Help Turning 4140

[HBilly1022]

To date all my limited experience with turning on the lathe has been with junk yard steel, assumed to be mild steel. I decided that it was time to use some more appropriate steel for making the pins on my Bobcat and got some 4140 and 4340. Today I am turning some 4140 and running into some issues I need guidance with.

It is 1.5" diameter that I am turning down to 1.260". I have the lathe set at 300RPM and using a carbide tool, feed rate of 0.007" with a DOC of 0.015". All starts out fine and the steel is peeling off in tight curls but pretty quickly it starts coming off in larger curls and starts wrapping around the work piece, toolpost, cutter and eventually getting into the chuck. I have to keep stopping the cut to clear this out of the way. I am dripping thread cutting lube on the steel, then spreading it with a brush. I've tried grabbing the steel curl and pulling it away from the cutting tip but it is too hot. It's coming off the cut a bright blue color.

What am I doing wrong and how to I correct this?

 

[Ulma Doctor]

perhaps a chip breaker is necessary, like this

 

[T Bredehoft]

The blue color says that (yes carbide can take it) the steel is heated to the point it is no longer stiff and won't break a chip. Try running at 200 rpm. The .007 and .015 are OK, you might try taking .030 depth. You want the chip to be too stiff to curl, so it breaks.

 

[HBilly1022]

The blue color says that (yes carbide can take it) the steel is heated to the point it is no longer stiff and won't break a chip. Try running at 200 rpm. The .007 and .015 are OK, you might try taking .030 depth. You want the chip to be too stiff to curl, so it breaks.

Excellent and thanks. I'll give that try after lunch.

 

[Wreck™Wreck]

Increase the DOC and feed rate to say .050-.075 and .012 respectively, if the chip is to thin it will never break, if the chip becomes to hot it will also never break so use flood coolant or slow the speed way down. A chip breaker insert only works when given sufficient DOC and Feeds.

 

[chips&more]

Please DO NOT grab the chips with your hand(s). That could send you to the hospital! Please use a tool, like pliers. And turn the machine off first…be safe. And for reference, the definitive chip from a carbide cutting operation is a blue colored chip in the shape of a figure 6. If you can get that chip, all is good.

 

[HBilly1022]

Thanks Wreck and chips. I finished the current job before reading these new posts. I sharpened the tool and tried a .030 DOC and had all sorts of problems. One issue was that the work got pushed into the chuck and away from the tailstock. I had the chuck tight but I'm pretty sure it was the way I sharpened the tool. So I fought with it a while longer and eventually got it cutting nicely again but could not get the chips to break unless I went to a very shallow DOC, like .005. I ended up using the higher turning speed (300 RPM) and a DOC of .025". That produced a very nice surface finish but I still got long strands of chips, just not as bad as earlier. Tomorrow I have some more turning to do and will try a faster feed, not sure that my little lathe will handle a .050 DOC though. I'll try a few variations on DOC and feed to see if I can find the sweet spot.

FYI, I don't grab the chips from the lathe anywhere near the work area. I use a small diameter wooden dowel to pull the long chip away from the lathe and then grab it with pliers. I was using my hands to pull the chip when it was about 4 feet from the lathe. Any closer and it was too hot to handle. If the chip got too fouled I shut the lathe down and used pliers to grab the stuff.