Advice on drilling 4140

[deepcreekdale]

Was working on a TRS lock plate casting this weekend and ran into some issues I would like some advice from you machinists types. The material is 4140, I was drilling with a # 10 bit at 2340 rpm, lots of cutting oil. The drill bit was a standard black oxide from McMasters/Carr. Bottom line, about 1/2 through the hole, it stopped cutting, the drill got real hot and the metal I was drilling got hard as could be. I was able to salvage the hole by coming in from the other side with a smaller bit and reaming. My question, was I using too high or low a speed? My drill speed chart says to use 3000 RPM for steel with a drill less than 1/4 inch. I was able to drill several other holes in the plate, mostly #29 with no problem, usually the plate seems easily machinable with no issues. I have had this happen a few times before, I wonder if I am doing something wrong or missing something. Or is this a characteristic of 4140 steel, it almost seemed to air harden.

[chubby]

Hi Dale, I feel you were drilling to fast, 4140 has nickle, slower speed and less angle on your drill point! hard metals slow,and less angle. softer metal brass, aluminum,and  such more angle.   Chubby

[Buffaload]

Me too. Cut your rpm in half.
Ed

[Scota4570]

Hi Dale, I feel you were drilling to fast, 4140 has nickle, slower speed and less angle on your drill point! hard metals slow,and less angle. softer metal brass, aluminum,and  such more angle.   Chubby

Soft metals, especially brass, bring the cutting edge of the flutes back to 90* to the work.  IF you don't it will grab the work.  IF you are holding on by hand it will probably hurt you.  I have never found much difference on the point angle. 

[FDR]

Try this.
https://www.yardstore.com/drilling-and-cutting/lubricants-and-cut-fluid/boelube

Best stuff since sliced bread for drilling steel.

Fred

[JPK]

Another factor can be too light of feed, that can work harden the steel as you experienced. Charts generally favor the max speeds and feeds for production work. One off jobs like you’re doing require a more modest approach.

[kudu]

Way too much rpm if your in a vise and on a mill or drill press 800 rpm would be about right drill a little back out OIL repeat.

[okieboy]

 There is no very simple answer to your question, but here is some "stuff". Machinists figure speed in Surface Feet per Minute, SPM. To get SPM, you take the circumference of the tool in inches, multiply by RPM and then divide by 12 to convert inches to feet. BUT, it is usually done the other way around, you know the recommended SPM for the particular material that you are cutting and solve for RPM.
 "The particular material", saying that a speed is recommended for "steel" doesn't really work, because there are hundreds of steels with different SFM starting points, and SFM recommendations are starting points.
For instance the SFM for 1018 (often referred to as mild steel) is about 100 SFM, but 4140 might be closer to 50 SFM. This is with high speed steel tools, with carbide the speeds would be higher.
 In metal cutting higher speed translates to higher heat and higher heat shortens tool life. With high speed steel tools it is generally better to use less speed rather than more speed. With carbide tools slower speed can be a problem because they are more brittle and can therefor be prone to chipping.
 The other problem is that 4140 (as well as some other metals, don't get me started about Hastelloy!) can be prone to something called "work hardening". When metals like this are plastic deformed they become harder. What this means in a cutting operation such as drilling is that when you start cutting you want to keep cutting. Letting off of you feed pressure such that the drill revolves with cutting (dwells) will work hard the metal ahead of the drill. That is likely what caused your problem.
 So, look up the recommended SPM for 4140, use less speed than the recommendation, and when you get a chip going, keep the chip going. 
 I hope this is some help to you.

[45-110]

the split point "tin" coated (gold) colored drill bits are way better than the black oxide variety. question....how do you know the material is 4140? my experience has been parts of this type are generally 8620 alloy? sometimes you just hit hard spots in parts and its slow going, sharp drill, lube, and slow the rpm's down. even if you get through the plate the hardness will be rough on the tap.

[G_T]

Agree way too much speed!

You didn't say if your bit started out sharp. There is a huge difference in how the bit will feed between sharp and dull. Dull generates more heat than cutting, and doesn't cut nearly as clean a hole. If you need much pressure on the bit, that's a good sign it is too dull for the job. Sharpen or replace.

Gerald

[deepcreekdale]

Thanks everybody, lots of good advice here. I will definitely slow the RPMs down going forward. 45-110, TRS says they use 4140 for their castings for all the parts except their springs and frizzens which they say are 6150. I am not a metallurgist or machinist, so I learn the hard way most of the time.

[JCKelly]

All good advice. You MUST go slower, much slower. Heavy feed (deeper cut) means less work hardening. HSS (High Speed Steel) bits are just fine.

