4140

What kind of milling machine? What kind (brand) of milling cutter? What style inserts? I know too many questions, need some pictures. One insert sticking out more than the rest can sometimes be corrected. Depends on the cutter. I would start by disassembling and cleaning real good then reassemble. Not fond of grinding one insert...
 
Just helping, like the Hobby Machinist forum always does:). You need another machine. You need a surface grinder. And then all your problems like in this thread will go away. You will get a great finish and easy control over dimension. Just trying to help. Go buy a surface grinder. It’s only money and room for it!…Dave
 
And a million bucks would be nice.

The list goes on
 
I like the million idea...

I reread the post and did not see what your depth of cut is.
 
I never wrote it. It varied. 015-.025"
 
When using carbide I tend to like the insert held at an angle where the corner is not the leading edge of the insert into the cut. If you took a square insert sitting flat on the table and tipped it (sideways, not front to back) to 30 or 45 degrees the side of the insert would meet the material first rather than the corner of the insert. I personally like this better. I don't like that corner out in front. This would not be good for cutting a square corner but I believe it to be easier on the cutter. The corner radius on the insert could be a problem if it is too small, (too fragile) for general work I like 1/32nd. If the job required larger radius so be it. The bigger the radius the more tool pressure. I like negative rake for hogging and positive for finish. The grade or hardness of carbide could be an issue also. Your depth of cut is kind of light and shouldn't be an issue unless the material is toughened (hardened) so something else could be up there.

4140 is a medium carbon steel. The last 2 digits being the carbon content. Up to about 20 is a low carbon steel (1020, 8620). The first 2 digits are the major alloying elements

For the most part I've only used Kennametal, Carbaloy, Valenite, or other American made inserts & milling cutters. Not a fan of chinese made goods in general so no experience there. I would recommend you browse the Kenametal website and look at some insert dimensions and nomenclature.
Yes they're expensive but they work...
 
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