18-8 stainless cross hole drilling issues

Hi - I am drilling 3/32" cross holes for a safety pin in 1/4-20 stainless steel threaded rods. I am also doing the same thing in 1/4-20 stainless hex head bolts. I am using a pneumatic Heinrich cross hole drill jig to hold the hardware on my drill press while I drill through them. Both the threaded rod and hex head bolts are 18-8 stainless steel. Drilling through the hex head bolt is relatively easy and I go through a 3/32" cobalt bit about every 35-50 bolts. However, I go through cobalt bits much quicker on the threaded rod. Sometimes I only get through drilling 5 rods before the bit breaks or just won't drill anymore. I'm averaging somewhere around 20-25 drillings before the bit breaks or just won't drill anymore. I am putting a drop or 2 of cutting oil every time a drill. Again, both the hex bolt and threaded rod are listed as 18-8 stainless. Does anyone have insight as why I go through many more drill bits on the threaded rod? Are there different types of 18-8 used on bolts versus threaded rod? I will add that I buy the bolts and threaded rods from 2 different suppliers and the hex head bolts have a bit more of a "shine" to them than the threaded rods.

Thanks!

Variations in the constituent elements (but still within the allowed ranges), differences in the thread rolling procedures resulting in more or less work hardening.

Also the bolts may have been hot-headed, which may have adjusted the metallurgy.

Regards.

Mike

Ideally the drilled hole should start in the V of the thread. In reality the drill may start anywhere on the thread. That could lead to drill deflection and breakage. If starting in the V the lips of the drill could be chipped by aggressively feeding the drill.

Drilling into a thread is a badly interrupted cut, the drill can bounce all over the place, the lips of the drill are likely engaging before the tip of the drill, which can break them off pretty quick. A good spot drill before the drill should take care of most of your issues.

We cross drill a 1/2"-20 threaded 304 stainless part. For years did them in the drill press (CNC knee mill) and had issues with holes wandering and had to chase the thread with a die afterwards. Now doing in 4th-axis on CNC mill. Spot, rotate 180 degrees, spot, drill 2/3rds of the way through, rotate 180 degrees, drill 2/3rds of the way through. Spot drill takes care of getting the threads out of the way cleanly and prevents drill wandering.

Perhaps you can use stop and a removeable locating pin to do the half way thing as well.

Thanks for the replies! In my limited knowledge I was guessing the relative ease of drilling the bolts vs the all-thread rods had something to do with the bolts being heated differently. In the end, it is still a mystery to me why the all-thread is so much more difficult to drill through than the bolt...I'm drilling through the threads on both, they're both 1/4-20, and they are both 18-8 stainless. Weird for sure!

We make a lot of our products out of 304 stainless. My understanding is that the spec for most commodity sorts of stainless is pretty broad. I often have issues in the lathe with some chunks of material being much harder to turn. We use insert drills and sometimes an insert will last 40 holes and sometimes it will last 2 holes. Luck of the draw, I guess.