.55 * 7500 * 4 which will give us ~995mm/m.
Your stepdown I’d probably keep without knowing the tool geometry (Since it did fine on the side entry cuts). Ideally you want to keep the flutes engaged so your deflection is more consistent across the cut. However, it might be safer/better to decrease or increase it depending on the helix of the flutes.
The alternative to the above is to get rid of the chip thinning (at least a 50% stepover) and radically reduce your stepdown. This is a requirement for slotting as you no longer have any chip thinning.
Going to stop there for now. Hope that’s useful. Let me know if there’s something I can help with.