Turning 4130 - Now 4340

[Kodin]

Quick question for those with lathes, and have successfully turned 4130 or similar materials...

I'm having a hard time getting decent finishes on this stuff with my Grizzly 13x40 "gunsmith" lathe. I'm using carbide insert tooling, and I've tried a few different tool geometries and chip breaker designs. I just can't seem to get a mirror finish no matter what I try. It all just turns out hazy. I've tried all the speeds between 70 and 1200 RPM for a 1.5" piece, (Haven't tried 2000 because it didn't seem to help,) and my best results were in the mid-to-higher speed range with DNMG 431 inserts with a "light finishing" rated chip-breaker. Also tried trigon inserts with a medium chip breaker, I think 332 size or something simliar. Any suggestions? Anyone else worked with this stuff and gotten good results? I think this stuff is "normalized" however not sure if it's annealed or hardened as well. I'll post some picture results later this week if anyone responds. :?

Oh, and depth-of-cut tried was kind of on the deeper side, I think deepest I bothered with was 0.200". Should I try a custom grind tool-steel or cemented carbide instead? How does this stuff single-point thread?

Thanks in advance!

 

[Farfle]

4140 likes the chips to come off screaming hot and smoking. Try going to ludicrous speed and medium feed with a slightly raduised tip carbide bit and it should behave better. The trick is that its very "sticky" and develops a "tooth" on the cutting face that builds up and breaks off every couple revolutions. The only way around it is to get the carbide hot enough to actively burn the tooth off, or sometimes you can use a moly based high pressure lubricant that may or may not be able to keep that tooth from sticking.

It does not like to single point thread well, the threads appear "torn" and ugly.

What you want is some 4340. That stuff is similar or better strength (depending on temper), and only sold in bar stock, but it machines beautifully even though its machineablilty index is lower. 4340 is harder and produces more heat when cut which achieves the "tooth burning" effect at more reasonable feeds and speeds.

 

[Kodin]

Yeah, from what I've read some of these machine better when hardened versus annealed. If I were to make a replacement motor shaft, would I be better off with or without heat-treatment? I'd prefer to be lazy and use the stuff annealed whatever I end up using, but if I have to HT it, so be it.

 

[Farfle]

Its up to you. All the chromoly alloys are really stout. If you had shaft breakage problems in the past, then bringing it to temper wouldnt be a bad thing.

 

[Kodin]

So looking closer, sounds like I need ~0.010 feed, and with a 1.5" piece, dial in as close to 533 RPM as possible, 0.100 depth of cut. I'm reading 625-800 SFM for optimal with modern insert designs. Go figure. I was running WAY slower feed rates.

 

[Kodin]

Quick update so far:

Moved from 4130 to 4340, first off... second:

Ordered some new DNGG carbide inserts to try varying geometries and chip-breaker designs. The Kyocera "light finishing" inserts look pretty promising so far...





You can still see the tearing pattern in the steel, but it's WAY better than most of the cuts I've done with other bits.

Having a nose-radius of 0.2 means they are going to have a very light cutting force, but won't really smooth at all; that said, I can operate at a feed speed/depth most would consider ridiculously inappropriate for 4340. I can get a finish like this with a ~1mm per rev lateral feed and ~0.005mm cutting depth, so I think I should be fine to use these for final dimension cuts. The only scary part is they don't start cutting well until I top out my lathe at 2000 RPM.

Interestingly, this is ~30.8mm 4340, at maybe a foot long, and the center still flexes out of the way of the bit enough to be larger by ~0.0030" or so. (My larger micrometers are only inch measurements).
Edit:
Never mind, I derped and realized the tailstock was still misaligned. Just spent at least an hour bringing it "close" to true, as well as finding a lateral misalignment of my gap bed. Gap bed is still not perfect, but it isn't enough to offset the carriage anymore like it was; I couldn't figure out why I was getting a repeated notch in my cuts at 3 inches. Tailstock is now aligned enough that I'm cutting within ~0.02mm end-to-end on a foot-long piece. It's close, but not "true". I'd say that's probably enough for now, may continue to tweak it tomorrow if I feel like wasting more time.

