1mm end mill keep breaking

[Kule Dong]

Recently I was trying to machine a 1mm, 120mm long with DOC of 0.5mm with a 1mm end mill. I'm running this on a 3-axis VMC.

Tool: 1mm carbide 4 flutes with TiSiN coating (claimed by the seller)
Material: Mild steel

First trial: 3500rpm, 15mm/min and the tool broke halfway. Inspected the end mill and it seems like the speed is too high causing the tool to heat up, the end appears dark and burnt.

Second trial: 2000rpm at 20mm/min, tool looks OK when I paused the machine to inspect but still broke.

Coolant was used on both trials.

I've used G-Wizard to get an idea on the speed and feed, it says that my chip load is too low and may cause rubbing, maybe this is the culprit? or I'm just not dialing the speed and feed correctly.

 

[Lnewqban]

It seems to me that the feed speed of 0.25 mm/s is too high for the bending capability of a 1 mm bit.

 

[EngMark]

Could you maybe use a shorter flute length?
With a 0.5 mm DOC the shortest flute length you can get is the best

Take multiple passes.?.

Maybe skip the flood coolant for such a shallow cut and use cutting oil applied directly on the intended cut area.
Once you make a first pass then you have a shallow depression that you can fill with oil to lubricate more effectively.

And if applicable to your situation, a 1 mm thick slitting saw blade will get it done faster.

"claimed by the seller" tends to make me think that you might have saved some money by getting the tool off amazon or an asian source or similar. If so I'd buy a first class tool(s) directly from a known good distributor. American, German, Japanese, Swiss, Canadian and not a low priced one. McMaster-Carr saves you the effort to do your own quality research.

And, this probably doesn't apply to you but I'll tell a little story here from a time that I worked in a shop with a lot of very low experience guys. The tendency was that once a small end mill broke they would reduce the spindle speed while keeping the feed the same thus increasing the chip load ("I must have been going too fast"). And some times after the second one broke they'd double down on reducing the spindle speed. More than a few times I'd find a bunch of small broken end mills in the garbage and usually stealthily hidden in a coffee cup or similar.

Just thoughts off the top of my head, not severely authoritative.

Mark