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 that. 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.
The thin cut might cause some work hardening, you'd want to keep the feedrate 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, you HAVE 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 materials.
I have done jobs with blue chips, most notably fly cutter work on large parts. 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, and they 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 tougher materials.
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 (Bridgeport).
Chuckle!
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.
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