# Drilling 4140



## Airpirate (Jan 20, 2018)

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


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## Bill Gruby (Jan 20, 2018)

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"


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## Airpirate (Jan 20, 2018)

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.


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## benmychree (Jan 20, 2018)

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.


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## JimDawson (Jan 20, 2018)

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.


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## Nogoingback (Jan 20, 2018)

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.


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## f350ca (Jan 20, 2018)

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


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## magicniner (Jan 20, 2018)

Have a look at this - 

https://www.cnccookbook.com/cnc-che...g+Techniques&utm_campaign=Newsletter+12042017

You are "Deep Drilling" and beyond a point peck drilling with almost full retraction to clear chips regularly is required and that point is earlier than you might expect. 
Have you checked feeds & speeds for your drill size? If your revs per minute are too low you will get grab .


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## 4GSR (Jan 20, 2018)

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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## Dan_S (Jan 20, 2018)

maybe i missed something, but 5" seems pretty deep for a 9/64" drill, that's 35 times the diameter.


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## EmilioG (Jan 21, 2018)

Straight flute gun drill.  I also have some long pilot tip Guhring cobalt drills. If you need one AirPirate, PM me. No charge.


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## epanzella (Jan 21, 2018)

When I'm drilling deep I'll only go about an inch with the pilot hole, then go 3/4 inch with the final size drill or at least a bigger bit. Then return to the pilot for an inch, then the bigger drill for an inch. The bigger hole provides chip clearance and also makes the relative depth of the pilot hole shallower. Bury a 9/64 bit 2 inches deep into a 9/64 hole and you're rolling the dice.


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## kd4gij (Jan 21, 2018)

What is the final size hole you need"


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## cjtoombs (Jan 21, 2018)

Can you drill from each end?  That would half the distance you need to drill the pilot hole.


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## magicniner (Jan 21, 2018)

cjtoombs said:


> Can you drill from each end?  That would half the distance you need to drill the pilot hole.



Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?


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## Wreck™Wreck (Jan 21, 2018)

magicniner said:


> Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?


I am not a hobbyist so what ever advice I might give is surely wrong.
However this is a tooling problem, either the machine itself or the tools chosen for the work. I would never expect a twist drill to go 5" deep and emerge on the other side in the center, this is asking a lot of such a roughing tool.


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## MarkM (Jan 21, 2018)

I wouldn t slow down the rpm s all that much and not use too big of a pilot hole and do it in stages.  Think of what happens when too large of a centre drill is used.  It bites in rather than shears on the cut and you end up with the drill flexing and will break on the rebound.   Also a hole that deep the material may heat up and expand just enough to bind up.  Have a couple peaces of metal as a heat sinks to help keep it cool.  One in cold water and switch them up.


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## magicniner (Jan 21, 2018)

Wreck™Wreck said:


> I am not a hobbyist so what ever advice I might give is surely wrong.
> However this is a tooling problem, either the machine itself or the tools chosen for the work. I would never expect a twist drill to go 5" deep and emerge on the other side in the center, this is asking a lot of such a roughing tool.



I make a Stainless Steel part with a 14mm bore and an OD which tapers from 16mm down to 14.4mm over 5 inches, I use an ordinary drill bit and a finish reamer, I do not use a pilot as I don't find that it helps when drilling deep holes. I do not scrap any parts ;-)


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## cjtoombs (Jan 21, 2018)

magicniner said:


> Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?



Nope.  They will miss by a small amount, but then again, as stated above, you're not going to drill 5" deep and get a straight hole with a twist drill anyway.  They should be close enough for garden variety work.  If you want a straight hole that deep, you need a gun drill and the ancilary items to operate it.


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## kd4gij (Jan 21, 2018)

I make pins for carinas from 4140 and drill a letter Q .332 drill up to 8" deep. Never drill a pilot hole.


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## Wreck™Wreck (Jan 21, 2018)

magicniner said:


> I make a Stainless Steel part with a 14mm bore and an OD which tapers from 16mm down to 14.4mm over 5 inches, I use an ordinary drill bit and a finish reamer, I do not use a pilot as I don't find that it helps when drilling deep holes. I do not scrap any parts ;-)


Never said that it can not be done merely that it rarely ends well, no pun intended, with twist drills.


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## Airpirate (Feb 15, 2018)

benmychree said:


> 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.


I may be drilling too slow. I checked a drill speed table and mistook SFM for RPM. A table I am referencing suggests 40-85 SFM for drilling.
I found a formula that specifies, (3.8197/drill size)X SFM = RPM, so for a 9/64 and 40 SFM I come out with 1046 RPM. This seems high can someone who has experience weigh in please? Pardon my ignorance.


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## Dan_S (Feb 15, 2018)

Airpirate said:


> I may be drilling too slow. I checked a drill speed table and mistook SFM for RPM. A table I am referencing suggests 40-85 SFM for drilling.
> I found a formula that specifies, (3.8197/drill size)X SFM = RPM, so for a 9/64 and 40 SFM I come out with 1046 RPM. This seems high can someone who has experience weigh in please? Pardon my ignorance.



I'd want to see the exact chart, before making a definitive statement.

However, a lot of the formulas listed in old texts, are for industrial work where they are trying to maintain the maximum material remove rate to tool wear ratio. They assume you have more than enough power and rigidity for the task at hand.


For a 9/64" jobber bit, I'd run it in the 600 to 700 rpm range personally.


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## cg285 (Feb 15, 2018)

kd4gij said:


> What is the final size hole you need"



+1


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## kd4gij (Feb 15, 2018)

I would drill a 1/4" or 5/16" pilot hole clearing chips often. then 1"


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## Airpirate (Feb 16, 2018)

Dan_S said:


> I'd want to see the exact chart, before making a definitive statement.
> 
> However, a lot of the formulas listed in old texts, are for industrial work where they are trying to maintain the maximum material remove rate to tool wear ratio. They assume you have more than enough power and rigidity for the task at hand.
> 
> ...


The chart I am using is found at www.hannibalcarbide.com/documents/reaming-speeds, this isntt the entire URL but it is current and the formula was found on several drill manufacturer's web pages.
I really appreciate your feedback and will drill at the speed range you state as it makes sense.
Thank you.


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## gasengin (Feb 16, 2018)

When the speed is too slow, it is really easy to break small drill bits.  Slow is not better.  I personally had a bad teacher on this when I was young (not a machinist).  He thought slower was better.  I broke lots of small bits until I learned about surface feet and how it relates to rpm.


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## Airpirate (Feb 17, 2018)

gasengin said:


> When the speed is too slow, it is really easy to break small drill bits.  Slow is not better.  I personally had a bad teacher on this when I was young (not a machinist).  He thought slower was better.  I broke lots of small bits until I learned about surface feet and how it relates to rpm.


So if the formula for the recommended SFM states 1046 RPM stick to that speed or close to it?


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