Need advice milling .079 guard slot in 1/8" O1

[Cardinal Knife]

my blade measures .079. I purchased a 5/64" 4 fluted end mill which measures .078125 (close I know but I didn't want to go smaller out of fear of breaking). It plunged ok and made it about 3/16" before it broke. Any tips here? Do I need to get the mill up into the collet as far as I can to reduce flex. Also, looking back, I was milling off to the side of the vice ( just remembered I took a photo I will try and post) so maybe the steel flexed or chattered a bit? It seemed like it was going smooth then it just broke. Also, I have no other reason for using O1 other than that's what I had available. Should I not be trying to mill a slot in tool steel?

 

[C.S. Graves]

I'm definitely not the most experienced machinist, but did you plunge deeper than .039"? Also, if I understand right, you cut towards the vice jaws? I probably would prefer to cut parallel to the jaws if possible, so that excess force has a slight chance of moving the workpiece instead of snapping your cutter.
What RPM were you using?

 

[Cardinal Knife]

I didn't measure plunge depth. I just plunged thru and started slotting. Why, is there a formula that I should be following? I'll order another mill and try slotting parallel as you suggest. Thanks

 

[Danketch]

Small mill in hardened steel plunging .125" and slotting?. If you weren't using flood coolant that prob. contributed. Also what speeds and feeds were you using. I don't have my work formulas here, but just referenced Garr's. I think you want to be around 100SFM with around .0005 chipload per tooth. Should be conservative and close enough. Also, if you drill out the ends of the slot first (with a smaller size drill) that relieves the stress a lot. I would def. take depth cuts as well, not plunge all the way through. Maybe .015-.020" depth of cut.

 

[TMIMITW]

I forget exactly what the rules of thumb is, but I would say generally to avoid trying to make cuts that are deeper than half the diameter of the tool you are using. I would probably go even shallower than that as I don't have much experience machining steel, much less tool steel. Other than that go slow.

 

[davek14]

Good advice. You only use 50% removal with an end mill so if you are going full width that is 1/2 the end mill deep. When you use tiny cutters you are in a whole new (baffling) world though. If you feed too slow you will dull the end mill and it will break and you will break it feeding too fast obviously.

I usually try using the proper feed and speeds and taking tiny DOC (depth of cuts) as has been recommended. .01" wouldn't be bad maybe.

Definitely reduce flex to minimum.

Not sure what the F&S on hardened steel would be. I'd hate to do it and would want a carbide cutter. And they're more brittle ☺. Stainless is (I think) 40-50 surface feet and .0005-.00025 chipload.

Here's an article
http://www.mmsonline.com/articles/hard-milling-by-the-numbers
to muddy the waters further. They recommend much higher F&S (but are likely using carbide, and larger tooling) but they do bear out the tiny DOC.
Axial cuts are full width and they recommend 5%-7% of the cutter. That would be .004"-.006".

 

[Cardinal Knife]

okay this is all good info and I thank all you of you. I am not a machinist, we bought an old bridgeport a while back when a local factory went out of business. We only use it a dozen times a year for stupid projects, so I never needed to really educate myself on the details, as you can see.

I don't remember the speed we're running it at btw