Drilling 4140

Airpirate

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I am trying to drill through 5 inches of 4140 steel. I am doing so on my Atlas horizontal mill at the lowest RPM.
I started a pilot hole of 9/64 using a cobalt drill. after going to the max length I changed over to a long HSS 9/64 bit.
I fed slowly and cleaned chips out regularly and then snap the bit broke off deep inside the project. Good thing it was just a practice piece.
larger bits feed very well as long as there is a pilot, trouble seems to be getting a pilot all the way through.
I ordered a long cobalt bit and hope to be successful with that but could use any suggestions.
thanks
 
Even at a slow RPM a drill going that deep need clearance and needs to cool. You got to the end of the twist, then it broke, right? If so you lost all clearance and the drill got hot and caught, then broke. It happens in a split second. I never run a drill to it's max depth.

"Billy G"
 
I was using an 8 inch long drill with 5 inches fluted. wasn't into the stock very far when it snapped close to the tip.
i suspect waste was piling up and bound the drill. I was going slow, feeding slow using cutting oil.
 
Perhaps you may have been running too slow and tended to feed too fast, 4140 is not all that hard, even in the commercial heat treated condition if it is annealed, not much speed reduction compared to CRS 1018 is required; one thing you could do is to drill the pilot hole as deep as is appropriate, then drill a larger hole to a depth less than the pilot hole to allow chips to work out and coolant/ cutting oil to work in; also it is good practice to withdraw the drills frequently to clear the chips out and re oil the drill bit If the drill is making any noise, it is definitely time to pull it out. Depending how large the finished hole is, I would grind a drill with a split point and not use a pilot hole, only a center or spotting drill to start the hole.
 
In deep holes, normally you have to peck drill. Drill about one drill diameter deep then pull all the way out and clear the chips, rinse, repeat until you get through. I don't remember how slow an Atlas mill will turn, but I would turn that that size bit at about 200 RPM in 4140.
 
Try drilling the diameters in stages. Let's say you got 2"deep before the pilot broke. Switch to the next larger drill (or final size) and drill a bit less
than 2" deep. Then switch back to the pilot and drill a bit deeper. By switching to the larger drill, you provide clearance for chips and reduce heat
and thus binding on the pilot. If you drill less than 2" with the larger drill, the pilot will follow it's own hole on the second pass.
Repeat as needed.
 
I agree with drilling in stages. Your pilot drill is producing chips the width of the cutting edge, the flutes aren't large enough to allow them to freely come out of the hole. The next bit is only cutting at the ends of the cutting edge so the chips are smaller and can follow the flute out.
I'd be running the 9/64 bit at 1200 rpm minimum, and hawg it into the work.

Greg
 
One of the things I learned many years ago. We used to drill 7/16" and 1/2" diameter holes 13-1/8" deep and a occasional 15" deep hole. We would run the part at about 300 RPM, Oh, we were cutting 4140-45 high heat treat material. Start with a normal length twist drill with average penetration rate. Change to a drill with flute length of about 6", then changed to the final length drill that had about 14" of flute length. Lots of good cutting oil mixed with tapping fluid like TapMagic. We would peck about 1/4" at a time, you knew when to retract and clean, oil, go back in. The drill would jump when the flutes were starting to load up, that was our clue when to pull out, which was about 1/4". We would keep the bed ways oiled for the tailstock the make it easier to back out. On the drill bits your using, be sure select one's with a fast helix, helps draw the chips back from the cut. Long drills with long helix will almost bind up in drilling. So pay attention to this. If your buying from Rex Supply there in Pharr, they should be able to get you what you need. I used to deal with them. You have to find it in their catalog, point it out to them and make sure they order what you want, not some of the junk they try to sell. Stick with a good name brand drill bit, too. Might cost you a little more, but well worth it in the long run.
 
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