metalmad said:
Hi Bob
I did not know that and I'm sorta hoping, I never have to drill another bit of Stainless in my life :big:
unfortunately all my drills are very cheap and nasty, but I was rather surprised today how bad they were
Pete
We're taught three basic types of drill bits at Boeing when we're in training, HSS, Cobalt, and Carbide. the latter two are used for titanium, depending on size of hole and user's preference, but titanium is expensive, so in training they have us drill stainless (which is actually harder). Here is what we learned in that class :
HSS can drill Stainless, but it's VERY finicky. Even with a brand new drill bit, chances are you'll break the bit 9 times out of 10 (in my case, I broke 40 before I got my first good hole, I'm told that's not uncommon). Once you learn the feel of the stainless, you get an idea of how much pressure and feed rate (VERY slow) to give it. Once you've got a pilot through (we pilot virtually every hole to .1285in , before bringing it higher) subsequent sizes go through much easier.
Cobalt is great for upsizing holes, but very fragile for pilots. Otherwise, same as above. feed and speed are key.
Carbide tends to be good for piloting, but overheats quickly if you upsize too much at once.
The trick to stainless, is that it work-hardens VERY easily. if you're not putting a constant smooth pressure on it, it'll work harden, and it'll be a real pain to punch a drill through it no matter how sharp or what it's made of. After my 40 bad drill bits, that one 'good' one got me 20 holes before my hand slipped on the drill and snapped the bit. Titanium is a bit more forgiving, but also requires VERY slow speeds, as the swarf can catch fire in a heartbeat if it gets hot enough. And it's almost impossible to put out a titanium fire with common methods.
In a Drill Press, I had much better success with stainless. constant pressure and a good slow RPM (350-500) and a good bit will chew through it with ease.
I'm not very familiar with how well it machines, but for drilling, there's my 5 cents. (i talk too much for it to only be 2 ;D )
