Micro Milling 4104HT

Greetings Bob, You really need to make sure to get all the chips removed so that there is no re-cutting of the chips going on. Especially with those tiny cutters. And the chip load needs to be the same for each flute as well as enough of a chip load to insure that the flute cuts instead of pushing away. So your tiny cutters need to run very true. I suggest you contact the cutter manufacturer for chip load requirements. I think you should also run coolant or cutting oil. If I was doing this job I would be using about a 10% water soluble coolant concentration. And I would try to make sure the coolant flushes away the chips. So aiming the coolant nozzle correctly can really help. Eric

I've been doing some simulations and with the smallest cutters I am still looking only 2-7 IPM depending on the cut at full 24K RPM. 7 is only for light step over with chip thinning turned on in the calculator. Maybe its time to invest in some 60K spindles just for this type of work.

I swear every day this gets scarier. I just got a request to cut some graphite molds. I know less about cutting graphite than I do about cutting steel. LOL. I may pass on that one. I have way to many jobs on the board right now already that are outside of my comfort zone.

If you haven't machined it, it's a piece of cake. Do you know what grade it is? Brand?

Most are very abrasive but you can go as fast as your spindle will allow. It machines more easily than magnesium. I machined a lot of thread-cutting electrodes from Poco 3 graphite a few decades ago Easy-peasy. Don't be afraid of it.

Wicked messy though. Graphite dust everywhere. I bought some equipment from a shop that did a lot of graphite machining and it was a dusty mess.

Despite the common understanding about graphite, the synthetic variety, used in the applications we're describing, is very abrasive. It's coarse and gritty, rather than slippery flakes, which is the typical structure of natural graphite.

I've been in shops that specialize in machining graphite EDM electrodes and they replace machines every two or three years. They often buy Fadals for that reason: they're cheap, and nothing else will last longer, unless the machine is built for it.

I marketed Roku-Roku for a couple of years. They were pioneers in building machining centers specifically made for machining graphite. They have a three-part approach to keeping graphite off the ways.

First, they have special way covers. Second, the area under the covers has pressurized air to keep graphite dust from entering. And third, they have a built-in high-volume vacuum system to suck the dust out of the complete machine enclosure.

As I said, I used to turn threading electrodes for an EDM company, and I'm serious about protecting the ways on my lathe. First, I don't machine much graphite. Second, I cover the ways with oil-soaked (WD-40-sprayed) newspaper, taped on tightly. Third, I cover areas with aluminum foil if I can't get newspaper around them. And fourth, I make a vacuusm plenum out of an old plastic milk bottle, attach a vacuum hose to it, and cut a hole in it that allows you to fit it closely around the cutting tool and the workpiece.

If you're going to do much of it, you really have to protect your machine. But machining it is really easy, requiring little force. It will wear HSS pretty quickly, however.

Sounds like for my shop the way to go might be to dedicate a cheap easily rebuildable & easily discardable small CNC router for the application. Put it in a box with light negative pressure (cheap shop vac) and call it a day.

I don't really use any HSS except for hand ground lathe bits, drill bits, and one specialty HSS lathe insert.

I have a little less than a small fortune in carbide mills on hand.

'Sounds reasonable.

Non-specialists use carbide. Specialists use diamond. Most machining of graphite today probably is done in EDM shops, where the volumes are not really large, and I've seen mostly carbide.

I used carbide on my SB 10L, and that machine does not like carbide for turning steel. But the forces are quite low when machining graphite -- if you try to take too big a bite it can crack -- and machine rigidity is not much of an issue, except for the brittleness of the workpiece.

Speeds are, basically, whatever ya' got. Feedrates are moderate, so the tool doesn't climb and crack the work.

If you want some tips, you can call Poco Graphite. They're very helpful and very expert.

When I milled carbon brushes for a Variac I found that I had to always cut into the edge, the material would crack off very easily where the cutting edge exited. OTOH climb milling wasn't a problem.

-jsw

Yeah, it can be a little tricky. A lot has to do with the quality of graphite you're using. Poco 3 is an industry standard for high-quality electrodes. It's more resistant to chipping and cracking than the cheaper stuff.

I should have asked what types of molds these are. Poco has some online articles about graphite molds.

I probably won't get this one with the graphite. I threw a price at them that covered my learning curve, and they said they were going to do some more research.