Going forward I'd like to see what I can do with 0.5 or 1.0 nose radius inserts, but for now I don't have spare cash I can throw at more tooling. There's absolutely no chip breaker on these, so you end up with huuuuge hairballs of chromoly building up no matter what you do, and near the left side of the shaft they can get caught in the lathe dog if you don't stop part way and clear them with long pliers in a safe manner. (Wear a hat, safety glasses, and a full face shield when you run any lathe at more than a few hundred RPM; that shit flies fast and whips you hard).

I have a couple other bits on their way as well, all of which have chip breakers and different geometries, but this is the first I've tried, and so far they seem to do a pretty nice job. Definitely glad these were only available in a 10-pack, as I'll probably end up using these somewhat frequently.

 

[Brake]

Thanks for posting your findings. I have that same lathe and like it a lot for a garage lathe.

 

[Kodin]

Brake, which model do you have? Mine's the Grizzly 13x40 "Gunsmith" lathe with foot brake.

 

[Chalo]

What are you making such that you need 4340 steel? That's not a nice material to work with. Trying to cut a magnetic sample ring out of an irregularly shaped piece of forged, hardened 4340 is one of the more memorable traumatic events of my machining career.

Regarding your changes in diameter between supports, you have to remember that all steel has the same stiffness, but hard tough steel like 4340 takes a lot of force to cut. It will deflect away from the tool much more than a free-machining steel or a mild steel.

 

[Kodin]

Interestingly, I don't seem to see any measureable difference there at this thickness. And the cutting bits I'm using are sharp enough that they seem to do the job _REALLY_ well. This lathe is capable of cutting mild steel in 1/4" depth-of-cut, and I've seen guys with custom tool grinds demonstrate 3/4 inch cuts once or twice for kicks. It's not rigidity or power I'm lacking, I simply don't have the lathe tuned yet. What I thought was flex was actually a completely mis-adjusted tailstock and gap bed from the factory. I never used the lathe for anything this precise, so I never noticed things were out-of-whack. The gap bed is still slightly off and will likely require scraping of paint and possibly shimming to get it exactly parallel in all degrees of freedom to the rest of the bed. Doesn't seem to be too much of a problem after my initial adjustment, but if you run a fingernail across the inverted-V surfaces, you can feel the difference in heights. Tailstock I managed to partially strip the head out of one of the grub screws, so I'll have to order replacements before I adjust it any truer. Also thinking about making a finger-gauge holder that I can mount on the QCTP. Regarding the tailstock offset, 0.02mm across a foot isn't a big deal considering the longest length of continuous diameter is going to be about 35mm long. The bearing seats will be done by draw-file or abrasive to final tolerance unless I get lucky on the last cut. Lastly, I'm switching from a piston-type tool post to a wedge-type, which was like night-and-day when I changed out the piston AXA on my 7x14. Hoping the same is true for the BXA on the Grizzly.

 

[Brake]

Sorry for the late reply. Mine is a G4003. I actually bought it from a gunsmith.

 

[Kodin]

So I got the wedge-type tool post, and found a problem; the Grizzly mounting bolt has a neck at the base that aligns the Grizzly toolpost, however it won't work on my wedge-style non-Grizzly model.

So I had to turn/mill a new T-nut for the compound out of the (supplied with the post) gigantic hardened steel block on the end of the mounting post:

Really digging the ER40/MT5 collet chuck other than that I had to bore it a bit larger diameter near the front so my post would mount well. My other chucks gets 10x the runout of the collets.

(Yes, it's actually reflective on all surfaces except the top. (I don't have a right-handed DNMG cutter holder yet, and my only other right-handed tool was really not at all the right geometry for the material I was cutting; Trigon inserts with huge radius chatter like hell on my lathe). I got a really nice finish on the flats with a 3-tooth face mill in the milling machine as well. (Reflection of my thumb evident on that cut)

...Aaaand done. Cleanest turning I've done like... ever.

New tool post is mounted, and the old post is still usable if I ever need to for any reason.

Now I'm just waiting on a right-handed DNMG holder to arrive...