Problems Milling 4140

E

ericc

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I was trying to cut a slot in a 1 and 3/4 4140 round. Actually, I am not sure what it is, but a friend gave it to me at a blacksmith's meet. One end was as-sheared with the green paint on it. The material cut with difficulty on the large band saw at the slowest speed with water soluble cutting fluid. It faced and cleaned up just fine in the lathe, but that was with inserted carbide tooling.

Using a 3/4 inch end mill turning at 400 rpm, there was a lot of difficulty cutting. At 0.020" depth of cut, as soon as the bit was pushed against the metal, it made a horrible squealing sound and the mill axis did not feed. In other words, even though the handled turned, the table did not move. This was in a different Techshop than I usually go to, so this mill was new to me, but it worked great on aluminum, including the end mill that was used. I tried backing off on the depth of cut, but at 0.005" the tool just skated and work hardened the part. I could cut the work hardened area, but with difficulty. This obviously did not cut, and I also tried a 5/8" end mill from the same set, and it had the same results at the same speed.

Looking at the Melin web site chart, it seems that 400 rpm is too fast for HSS at 3/4". Could this have been the problem. Also, the feed per tooth is very small, calling for a feed of only a couple inches per minute. I did not use the power feed and must have been feeding faster than that.

Would this material cut better with a name brand end mill, perhaps a cobalt one instead? I suspect that there was a mill issue as well, since only my sad Craftsman 109 will jump and stall due to the worn leadscrew and nut. Once the feed becomes jumpy, the machine is telling you to slow down, but if it is wound slowly, it does not seem to move, and I am afraid of a sudden big jump with all kinds of unpleasant rapid fire things happening without warning.

Any suggestions? I am thinking of doing a sub critical anneal at 1100 F in case the steel is pre-hardened or worse. Would that help, or maybe a fly-cutter type butt to see if it will cut with a cobalt lathe tool? Thanks!
 
250 rpm, or 50 surface feet per minute on 4140 is not a bad figure, 400 probably smoked the tips of the end mill. Feed per tooth is nothing compared to RPM, .001 to .004 is usually good per tooth, too little isn't good, generates too much heat for the metal removed. Remember, the chips are carrying away heat.
 
T Bredehoft said:
250 rpm, or 50 surface feet per minute on 4140 is not a bad figure, 400 probably smoked the tips of the end mill. Feed per tooth is nothing compared to RPM, .001 to .004 is usually good per tooth, too little isn't good, generates too much heat for the metal removed. Remember, the chips are carrying away heat.
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I always get into the ball park for rpm using SFM*4/Dia, then divide the result by the number of teeth on my cutter. I reduce the speed by a bit for tough materials like steel, and often go up a bit for softer materials like Aluminum. I then try to add a dose of common sense.
In your case 100*4/.75 = 533
So I would start a 2flute cutter at about 250RPM and see how it cuts.
I adjust feed rate until the chip looks right for what I am doing. The chip should pull away the heat, so I try to find that balance between cutting into fresh cool material and not having the chip linger too long in the flute (overheats the mill).

I find trying to do rpm calculations using stuff like chip load to be burdensome and I do not get a solution any better for actually machining than just using the quick and dirty method.
 
As a machine operator one of my tasks was to cut stock for projects. I was taught you can't go by the paint on the ends. Some wholesale companies would paint/repaint them for inventory control. I had the same steel come in from different suppliers with several different paint codes.
 
The green end does not mean much. As Bill C says, it seems each vendor has their own color codes. In my area, most 4140 comes from one vendor and it has purple ends. I think what you have there is a true piece of unknowdium. Unless you have a full metallurgical lab at your disposal, a spark test on the grinder is probably the best way to get in the ball park for grade. https://en.wikipedia.org/wiki/Spark_testing

When starting to machine an unknown piece of steel, I normally give it a quick file test to see how it cuts. Then will start cutting at slower spindle speeds, then bring up the speed until the machine and tool bit is happy. In this case, you already know you have a pretty tough piece of material from cutting it in the band saw, so that would dictate starting out much slower than normal.
 
Sometimes the first couple of inches on a bar can give you havoc trying to cut on. Just depends on how it is treated after rolling to shape at the mill. Most places I've worked at, they would cut the first inch or so off the bar before cutting pieces for an order.
 
Yep, 4140 can be difficult at times. If this is the end of the bar, it cools faster in production and therefore can heat treat itself, so to speak.
 
Hi. Thanks fellas for the suggestions. Now that I look at the online charts, it was too fast both on the feed and the rpm's. I'll back off to 200-250 and slow down the feed. And start with 0.025 depth of cut. I sub-critical annealed the mangled bar end and it is noticeably softer to a file.

The bar is almost certainly 4140. I have standards and bounds and pretty good practical knowledge of spark testing. It is not 1050 or 1060, and definitely not S-7 or A-2. Nothing else is even close. It was machined on the sawed end. The sheared end will be used for forging.
 
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