Yeah, more jobs can get scarier. Especially when new materials are considered. Graphite is one material I myself would avoid because it is so abrasive and dirty. I know this because I have machined it before. Because you are machining with small cutters at high speeds you might be interested in this web site:

. Then again you may already know about it. If you don't then if you visit the site you will discover a good source of knowledge regarding high rpm cutting with tiny tools. I called them once with a question and had a quite informative discussion and so learned some good stuff. Cheers, Eric

Yeah, Ron is a really good guy and pretyt knowledgable about small cutters. Ages ago was using his collets in a Bosch Colt trim router for milling aluminum. I burned a few of those routers. Still have one mounted on my little CNC router.

As far as graphite, as I mentioned in another post, my plan if I had gotten that job was to use my little (almost throw away) Chinese noodle (limp like a noddle) router. If I ate the whole thing up a decent size job would pay for another one. On my short list of plans is to make a little better frame and move what's good on that router over to the new frame. I'd lay the machine out on my surface plate and I'd be able to replace any of the cheap parts even faster and easier than it is now. I already have it in a box. I could slap some doors on it, and put some vacuum in there with a shop vac to keep the graphite out of the shop. I had doors on the box before, and they are laying around somewhere.

Only part that would not be super cheap to replace quickly and easily is the YZ motors. The X can actually be outside of the box when I rebuild it.

Well I just started breaking down stock for this job. I don't know how its going to micro machines, but regular machining should go just fine based on the way it saw cuts.

I spent most of Thursday and Friday setting up roller tables so I could run the large pieces of bar stock into my bandsaw, and roughing out blanks. I liked the way it saw cut.

Today I am truing up blanks on the Hurco mill for further machining. Cut starts sound a little hard on my face mill, but the surface finish on the final pass is really nice.

Higher RPM with the same feed reduced the pounding, and of course improved the finish even on the roughing passes. Flood coolant seems to work about the same as cutting oil, so I went with flood coolant since I don't have to stand there with a brush. The face mill does not have only has TiN coated inserts. They definitely did not work well dry. It cut, but the sound made the hair on the back of my neck stand on end.

I am not looking forward to the sound of the detailed machining of the embossing portions on the smaller mills at "high" speed, but the stuff machines really nicely at lower RPMs on the big mill. My finish pass isn't mirror smooth, but its close enough I know I could make it that way if I needed to.

I like machining both 4140HT and 4340HT. Nice chips and the hot chips smell sweet. You will need to make sure the chip load is high enough to get under the surface hardened slightly by the previous pass. I used to turn lots of both metals and roughing passes using negative rake inserts worked well to break the chip and to keep most of the heat of the metal removal in the chip. When turning to final dimensions where I had to hold a couple tenths and of course get the finish required by the close tolerances I used very sharp positive rake carbide tools. Usually inserts modified by hand grinding with diamond wheels and brass laps charged with diamond. But sometimes brazed carbide tools too, also hand ground. I used high sulfur cutting oil for the final high tolerance, low material removal cuts. I know milling is different because of the interrupted cuts and the way the chip changes thickness during the cut. But really sharp cutters are important as is getting the chips away from the cut so as to avoid re-cutting of the chips. Eric

How much can you get capturing and reselling the stuff?

About zero. Solid blocks of graphite are expensive (energy-intensive) to make. Graphite dust is roughly the same material as carbon fly ash, which they scrape out of the smokestacks of coal-burning electrical generating plants.

Ok, roughing now with a 1/2" 6 Flute and its going as good as I can expect with my "not as rigid as it once was" KMB1. 0.8" DOC, 5% Step over, conservative trochoidal approach. Its barking a little bit, but the wall looks pretty good, and this is the roughing pass. I did the math at 10% full depth stepover, and got to much deflection. Right now its only at about 1HP load, but its removing a lot of material. I think I could feed a lot faster, but at the risk of having to leave room for two finish passes. The flood is really doing the trick. If it wasn't for deflection I would rough faster. The chips are just brown. Not even close to blue.

I am starting to like this material. Not sure what I was afraid of. I may be done in a week. I was planning three. Well I may be done in a week if the pieces I ordered for the largest die arrive soon.

Nah. I don't want to curse it. Three weeks it is. Maybe four. LOL. I still don't know how the fine details of the embossing portion of the dies will machine.