I am a metallurgist, not a machinist. Job for decades required me to provide machining information regarding high nickel alloys - and you thought 4140 was a pain - usually slower speeds did the trick. Much, much slower speed than people who machine steel are used to.

Worked a while in Stainless Research, heard the salesman ask our machining guy - "Say something in machining, Al".

"Feeds and Speeds" That was it.

[John Archer]

Slow speed like everyone says.....water for lube. An old mechanic’s trick.

John.

[Clint]

Cast steel parts are notorious for hard spots. Some of it may be from odd crystallization on cooling and a lot of the hardness is from "chilling". That is when small parts lose heat fast enough, in the mold, to  locally harden harden. Frizzen toes are often villains. All new castings that come to my shop are put into a piece of steel tubing and brought up to red heat in a small propane forge. The forge is turned off and allowed to cool completely. Putting a lock together from castings is not for the faint of heart and it is well advised to be careful in preparation. I use a second heat treat before filing and drilling by heating all of the pieces in an electric kiln to 900 degrees F. and cooling slow. Now I can drill file straighten and thread with a large degree of comfort. All of the advise about drilling upwind of my comments will help. I also always brand new drills and taps for each lock. Have Fun!

[Clark Badgett]

Cut 4140 quite frequently. It cuts like a dream, love turning it and milling, but not so crazy about drilling it. Like said above drop your RPM to around 1000, keep water or coolant on it and just keep going and if you peck drill, get that drill completely out of the metal.

[flehto]

For a #10 drill {.194}, a spindle speed of 700-800 RPM is good. ...and should be used on all steels. As a toolmaker building stamping dies, many were made from D-2 air hardening tool steel which has a chromium content of 10% and will dull a tap or drill in short order if the RPM is too high.

Had a neighbor who one day was trying to drill some holes in his car bumper for mounting a trailer hitch. He and his wife were both pushing the hole shooter to no avail. I finally walked  over, asked if I could  see the drill  and as suspected, the heel of the drill point was preventing the cutting edge form contacting. Resharpened the drill and they were both amazed at how easy the holes went in.

Too high an RPM for larger drills and too low of an RPM for smaller drills hinder the drilling of steel or worse in the case of smaller drills being broken. .....Fred

[Bob Roller]

Cast steel parts are notorious for hard spots. Some of it may be from odd crystallization on cooling and a lot of the hardness is from "chilling". That is when small parts lose heat fast enough, in the mold, to  locally harden harden. Frizzen toes are often villains. All new castings that come to my shop are put into a piece of steel tubing and brought up to red heat in a small propane forge. The forge is turned off and allowed to cool completely. Putting a lock together from castings is not for the faint of heart and it is well advised to be careful in preparation. I use a second heat treat before filing and drilling by heating all of the pieces in an electric kiln to 900 degrees F. and cooling slow. Now I can drill file straighten and thread with a large degree of comfort. All of the advise about drilling upwind of my comments will help. I also always brand new drills and taps for each lock. Have Fun!

"Putting a lock together from castings is not for the faint of heart".I have never tried to on locks that use undersize antique
parts like tumblers and sears is a bad idea and don't recall trying any of the others and make my own bits and pieces for
better or worse. Under size parts copied from a relic are not for anyone that shoots a lot and area nuisance to try to repair.
Cocks or hammers,plates and frizzens are OK usually but even they can present problems.I well remember cast lock plates that
shattered like glass and couldn't be drilled even if straightening worked.Try to file cut a square hole in a hammer in some of those
older castings.Speed and feed is the combination but they can be elusive.

Bob Roller

[jerrywh]

You need to read up on drill speeds and drilling different materials. The speed you were using is about right for wood. If nothing else remember this as a rule.Wwhen drilling tough steel you can't go too slow.  Going too fast is auto failure.

[hudson]

When consulting charts for recommended surface feet per minute (drill rpm) those numbers are with the correct cutting fluid.

[deepcreekdale]

Just want to thank everybody for the good advice. I was doing some drilling this morning, went as slow as I could, lots of cutting oil and it went perfectly, cut like warm butter. The chart I was using was the one that came with the Delta  drill press I was using, and I have a feeling when they were talking about "steel" they meant mild steel that is fairly thin. I will know now to be calculating my SPM now, thanks  okieboy and you were right also about the metal hardening ahead of the bit when it slowed. By the way , the lock plate I am working on is for a Patiilla or miquelet lock from Rifle Shoppe. It is a bit thicker than the ones you would find on English/German locks. As an aside, once you play with one of those locks and see how they work, you won't want to fool with English/German locks any more although their looks take some getting used to.