Why is cold rolled steel so difficult to part?

[beanbag]

I have an insert parting tool that works great on aluminum, hot rolled 1018 steel, and even stainless. But cold rolled is a no go. It tends to chatter a lot, not cut, and then go "crunk" and the insert breaks.

[Jaakko Fagerlund]

Too much surface speed or too light feed if it chatters.

[vpt]

I make up lots of these grease adapters now and then. I have used it all from cold rolled, hot rolled, mystery metal for them. I gave up on the parting and just cut them off long in the bandsaw and then clean them up in the lathe after cutting. Cold rolled does seem "grabby" when turning and especially parting.

[DR]

It could also have to do with the steel. The current steels made in this country are primarily re-melt and you never know what's in it.

I've been buying cold roll (1018) for 30 some years and what we get these days is no where near as easy to machine as the old stuff. I had one lot of 5/8" round bars that were litterally impossible to machine, even on the CNC's yet the test reports indicated it met the 1018 spec.

[PixMan]

Never have any problems with it myself. If you're using carbide insert parting tools, you probably aren't running it fast enough. The first cure to chatter with an insert is counter to natural instinct....give it more feed.

[Rustybolt]

cold rolled encompasses a lot of alloys. Which one are you using? Perhaps switch to leaded stock?

[bborr01]

Never have any problems with it myself. If you're using carbide insert parting tools, you probably aren't running it fast enough. The first cure to chatter with an insert is counter to natural instinct....give it more feed.

+1 on that. Cold rolled is more difficult to part than some other materials though.

Brian

[projectnut]

Way back in school they taught us parting is a 1/4 speed "special operation". Meaning that at whatever speed you turn the material it should be parted at 1/4 of that speed with lots of cutting oil.

[lakeside53]

I find it (CR1018 etc) parts easily. Get the speed correct (same sfm as for profiling) and use cutting oil dripped onto the cut/tool continously. Use power feed if possible, and if manual feed aggressively and don't back off.

Are you actually getting 300-600sfm for your carbide? That's 1900 rpm at 500sfm with 1 inch material on the outside!. If not, switch to HSS - the speed and more important to most tentative parters.. the feed will be much lower.

[beanbag]

give it more feed.

This is how I break inserts in cold rolled. Push, hmm it's not going, push harder, crunk.

[Evan]

1018 mild steel is a "low manganese" steel. That means it may have up to almost 1% manganese to toughen and harden it. The spec calls for 0.6 to 0.9% manganese. Any steels with less than 1% other metal is considered a low alloy steel or carbon steel. The manganese is the reason that 1018 is more difficult to machine than many of the harder and hardenable alloys. 1018 is also hardenable to a significant degree and work hardens considerably. The term "low manganese" doesn't mean it is lacking manganese. To the contrary, it means it is alloyed with manganese but less than 1%. The machinablity is a relative matter. You cannot compare the machinability of carbon steels to high alloy steels or other metals. Each is graded for machinability within its own group. 1018 is actually considered to have "good machinability" compared to many other steels.

Hot rolled is annealed, cold rolled is work hardened with inbuilt strains that cause it to move and pinch as the strains are disrupted by machining. Even 1018 can be heat treated and quenched to a significant degree. By using a brine quench it can be hardened to around RC40 or a little more. I have done that many times and it improves surface machinability considerably. Annealed 1018 has a tendency to tear because of the work hardening of the manganese.

For better machinability use 1020 steel instead of 1018. It has slightly higher carbon and only .6% manganese maximum.

[MaxxLagg]

The biggest problem is it's relatively high ductility and the changing Surface feet per minute as you progressively near the center. If your lathe is variable speed try manually feeding it in as you increase the rpm as you near the center.

[lakeside53]

This is how I break inserts in cold rolled. Push, hmm it's not going, push harder, crunk.

In addition to the correct sfm, make sure you tooling is right on center, and not "a bit lower".

What type of lathe are we taling about?

[John Stevenson]

This is just my take on it, I don't have any proof or documentation to back this up, only my own experience.

Hot rolled as the name implies is hot rolled and then allowed to cool, how fast it cools is related to size.

Now a while ago i had to make some not critical specific pins from 2" round steel, quite a few of them. To cut costs I priced these up out of Bright and black [ hot rolled ]. Black was virtually half the price.

Expecting loads of tearing etc I was very surprised to find it machined better than bright, higher speed and more aggressive cut,
Following on from this I also bought some 30mm bar and that was OK, then did an experiment to see ust how far you could go, Got 20mm bar [ 3/4" ] machining OK but 16mm [5/8"] machined like crap.

So I reckon that the larger the bar, the longer it takes to cool and the better it machines, but can't prove this other than by my own trials.

[Jaakko Fagerlund]

Are you actually getting 300-600sfm for your carbide? That's 1900 rpm at 500sfm with 1 inch material on the outside!. If not, switch to HSS - the speed and more important to most tentative parters.. the feed will be much lower.

Seems like you haven't used carbide parting off tool, otherwise you would know it cuts even at 20 m/min. And funny thing is, it still cuts even near the center of the workpiece.