That's often a sign of overspeed, even with free machining stainless grades. It can also be a sign of a rounded (dull) cutting edge, which is death in stainless.
I'm not inclined to agree with that, and none of my experiences with stainless (more than 60 years worth) supports that idea. Too slow isn't efficient, but it eliminates a lot of problems with material that is prone to work hardening, which is true of most stainless alloys. When in doubt, slow down. It is higly unusual to improve conditions by increasing speed when drilling in stainless. That's assuming you're not running unreasonably slow.I do remember using large drills in stainless and if you went too slow you could create hard areas.
I also remember some raw stainless coming with hard sport but that was 45 years ago.
A commonly reported phenomenon. I suspect there's a little truth to it, but it's not been the norm for me. Keep in mind, the 300 series of stainless is NOT heat treatable, so there's little reason for hard spots to be found within the material. Doesn't mean it can't happen, though. Just not common.
Slightly lower rpm most likely would have solved the problem. Reducing speed for stainless is paramount to success. Also, drills for stainless should be razor sharp, so cutting pressure is as low as it can be (that helps avoid work hardening, which is easy to do in stainless).I tried 2 different taps and they felt about the same so maybe it was my drill speed. It was a 3mm drill at around 1135 rpm.
In regards to the taps, it's not all that difficult to determine if they're sharp or not. For tough materials (stainless in the annealed state is not hard---but it is tough), if the cutting edges of the tap show the least bit of shininess or rounding, they can safely be considered as dull. You have to look where they cut, not on the flutes above the cutting edges, though. That portion of a tap does very little aside from helping guide the tap.
The bulge you speak of tends to send up a red flag for me. When your cutting tools are keenly sharp, you shouldn't get that. If you start out with sharp tools but the surface speed is beyond reason, the tool has a tendency to lose the keen edge, which raises cutting pressure and can yield deformation in the proximity of the cut. Without being there to see what was happening, I'm only second guessing, so please don't take what I say as gospel. It is only to provide food for thought, perhaps leading you to a viable conclusion. One thing I know----303 stainless, be it S or Se, is exceedingly nice to machine and will yield good surface finishes as well as clean cuts, assuming you operate within acceptable parameters. Those parameters are far looser than those for other grades of 300 series stainless, excepting 302, which is also free machining.I used a center drill of maybe 3/16 diameter and it did make some noise as the larger diameter hit the stainless. In fact, I think I could see a visible bulge just outside. It was at the same speed so what could I have done differently?
Heh! It's obvious you are not a CNC operator. They'll argue with an old school machinist almost endlessly that a center drill isn't a good starter for drilling. They're reasoning is often that a starter drill typically provides the same angle as the drill point, so the drill starts exactly where it should. Too bad old school machinist, who worked and won WW II with their bad choice of using center drills for starting holes didn't know they didn't work.I have never heard of a starter drill. How is it different from a center drill?
Starting drills are made relatively short and have an exceedingly thin web at the point, so there's almost no surface that doesn't cut. The short length and tapered web offer greater rigitity, so they don't deflect easily. That's very unlike a common twist drill, which, unless it has a split point, the center (the web) does NOT cut. The web causes a twist drill to easily move away from the target unless a means is provided for the drill to start where aimed. The reason for that is the chisel point (the web portion of the twist drill) displaces metal (which demands excessive drilling pressure) which is then cut and removed by the cutting lips. With a split point, assuming it's properly ground, that is no longer true, although the rake angle of a split point may be only 0°, while the cutting lips enjoy a much greater positive rake (determined by the lead, or helix angle of the flutes). A center drill, as you likely know, has a chisel point, but the reduced diameter of that point is so short that it doesn't deflect easily, so they're inclined to start where aimed, in spite of the chisel point. They do an admirable job of starting drills, assuming you allow them to start on location. It's good to start out with light pressure long enough for a shallow dimple to form before applying drilling pressure. Once properly started, they perform their duty just fine, with the small hole providing relief for the web of the follow-up drill. That's real obvious as you can always tell when you've drilled beyond the tip depth by the increase in cutting pressure.
Starter drills are generally provided in common fraction sizes (1/8", ¼", 3/8" and so forth) so they cover a useful range of sizes. One stops at the desired depth to accommodate the target drill size, so large starter drills work for smaller drills.