I recently I milled a piece of 3/8" steel bar that I pulled out of my
junk pile. It wasn't cutting very well and on inspection I saw that the
cutter was dull and put on another one. Very shortly it stopped cutting
and was found to be dull. This stock had cut and drilled easily, so it
wasn't hardened. I concluded that there must have been a bad spot in
the stock, an inclusion of something hard. Does that happen?
If so, how do I avoid it? If I buy stock from a place like OnlineMetals
can I have any confidence that it won't have this problem? I'd hate to
throw out my carefully acquired junk but buying new stock would be
cheaper than buying new cutters.
Thanks for all the replies - a lot of good information (as one would
expect from RCM).
As for identifying the type of steel, I'm a 2-type guy: hard and mild.
If the steel fails the file test, I don't use it. The stuff that I make
just doesn't need "hard" (well, occasionally I've used oil-hardening
that I've hardened AFTER machining).
Thanks for confirming my suspicion about the non-homogeneous stock (that
would be heterogeneous, right?). I.e., that mild stock can have hard
inclusions in it. I guess the only way to avoid it is to buy "fresh"
stock (not scrap yard stuff). As much as I hate to give up the nice
collection of stock that I got at the dump, it just isn't worth it if
I'm going to ruin cutters on it. Worse, if it means that I'll put
significant time into a part only to be unable to finish it because of
I'm thinking that I should make a list of the stock I'm likely to use
and just buy it to have around. And buy nice stuff (12L14 comes to
mind). Then throw out my dump stock. Life's too short for that kind of
aggravation and I don't use so much stock that it matters.