I was out playing with my new-to-me mill and was having difficulty finding
appropriate cutting speeds for TiN coated end mills in either my old
Machinery's Handbook or more modern Shop Practices book. Both show speeds
HSS and Cobalt, but not TiN coated. Any rules of thumb for these bits? For
home shop use, should I follow the speeds for HSS?
I suspect I ran my half inch end mill at a speed that was too high. I was
taking small cuts on what in retrospect I think was harder steel than I'd
anticipated, and I got glowing red "sparks", smoking cutting oil, and a loud
and unattractive screeching noise. It sounded like the end mill trying to
RUB the metal off rather than cut it off. The end mill looks like it's seen
better days... Probably not my finest moment as a budding machinist.
If you wan to get life out of your cutting tools, pay close attention to the
color of the chips coming off. If they are a light yellow, you don't want
to run any faster, and it's a good idea to back off slightly, or to use a
vapor (spray) mist if you have one. Blue is out of the question, and
sparks are *way* too late.
Best to remember that TiN is a "COATING". You will have to set your
speeds according to the actual material of the end mill, and live with
The TiN helps prevent the material being cut from sticking in the
flutes (a bit) but mostly it makes the tools shiny and golden,
therefore, they must be better, right?
Thanks Harold. I figured you'd get a kick out of the picture of me standing
there in front of this squealing machine with sparks popping off it
thinking, "Hmmm, this can't be good."
I know I was turning too fast, but did the fact that I was taking a light
cut (.005") make the situation worse? Could I have work hardening the metal
or is this tough to say without knowing the material I was milling? I was
trimming the "table slot keys" that stuck out of the bottom on a new vise.
I'm thinking that these might be pretty hard...?
Afew months ago I was running some of my favorite material on the VMC. Yes!
304!! I had 2 3/8 end mills in the rack, 1 carbide and 1 HSS. My intent was to
use the carbide so speed/feed was punched in accordingly. Flood coolant spewing
all over the critical zone was not much help. An orange glow from within a flood
of coolant is also a sign of **WAY** too late.
Humbled back to Earth on occasion....
The thin cut might cause some work hardening, you'd want to keep the
up to reduce that somewhat. The screeching is almost always a sign your
feedrate is too low, and the tool is rubbing, not cutting. Sometimes,
to take fairly thin cuts, but if the tool RPM and feedrate are proper
for the tool
and material, that shouldn't cause trouble except with the most stubborn
I have done jobs with blue chips, most notably fly cutter work on large
I may have to touch up the edge on the fly cutter bit after one piece,
but that is
no big headache. I use some heavy cobalt lathe bits in the fly cutter,
can take a lot of abuse.
What do I do if I want to take a small cut, but I want a nice finish?
In other words, with aluminum, my tendency has been to slow the feed
rate to get a better finish, but it sounds like that may not work on
Knowing what you were machining when you had your trouble explains the
reaction you experienced (assuming the keys were hardened, anyway)..
Hardened items don't lend themselves to machining, especially when they get
above 45 Rc. You may have had a great deal of difficulty even when
running slowly. In your situation, consider your trouble was not so much
the speed, but the material you were machining.
Tough materials will respond fine to light cuts and slow feeds, and in fact
may be the best way to remove metal. I offer 304 stainless as an example.
A light feed with positive rake will keep chips coming off, where a heavy
feed rate will generally yield premature tip failure (somehow I shifted
gears and got you to a lathe, but the principle is the same).
When machining tough material, the key to success is keeping a sharp edge,
so the tool doesn't scuff, but cuts instead. Once a tool starts dragging
because the edge is gone, it's a tiny leap from dull to burned. If you use
a sharp end mill, you should be able to take light cuts with good results.
Once you've used an end mill for roughing, the chance it will make a good
finishing tool is greatly diminished.
What can, and will, kill the tool is to stop feed and leave the cutter in
contact with the work while it's still turning. Idling cutting tools that
are still in contact can be a recipe for failure, depending on the nature of
the material being machined. Stainless, chrome moly, various other
alloys don't like idling tools. Aluminum doesn't care.
From this you can conclude that if you want a nice finish, make sure you use
a sharp cutter, take the light cut you desire, lubricate well, and climb
mill the final pass. Your mill must be tight, and you have to use caution
when going into inside corners when doing so, however. Always use extra
caution when climb milling.
So how do I stop a cut mid run without trashing the tool? Stop the
feed and raise the spindle or lower the knee at the same time? I've
been running the spindle all the way up and using the knee to position
the work - maybe I should run the spindle down a little bit so I can
relieve cutting pressure in a hurry if I need to...
Thanks Harold. Any time you're in San Francisco and want to stop by
and give me lessons, feel free. ;) Of course, after a couple more
months with this NG, I'll know everything I need to know, right? I
mean, how long could it take?
I've always made it a practice to work against a spindle stop so the spindle
can be retracted, if for no other reason, measuring purposes. If the work
is ultra-critical, I use an indicator on the quill. Stopping a cutter in
a cut is NEVER recommended. The biggest problem is that spindles don't
always just stop, they tend to ever-so-slighlty turn backwards as they quit
moving. That usually kills the cutting edge. You've likely noticed that
cutting tools hate life when they get run in reverse. It takes almost no
effort to destroy that tiny surface that makes the difference between a tool
that is sharp, or cuts, and one that isn't, or doesn't cut well. By the
way, I usually move the cutter away from any contact before raising the
spindle, depending on the nature of the work at hand. It's easy enough to
go back to the same marks, and if you do it in the corner, you don't have
any blending issues. It's worked for me for years. And years.
The typical spindle stop on most drop spindle mills is a PITA to relocate,
so I have one of those snap types that can be moved in an instant. I
highly recommend one. Mine has served me as well as the factory stop
Well, if you can sort the good from the bad, both of which are well
dispensed here, and couple it with a few years of 60 hr/weeks on the
machine, shouldn't take you too long to get up to the speed of those of us
that have done exactly that!
Telling it straight-------any damned fool can make chips---but it takes
years of GOOD experience, not just experience, to do good work, in a timely
fashion. Stick with us, we'll make a fine machinist of you.