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Carbide vs HSS


Jan 12, 2003
2 ways on steel, very sharp tools, still get a dull finish but should help get rid of the bumps/lines and create a good smoother finish.

With carbide you have to use high SFM, Dry if you can, and as much feed as you can push while staying in the required finish. Can be a pain to do it on manual equipment though. If you can push enough speed a slightly honed edge can actually help, but I still hate working with 1018 steel anyway. I'll take stainless any day.


Nov 29, 2007
North Alabama
Use an uncoated carbide insert with a ground up-sharp edge. WNGP-431 or 432 will do it. To clear up some things about coatings: CVD will not adhere correctly over a dead sharp edge, PVD coatings will. You can't rely on the color to tell you which is which. There are now gold and grey coatings of both types.
Good luck with it.


Cast Iron
Dec 6, 2016
Xinjhuan District, New Taipei City
I'm going to chime in here, I been machining annealed 4140 and was having trouble getting a good finish as well until I found the following:

Chipbreaker geometry matters. Look for something like this:

I used that very insert and was able to get a very good finish at 1000+rpm, and low (about 0.1mm) depth of cut.

Chances are your insert looked something like this:

These type of geometries require at least 0.5mm depth of cut before it will give you a good finish. They really are "light cut/half roughing" inserts.

I've also had good luck with this geometry:


Yes this is a CCMT but they make WNMG inserts with this type of geometries too, and they work, allowing you to take finish passes of less than 0.1mm. Keep the feed rate low too, and make sure you brush the chips away. It won't be chips but strings, and do not let them rub onto the workpiece or else it will ruin the finish.