Help with turning 4140

[Investigator]

I'm trying to turn some 4140 bar on my 12" logan lathe. The surface finish looks rough, literally. Looking at it up close it looks like the metal is tearing off. The 4140 is supposed to be annealed. It is 2" round bar, I am turning at 250rpm. I am using HSS bits for cutting. I am turning down the OD, as well as making an internal 60* cone. I have tried lighter cuts, and deeper cuts. The RPM I reached by reading the Niagra Tools slide rule they send out.

It turns and cuts, the shavings are very tight spirals, sometimes 2 inches long. I am feeding at the slowest feed rate on the lathe.

What can I do to make it better?

 

[benmychree]

Are you using a cutting oil or other lubricant? using the lowest feed rate may not be the best thing. I use Tap Magic as a cutting oil on nearly everything so far as steel is concerned, it can make a huge difference. Make sure your tools are on center, and too slow of a cutting speed is not good. 4140 ann is not difficult to cut compared to mild steel.

 

[Alan H.]

Make sure your bit is ground properly. Try bumping the speed up to 400 rpm and as already said use a mist coolant or oil.

Can you post a photo of the HSS bit showing its grind?

The 4140 should have a good finish. I like turning it a lot.

 

[dlane]

Higher Rpms ,radius cutter, smaller= faster speed, where was steel made , pics?

 

[Technical Ted]

You stated is was supposed to be annealed... What color are your chips? This will help you determine how hard the 4140 actually is... If running HSS it's probably best if they come off a straw yellow or silver rather than blue/purple. I have run my best, high quality HSS lathe tool bits (usually with Cobalt added) at higher speeds and gotten away with it, but if you're using a low/normal quality HSS then you might be loosing the cutting edge if running too fast. If using an import HSS, you're most likely better off running even slower yet.

Also, when grinding your cutting tool, the harder the material the less top rake you want. Another thing is that, typically, the cutting speeds most charts list are suggested maximum speeds. I always use 60-80% of those speeds and adjust from there. I'm not running production and I like my tools to last a little longer. Sometimes, running slower helps to break the chips because they have time to cool off and become more brittle. Of course, sometimes you need to run faster to break them. Go figure... I hate long stringy chips. As others have suggested, cutting oil or coolant will help as well.

YMMV,
Ted

 

[Investigator]

Thanks for the replies. I am using a blank I ground myself, I got the blank when I bought the lathe. It is marked Fagersta WKE 4 Sweden. I am using non-detergent 30W oil as it turns.

 

[Investigator]

The chips come off in tight curls and are uniformly dull silver in color

 

[cg285]

using the lowest feed rate may not be the best thing. Make sure your tools are on center, and too slow of a cutting speed is not good. 4140 ann is not difficult to cut compared to mild steel.

+1, + more doc if possible

 

[Investigator]

DOC was usually .030" to .050"
What would be the recommendation for DOC and feed rate?

 

[pdentrem]

Your depth of cut is fine. RPM per the charts for 2” is between 136 - 382. Everything else will depend on the tool geometry. A HSS tool has to be sharp. I ground mine with at least 10 degrees of side angle and a chip breaker, fairly square to the work and a small nose radius. As I was only roughing the shafts prior to finish grinding to size, surface finish was not taken into consideration. A finish pass would of been required to make the surface smoother.