Finishing floor of pocket, which is better: SQ end or bull nose...
Yeah, I know, try it and see. I have tried it and it seems to vary depending. I have not run a "controlled" experiment, like five parts with this and five parts with that. So I am wondering if this is very well known territory and I just need to be told the answer. Material is aluminum and sometimes stainless steel. By bull nose I mean something with a .015 to .060 corner radius, vs. square. For these pockets I have to use a bull nose at the edge, because a corner radius is required at the wall. It makes sense then to use a bull nose for the entire pocket.
I have two competing theories: one, that the rounded corner will smooth the transition between stepovers, and two, that the rounded corner will produce significant axial forces, exaggerating the transition at the stepover. Maybe it depends on the DOC of the finished path vs. corner radius?
I think it has more to do with the way the bottom is ground than it does with the corner radius. Someone with more knowledge than me on grinding endmills may have a better explanation. I was running a mold a while back using a .375 endmill with a .015 corner radius and getting a poor finish on the bottom. An oldtimer that works for the company I was doing the mold for took my endmills and ground the bottom at a different angle or put some sort of relief on them and the bottom finish improved dramatically. I have certain inserted cutters with aluminum that will leave an awesome bottom finish and others that look similar that are terrible at it. I spoke with my rep and he said it was the relief on the bottom of the insert and it's angle.
That is sort of what I think I am seeing. Even with two end mills from the same supplier, same pn., I seem to see some differences. There are always a lot of variables though: different piece of metal, different support for the pocket, etc.
Hmm... thought there would be more opinions on this one. What do you all use for this? SQ end or radius?
It varies. Does that answer your question?
I have used both and had different results. Sometimes if I leave about .003 on the bottom of my cavity and the program isn't too long to where you lose your corners I achieve a really nice finish with a square tool. The longer the tool is in the cut I seem to get a better finish with the corner radius. Compared to the one that used to have a square corner, but has now lost it's edge 15 min ago and the finish sucks to polish out.
You can get a really nice surface with a new endmill if you, carefully stone by hand or cutter grind, a flat edge on the flute ends perpendicular to the endmill's axis with the needed relief . The flat doesn't need to be much longer than the distance the endmill travels between flutes as it rotates. This removes the scratchy surface left by the sharp endmill "points," on the tool's circumference.
I actually get some of the best finishes on floors with a .005-.01 x 45 deg chamfer on the bottom of the tool. When using a bullnose endmill on hard material, there is a considerable amount of axial force, I have a rather whimpy high speed spindle, so I would imagine this is the reason in my case.
It's all in the fishtail...not the radius from my experience.
I was told long ago that most fishtail on the bottom of a tool was anywhere from 1.5 to 2.5 deg. depending on the tool vendors specs.
By increasing it to 5 or 7 deg. it doesn't trap the chip between the work and the tool, allowing the tool to cut free and not drag the chip around which gives the poor finish.
For aluminum, sharp corner.
For steel, small radius.