Advice for machining 4140 needed

Jump right in. It is beautiful stuff to machine. When turned it looks almost like chrome.

Ron Thompson On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA

The ultimate result of shielding men from the effects of folly is to fill the world with fools.

--Herbert Spencer, English Philosopher (1820-1903)

I agree. Very nice finish from an indexable facemill. I think 4140 tends to require more power though. You'll probably notice your machine straining a bit compared to a mild steel.. Not a big deal.

As for cutting speeds, I'd start with around 60SFPM (HSS of course) for turning. Less for drilling and milling, although it really depends on the setup, tool and machine. Coolant is of course a great idea.

Regards,

Robin

Feeds and speeds somewhat lower than for softer steels, maybe some cutting oil, keep your tool sharp, no problem. Breaking the sharp point of the tool maybe .020" will avoid a lot of frustration, sharp points don't last long. Milling speeds somewhat lower than what "looks about right", err to the low side, don't be in a rush with it. Rush means "burn tools". I'd rather work 4140 than most of the plain CRS. Much easier to get a good finish than 1018, and doesn't seem to want to hog if things aren't just right. Grinds and polishes beautifully, even better if hardened.

See other posts for the machining side. Consider heat treating the resulting parts. 4140 will heat treat up to the 130kpsi to 150kpsi range without losing too much ductility. You can push that number way higher but it gets fairly brittle in those ranges.

We use it for off road vehicle axles, run it up to about 140kpsi.

charles wrote:

Good advice. Keep rpm down, but don't approach the cut fearfully. Make a chip, bad news if you don't. Cutting oil is a good thing, and major differences exist among different oils. When you use the right stuff, you can hear and feel the machine being happier. MolyDee is one of the better ones. 1018 sucks IMO. A nicely finished part from 4140 can be dazzling.

michael

Thanks all for the advice. I'm going to order some inserts based from you recommendations from MSC on Monday. The shape we have to machine is thankfully simple, however the tolerances are critical. The mounting holes needs to be lapped to + or - .0001" needless to say I don't plan on getting it right on the first shot.

This makes it sound like 4140 is as easy to machine as aluminum.

Anything is easy, once you know how.

Leonardo of Lurk

Lennie the Lurker

I remember the first time I turned 4140. It was a bit interesting. I have since turned several model engine crankshafts from 4140 and I don't get the mirror finish these guys are talking about. I can get a decent finish with a light finish cut and a very sharp cobalt HSS toolbit. Polished with 600 sandpaper and crocus cloth to get very shinny. Heavy cuts leave a sandpaper finish.

I got my 4140 from metal express. Maybe it is annealed and I need to start buying the pre hardened stuff. However I found this stock to be very tough and I cringe at the thought of trying something harder!

chuck

Try an indexable tool. If you're not going so fast and hard that it is perhaps "scary" then you're not going fast enough...

Regards,

Robin

Yes; stiff lathe, decent power, large radii on carbide, 400-600 sfm, +.075" doc, .005-.0075 feed, you want to see a light blue chip. That will get you started. YMMV

Scott

[ ... some attributions missing here ... ]

You don't say what lathe you are using. I think that you want a beefy lathe to do the 4140, and yours may just be too light.

Also -- good insert tooling will help greatly. Ideally, the uncoated (no golden TiN coating) with as sharp an edge as possible.

Good Luck, DoN.

- Nothing sharp tooling or good feed/speed/lube can't fix though, right? Been dreaming of turning 9" of that torsion bar into a spindle...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Hmm ... below a certain rigidity, you can't get the speed high enough or the feed heavy enough to get the best finish, so you may be stuck doing as the other did -- rough finish to a few thousandths over, then a very fine cut, followed by abrasive polishing.

I was already suggesting sharp (carbide) tooling for the finish cuts -- that without the TiN coatings which tends to round the edges of the insert and reduce sharpness. My best sharp tooling is a diamond shaped tooling for my Compact-5/CNC. Nothing of a size to fit my

12x24" Clausing is really sharp.

Normal HSS would probably dull too quickly on even annealed

4140, let alone already hardened. A good cobalt steel might work, and I suspect that the magic (and nearly unobtainable) Stellite would do a superb job.

Once you have your machine made and in service, you will be in a position to discover the limitations produced by insufficient rigidity, so make a separate set of patterns to the shrink dimensions for cast iron for the second pass at things. :-)

Enjoy, DoN.

There isn't a whole lot of difference in the HT stuff, just slow down a little more. I don't use carbides, just HSS, maybe cobalt if I've got it, but I try to keep the tool sharp. Found that the final cut should be no less than .010 per side for good finish, and light feed with maybe .030" radius on the tip helps. If I _need_ a high finish, I'll grind, may add another operation, but it's normally faster than polishing from the turned finish. After grinding, one can start with

320 grit and end up with 1200 or finer, but there are limits as to what is practical and what is anal. You might want to try 41L40, machines a little better, otherwise not a lot of difference.

Some time ago I picked up a bar end at the scrapyard, faded marking I thought was "4140HT" turned out to be 4340HT. A real education in slowing down followed.

(Dumpster diving at Metal Express can pay off, when you don't get caught.)

($84 for a 34" piece of 2 1/2" square chunk of 1018. Ouch!!)

At the time I was using an atlas 12x36; which is not a particularly rigid lathe, BUT I think the part is more limiting. The part was a model engine crankshaft that is turned to 1/2 inch diameter and about 10 inchs long. No way can you take heavy cuts on that and hope to achieve good results.

I now have a much better rockwell 11x24, which would give me lots more setup options, but I don't need to make a crank anytime soon.

chuck

Hmm... I see...

Heh, I found a grooving tool floating around in one of Dad's old tool boxes... seems kinda sharp (we have a diamond stone anyway so that's not an issue).

Ouch, wouldn't think it would have much carbide or something in it even when annealed though... Hmm... to the Google-mobile!

the bottom shows layered pearlite. Relatively hard I suppose.

Or it's just that the stuff is hard, and so much wear at the cutting point just doesn't work for long...

Or just plain ol' carbide. But where to get it without leaving town... (Heck, where to get HSS bits without leaving town. Humph.)

Heheh. Already I think I'm going to load this up with either cement or lead, in the bed's cells at least. Some weight in the head and tailstock would be nice but those won't be as easy to mount.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @